Photoacoustic Imaging: Principles and Advances

**IV**

cooperation from the staff at IntechOpen: Ms. Klara Mestrovic, Commissioning Editor; Ms. Nina Kalinic Babic, Author Service Manager; and Mr. Ivan Butkovic,

> **Reda R. Gharieb** Assiut University,

> > Egypt

Author Service Manager.

**Chapter 1**

**Abstract**

Photoacoustic Imaging for

Cancer Diagnosis: A Breast

Photoacoustic (PA) imaging utilizes laser pulses to deliver energy to an examined object for the generation of ultrasonic waves. Thus, it provides a noninvasive and nonionizing imaging modality. Therefore, it has found clinical use for cancer diagnosis in different organs, e.g., breast, prostate, and thyroid nodules. It offers morphological, functional, and molecular imaging. Moreover, the oxygen saturation in a body can be computed by calculating the wavelength-dependent light absorption coefficients at two different wavelengths. In this chapter, the

**Keywords:** photoacoustic imaging, backprojection, breast tumor, ARC-shaped

Photoacoustic (PA) imaging is an emerging diagnostic modality that gets advantage of the optically induced ultrasonic signals in tissues [1–5]. PA imaging in cancer diagnosis relies on the enhanced optical absorption of tumors and the relatively high optical transparency of normal tissues in addition to low acoustic distortion and attenuation of tissues [6]. Cancer cells gradually develop a dense microvascular network, which appears to be a marker that a tumor is aggressively growing and subject to metastasis. Furthermore, not only the amount of blood is substantially higher in malignant tumors compared to normal tissue, but also the blood in malignant tumors tends to be less oxygenated blood than the one in normal

tissue. Those abnormalities of blood amount and plus being less oxygenated increase the optical absorption of the cancer tissue in the near-infrared (NIR)

PA imaging has been combined with ultrasound imaging and augmented with molecular targeted contrast agents [7, 8]. This makes this hybrid modality capable of imaging cancer at the cellular and molecular level, thus opening diverse opportunities to improve diagnosis of tumors, detect circulating tumor cells, and identify

PA imaging consists of shining the object of interest, e.g., prostate/breast by a laser pulse; the light energy absorbed by the object tissues causes a thermal induced mechanical vibration within the tissues. The intensity of the mechanical vibration is

principle of the PA imaging is introduced for the present book.

Tumor Example

*Reda R. Gharieb*

ultrasonic detectors

**1. Introduction**

region.

**3**

metastatic lymph nodes.

## **Chapter 1**

## Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example

*Reda R. Gharieb*

## **Abstract**

Photoacoustic (PA) imaging utilizes laser pulses to deliver energy to an examined object for the generation of ultrasonic waves. Thus, it provides a noninvasive and nonionizing imaging modality. Therefore, it has found clinical use for cancer diagnosis in different organs, e.g., breast, prostate, and thyroid nodules. It offers morphological, functional, and molecular imaging. Moreover, the oxygen saturation in a body can be computed by calculating the wavelength-dependent light absorption coefficients at two different wavelengths. In this chapter, the principle of the PA imaging is introduced for the present book.

**Keywords:** photoacoustic imaging, backprojection, breast tumor, ARC-shaped ultrasonic detectors

## **1. Introduction**

Photoacoustic (PA) imaging is an emerging diagnostic modality that gets advantage of the optically induced ultrasonic signals in tissues [1–5]. PA imaging in cancer diagnosis relies on the enhanced optical absorption of tumors and the relatively high optical transparency of normal tissues in addition to low acoustic distortion and attenuation of tissues [6]. Cancer cells gradually develop a dense microvascular network, which appears to be a marker that a tumor is aggressively growing and subject to metastasis. Furthermore, not only the amount of blood is substantially higher in malignant tumors compared to normal tissue, but also the blood in malignant tumors tends to be less oxygenated blood than the one in normal tissue. Those abnormalities of blood amount and plus being less oxygenated increase the optical absorption of the cancer tissue in the near-infrared (NIR) region.

PA imaging has been combined with ultrasound imaging and augmented with molecular targeted contrast agents [7, 8]. This makes this hybrid modality capable of imaging cancer at the cellular and molecular level, thus opening diverse opportunities to improve diagnosis of tumors, detect circulating tumor cells, and identify metastatic lymph nodes.

PA imaging consists of shining the object of interest, e.g., prostate/breast by a laser pulse; the light energy absorbed by the object tissues causes a thermal induced mechanical vibration within the tissues. The intensity of the mechanical vibration is proportional to the light absorption coefficient of different region within the tissues. The mechanical vibration generates ultrasonic waves that propagate outward the object [1, 6, 9]. Theses ultrasonic signals can be recorded at different locations on the object surface and are used to construct an image for the absorption coefficient of the tissue region within the object. This distribution of the light absorption coefficient within the object demonstrates the tumor location and shape. The image construction, which provides an inverse problem solution, can be done analytically or by computed tomography through different backprojection methods [10].

**3. Advantages and disadvantages of PA imaging**

*DOI: http://dx.doi.org/10.5772/intechopen.92084*

*Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example*

spatial resolution than ultrasonic imaging; and

1.depth of penetration and distribution of light are issues.

radius *r*<sup>0</sup> in the 2D space due to a laser pulse is given by [13]

tromagnetic absorption property of the medium at *r*, given *p r*ð Þ 0, *t :*

*p r*ð Þ¼ 0, *t*, *φ*

the ultrasonic signals and the final image.

**4. Ultrasound signals and backprojection**

1.nonionizing imaging since it relies on just NIR light (optical radiation) and

2.high contrast and good spatial resolution. The high contrast is due to the use of the optical contrast absorption and the good spatial resolution is due to the use of ultrasonic signals resolution. Thus, it is a single hybrid imaging modality;

3.provides better depth of penetration than different optical imaging and better

4.nanoparticles can be used to enhance the tissue heating process and therefore

As illustrated by **Figure 2**, the ultrasonic pressure wave received at a circle of

*∂ ∂t*

where *vs* is the speed of the acoustic waves; *B* is the coefficient of volumetric thermal expansion; *C* is the specific heat capacity; *I*<sup>0</sup> is a scaling factor proportional to the incident radiation intensity; and *A r*ð Þ describes the to-be-reconstructed elec-

*The acoustic pressure wave p r*ð Þ 0, *t detected on an ARC of radius r*<sup>0</sup> *and angle φ due to the electromagnetic*

<sup>∮</sup> j j¼ *<sup>r</sup>*0�*<sup>r</sup> vst*

*A r*ð Þ ∣*r*<sup>0</sup> � *r*∣

*dr* (1)

*vsBI*<sup>0</sup> 4*πC*

Advantages include:

Disadvantages include:

**Figure 2.**

**5**

*absorption A r*ð Þ*:*

ultrasound;

In the next subsections, a review of a PA imaging system is presented. The PA imaging for the breast cancer detection is used as an example. The review highlights the array of the ultrasonic detectors, the received signals, and the formation of the image using a weight and sum backprojection algorithm.

#### **2. General PA imaging system**

**Figure 1** shows a block diagram of the photoacoustic imaging principle. As shown, laser pulse is used to deliver optical energy to an absorptive object, e.g., breast/prostate tissues, whole body of a small animal. The tissues absorb some energy from the laser light and get heated up. After heating, the tissues expand or vibrate in order to release the absorbed heat energy. This sudden and fast expansion of tissues in turn causes sound waves to be generated and they are in the frequency range of Mega Hertz (Ultrasound). These ultrasonic waves are recorded using a set of piezoelectric detectors. The time taken to receive ultrasound after the laser pulse has been given is used to find the location of the tissues which vibrate. Thus, the ultrasonic signals can be handled so that to provide a morphological image in twoor three-dimensional for the absorptive object. The image formation is accomplished utilizing different backprojection methods [10]. These methods take into account the light distribution, the geometry of the ultrasonic detectors array, and the directivity pattern of each detector. In [11], the recent advancement of lightemitting diode (LED) technology has shown to provide a less expensive and more safety source of light to replace the class-IV laser systems. Recently, integrating a microrobotic system and PA imaging has enabled deep imaging and precise control of the micromotors in vivo [12].

Near-infrared (NIR) light (wavelength range of 700–2500 nanometers) is used as a laser pulse to deliver energy to the tissues. This is because of the fact that NIR can penetrate skin since the skin absorbs very less light at longer wavelengths. However, as NIR skips past the skin to deeper tissues, water and other components in deeper tissues absorb NIR and therefore get heated up and release the ultrasonic waves.

**Figure 1.** *A general block diagram of PA imaging principle.*

*Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example DOI: http://dx.doi.org/10.5772/intechopen.92084*

### **3. Advantages and disadvantages of PA imaging**

Advantages include:

proportional to the light absorption coefficient of different region within the tissues. The mechanical vibration generates ultrasonic waves that propagate outward the object [1, 6, 9]. Theses ultrasonic signals can be recorded at different locations on the object surface and are used to construct an image for the absorption coefficient of the tissue region within the object. This distribution of the light absorption coefficient within the object demonstrates the tumor location and shape. The image construction, which provides an inverse problem solution, can be done analytically or by computed tomography through different backprojection

In the next subsections, a review of a PA imaging system is presented. The PA imaging for the breast cancer detection is used as an example. The review highlights the array of the ultrasonic detectors, the received signals, and the formation of the

**Figure 1** shows a block diagram of the photoacoustic imaging principle. As shown, laser pulse is used to deliver optical energy to an absorptive object, e.g., breast/prostate tissues, whole body of a small animal. The tissues absorb some energy from the laser light and get heated up. After heating, the tissues expand or vibrate in order to release the absorbed heat energy. This sudden and fast expansion of tissues in turn causes sound waves to be generated and they are in the frequency range of Mega Hertz (Ultrasound). These ultrasonic waves are recorded using a set of piezoelectric detectors. The time taken to receive ultrasound after the laser pulse has been given is used to find the location of the tissues which vibrate. Thus, the ultrasonic signals can be handled so that to provide a morphological image in twoor three-dimensional for the absorptive object. The image formation is accomplished utilizing different backprojection methods [10]. These methods take into account the light distribution, the geometry of the ultrasonic detectors array, and the directivity pattern of each detector. In [11], the recent advancement of lightemitting diode (LED) technology has shown to provide a less expensive and more safety source of light to replace the class-IV laser systems. Recently, integrating a microrobotic system and PA imaging has enabled deep imaging and precise

Near-infrared (NIR) light (wavelength range of 700–2500 nanometers) is used as a laser pulse to deliver energy to the tissues. This is because of the fact that NIR can penetrate skin since the skin absorbs very less light at longer wavelengths. However, as NIR skips past the skin to deeper tissues, water and other components

in deeper tissues absorb NIR and therefore get heated up and release the

image using a weight and sum backprojection algorithm.

*Photoacoustic Imaging - Principles, Advances and Applications*

**2. General PA imaging system**

control of the micromotors in vivo [12].

*A general block diagram of PA imaging principle.*

ultrasonic waves.

**Figure 1.**

**4**

methods [10].


Disadvantages include:

1.depth of penetration and distribution of light are issues.

## **4. Ultrasound signals and backprojection**

As illustrated by **Figure 2**, the ultrasonic pressure wave received at a circle of radius *r*<sup>0</sup> in the 2D space due to a laser pulse is given by [13]

$$p(r\_0, t, \rho) = \frac{v\_\imath B I\_0}{4\pi \mathcal{C}} \frac{\partial}{\partial t} \oint\_{|r\_0 - r| = v\_\imath t} \frac{A(r)}{|r\_0 - r|} dr \tag{1}$$

where *vs* is the speed of the acoustic waves; *B* is the coefficient of volumetric thermal expansion; *C* is the specific heat capacity; *I*<sup>0</sup> is a scaling factor proportional to the incident radiation intensity; and *A r*ð Þ describes the to-be-reconstructed electromagnetic absorption property of the medium at *r*, given *p r*ð Þ 0, *t :*

#### **Figure 2.**

*The acoustic pressure wave p r*ð Þ 0, *t detected on an ARC of radius r*<sup>0</sup> *and angle φ due to the electromagnetic absorption A r*ð Þ*:*

For 2D image formation using the recorded acoustic pressure *p r*ð Þ 0, *t* , the approximate inverse solution is given by [14]

$$A(r) = -\frac{r\_0^2 C}{4\pi B I\_0 v\_s^4} \int\_{\rho} \frac{1}{t} \frac{\partial p(r\_0, t)}{\partial t} \Big|\_{t=|r\_0 - r| / v\_s} d\rho \tag{2}$$

We can also restrict the projected location ð Þ *<sup>x</sup>*, *<sup>y</sup>* for only ð Þ *<sup>x</sup>*, *<sup>y</sup>* : j j *<sup>θ</sup>k*ð Þ *<sup>x</sup>*, *<sup>y</sup>* <sup>≤</sup> *<sup>π</sup>*

*Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example*

*G x*ð Þ¼ , *<sup>y</sup>* <sup>X</sup>

sensors, that is

**Figure 5.**

**7**

**Figure 4.**

**5. PA signal and image processing**

*DOI: http://dx.doi.org/10.5772/intechopen.92084*

the *M* sensors, the back projection is done by the summation of (4) over the *M*

*M*

*k*¼1

In PA imaging, ultrasonic signals are received in noise; thus, small objects become buried in noise. Hence, prior to image formulation, different signal processing techniques can be used to remove the noise, enhancing the signal-tonoise ratio. Filtering in addition to principal component analysis (PCA) are widely used techniques [15–24]. Also, multiresolution analysis, utilizing wavelet analysis and subband decomposition, of PA signals can be used to obtain images at different

scales [17, 20–24]. Furthermore, post-image formulation; image processing methods can be used to enhance the visualization of the image. These methods include contrast enhancement, edge detection, segmentation, and pattern

*Image formation by the back projection algorithm in (4) and (5). It is obvious that the tumor is extracted. Notice that the problem here is simplified, no noise is assumed, the light distribution is assumed uniform within*

*Signals received at sensors number 1 (most top-left side), 32 and 64 (most top-right side) in the system geometry of Figure 3. The simulated tumor is a uniform circular disk of radius 0.25 cm centered at* ð Þ �2, �2 *. The set of*

*the overall examined object, which is not practically a simple task.*

*the signals are used for the construction of the image in Figure 5.*

2

� �. For

*Gk*ð Þ *x*, *y* (5)

This implies that first, compute time-derivative of the pressure wave, divide the resultant by *t*, and finally, integrate over the ARC angle. Notice that *r* is determined by the time the wave taken to travel from the point *r* to *r*0*:*

Thus, practically to detect the ultrasonic signals released form the absorptive object excited by the laser pulse, a set of the piezoelectric sensors are used. These sensors are arranged in an array of a certain geometry either linear, surface, curvature, etc. The number of sensors, the geometry of the array, and the directivity of each sensor, the ability of the sensor to receive a signal from only a certain direction, are important factors for the image quality. Each sensor is followed by a lownoise amplifier to amplify the detected signal. **Figure 3** shows the geometry of an ARC-shaped array of 64 sensors that has been adopted for breast cancer imaging [6]. These sensors are uniformly distributed on an angle of 166°. All the sensors have the directivity, the maximum gain, to the center of the ARC, the point 0, 0 ð Þ. Thus, the recorded signal by the *k*th sensor in time-domain can be expressed by

$$w\_k(t), t = 0, T, \mathcal{Z}T, \mathcal{Y}T, \dots \tag{3}$$

where *T* is the sampling time.

Assuming uniform light distribution within the tissue, the back projection of the kth sensor signal is given by

$$\mathbf{G}\_{k}(\mathbf{x},\mathbf{y}) = \overline{v\_{k}} \begin{pmatrix} R\left(\mathbf{x} - \mathbf{x}\_{k,\cdot}\mathbf{y} - \mathbf{y}\_{k,\cdot}\right) \\ v\_{\mathbf{s}} \\ -4 < \mathbf{y} < \mathbf{0} \end{pmatrix} R\left(\mathbf{x} - \mathbf{x}\_{k,\cdot}\mathbf{y} - \mathbf{y}\_{k,\cdot}\right) \cos\left(\theta\_{k}(\mathbf{x},\mathbf{y})\right), \quad -7 < \mathbf{x} < 7, \mathbf{y} \tag{4}$$

where *vk* is the velocity potential computed by the time-integration of the received pressure signals; and *vs* ¼ 1540 m*=* sec is the speed of sound in tissues.

**Figure 3.** *ARC-shaped PA ultrasonic detecting array.*

We can also restrict the projected location ð Þ *<sup>x</sup>*, *<sup>y</sup>* for only ð Þ *<sup>x</sup>*, *<sup>y</sup>* : j j *<sup>θ</sup>k*ð Þ *<sup>x</sup>*, *<sup>y</sup>* <sup>≤</sup> *<sup>π</sup>* 2 � �. For the *M* sensors, the back projection is done by the summation of (4) over the *M* sensors, that is

$$\mathbf{G}(\mathbf{x}, \boldsymbol{\upchi}) = \sum\_{k=1}^{M} \mathbf{G}\_{k}(\mathbf{x}, \boldsymbol{\upchi}) \tag{5}$$

#### **5. PA signal and image processing**

In PA imaging, ultrasonic signals are received in noise; thus, small objects become buried in noise. Hence, prior to image formulation, different signal processing techniques can be used to remove the noise, enhancing the signal-tonoise ratio. Filtering in addition to principal component analysis (PCA) are widely used techniques [15–24]. Also, multiresolution analysis, utilizing wavelet analysis and subband decomposition, of PA signals can be used to obtain images at different scales [17, 20–24]. Furthermore, post-image formulation; image processing methods can be used to enhance the visualization of the image. These methods include contrast enhancement, edge detection, segmentation, and pattern

**Figure 4.**

For 2D image formation using the recorded acoustic pressure *p r*ð Þ 0, *t* , the

*<sup>∂</sup>p r*ð Þ 0, *<sup>t</sup> ∂t*

This implies that first, compute time-derivative of the pressure wave, divide the resultant by *t*, and finally, integrate over the ARC angle. Notice that *r* is determined

Thus, practically to detect the ultrasonic signals released form the absorptive object excited by the laser pulse, a set of the piezoelectric sensors are used. These sensors are arranged in an array of a certain geometry either linear, surface, curvature, etc. The number of sensors, the geometry of the array, and the directivity of each sensor, the ability of the sensor to receive a signal from only a certain direction, are important factors for the image quality. Each sensor is followed by a lownoise amplifier to amplify the detected signal. **Figure 3** shows the geometry of an ARC-shaped array of 64 sensors that has been adopted for breast cancer imaging [6]. These sensors are uniformly distributed on an angle of 166°. All the sensors have the directivity, the maximum gain, to the center of the ARC, the point 0, 0 ð Þ. Thus, the recorded signal by the *k*th sensor in time-domain can be expressed by

Assuming uniform light distribution within the tissue, the back projection of the

A*R x* � *xk*,, *y* � *yk*, � �

where *vk* is the velocity potential computed by the time-integration of the received pressure signals; and *vs* ¼ 1540 m*=* sec is the speed of sound in tissues.

1

� � � � *t*¼j j *r*0�*r =vs*

*vk*ð Þ*t* , *t* ¼ 0, *T*, 2*T*, 3*T*, … (3)

*dφ* (2)

cosð Þ *θk*ð Þ *x*, *y* , � 7 < *x*< 7,

(4)

0*C* 4*πBI*0*v*<sup>4</sup> *s* ð *φ* 1 *t*

approximate inverse solution is given by [14]

where *T* is the sampling time.

� 4<*y*<0

*R x* � *xk*,, *y* � *yk*, � �

*vs*

kth sensor signal is given by

0 @

*Gk*ð Þ¼ *x*, *y vk*

**Figure 3.**

**6**

*ARC-shaped PA ultrasonic detecting array.*

*A r*ð Þ¼� *<sup>r</sup>*<sup>2</sup>

*Photoacoustic Imaging - Principles, Advances and Applications*

by the time the wave taken to travel from the point *r* to *r*0*:*

*Signals received at sensors number 1 (most top-left side), 32 and 64 (most top-right side) in the system geometry of Figure 3. The simulated tumor is a uniform circular disk of radius 0.25 cm centered at* ð Þ �2, �2 *. The set of the signals are used for the construction of the image in Figure 5.*

#### **Figure 5.**

*Image formation by the back projection algorithm in (4) and (5). It is obvious that the tumor is extracted. Notice that the problem here is simplified, no noise is assumed, the light distribution is assumed uniform within the overall examined object, which is not practically a simple task.*

recognition [4]. Deep learning is expected to play a good role in extracting more features PA imaging. In [25], deep learning is used to construct PA image from sparse data (**Figure 4**).

**References**

[1] Oraevsky AA, Jacques SL, Tittel FK.

*DOI: http://dx.doi.org/10.5772/intechopen.92084*

*Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example*

Photons Plus Ultrasound: Imaging and

[9] Wang LV. Tutorial on photoacoustic microscopy and computed tomography. IEEE Journal of Selected Topics in Quantum Electronics. 2008;**14**(1):171-179

[10] Anastasio MA. Advancements in image reconstruction for photoacoustic computed tomography. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 108780,

[11] Hariri A et al. The characterization of an economic and portable LED-based photoacoustic system to facilitate molecular imaging. Photoacoustics.

[12] Wu Z et al. A microrobotic system guided by photoacoustic computed tomography for targeted navigation in intestines in vivo. Science robotics.

[13] Xu Y, Wang LV, Ambartsoumian G,

tomography. Medical Physics. 2004;**31**

[14] Xu M, Wang LV. Universal backprojection algorithm for photoacoustic computed tomography. Physical Review

[15] Wang Y, Xing D, Zeng Y, Chen Q. Photoacoustic imaging with deconvolution algorithm. Physics in Medicine and

[16] Manohar S, Dantuma M. Current and future trends in photoacoustic breast imaging. Photoacoustics. 2019;**16**

[17] Zeng L, Xing D, Gu H, Yang D, Yang S, Xiang L. High antinoise photoacoustic tomography based on a

E. 2005;**71**(1):016706-1:7

Biology. 2004;**49**:3117-3124

Kuchment P. Reconstructions in limited‐view thermoacoustic

Sensing. 2008

March 2019. 2019

2018;**9**:10-20

2019;**4**(32):1-11

(4):724-733

(100134):1-27

properties by piezoelectric detection of

[2] Shi J et al. High-resolution highcontrast mid-infrared imaging of fresh biological samples with ultravioletlocalized photoacoustic microscopy. Nature Photonics. 2019;**13**(9):609-615

[3] Lin L, Wang LV. Single-breath-hold photoacoustic computed tomography of the breast. Nature Communications.

[4] Wang B, Su JL, Amirian J, Litovsky SH, Smalling R, Emelianov S. Detection of lipid in atherosclerotic vessels using ultrasound-guided spectroscopic intravascular photoacoustic imaging. Optics Express. 2010;**18**(5):4889-4897

[5] Mallidi S, Luke GP, Emelianov S. Photoacoustic imaging in cancer detection, diagnosis, and treatment guidance. Trends in Biotechnology.

[6] Ermilov S et al. Detection and noninvasive diagnostics of breast cancer with 2-color laser optoacoustic imaging system. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 6437. 2007. pp. 1-11

[7] Mehrmohammadi M, Yoon SJ, Yeager D, Emelianov SY. Photoacoustic imaging for cancer detection and staging. Current Molecular Imaging.

[8] Yaseen MA, et al. Hybrid

optoacoustic and ultrasonic imaging system for detection of prostate malignancies. In: Proceedings of SPIE.

Determination of tissue optical

laser-induced stress waves. In: Proceedings of SPIE (The Society of Photo-Optical Instrumentation Engineers). Photons Plus Ultrasound: Imaging and Sensing, Vol. 1882, January

1993. 1993. pp. 86-101

2018;**9**(2352):1-9

2011;**29**(5):213-221

2013;**2**(1):89-105

**9**

## **6. Conclusion**

Simply saying, PA imaging utilizes the ultrasonic signal sent by the examined object to formulate an image for the distribution of the NIR light absorption coefficient within this object. This distribution differentiates abnormal tissues (malignant) from the normal one. Utilizing the absorption coefficients due to two different wavelengths provides information about the oxygen saturation. Hybrid modalities by combing PA image and pure ultrasound image offer imaging at cellular and molecular level. The challenges associated with the PA imaging modality are to deliver the examined object with a uniform distributed light independent of the depth and shape of this object and to reconstruct an online three-dimensional image for image-guided biomedical applications.

## **Author details**

Reda R. Gharieb Faculty of Engineering, Assiut University, Egypt

\*Address all correspondence to: rrgharieb@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Photoacoustic Imaging for Cancer Diagnosis: A Breast Tumor Example DOI: http://dx.doi.org/10.5772/intechopen.92084*

## **References**

recognition [4]. Deep learning is expected to play a good role in extracting more features PA imaging. In [25], deep learning is used to construct PA image from

Simply saying, PA imaging utilizes the ultrasonic signal sent by the examined object to formulate an image for the distribution of the NIR light absorption coefficient within this object. This distribution differentiates abnormal tissues (malignant) from the normal one. Utilizing the absorption coefficients due to two different wavelengths provides information about the oxygen saturation. Hybrid modalities by combing PA image and pure ultrasound image offer imaging at cellular and molecular level. The challenges associated with the PA imaging modality are to deliver the examined object with a uniform distributed light independent of the depth and shape of this object and to reconstruct an online three-dimensional

sparse data (**Figure 4**).

image for image-guided biomedical applications.

*Photoacoustic Imaging - Principles, Advances and Applications*

Faculty of Engineering, Assiut University, Egypt

provided the original work is properly cited.

\*Address all correspondence to: rrgharieb@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

**6. Conclusion**

**Author details**

Reda R. Gharieb

**8**

[1] Oraevsky AA, Jacques SL, Tittel FK. Determination of tissue optical properties by piezoelectric detection of laser-induced stress waves. In: Proceedings of SPIE (The Society of Photo-Optical Instrumentation Engineers). Photons Plus Ultrasound: Imaging and Sensing, Vol. 1882, January 1993. 1993. pp. 86-101

[2] Shi J et al. High-resolution highcontrast mid-infrared imaging of fresh biological samples with ultravioletlocalized photoacoustic microscopy. Nature Photonics. 2019;**13**(9):609-615

[3] Lin L, Wang LV. Single-breath-hold photoacoustic computed tomography of the breast. Nature Communications. 2018;**9**(2352):1-9

[4] Wang B, Su JL, Amirian J, Litovsky SH, Smalling R, Emelianov S. Detection of lipid in atherosclerotic vessels using ultrasound-guided spectroscopic intravascular photoacoustic imaging. Optics Express. 2010;**18**(5):4889-4897

[5] Mallidi S, Luke GP, Emelianov S. Photoacoustic imaging in cancer detection, diagnosis, and treatment guidance. Trends in Biotechnology. 2011;**29**(5):213-221

[6] Ermilov S et al. Detection and noninvasive diagnostics of breast cancer with 2-color laser optoacoustic imaging system. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 6437. 2007. pp. 1-11

[7] Mehrmohammadi M, Yoon SJ, Yeager D, Emelianov SY. Photoacoustic imaging for cancer detection and staging. Current Molecular Imaging. 2013;**2**(1):89-105

[8] Yaseen MA, et al. Hybrid optoacoustic and ultrasonic imaging system for detection of prostate malignancies. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing. 2008

[9] Wang LV. Tutorial on photoacoustic microscopy and computed tomography. IEEE Journal of Selected Topics in Quantum Electronics. 2008;**14**(1):171-179

[10] Anastasio MA. Advancements in image reconstruction for photoacoustic computed tomography. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 108780, March 2019. 2019

[11] Hariri A et al. The characterization of an economic and portable LED-based photoacoustic system to facilitate molecular imaging. Photoacoustics. 2018;**9**:10-20

[12] Wu Z et al. A microrobotic system guided by photoacoustic computed tomography for targeted navigation in intestines in vivo. Science robotics. 2019;**4**(32):1-11

[13] Xu Y, Wang LV, Ambartsoumian G, Kuchment P. Reconstructions in limited‐view thermoacoustic tomography. Medical Physics. 2004;**31** (4):724-733

[14] Xu M, Wang LV. Universal backprojection algorithm for photoacoustic computed tomography. Physical Review E. 2005;**71**(1):016706-1:7

[15] Wang Y, Xing D, Zeng Y, Chen Q. Photoacoustic imaging with deconvolution algorithm. Physics in Medicine and Biology. 2004;**49**:3117-3124

[16] Manohar S, Dantuma M. Current and future trends in photoacoustic breast imaging. Photoacoustics. 2019;**16** (100134):1-27

[17] Zeng L, Xing D, Gu H, Yang D, Yang S, Xiang L. High antinoise photoacoustic tomography based on a

**Chapter 2**

**Abstract**

tomography (PAT)

**1. Introduction**

atmospheric pollutants [2].

**11**

**1.1 Photophone and the spectrophone**

*Surya Narayan Thakur*

tissue and internal organs in small animals.

Photoacoustic Spectroscopy of

Gaseous and Condensed Matter

A brief historical account of photoacoustic (PA) effect is followed by a simple mathematical model for the generation of PA signals due to nonradiative transitions in atoms and molecules. Some experimental setups, with microphone and piezoelectric transducers, are described for recording PA spectra of gaseous, solid, and liquid samples. Applications of PA spectroscopy in the investigation of harmful chemicals are presented with illustrative examples. The principle of photoacoustic imaging (PAI) is discussed along with examples of molecular imaging of biological

**Keywords:** photoacoustic effect, nonradiative decay, microphone, piezoelectric transducer, photoacoustic cell, aerosols, explosives, actinides, hyperspectral imaging, molecular imaging, photoacoustic microscopy (PAM), photoacoustic

Photoacoustic imaging (PAI) is a novel method of obtaining spectral images of

Bell succeeded in wireless audio communication about two decades before the radio transmission. He used the newly discovered selenium cell in the receiver in view of selenium's property to react to modulated intensity of sunlight incident on it, as the resistance of selenium crystal depends on the incident light. A flexible mirror was attached at the speaking end of the photophone that created slight

chemical constituents of a sample or a scene, to gain valuable insight into its structure and dynamics. It is based on the technique of photoacoustic spectroscopy (PAS) and covers the entire spectral range from the ultraviolet to the infrared. When light is incident on a sample, photons can be either absorbed, transmitted, or reflected, and the PAS technology focuses on the amount of absorption and its subsequent release as heat. PAS is an extremely sensitive detection technique as it can detect molecular concentrations below the parts per billion (ppb) level. This technique emerged from the discovery of the photoacoustic effect by Graham Bell in 1880 during his attempt to transmit sound over a beam of sunlight [1]. However, it remained dormant for almost a century until the advent of tunable lasers in the 1970s and was successfully used by Kreuzer and Patel for the trace detection of

modified filtered backprojection algorithm with combination wavelet. Medical Physics. 2007;**34**(2):556-563

[18] Li PC, Wei CW, Sheu YL. Subband photoacoustic imaging for contrast improvement. Optics Express. 2008; **16**(25):20215-20226

[19] Gao F, Feng X, Zhang R, Liu S, Zheng Y. Adaptive photoacoustic sensing using matched filter. IEEE Sensors Letters. 2017;**1**(5):1-3

[20] Holan SH, Viator JA. Automated wavelet denoising of photoacoustic signals for circulating melanoma cell detection and burn image reconstruction. Physics in Medicine and Biology. 2008;**53**:N227-N236

[21] Zalev J, Kolios MC. Detecting abnormal vasculature from photoacoustic signals using waveletpacket features. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 7899. 2011. p. 78992M-1:15

[22] Zhou M, Xia H, Lan H, Duan T, Zhong H, Gao F. Wavelet de-noising method with adaptive threshold selection for photoacoustic tomography. In: Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, July 2018. 2018. pp. 4796-4799

[23] Zhou M et al. A noise reduction method for photoacoustic imaging in vivo based on EMD and conditional mutual information. IEEE Photonics Journal. 2019;**11**(1):1-10

[24] Frikel J, Haltmeier M. Efficient regularization with wavelet sparsity constraints in photoacoustic tomography. Inverse Problems. 2018; **34**(2):1-28

[25] Antholzer S, Haltmeier M, Schwab J. Deep learning for photoacoustic tomography from sparse data. Inverse Problems in Science and Engineering. 2019;**27**(7):987-1005

## **Chapter 2**

modified filtered backprojection algorithm with combination wavelet. Medical Physics. 2007;**34**(2):556-563

**16**(25):20215-20226

[18] Li PC, Wei CW, Sheu YL. Subband photoacoustic imaging for contrast improvement. Optics Express. 2008;

*Photoacoustic Imaging - Principles, Advances and Applications*

[19] Gao F, Feng X, Zhang R, Liu S, Zheng Y. Adaptive photoacoustic sensing using matched filter. IEEE Sensors Letters. 2017;**1**(5):1-3

[20] Holan SH, Viator JA. Automated wavelet denoising of photoacoustic signals for circulating melanoma cell

reconstruction. Physics in Medicine and

detection and burn image

Biology. 2008;**53**:N227-N236

[21] Zalev J, Kolios MC. Detecting abnormal vasculature from

photoacoustic signals using waveletpacket features. In: Proceedings of SPIE. Photons Plus Ultrasound: Imaging and Sensing, Vol. 7899. 2011. p. 78992M-1:15

[22] Zhou M, Xia H, Lan H, Duan T, Zhong H, Gao F. Wavelet de-noising method with adaptive threshold

In: Proceedings of the Annual

Journal. 2019;**11**(1):1-10

constraints in photoacoustic

**34**(2):1-28

**10**

selection for photoacoustic tomography.

International Conference of the IEEE Engineering in Medicine and Biology Society, July 2018. 2018. pp. 4796-4799

[23] Zhou M et al. A noise reduction method for photoacoustic imaging in vivo based on EMD and conditional mutual information. IEEE Photonics

[24] Frikel J, Haltmeier M. Efficient regularization with wavelet sparsity

tomography. Inverse Problems. 2018;

[25] Antholzer S, Haltmeier M, Schwab J.

Deep learning for photoacoustic tomography from sparse data. Inverse Problems in Science and Engineering.

2019;**27**(7):987-1005

## Photoacoustic Spectroscopy of Gaseous and Condensed Matter

*Surya Narayan Thakur*

## **Abstract**

A brief historical account of photoacoustic (PA) effect is followed by a simple mathematical model for the generation of PA signals due to nonradiative transitions in atoms and molecules. Some experimental setups, with microphone and piezoelectric transducers, are described for recording PA spectra of gaseous, solid, and liquid samples. Applications of PA spectroscopy in the investigation of harmful chemicals are presented with illustrative examples. The principle of photoacoustic imaging (PAI) is discussed along with examples of molecular imaging of biological tissue and internal organs in small animals.

**Keywords:** photoacoustic effect, nonradiative decay, microphone, piezoelectric transducer, photoacoustic cell, aerosols, explosives, actinides, hyperspectral imaging, molecular imaging, photoacoustic microscopy (PAM), photoacoustic tomography (PAT)

## **1. Introduction**

Photoacoustic imaging (PAI) is a novel method of obtaining spectral images of chemical constituents of a sample or a scene, to gain valuable insight into its structure and dynamics. It is based on the technique of photoacoustic spectroscopy (PAS) and covers the entire spectral range from the ultraviolet to the infrared. When light is incident on a sample, photons can be either absorbed, transmitted, or reflected, and the PAS technology focuses on the amount of absorption and its subsequent release as heat. PAS is an extremely sensitive detection technique as it can detect molecular concentrations below the parts per billion (ppb) level. This technique emerged from the discovery of the photoacoustic effect by Graham Bell in 1880 during his attempt to transmit sound over a beam of sunlight [1]. However, it remained dormant for almost a century until the advent of tunable lasers in the 1970s and was successfully used by Kreuzer and Patel for the trace detection of atmospheric pollutants [2].

#### **1.1 Photophone and the spectrophone**

Bell succeeded in wireless audio communication about two decades before the radio transmission. He used the newly discovered selenium cell in the receiver in view of selenium's property to react to modulated intensity of sunlight incident on it, as the resistance of selenium crystal depends on the incident light. A flexible mirror was attached at the speaking end of the photophone that created slight

deviation of the beam of light reaching the receiver end. This led to variation of intensity at the selenium receiver, which acted like an optical version of the electric coil in the telephone receiver, converting the intensity modulated light back into sound. Bell performed many experiments and observed that sound waves were also produced directly from a solid sample when exposed to a periodically modulated beam of sunlight as illustrated in **Figure 1**. A hearing tube, whose other end was tightly attached to the open end of a transparent glass test tube with sample placed at its closed end, could be used as a photophone. When a beam of sunlight focused on the sample was rapidly interrupted with a rotating slotted wheel at an audible frequency, the intensity of sound in the hearing tube was dependent on the type of material. The loudest sound was heard when the sample was carbon black, leading to the conclusion that photoacoustic effect was caused by the absorbed light energy which subsequently heats the sample.

temperature. (3) The expansion and contraction following these temperature

The heat generation following optical absorption is caused by internal motions in molecules or those of the matrix in which atoms are imbedded in condensed matter. In the quantum mechanical description, an excited molecular state reached by the optical absorption has two channels of relaxation. The radiative decay leads to optical emission, whereas the nonradiative decay causes heating. Thus a molecule, optically excited to a vibronic or a rovibrational state, loses a part of its excitation energy as heat leading to the photoacoustic signal. The photoacoustic spectrum is similar to the absorption spectrum, but its intensity at the exciting wavelength is proportional to the product of the absorption coefficient and the probability of

**Figure 3** shows the energy level diagram, in the Born-Oppenheimer approximation, of a typical organic molecule with an even number of electrons where total internal energy E ¼ Ee þ Ev þ Er. The rotational energy levels are not shown for the sake of simplicity, and only one of the 3N-6 (or 3N-5) normal modes for a nonlinear

*Typical energy level diagram of an organic molecule where radiative processes A, F, and P stand for absorption, fluorescence, and phosphorescence, respectively. The nonradiative processes IC and ISC refer to internal*

changes lead to periodic pressure variation to generate sound.

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

**2. Nonradiative transitions and PA signal generation**

nonradiative decay of the excited state.

*conversion (IC) and intersystem crossing (ISC), respectively.*

*Alexander Graham Bell and his spectrophone.*

**Figure 2.**

**Figure 3.**

**13**

During Bell's visit to England in 1880, John Tyndall performed the photoacoustic experiment in gases, and although the photoacoustic effect was confirmed, Tyndall was of the view that it was caused mainly by the radiant heat [3]. Bell was driven by rare intellectual curiosity to learn, and it led him to invent the spectrophone to find out the wavelengths that were more efficient for the radiant heat [4]. For this purpose he converted a prism spectroscope into a spectrophone by replacing the eyepiece of the telescope with a hearing tube in which a thin wire mesh coated with lampblack was fitted in the position of the cross wires (see **Figure 2**). When the incident sunlight was interrupted by a mechanical chopper, the hearing tube produced sound whose frequency was equal to the periodic intermittence of light. The loudness of sound, however, varied in accordance with the intensity of the solar spectrum, being maximum in the green-yellow region and decreasing at both the red and violet ends, and observations were made by fixing the position of the telescope in different spectral regions of the solar spectrum. These observations confirmed the fact that the photoacoustic effect is due to optical absorption, since lampblack totally absorbs light at each wavelength. On the basis of his observations, Bell made the prophetic statement about the great importance of photoacoustic spectroscopy in the infrared.

The interaction between light and matter giving rise to photoacoustic effect has three distinct features. (1) The absorbed energy of optical radiation is converted into heat. (2) At the site of optical absorption, there is a temporal rise and fall of

#### **Figure 1.**

*Illustration of photoacoustic effect with periodically chopped light incident on carbon black sample cell fitted with a hearing tube.*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

**Figure 2.** *Alexander Graham Bell and his spectrophone.*

deviation of the beam of light reaching the receiver end. This led to variation of intensity at the selenium receiver, which acted like an optical version of the electric coil in the telephone receiver, converting the intensity modulated light back into sound. Bell performed many experiments and observed that sound waves were also produced directly from a solid sample when exposed to a periodically modulated beam of sunlight as illustrated in **Figure 1**. A hearing tube, whose other end was tightly attached to the open end of a transparent glass test tube with sample placed at its closed end, could be used as a photophone. When a beam of sunlight focused on the sample was rapidly interrupted with a rotating slotted wheel at an audible frequency, the intensity of sound in the hearing tube was dependent on the type of material. The loudest sound was heard when the sample was carbon black, leading to the conclusion that photoacoustic effect was caused by the absorbed light energy

*Photoacoustic Imaging - Principles, Advances and Applications*

During Bell's visit to England in 1880, John Tyndall performed the photoacoustic experiment in gases, and although the photoacoustic effect was confirmed, Tyndall was of the view that it was caused mainly by the radiant heat [3]. Bell was driven by rare intellectual curiosity to learn, and it led him to invent the spectrophone to find out the wavelengths that were more efficient for the radiant heat [4]. For this purpose he converted a prism spectroscope into a spectrophone by replacing the eyepiece of the telescope with a hearing tube in which a thin wire mesh coated with lampblack was fitted in the position of the cross wires (see **Figure 2**). When the incident sunlight was interrupted by a mechanical chopper, the hearing tube produced sound whose frequency was equal to the periodic intermittence of light. The loudness of sound, however, varied in accordance with the intensity of the solar spectrum, being maximum in the green-yellow region and decreasing at both the red and violet ends, and observations were made by fixing the position of the telescope in different spectral regions of the solar spectrum. These observations confirmed the fact that the photoacoustic effect is due to optical absorption, since lampblack totally absorbs light at each wavelength. On the basis of his observations, Bell made the prophetic statement about the great importance of photoacoustic

The interaction between light and matter giving rise to photoacoustic effect has three distinct features. (1) The absorbed energy of optical radiation is converted into heat. (2) At the site of optical absorption, there is a temporal rise and fall of

*Illustration of photoacoustic effect with periodically chopped light incident on carbon black sample cell fitted*

which subsequently heats the sample.

spectroscopy in the infrared.

**Figure 1.**

**12**

*with a hearing tube.*

temperature. (3) The expansion and contraction following these temperature changes lead to periodic pressure variation to generate sound.

## **2. Nonradiative transitions and PA signal generation**

The heat generation following optical absorption is caused by internal motions in molecules or those of the matrix in which atoms are imbedded in condensed matter. In the quantum mechanical description, an excited molecular state reached by the optical absorption has two channels of relaxation. The radiative decay leads to optical emission, whereas the nonradiative decay causes heating. Thus a molecule, optically excited to a vibronic or a rovibrational state, loses a part of its excitation energy as heat leading to the photoacoustic signal. The photoacoustic spectrum is similar to the absorption spectrum, but its intensity at the exciting wavelength is proportional to the product of the absorption coefficient and the probability of nonradiative decay of the excited state.

**Figure 3** shows the energy level diagram, in the Born-Oppenheimer approximation, of a typical organic molecule with an even number of electrons where total internal energy E ¼ Ee þ Ev þ Er. The rotational energy levels are not shown for the sake of simplicity, and only one of the 3N-6 (or 3N-5) normal modes for a nonlinear

#### **Figure 3.**

*Typical energy level diagram of an organic molecule where radiative processes A, F, and P stand for absorption, fluorescence, and phosphorescence, respectively. The nonradiative processes IC and ISC refer to internal conversion (IC) and intersystem crossing (ISC), respectively.*

(or linear) molecule with N atoms is depicted for the various electronic states. In a large molecule like benzene, nonradiative decay from excited states occurs even in the vapor phase. Michael Kasha pioneered the investigations on nonradiative transitions, and it was found that radiative emission occurs only from the lowest excited state of any multiplicity [5]. The concept of triplet state (T) was first given by Lewis and Kasha [6] to explain the origin of phosphorescence emission in organic or inorganic molecules. Internal conversion (IC) refers to the nonradiative transfer of a molecule from an excited vibronic state to a lower vibronic state of the same multiplicity, whereas intersystem crossing (ISC) corresponds to nonradiative transition between a singlet (S) and a triplet (T) state. Lewis and Kasha formulated a fundamental law concerning fluorescence and phosphorescence through intersystem crossing [6].

#### **2.1 Generation of photoacoustic signal in gases**

The rate of radiative transition **r**ij between two states of a molecule Ei and <sup>E</sup> <sup>j</sup>ð Þ <sup>&</sup>lt;Ei in terms of Einstein coefficient of stimulated Bij and spontaneous Aij emission is of the form

$$\mathbf{r}\_{\ddot{\mathbf{i}}} = \rho(\nu)\mathbf{B}\_{\dddot{\mathbf{i}}\dot{\mathbf{j}}} + \mathbf{A}\_{\dddot{\mathbf{j}}} \tag{1}$$

We define the radiative and nonradiative lifetimes to be τ*r* ¼ 1*=*A10 and τ*c* ¼ 1*=*c10, respectively, and the total lifetime τð Þ 1*=*τ ¼ 1*=*τ*r* þ 1*=*τ*c* , to get the following expression for the rate of change of excited state population:

> dn1*=*dt ¼ ρ νð ÞB01ð Þ� n0 � n1 ð Þ 1*=*τ*r* þ 1*=*τ*c* n1 ¼ ρ νð ÞB01ð Þ� n0 � n1 n1*=*τ

In a similar manner, we find the following expression for the rate of change of

In a photoacoustic experiment, we assume the incident light intensity I to vary

0 ¼ 2ρ νð ÞB01ðn0 � N þ n0Þ � 2 Nð Þ � n0 *=*τ ¼ 4ρ νð ÞB01n0 � 2Nðρ νð ÞB01 þ 1*=*τÞ þ 2n0*=*τ

The spectral radiant energy density is directly proportional to the intensity I of the light source, and we define a constant B ¼ ρ νð ÞB01*=*I so that Eq. (7) takes the

Mechanical chopping of the light source at a frequency ω can be expressed as the periodic "on" and "off" of the intensity by I ¼ I0cosωt for simplicity, and we get the

If the gas is very weakly absorbing, we assume that most of the molecules are in state E0 or n0>>n1. Hence from Eqs. (6) and (7), BI< <1*=*τ so that Eq. (9) takes the

For a molecular gas, its kinetic energy (K) has to be considered to get the total

dU*=*dt <sup>¼</sup> dn1*=*dt E1 <sup>þ</sup> dK*=*dt <sup>¼</sup> ½ � ρ νð ÞB01ð Þ� n0 � n1 ð Þ A10 <sup>þ</sup> c10 n1 E1 <sup>þ</sup> dK*=*dt

slowly so that we may consider the upper and lower state population density changes to be an adiabatic interchange. Under this approximation we can set the left-hand side of Eq. (5) to zero. Since the total molecular density for the two-level

dn0*=*dt ¼ ρ νð ÞB01ð Þþ n1 � n0 n1*=*τ (4)

dn1*=*dt � dn0*=*dt ¼ 2ρ νð ÞB01ð Þ� n0 � n1 2n1*=*τ (5)

Similarly n1 ¼ Nρ νð ÞB01*=*ð Þ 2ρ νð ÞB01 þ 1*=*τ (7)

n1 ¼ NBI*=*ð Þ 2BI þ 1*=*τ (8)

n1 ¼ τNBI0cosωt (10)

U ¼ n1E1 þ K (11)

n1 ¼ NBI0cosωt*=*ð Þ 2B10cosωt þ 1*=*τ (9)

ground state population density:

following form:

following form:

**15**

Hence from Eqs. (3) and (4), we get

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

system is N ¼ n1 þ n0, Eq. (5) takes the following form:

following expression for the excited state population density:

internal energy density U of the two-level system leading to

The rate of change of energy is given by

Hence n0 ¼ N½ � ρ νð ÞB01 þ 1*=*τ *=*ð Þ 2ρ νð ÞB01 þ 1*=*τ

(3)

(6)

(12)

where Bij ¼ Bji and ρ νð Þ is the spectral energy density at the frequency ν of the transition. For a two-level system of energy E0 and E1 shown in **Figure 4**, we have B01 ¼ B10 and A01 ¼ 0, since spontaneous emission from a state of lower energy to one of higher energies does not occur. If cij represents the probability of a nonradiative (collisional) transition between a pair of states, we assume the collisional excitation from E0 to E1 to be very small so that c01 ¼ 0.

Suppose the number density of molecules in the ground and excited states of **Figure 4** is n0 and n1, respectively. In view of the above description of the rate of change, the number density in the excited state by radiative and nonradiative transitions is given by

$$\begin{split} \mathbf{dn}\_1 / \mathbf{dt} &= (\mathbf{r}\_{01} + \mathbf{c}\_{01}) \mathbf{n}\_0 \cdot (\mathbf{r}\_{10} + \mathbf{c}\_{10}) \mathbf{n}\_1 = \rho(\nu) \mathbf{B}\_{01} \mathbf{n}\_0 - (\rho(\nu) \mathbf{B}\_{10} + \mathbf{A}\_{10} + \mathbf{c}\_{10}) \mathbf{n}\_1 \\ &= \rho(\nu) \mathbf{B}\_{01} (\mathbf{n}\_0 - \mathbf{n}\_1) - (\mathbf{A}\_{10} + \mathbf{c}\_{10}) \mathbf{n}\_1 \end{split} \tag{2}$$

#### **Figure 4.**

*Two-level molecular model showing the rate of radiative rij and nonradiative cij transitions between energy states E0 and E1.*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

(or linear) molecule with N atoms is depicted for the various electronic states. In a large molecule like benzene, nonradiative decay from excited states occurs even in the vapor phase. Michael Kasha pioneered the investigations on nonradiative transitions, and it was found that radiative emission occurs only from the lowest excited state of any multiplicity [5]. The concept of triplet state (T) was first given by Lewis and Kasha [6] to explain the origin of phosphorescence emission in organic or inorganic molecules. Internal conversion (IC) refers to the nonradiative transfer of a molecule from an excited vibronic state to a lower vibronic state of the same multiplicity, whereas intersystem crossing (ISC) corresponds to nonradiative transition between a singlet (S) and a triplet (T) state. Lewis and Kasha formulated a

fundamental law concerning fluorescence and phosphorescence through

The rate of radiative transition **r**ij between two states of a molecule Ei and

where Bij ¼ Bji and ρ νð Þ is the spectral energy density at the frequency ν of the transition. For a two-level system of energy E0 and E1 shown in **Figure 4**, we have B01 ¼ B10 and A01 ¼ 0, since spontaneous emission from a state of lower energy to

nonradiative (collisional) transition between a pair of states, we assume the colli-

Suppose the number density of molecules in the ground and excited states of **Figure 4** is n0 and n1, respectively. In view of the above description of the rate of change, the number density in the excited state by radiative and nonradiative

dn1*=*dt ¼ ð Þ r01 þ c01 n0� ð Þ r10 þ c10 n1 ¼ ρ νð ÞB01n0 � ð Þ ρ νð ÞB10 þ A10 þ c10 n1

one of higher energies does not occur. If cij represents the probability of a

sional excitation from E0 to E1 to be very small so that c01 ¼ 0.

¼ ρ νð ÞB01ð Þ� n0 � n1 ð Þ A10 þ c10 n1

*Two-level molecular model showing the rate of radiative rij*

and spontaneous Aij

**r**ij ¼ ρ νð ÞBij þ Aij (1)

*and nonradiative cij*

(2)

*transitions between energy*

**2.1 Generation of photoacoustic signal in gases**

*Photoacoustic Imaging - Principles, Advances and Applications*

E <sup>j</sup>ð Þ <Ei in terms of Einstein coefficient of stimulated Bij

intersystem crossing [6].

emission is of the form

transitions is given by

**Figure 4.**

**14**

*states E0 and E1.*

We define the radiative and nonradiative lifetimes to be τ*r* ¼ 1*=*A10 and τ*c* ¼ 1*=*c10, respectively, and the total lifetime τð Þ 1*=*τ ¼ 1*=*τ*r* þ 1*=*τ*c* , to get the following expression for the rate of change of excited state population:

$$\begin{split} \mathbf{dn}\_1 / \mathbf{dt} &= \rho(\nu) \mathbf{B}\_{01} (\mathbf{n}\_0 - \mathbf{n}\_1) - (\mathbf{1}/\pi r + \mathbf{1}/\pi c) \mathbf{n}\_1 \\ &= \rho(\nu) \mathbf{B}\_{01} (\mathbf{n}\_0 - \mathbf{n}\_1) - \mathbf{n}\_1 / \pi \end{split} \tag{3}$$

In a similar manner, we find the following expression for the rate of change of ground state population density:

$$\mathbf{dn}\_0/\mathbf{dt} = \rho(\nu)\mathbf{B}\_{01}(\mathbf{n}\_1 - \mathbf{n}\_0) + \mathbf{n}\_1/\mathbf{\tau} \tag{4}$$

Hence from Eqs. (3) and (4), we get

$$\mathbf{dn}\_1/\mathbf{dt} - \mathbf{dn}\_0/\mathbf{dt} = \mathbf{2}\rho(\nu)\mathbf{B}\_{01}(\mathbf{n}\_0 - \mathbf{n}\_1) - \mathbf{2}\mathbf{n}\_1/\mathbf{\tau} \tag{5}$$

In a photoacoustic experiment, we assume the incident light intensity I to vary slowly so that we may consider the upper and lower state population density changes to be an adiabatic interchange. Under this approximation we can set the left-hand side of Eq. (5) to zero. Since the total molecular density for the two-level system is N ¼ n1 þ n0, Eq. (5) takes the following form:

$$\begin{aligned} \mathbf{0} &= 2\rho(\nu)\mathbf{B}\_{01}(\mathbf{n}\_0 - \mathbf{N} + \mathbf{n}\_0) - 2(\mathbf{N} - \mathbf{n}\_0)/\tau = 4\rho(\nu)\mathbf{B}\_{01}\mathbf{n}\_0 - 2\mathbf{N}(\rho(\nu)\mathbf{B}\_{01} + \mathbf{1}/\tau) + 2\mathbf{n}\_0/\tau \\ \mathbf{H} &\mathbf{E} \mathbf{n} \mathbf{e} \qquad \mathbf{n}\_0 = \mathbf{N}[\rho(\nu)\mathbf{B}\_{01} + \mathbf{1}/\tau]/(2\rho(\nu)\mathbf{B}\_{01} + \mathbf{1}/\tau) \end{aligned}$$

$$(6)$$

$$\text{Similarly} \ n\_1 = \text{N}\rho(\nu)\text{B}\_{01}/(2\rho(\nu)\text{B}\_{01} + \text{1}/\pi) \tag{7}$$

The spectral radiant energy density is directly proportional to the intensity I of the light source, and we define a constant B ¼ ρ νð ÞB01*=*I so that Eq. (7) takes the following form:

$$\mathbf{n}\_1 = \mathbf{NBI}/(2\mathbf{BI} + \mathbf{1}/\pi) \tag{8}$$

Mechanical chopping of the light source at a frequency ω can be expressed as the periodic "on" and "off" of the intensity by I ¼ I0cosωt for simplicity, and we get the following expression for the excited state population density:

$$\mathbf{n}\_1 = \text{NBI}\_0 \mathbf{cos} \text{ot/} (2\mathbf{B} \mathbf{1}\_0 \mathbf{cos} \text{ot} + \mathbf{1}/\mathbf{\tau}) \tag{9}$$

If the gas is very weakly absorbing, we assume that most of the molecules are in state E0 or n0>>n1. Hence from Eqs. (6) and (7), BI< <1*=*τ so that Eq. (9) takes the following form:

n1 ¼ τNBI0cosωt (10)

For a molecular gas, its kinetic energy (K) has to be considered to get the total internal energy density U of the two-level system leading to

$$\mathbf{U} = \mathbf{n}\_1 \mathbf{E}\_1 + \mathbf{K} \tag{11}$$

The rate of change of energy is given by

$$\mathbf{d}\mathbf{U}/\mathbf{dt} = \left(\mathbf{d}\mathbf{n}\_1/\mathbf{d}\mathbf{t}\right)\mathbf{E}\_1 + \mathbf{d}\mathbf{K}/\mathbf{dt} = \left[\rho(\nu)\mathbf{B}\_{01}(\mathbf{n}\_0 - \mathbf{n}\_1) - (\mathbf{A}\_{10} + \mathbf{c}\_{10})\mathbf{n}\_1\right]\mathbf{E}\_1 + \mathbf{d}\mathbf{K}/\mathbf{dt} \tag{12}$$

This change of energy is equal to the difference between the absorbed and radiated optical energy so that

$$\begin{aligned} \mathbf{d} \mathbf{U}/\mathbf{dt} &= (\mathbf{r}\_{01}\mathbf{n}\_0 - \mathbf{r}\_{10}\mathbf{n}\_1)\mathbf{E}\_1 \\ &= [\rho(\nu)\mathbf{B}\_{01}(\mathbf{n}\_0 - \mathbf{n}\_1) - \mathbf{A}\_{10}\mathbf{n}\_1]\mathbf{E}\_1 \end{aligned} \tag{13}$$

We know that A01 ¼ 0 and c01 ¼ 0; hence from Eqs. (12) and (13), we get the following relation:

$$\mathbf{d} \mathbf{K} / \mathbf{d} \mathbf{t} = \mathbf{c}\_{10} \mathbf{n}\_1 \mathbf{E}\_1 \tag{14}$$

Since the volume of the photoacoustic cell does not change in the experiment, and thermodynamic evaluation of the change in kinetic energy at constant volume gives

$$\begin{split} \mathbf{dK} &= (\delta \mathbf{K} / \delta T)\_{\mathbf{V}} \mathbf{dT} + (\delta \mathbf{K} / \delta V)\_{\mathbf{T}} \mathbf{dV} \\ &= (\delta \mathbf{K} / \delta T)\_{\mathbf{V}} \mathbf{dT} \end{split} \tag{15}$$

Since the specific heat capacity of gas CV ¼ ð Þ δ*K=*δ*T* <sup>V</sup>, by integrating Eq. (15) we get

$$\mathbf{K} = \mathbf{C}\mathbf{V} + \mathbf{f}(\mathbf{V}) \tag{16}$$

transitions. The acoustic signal produced in the coupling gas is due to periodic heat flow from the sample. Rosencwaig-Gersho model for PA signal generation depends on the optical as well as thermal properties of the solid sample [7]. The periodic heat flow from the solid sample heats a thin layer of coupling gas whose thickness depends on the frequency of chopping of the incident light (about 1 mm at 100 Hz). This layer (shown in red in **Figure 5**) may be imagined as a vibratory gas piston that creates the acoustic signal detected by the microphone. The intensity of light of wavelength λ transmitted through thickness "x" of the solid sample of absorption

*Photoacoustic cell for a solid sample. Thermal waves originate from the point of absorption and travel toward*

*the solid–gas interface to periodically heat a thin layer of gas (shown in red).*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

where I0 is the incident intensity for wavelength λ, and if ω is the modulation

The temperature variation in the gas dies out within a thickness of 2πμ from the

frequency of the incident radiation, the temporal variation of I0 is given by

surface of the solid sample where μ is thermal diffusion length of the gas.

**3. Experimental methods in photoacoustic spectroscopy (PAS)**

temperature fluctuations in liquid and gaseous samples. In such cases,

photoacoustic spectroscopy is carried out by detection of thermal lens formation

The temperature and pressure changes involved in the process of PA signal generation are extremely small, typically a micro- to millidegree and nano- to microbar, respectively. This was the reason that the field of PAS remained dormant till the advent of tunable laser sources and of sensitive audio detectors. A piezoelectric transducer or a sensitive microphone serves as the acoustic detector in photoacoustic cells used during the laboratory experiments. Recent developments in the field of miniaturization and the related progress in computer software have made it possible to use tiny quartz-based acoustic detection devices like cantilever and crystal tuning forks. The photoacoustic spectrometer using miniature lasers fitted with these novel detectors can be easily packed in a small box and can be used in the laboratory or in the field for standoff detection of hazardous materials. The heat generated by the photoacoustic effect produces density changes caused by

*=*

I0ðÞ¼ t <sup>1</sup>

<sup>I</sup>ð Þ¼ <sup>λ</sup> I0 <sup>e</sup>�<sup>β</sup> <sup>x</sup> (21)

<sup>2</sup> ð ÞI0ð Þ 1 þ cosωt (22)

coefficient β is given by

**Figure 5.**

**17**

where f Vð Þ is a constant of integration.

The pressure of the ideal gas P ¼ NkBT, where N is the number density of molecules, kB is the Boltzmann constant, and T is the temperature. Thus substituting for T from Eq. (16) in the expression for P, we get

$$\mathbf{P} = \mathbf{N} \mathbf{k}\_{\mathbf{B}} [\mathbf{K} - \mathbf{f}(\mathbf{V})] / \mathbf{C}\_{\mathbf{V}} \tag{17}$$

Since f Vð Þ is constant and the pressure wave (sound) is given by δ*P=*δ*t*, we get the following relation from Eq. (17) in the light of Eq. (14):

$$\begin{split} \delta P/\delta t &= (\mathbf{N} \mathbf{k}\_{\mathbf{B}}/\mathbf{C}\_{\mathbf{V}}) \mathbf{d} \mathbf{K}/\mathbf{dt} \\ &= (\mathbf{N} \mathbf{k}\_{\mathbf{B}}/\mathbf{C}\_{\mathbf{V}}) \mathbf{c}\_{\mathbf{10}} \mathbf{n}\_{\mathbf{1}} \mathbf{E}\_{\mathbf{1}} \end{split} \tag{18}$$

Substituting for n1 from Eq. (10) and using the relation c10 ¼ 1*=*τ*c*, Eq. (18) reduces to

$$\text{\ $}P/\text{\$ }t = \left(\text{k}\_{\text{B}} \text{N}^{2} \text{E}\_{1}/\text{C}\_{\text{V}}\right) \left(\text{\textdegree/}\text{\textdegree c}\right) \text{BI}\_{0} \text{coso} \text{ot} \tag{19}$$

Integrating Eq. (19) we get the following expression for photoacoustic signal, which is detected by a sensitive microphone:

$$\mathbf{P(t)} = \left(\mathbf{k\_{B}N^{2}E\_{1}/C\_{V}}\right)\left(\mathbf{r/\pi}c\right)\mathbf{B}\mathbf{I}\_{0}\mathbf{sinot}\tag{20}$$

#### **2.2 Generation of photoacoustic signal in solids**

Let us consider a cylindrical photoacoustic cell shown in **Figure 5** where the light-absorbing solid sample is surrounded by optically transparent gas (air) on the front side and the backing material is a poor conductor of heat. The absorption of light, of a particular wavelength in the sample, generates heat by nonradiative

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

**Figure 5.**

This change of energy is equal to the difference between the absorbed and

¼ ½ � ρ νð ÞB01ð Þ� n0 � n1 A10n1 E1

dK*=*dt ¼ c10n1 E1 (14)

K ¼ CVT þ f Vð Þ (16)

P ¼ NkB½ � K � f Vð Þ *=*CV (17)

ð Þ <sup>τ</sup>*=*τ*<sup>c</sup>* BI0cosω<sup>t</sup> (19)

ð Þ <sup>τ</sup>*=*τ*<sup>c</sup>* BI0sinω<sup>t</sup> (20)

We know that A01 ¼ 0 and c01 ¼ 0; hence from Eqs. (12) and (13), we get the

Since the volume of the photoacoustic cell does not change in the experiment, and thermodynamic evaluation of the change in kinetic energy at constant volume

dK ¼ ð Þ *δK=δT* <sup>V</sup>dT þ ð Þ δ*K=*δ*V* <sup>T</sup>dV

Since the specific heat capacity of gas CV ¼ ð Þ δ*K=*δ*T* <sup>V</sup>, by integrating Eq. (15)

The pressure of the ideal gas P ¼ NkBT, where N is the number density of molecules, kB is the Boltzmann constant, and T is the temperature. Thus substitut-

Since f Vð Þ is constant and the pressure wave (sound) is given by δ*P=*δ*t*, we get

¼ ð Þ NkB*=*CV c10n1E1

δ*P=*δ*t* ¼ ð Þ NkB*=*CV dK*=*dt

Substituting for n1 from Eq. (10) and using the relation c10 ¼ 1*=*τ*c*, Eq. (18)

E1*=*CV

Integrating Eq. (19) we get the following expression for photoacoustic signal,

E1*=*CV

Let us consider a cylindrical photoacoustic cell shown in **Figure 5** where the light-absorbing solid sample is surrounded by optically transparent gas (air) on the front side and the backing material is a poor conductor of heat. The absorption of light, of a particular wavelength in the sample, generates heat by nonradiative

¼ ð Þ δ*K=*δ*T* <sup>V</sup>dT

where f Vð Þ is a constant of integration.

ing for T from Eq. (16) in the expression for P, we get

the following relation from Eq. (17) in the light of Eq. (14):

<sup>δ</sup>*P=*δ*<sup>t</sup>* <sup>¼</sup> kBN<sup>2</sup>

P tðÞ¼ kBN<sup>2</sup>

which is detected by a sensitive microphone:

**2.2 Generation of photoacoustic signal in solids**

(13)

(15)

(18)

dU*=*dt ¼ ð Þ r01n0 � r10n1 E1

*Photoacoustic Imaging - Principles, Advances and Applications*

radiated optical energy so that

following relation:

gives

we get

reduces to

**16**

*Photoacoustic cell for a solid sample. Thermal waves originate from the point of absorption and travel toward the solid–gas interface to periodically heat a thin layer of gas (shown in red).*

transitions. The acoustic signal produced in the coupling gas is due to periodic heat flow from the sample. Rosencwaig-Gersho model for PA signal generation depends on the optical as well as thermal properties of the solid sample [7]. The periodic heat flow from the solid sample heats a thin layer of coupling gas whose thickness depends on the frequency of chopping of the incident light (about 1 mm at 100 Hz). This layer (shown in red in **Figure 5**) may be imagined as a vibratory gas piston that creates the acoustic signal detected by the microphone. The intensity of light of wavelength λ transmitted through thickness "x" of the solid sample of absorption coefficient β is given by

$$\mathbf{I}(\lambda) = \mathbf{I}\_0 \mathbf{e}^{-\beta \mathbf{x}} \tag{21}$$

where I0 is the incident intensity for wavelength λ, and if ω is the modulation frequency of the incident radiation, the temporal variation of I0 is given by

$$\mathbf{I}\_{0}(\mathbf{t}) = (\mathbf{1}\prime\_{2})\,\mathbf{I}\_{0}(\mathbf{1}+\mathbf{cos}\mathbf{ot})\tag{22}$$

The temperature variation in the gas dies out within a thickness of 2πμ from the surface of the solid sample where μ is thermal diffusion length of the gas.

#### **3. Experimental methods in photoacoustic spectroscopy (PAS)**

The temperature and pressure changes involved in the process of PA signal generation are extremely small, typically a micro- to millidegree and nano- to microbar, respectively. This was the reason that the field of PAS remained dormant till the advent of tunable laser sources and of sensitive audio detectors. A piezoelectric transducer or a sensitive microphone serves as the acoustic detector in photoacoustic cells used during the laboratory experiments. Recent developments in the field of miniaturization and the related progress in computer software have made it possible to use tiny quartz-based acoustic detection devices like cantilever and crystal tuning forks. The photoacoustic spectrometer using miniature lasers fitted with these novel detectors can be easily packed in a small box and can be used in the laboratory or in the field for standoff detection of hazardous materials. The heat generated by the photoacoustic effect produces density changes caused by temperature fluctuations in liquid and gaseous samples. In such cases, photoacoustic spectroscopy is carried out by detection of thermal lens formation

using a probe laser. In the present case, however, we will confine to the detection of acoustic vibrations in the analysis of PA spectra.

laser light was varied between 27 and 1000Hz, and the resulting PA signals are

The photoacoustic spectrum of iodine vapor recorded using a Nd:YAG laser pumped tunable dye laser in the presence of atmospheric air and at 15 Torr is shown in **Figure 7**. Dye laser pulses used for the measurements were of 7 ns duration, 0.05 nm bandwidth, and 2mJ*=*pulse energy at a pulse repetition rate of 10 Hz. It is seen from the spectrum recorded with low air pressure in **Figure 7b** (upper half) that the relative intensity of vibronic bands monotonically increases in going up to the dissociation limit where I2 molecule dissociates into a ground-state 2P3*<sup>=</sup>*<sup>2</sup>

*Photoacoustic spectra of I2 vapor in the wavelength region 492–552 nm (20,300–18,100 cm*�<sup>1</sup>Þ*. The little downward arrow indicates the wavelength of exciting radiation that dissociates I2 into two iodine atoms*

shown in **Figure 6(b–d)** [8].

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

**Figure 7.**

**19**

*(20,043 cm*�<sup>1</sup>Þ*.*

## **3.1 Photoacoustic cells for gaseous samples**

One of the simplest PA cells for measurements on gases and vapors is made from a Pyrex tube fitted with quartz windows at the two ends. The length of the tube and location of the microphone are chosen to maximize the PA signal using the resonance of sound generated by the modulated light beam. The design of such a cell made from 62.5-cm-long Pyrex tube of 2.5 cm diameter and fitted with quartz windows at Brewster's angle is shown in **Figure 6(a)**. The cell was resonant at 335 Hz with maximum signal in the middle port for the microphone, and it was resonant at 669 Hz, rendering maximum signal at two ports symmetrically located on either side of the middle port. The microphone ports correspond to the positions of three possible antinodes of the stationary acoustic waves formed at the two resonant frequencies. Acoustic isolation was achieved by putting the cell in a wooden box filled with sand. Two of the ports, not in use during the measurement, were sealed using O-rings and flat Teflon disks. PA measurements were carried out with iodine vapor at room temperature in the presence of air at atmospheric pressure using 20 milliwatt Argon laser light at 514.5 nm. The chopping frequency of the

#### **Figure 6.**

*Longitudinally resonant PA cell with three ports for microphone (a). PA signal for resonance at 335 Hz (b) and those for resonance at 669 Hz (c) and (d).*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

using a probe laser. In the present case, however, we will confine to the detection of

One of the simplest PA cells for measurements on gases and vapors is made from a Pyrex tube fitted with quartz windows at the two ends. The length of the tube and location of the microphone are chosen to maximize the PA signal using the resonance of sound generated by the modulated light beam. The design of such a cell made from 62.5-cm-long Pyrex tube of 2.5 cm diameter and fitted with quartz windows at Brewster's angle is shown in **Figure 6(a)**. The cell was resonant at 335 Hz with maximum signal in the middle port for the microphone, and it was resonant at 669 Hz, rendering maximum signal at two ports symmetrically located on either side of the middle port. The microphone ports correspond to the positions of three possible antinodes of the stationary acoustic waves formed at the two resonant frequencies. Acoustic isolation was achieved by putting the cell in a wooden box filled with sand. Two of the ports, not in use during the measurement, were sealed using O-rings and flat Teflon disks. PA measurements were carried out with iodine vapor at room temperature in the presence of air at atmospheric pressure using 20 milliwatt Argon laser light at 514.5 nm. The chopping frequency of the

*Longitudinally resonant PA cell with three ports for microphone (a). PA signal for resonance at 335 Hz (b)*

acoustic vibrations in the analysis of PA spectra.

*Photoacoustic Imaging - Principles, Advances and Applications*

**3.1 Photoacoustic cells for gaseous samples**

**Figure 6.**

**18**

*and those for resonance at 669 Hz (c) and (d).*

laser light was varied between 27 and 1000Hz, and the resulting PA signals are shown in **Figure 6(b–d)** [8].

The photoacoustic spectrum of iodine vapor recorded using a Nd:YAG laser pumped tunable dye laser in the presence of atmospheric air and at 15 Torr is shown in **Figure 7**. Dye laser pulses used for the measurements were of 7 ns duration, 0.05 nm bandwidth, and 2mJ*=*pulse energy at a pulse repetition rate of 10 Hz. It is seen from the spectrum recorded with low air pressure in **Figure 7b** (upper half) that the relative intensity of vibronic bands monotonically increases in going up to the dissociation limit where I2 molecule dissociates into a ground-state 2P3*<sup>=</sup>*<sup>2</sup> 

#### **Figure 7.**

*Photoacoustic spectra of I2 vapor in the wavelength region 492–552 nm (20,300–18,100 cm*�<sup>1</sup>Þ*. The little downward arrow indicates the wavelength of exciting radiation that dissociates I2 into two iodine atoms (20,043 cm*�<sup>1</sup>Þ*.*

iodine atom and an excited-state 2P1*<sup>=</sup>*<sup>2</sup> iodine atom. In **Figure 7a** for the spectrum recorded at atmospheric air, the relative intensity of vibronic bands monotonically decreases after maximum intensity at 19,225 cm�<sup>1</sup> This reduction in intensity of the PA spectrum has been explained in terms of energy transfer from excited 2P1*<sup>=</sup>*<sup>2</sup> iodine atoms to O2 molecules following the dissociation of I2 [9].

PA cell. An instrument of this type can directly measure light absorption by aerosols over the entire range of sunlight entering the atmosphere. This type of PA cell has been used with a single laser as well as with two lasers of different wavelengths

PA cells fitted with microphone, for recording PA spectra of solid samples, have been routinely used in the laboratory for almost four decades. One of the important aspects of homemade cells is to choose a material for effective shielding from extraneous sound. The design of a nonresonant PA cell is schematically shown in **Figure 9**. The main body of the cell has been constructed from a single block of aluminum with a cavity made from the bottom side for fixing the microphone along with its preamplifier. A cavity is made on the top of the block to put the sample cuvette whose open end is in the same horizontal plane as the microphone surface. A flat aluminum plate with double quartz windows in front of the sample cuvette is tightly fixed at the top of the main body with a very thin, suitably cut rubber sheet to make the chamber airtight. The thickness of the air duct connecting the sample and microphone is about 1 mm, and its total volume is less than 1 cc. The exterior dimensions of the stainless steel sample cuvettes are identical so as to tightly fit into their designated cavity. The sample cuvettes are, however, of varying depths to make measurements on powder samples of different thicknesses. Carbon black is used as the standard sample for recording the power spectrum of the excitation source of light to normalize the PA signals of the sample under investigation.

The PA spectra of powder samples of RDX and TNT recorded with a PA cell of

the above type are shown in **Figure 10**. The source of excitation used in these experiments was a rotational line tunable cw CO2 laser, and the window, in front of

*Design of nonresonant PA cell. (1) the main body of aluminum, (2) microphone and preamplifier chamber,*

the sample in the PA cell, was made of flat ZnSe plate. The PA signals were measured manually from the lock-in amplifier and normalized using the power meter reading for each laser line. Microgram quantities of powder samples were used in the spectral measurements which exhibit the characteristic vibrational

impacting the same gaseous sample [10, 11].

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

**3.2 Photoacoustic cells for solid samples**

**Figure 9.**

**21**

*and (3) sample cuvette.*

To make photoacoustic measurements on a flowing gas sample, such as in the case of pollution monitoring, one needs a different type of acoustic resonant PA cell as schematically illustrated in **Figure 8**. The U-shaped cell has a total length L ¼ λ, corresponding to the acoustic wavelength generated by the periodically modulated laser. The horizontal section of the tube is of length λ*=*2 and two vertical sections at each end of length λ*=*4. At the resonance frequency, a stationary wave is formed with antinodes at the two ends as well as at the center of the horizontal section and nodes at the two bent corners (see **Figure 8**). The laser as well as gas inlet and outlet, in the PA cell, is located near the nodes, which form regions of low pressure, of the stationary acoustic wave to prevent pressure fluctuations in PA signal at the antinodes. A microphone is fixed near the open end of one of the vertical arms, to detect the PA signal. A piezoelectric disk, fixed at the end of the other vertical arm, is used for detection of resonance frequency of the PA cell and the resonator quality factor to calibrate the system. There is a phase difference of 180° between high pressure points of the stationary acoustic wave at the center of the horizontal section, where heat is generated, and the two ends where microphone and piezoelectric signals are measured. The flow rate of the sample in the PA cell is controlled by a very narrow orifice at the outlet end, such that the pressure inside the PA cell remains steady. The special narrow hole causes a pressure drop of at least 50% on the outlet side compared to pressure inside the PA cell, while the airflow velocity through the orifice reaches the speed of sound. Since the air traveling through the orifice is moving out at the speed of sound, any sound from the pump traveling toward the PA cell, also at the same speed, meets a barrier in air, moving in the opposite direction, and it goes no further. This arrangement blocks the noise from the pump entering the PA cell and cluttering the PA signal.

The airborne particulate matter (aerosols) remains suspended in air inside the PA cell shown in **Figure 8** and has enough time to absorb radiation from the tunable laser beam. The absorbed optical energy is transferred as heat to the surrounding air, before the next laser pulse arrives, to build the stationary pressure wave in the

#### **Figure 8.**

*The U-shaped resonant PA cell for detection of PA signal in a flowing air sample with special orifice for controlling the pressure inside the cell.*

PA cell. An instrument of this type can directly measure light absorption by aerosols over the entire range of sunlight entering the atmosphere. This type of PA cell has been used with a single laser as well as with two lasers of different wavelengths impacting the same gaseous sample [10, 11].

## **3.2 Photoacoustic cells for solid samples**

iodine atom and an excited-state 2P1*<sup>=</sup>*<sup>2</sup>

*Photoacoustic Imaging - Principles, Advances and Applications*

iodine atom. In **Figure 7a** for the spectrum

recorded at atmospheric air, the relative intensity of vibronic bands monotonically decreases after maximum intensity at 19,225 cm�<sup>1</sup> This reduction in intensity of the PA spectrum has been explained in terms of energy transfer from excited 2P1*<sup>=</sup>*<sup>2</sup>

To make photoacoustic measurements on a flowing gas sample, such as in the case of pollution monitoring, one needs a different type of acoustic resonant PA cell as schematically illustrated in **Figure 8**. The U-shaped cell has a total length L ¼ λ, corresponding to the acoustic wavelength generated by the periodically modulated laser. The horizontal section of the tube is of length λ*=*2 and two vertical sections at each end of length λ*=*4. At the resonance frequency, a stationary wave is formed with antinodes at the two ends as well as at the center of the horizontal section and nodes at the two bent corners (see **Figure 8**). The laser as well as gas inlet and outlet, in the PA cell, is located near the nodes, which form regions of low pressure, of the stationary acoustic wave to prevent pressure fluctuations in PA signal at the antinodes. A microphone is fixed near the open end of one of the vertical arms, to detect the PA signal. A piezoelectric disk, fixed at the end of the other vertical arm, is used for detection of resonance frequency of the PA cell and the resonator quality factor to calibrate the system. There is a phase difference of 180° between high pressure points of the stationary acoustic wave at the center of the horizontal section, where heat is generated, and the two ends where microphone and piezoelectric signals are measured. The flow rate of the sample in the PA cell is controlled by a very narrow orifice at the outlet end, such that the pressure inside the PA cell remains steady. The special narrow hole causes a pressure drop of at least 50% on the outlet side compared to pressure inside the PA cell, while the airflow velocity through the orifice reaches the speed of sound. Since the air traveling through the orifice is moving out at the speed of sound, any sound from the pump traveling toward the PA cell, also at the same speed, meets a barrier in air, moving in the opposite direction, and it goes no further. This arrangement blocks the noise from

iodine atoms to O2 molecules following the dissociation of I2 [9].

the pump entering the PA cell and cluttering the PA signal.

**Figure 8.**

**20**

*controlling the pressure inside the cell.*

The airborne particulate matter (aerosols) remains suspended in air inside the PA cell shown in **Figure 8** and has enough time to absorb radiation from the tunable laser beam. The absorbed optical energy is transferred as heat to the surrounding air, before the next laser pulse arrives, to build the stationary pressure wave in the

*The U-shaped resonant PA cell for detection of PA signal in a flowing air sample with special orifice for*

PA cells fitted with microphone, for recording PA spectra of solid samples, have been routinely used in the laboratory for almost four decades. One of the important aspects of homemade cells is to choose a material for effective shielding from extraneous sound. The design of a nonresonant PA cell is schematically shown in **Figure 9**. The main body of the cell has been constructed from a single block of aluminum with a cavity made from the bottom side for fixing the microphone along with its preamplifier. A cavity is made on the top of the block to put the sample cuvette whose open end is in the same horizontal plane as the microphone surface. A flat aluminum plate with double quartz windows in front of the sample cuvette is tightly fixed at the top of the main body with a very thin, suitably cut rubber sheet to make the chamber airtight. The thickness of the air duct connecting the sample and microphone is about 1 mm, and its total volume is less than 1 cc. The exterior dimensions of the stainless steel sample cuvettes are identical so as to tightly fit into their designated cavity. The sample cuvettes are, however, of varying depths to make measurements on powder samples of different thicknesses. Carbon black is used as the standard sample for recording the power spectrum of the excitation source of light to normalize the PA signals of the sample under investigation.

The PA spectra of powder samples of RDX and TNT recorded with a PA cell of the above type are shown in **Figure 10**. The source of excitation used in these experiments was a rotational line tunable cw CO2 laser, and the window, in front of the sample in the PA cell, was made of flat ZnSe plate. The PA signals were measured manually from the lock-in amplifier and normalized using the power meter reading for each laser line. Microgram quantities of powder samples were used in the spectral measurements which exhibit the characteristic vibrational

#### **Figure 9.**

*Design of nonresonant PA cell. (1) the main body of aluminum, (2) microphone and preamplifier chamber, and (3) sample cuvette.*

bands of RDX and TNT [12]. The solid circles in the upper half of **Figure 10** represent the normalized PA signals for the pure explosive powders in the rotational line tunable CO2 laser wavelength range from 9.6 to 10*:*6μm. The samples were diluted by uniformly mixing 10 micrograms of each explosive with 8 grams of SiO2, which does not have any absorption in this region. Thus, the concentrations of the two explosives correspond to a few ppm, exhibiting four persistent bands of RDX and five of TNT with appreciable intensity as shown in the lower part of **Figure 10**.

#### **3.3 PA spectroscopy of contaminated water**

PA instrumentation for detection of liquid samples is somewhat complicated. A schematic diagram of the experimental setup for detection of harmful and dangerous pollutants in water is shown in **Figure 11**. The tunable dye laser beam, pumped by an excimer laser or a Nd:YAG laser, is focused into a 600 micron core multimode optical fiber for investigation on remotely located samples. The polluted water is kept in a quartz cuvette which is acoustically coupled to a piezoelectric transducer. The light exiting from the optical fiber is collimated into the quartz cuvette by means of a 10X microscope objective. The pulse to pulse energy fluctuation, in the light exiting from the cuvette, is monitored by a photodiode assembly, to ratio the PA spectra with the dye laser power profile. The PA signal and the photodiode signal are fed to two different gates of the boxcar, and the normalized spectrum is recorded by the computer by scanning the dye laser wavelength. The quartz cuvette with its piezoelectric transducer, the microscope objective, and the photodiode are fixed on a small portable platform which can be easily positioned within the glovebox for handling radioactive samples. Klenze et al. [13] have determined ultralow concentrations of Am3<sup>þ</sup> and Pu<sup>4</sup><sup>þ</sup> in aqueous solutions with excimer laser-pumped dye laser, while Nd : YAG-pumped dye laser system was used by Russo et al. [14] to record PA spectra of aqueous solutions containing different concentrations of Pr<sup>3</sup><sup>þ</sup> and Am<sup>3</sup><sup>þ</sup> . Kim [15] has investigated the problem of actinide colloid generation in groundwater which plays a critical role in geochemical interaction and migration of actinides.

**3.4 Quartz tuning fork for PA detection**

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

**Figure 11.**

**23**

produced by the piezoelectricity generates the PA signal.

heat produced by optical absorption at the surface.

In a PA cell, the acoustic energy is accumulated in a resonant cavity, but the principle of PA detection by a quartz tuning fork (QTF) involves the accumulation of the acoustic energy in a sharply resonant acoustic transducer [16–18]. Crystal quartz is an easy material for such a transducer because of its low loss piezoelectric property, and QTFs can be designed to resonate at any frequency between 4 Hz and 200 kHz. The widely used QTF, manufactured for use in electronic clocks as frequency standard, resonates at 32,768 Hz in vacuum. The detection of PA signal by QTF is based on the piezoelectric effect produced by acoustic waves at the resonant frequency that mechanically bend its prongs. In the PA spectroscopy of gaseous sample, the interaction between the modulated laser beam and a trace gas generates acoustic waves that mechanically bend the QTF prongs. The voltage

*Schematic PA spectroscopy system for trace detection of chemical species in polluted water.*

When a laser beam is focused at the center between the two prongs of the QTF placed in a gaseous sample, the absorbed optical energy converted into heat generates a weak acoustic pressure wave. When the laser beam is modulated at half the QTF resonant frequency (f), the pressure wave makes the two prongs move apart two times during each acoustic cycle. In this situation the QTF detects sound oscillations at the second harmonic of the modulation frequency due to two absorption events during each modulation period. The laser light is modulated at " f*=*2" and PA signal demodulated by lock-in amplifier at " f" as shown in **Figure 12**. The PA spectrum is recorded by varying the wavelength of the tunable laser. The use of QTF for solid phase PA detection in the laboratory requires a very thin film of the molecular sample to be adsorbed on the outer surface of one of the prongs. The absorbed laser light heats the sample, generating an acoustic wave at the prong's surface interface with air. When the frequency of repetition of the incident laser pulse coincides with the mechanical resonant frequency of the QTF, the localized pressure variation sets the latter into vibration. The amplitude of this vibration and the resulting piezoelectric voltage are proportional to the amount of

An experimental arrangement using the above concept is schematically shown in **Figure 13** using a quantum cascade laser (QCL). The large wavelength coverage in

#### **Figure 10.**

*CO*<sup>2</sup> *laser-excited PA spectra of RDX and TNT powders. The persistent vibrational bands of the two molecules are seen for highly diluted samples in the lower half of the figure.*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

bands of RDX and TNT [12]. The solid circles in the upper half of **Figure 10** represent the normalized PA signals for the pure explosive powders in the rotational line tunable CO2 laser wavelength range from 9.6 to 10*:*6μm. The samples were diluted by uniformly mixing 10 micrograms of each explosive with 8 grams of SiO2, which does not have any absorption in this region. Thus, the concentrations of the two explosives correspond to a few ppm, exhibiting four persistent bands of RDX and five of TNT with appreciable intensity as shown in the lower part of **Figure 10**.

PA instrumentation for detection of liquid samples is somewhat complicated. A schematic diagram of the experimental setup for detection of harmful and dangerous pollutants in water is shown in **Figure 11**. The tunable dye laser beam, pumped by an excimer laser or a Nd:YAG laser, is focused into a 600 micron core multimode optical fiber for investigation on remotely located samples. The polluted water is kept in a quartz cuvette which is acoustically coupled to a piezoelectric transducer. The light exiting from the optical fiber is collimated into the quartz cuvette by means of a 10X microscope objective. The pulse to pulse energy fluctuation, in the light exiting from the cuvette, is monitored by a photodiode assembly, to ratio the PA spectra with the dye laser power profile. The PA signal and the photodiode signal are fed to two different gates of the boxcar, and the normalized spectrum is recorded by the computer by scanning the dye laser wavelength. The quartz cuvette with its piezoelectric transducer, the microscope objective, and the photodiode are fixed on a small portable platform which can be easily positioned within the glovebox for handling radioactive samples. Klenze et al. [13] have determined ultralow concentrations of Am3<sup>þ</sup> and Pu<sup>4</sup><sup>þ</sup> in aqueous solutions with excimer laser-pumped dye laser, while Nd : YAG-pumped dye laser system was used by Russo et al. [14] to record PA spectra of aqueous solutions containing different

colloid generation in groundwater which plays a critical role in geochemical inter-

*CO*<sup>2</sup> *laser-excited PA spectra of RDX and TNT powders. The persistent vibrational bands of the two molecules*

*are seen for highly diluted samples in the lower half of the figure.*

. Kim [15] has investigated the problem of actinide

**3.3 PA spectroscopy of contaminated water**

*Photoacoustic Imaging - Principles, Advances and Applications*

concentrations of Pr<sup>3</sup><sup>þ</sup> and Am<sup>3</sup><sup>þ</sup>

action and migration of actinides.

**Figure 10.**

**22**

**Figure 11.** *Schematic PA spectroscopy system for trace detection of chemical species in polluted water.*

#### **3.4 Quartz tuning fork for PA detection**

In a PA cell, the acoustic energy is accumulated in a resonant cavity, but the principle of PA detection by a quartz tuning fork (QTF) involves the accumulation of the acoustic energy in a sharply resonant acoustic transducer [16–18]. Crystal quartz is an easy material for such a transducer because of its low loss piezoelectric property, and QTFs can be designed to resonate at any frequency between 4 Hz and 200 kHz. The widely used QTF, manufactured for use in electronic clocks as frequency standard, resonates at 32,768 Hz in vacuum. The detection of PA signal by QTF is based on the piezoelectric effect produced by acoustic waves at the resonant frequency that mechanically bend its prongs. In the PA spectroscopy of gaseous sample, the interaction between the modulated laser beam and a trace gas generates acoustic waves that mechanically bend the QTF prongs. The voltage produced by the piezoelectricity generates the PA signal.

When a laser beam is focused at the center between the two prongs of the QTF placed in a gaseous sample, the absorbed optical energy converted into heat generates a weak acoustic pressure wave. When the laser beam is modulated at half the QTF resonant frequency (f), the pressure wave makes the two prongs move apart two times during each acoustic cycle. In this situation the QTF detects sound oscillations at the second harmonic of the modulation frequency due to two absorption events during each modulation period. The laser light is modulated at " f*=*2" and PA signal demodulated by lock-in amplifier at " f" as shown in **Figure 12**. The PA spectrum is recorded by varying the wavelength of the tunable laser.

The use of QTF for solid phase PA detection in the laboratory requires a very thin film of the molecular sample to be adsorbed on the outer surface of one of the prongs. The absorbed laser light heats the sample, generating an acoustic wave at the prong's surface interface with air. When the frequency of repetition of the incident laser pulse coincides with the mechanical resonant frequency of the QTF, the localized pressure variation sets the latter into vibration. The amplitude of this vibration and the resulting piezoelectric voltage are proportional to the amount of heat produced by optical absorption at the surface.

An experimental arrangement using the above concept is schematically shown in **Figure 13** using a quantum cascade laser (QCL). The large wavelength coverage in

property. In the following sections, we will present examples of point detection as

Ethylene Cð Þ 2H4 is a well-known emission fingerprint from vehicle exhausts and its reaction with nitric oxides under solar UV radiation produces ozone. Altuzar et al. [21] have used a CO2 laser with PA cell located inside the laser cavity to profit from the intracavity laser power. A mechanical chopper modulated the intensity of the laser beam at a frequency that matches with the acoustic resonant frequency of the PA cell. The 10P14 line of CO2 laser is in exact resonance with the Q branch of the ν<sup>7</sup> band of ethylene. Air samples from different locations in Mexico city were collected from 6 to 9 am in the morning and from 12 to 15 pm in the afternoon. The raw samples, collected in stainless steel vessels, went through the process of removing CO2 and water vapor before being transferred into the PA cell. The results of ethylene concentration analysis exhibit higher values in samples collected in the morning and lower values for the afternoon samples. The increased concentration of ethylene is attributed to emission from vehicles when the morning traffic is heavy. The decrease of ethylene concentration in the afternoon samples has two possible causes. Firstly, there is expansion in the thickness of atmospheric air, due to heating by sunlight leading to dilution of ethylene. Secondly, ethylene reacts with nitrogen oxides in the presence of solar UV radiation to produce ozone and other oxidants, causing a decrease in its concentration. It is to be emphasized that PA technique has the advantage of performing online measurements with excellent time resolution.

well as standoff detection of chemical compounds using PA spectroscopy.

**4.2 PA spectroscopy of gases emanating from the human body**

The smells emanating from various parts of the body are unique to an individual, made up of specific chemical compounds that vary depending on age, diet, metabolism, and health. Near-IR diode laser at 1*:*53μm has been used in PA detection of ammonia concentrations in ppmV and ppbV range by several workers [18, 22]. Although NH3 is biosynthesized through normal amino acid metabolism, it is toxic in high concentrations, and its amount in blood gets elevated due to liver dysfunction. Compact quantum cascade lasers (QCL) may prove to be an important mid-IR

Trace level detection of nitric oxide has many applications in medicine, biology, and environmental science. CO laser was the first to be used by Kreuzer and Patel [2] for PA detection of NO concentrations of 0.01 ppmV. Since its first detection in exhaled air [23], NO has been found to be a sensitive marker for asthmatic airway inflammation [24]. A QCL-based PA cell has been developed by Elia et al. [25], while Spagnolo et al. have reported a minimum NO concentration limit of 15 ppbV [26]. A CO2 laser has been used as the source of PA excitation by Harren et al. [27] to detect the emission of ethylene from the human breath and skin under exposure to UV radiation. In this experiment a small amount of exhaled air was cleaned for its content of CO2, water vapor, and other spectroscopically interfering gases, before introducing it into the PA cell. The 10P14 emission line of CO2 laser was used to

Morphine is the prototype narcotic drug, and it is the standard against which all other opioids are tested. An acetylated form of morphine, almost two times more

**4.1 PA spectroscopy of ethylene**

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

source for portable ammonia sensors.

detect a lower limit of 6 ppbV ethylene in nitrogen.

**4.3 PA spectroscopy of dangerous drugs**

**25**

#### **Figure 12.**

*Experimental setup for gas phase PA spectroscopy with QTF detector. The excitation diode laser source is currently modulated at half of the QTF resonant frequency (f).*

#### **Figure 13.**

*Schematic experimental arrangement for solid phase PA spectroscopy using QTF detector with sample adsorbed on one of the QTF prongs. The function generator controls the pulse repetition rate of the quantum cascade laser (QCL) to be in resonance with the symmetric vibration of the QTF.*

the mid-IR region combined with narrow linewidth and powering up to tens of mW of QCL proves ideal for trace detection of molecules [19]. This system can be employed in trace detection of several molecular species adsorbed on the surface of the prong. A variant of this system has been used in detection of adsorbed chemicals on surfaces remotely located from the laser [20].

## **4. Photoacoustic detection of harmful chemicals**

PA spectroscopy has been widely used in chemical sensing applications in environmental science and medical diagnostics. It is useful in rapid detection of illicit drugs, nerve agents, and hazardous biological materials. In a typical hospital environment, there is a need for evaluation of anesthetic gaseous components. Although hospital staff are exposed to much lower anesthetic concentration than the patients, this exposure extends over many years. Under inadequate hygiene conditions, people working in hospitals or factories often complain of headaches and fatigue due to traces of harmful gases in the environment. Illicit drug trafficking poses many challenges for detection of dangerous chemicals that threaten life and

property. In the following sections, we will present examples of point detection as well as standoff detection of chemical compounds using PA spectroscopy.

## **4.1 PA spectroscopy of ethylene**

Ethylene Cð Þ 2H4 is a well-known emission fingerprint from vehicle exhausts and its reaction with nitric oxides under solar UV radiation produces ozone. Altuzar et al. [21] have used a CO2 laser with PA cell located inside the laser cavity to profit from the intracavity laser power. A mechanical chopper modulated the intensity of the laser beam at a frequency that matches with the acoustic resonant frequency of the PA cell. The 10P14 line of CO2 laser is in exact resonance with the Q branch of the ν<sup>7</sup> band of ethylene. Air samples from different locations in Mexico city were collected from 6 to 9 am in the morning and from 12 to 15 pm in the afternoon. The raw samples, collected in stainless steel vessels, went through the process of removing CO2 and water vapor before being transferred into the PA cell. The results of ethylene concentration analysis exhibit higher values in samples collected in the morning and lower values for the afternoon samples. The increased concentration of ethylene is attributed to emission from vehicles when the morning traffic is heavy. The decrease of ethylene concentration in the afternoon samples has two possible causes. Firstly, there is expansion in the thickness of atmospheric air, due to heating by sunlight leading to dilution of ethylene. Secondly, ethylene reacts with nitrogen oxides in the presence of solar UV radiation to produce ozone and other oxidants, causing a decrease in its concentration. It is to be emphasized that PA technique has the advantage of performing online measurements with excellent time resolution.

#### **4.2 PA spectroscopy of gases emanating from the human body**

The smells emanating from various parts of the body are unique to an individual, made up of specific chemical compounds that vary depending on age, diet, metabolism, and health. Near-IR diode laser at 1*:*53μm has been used in PA detection of ammonia concentrations in ppmV and ppbV range by several workers [18, 22]. Although NH3 is biosynthesized through normal amino acid metabolism, it is toxic in high concentrations, and its amount in blood gets elevated due to liver dysfunction. Compact quantum cascade lasers (QCL) may prove to be an important mid-IR source for portable ammonia sensors.

Trace level detection of nitric oxide has many applications in medicine, biology, and environmental science. CO laser was the first to be used by Kreuzer and Patel [2] for PA detection of NO concentrations of 0.01 ppmV. Since its first detection in exhaled air [23], NO has been found to be a sensitive marker for asthmatic airway inflammation [24]. A QCL-based PA cell has been developed by Elia et al. [25], while Spagnolo et al. have reported a minimum NO concentration limit of 15 ppbV [26].

A CO2 laser has been used as the source of PA excitation by Harren et al. [27] to detect the emission of ethylene from the human breath and skin under exposure to UV radiation. In this experiment a small amount of exhaled air was cleaned for its content of CO2, water vapor, and other spectroscopically interfering gases, before introducing it into the PA cell. The 10P14 emission line of CO2 laser was used to detect a lower limit of 6 ppbV ethylene in nitrogen.

#### **4.3 PA spectroscopy of dangerous drugs**

Morphine is the prototype narcotic drug, and it is the standard against which all other opioids are tested. An acetylated form of morphine, almost two times more

the mid-IR region combined with narrow linewidth and powering up to tens of mW of QCL proves ideal for trace detection of molecules [19]. This system can be employed in trace detection of several molecular species adsorbed on the surface of the prong. A variant of this system has been used in detection of adsorbed chemicals

*Schematic experimental arrangement for solid phase PA spectroscopy using QTF detector with sample adsorbed on one of the QTF prongs. The function generator controls the pulse repetition rate of the quantum cascade laser*

*Experimental setup for gas phase PA spectroscopy with QTF detector. The excitation diode laser source is*

PA spectroscopy has been widely used in chemical sensing applications in environmental science and medical diagnostics. It is useful in rapid detection of illicit drugs, nerve agents, and hazardous biological materials. In a typical hospital environment, there is a need for evaluation of anesthetic gaseous components. Although hospital staff are exposed to much lower anesthetic concentration than the patients, this exposure extends over many years. Under inadequate hygiene conditions, people working in hospitals or factories often complain of headaches and fatigue due to traces of harmful gases in the environment. Illicit drug trafficking poses many challenges for detection of dangerous chemicals that threaten life and

on surfaces remotely located from the laser [20].

*(QCL) to be in resonance with the symmetric vibration of the QTF.*

*currently modulated at half of the QTF resonant frequency (f).*

*Photoacoustic Imaging - Principles, Advances and Applications*

**Figure 13.**

**24**

**Figure 12.**

**4. Photoacoustic detection of harmful chemicals**

**Figure 14.** *CO*<sup>2</sup> *laser-excited PA spectra of powders of heroin (on the left) and morphine (on right).*

potent than morphine itself, is known as heroin. Animal and human studies and clinical experience back up the contention that morphine is one of the most euphoric drugs on earth. Both morphine and heroin are used for pain medication, but both are addictive and identified as illegal drugs.

sample. A typical value of DT <sup>¼</sup> <sup>1</sup>*:*<sup>4</sup> � <sup>10</sup>�3cm2*=*s for most tissue [29] gives <sup>δ</sup><sup>T</sup> <sup>¼</sup> 0*:*05μm for a laser pulse of τ<sup>p</sup> ¼ 5 ns. Duration of thermal diffusion is given by

*Schematic illustration of the principle and processes involved in photoacoustic image formation.*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

sample. Thus for L ¼ 15μm, we get τth ¼ 0*:*4 ms. If "v" is the velocity of sound in tissue, the time taken by the stress to transit a sample length L in the heated region is given by τ<sup>s</sup> ¼ L*=*v. Assuming v ¼ 1*:*5mm*=*μs we get τ<sup>s</sup> ¼ 10 ns. This simple calculation shows that we would achieve a spatial resolution of 15μm in the PA image if τ<sup>s</sup> ¼ 10 ns and τth ¼ 0*:*4 ms. Thus we find that the process of heat generation by a 5 ns laser pulse would satisfy the two conditions for generating PA signals

Photoacoustic tomography (PAT) and photoacoustic microscopy (PAM) are the two methods of PA imaging. In the PAT mode, an expanded laser beam illuminates the whole sample, and laser photons are absorbed at various points in the sample generating ultrasonic waves. PAT acquires depth-dependent information by timeof-flight measurements of the acoustic waves. An ultrasonic transducer placed outside the sample detects the PA signal, which is measured either by moving a single transducer around the sample or by using an array of transducers. PA image is obtained from the data set of PA signals by using appropriate reconstruction algorithms in the computer. In the case of PAM, the laser beam is focused into a tiny volume, and ultrasonic waves from this localized region are imaged by the detector. To obtain a 3D image, transverse sections at different depths in the sample are scanned in two dimensions. The axial resolution of the image can be as good as the optical resolution ð Þ <1μm where the depth information is determined by the time

Fluorescence microscopy is an effective tool in thin biological samples like single-celled organisms, but with slightly thicker samples, it becomes difficult to know where exactly the fluorescence originates. In a complex organism, like zebra fish, it is crucial to image deeper and deeper while the organism is kept alive. Fluorescent light emerging from the point of absorption suffers multiple scattering in the tissue on its way to the optical detector. This leads to loss of information on the origin and propagation path of the fluorescent light, giving rise to a blurred

**5.1 Photoacoustic microscopy and photoacoustic tomography**

*=*4DT where L is the radius of the spherical shell of heat propagation in the

<sup>τ</sup>th <sup>¼</sup> L2

**Figure 15.**

efficiently.

of flight of ultrasonic waves.

**27**

**5.2 Photoacoustic microscopy of zebra fish**

Microgram quantities of powders of morphine and heroin were used in a PA cell shown in **Figure 9** and fitted with ZnSe window. Rotational line tunable CO2 laser was used as the excitation source for recording the PA spectra of the two compounds shown in **Figure 14**. The black dots in the figures represent the manually measured PA signals from the lock-in amplifier normalized by laser power at each wavelength. The spectral peaks correspond to the vibrational bands of morphine and heroin in the wavelength range 9.6 to 10.6 micron [28].

#### **5. Photoacoustic imaging**

Photoacoustic imaging is an emerging technique that combines the high resolution of light and deep imaging capability of ultrasound. It is similar to hyperspectral imaging except for the fact that optical sensors are replaced by ultrasonic detectors that convert the sound waves into images. It has many applications as a noninvasive technique in medicine to produce molecular images of internal organs. It is based on the rapid production of heat, when the optical energy from a nanosecond laser pulse is absorbed in the tissue, causing thermal expansion and the generation of ultrasonic waves. The processes involved in the image formation are schematically illustrated in **Figure 15**, which show that it is a hybrid technique making use of optical absorption and ultrasonic wave propagation.

There are two basic conditions for efficient generation of the PA signal for imaging. The condition of "thermal confinement" requires the laser pulse duration τ<sup>p</sup> to be shorter than temporal duration τth of thermal diffusion from the heated volume. This condition implies that there is negligible heat diffusion during the laser pulse so that the PA image essentially reveals the initial absorbed energy distribution in the sample. The second condition of "stress confinement" requires that time τ<sup>s</sup> for stress to transit the heated volume should be longer than the laser pulse duration τp.

It can be shown that a nanosecond laser pulse impacting a biological tissue sample satisfies the conditions for PA imaging. Thermal diffusion length during the laser pulse is given by δ<sup>T</sup> ¼ 2 τpDT <sup>1</sup>*=*<sup>2</sup> where DT is the thermal diffusivity of the

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

**Figure 15.**

potent than morphine itself, is known as heroin. Animal and human studies and clinical experience back up the contention that morphine is one of the most euphoric drugs on earth. Both morphine and heroin are used for pain medication,

*CO*<sup>2</sup> *laser-excited PA spectra of powders of heroin (on the left) and morphine (on right).*

Microgram quantities of powders of morphine and heroin were used in a PA cell shown in **Figure 9** and fitted with ZnSe window. Rotational line tunable CO2 laser was used as the excitation source for recording the PA spectra of the two compounds shown in **Figure 14**. The black dots in the figures represent the manually measured PA signals from the lock-in amplifier normalized by laser power at each wavelength. The spectral peaks correspond to the vibrational bands of morphine

Photoacoustic imaging is an emerging technique that combines the high resolution of light and deep imaging capability of ultrasound. It is similar to hyperspectral imaging except for the fact that optical sensors are replaced by ultrasonic detectors that convert the sound waves into images. It has many applications as a noninvasive technique in medicine to produce molecular images of internal organs. It is based on the rapid production of heat, when the optical energy from a nanosecond laser pulse is absorbed in the tissue, causing thermal expansion and the generation of ultrasonic waves. The processes involved in the image formation are schematically illustrated in **Figure 15**, which show that it is a hybrid technique making use of optical

There are two basic conditions for efficient generation of the PA signal for imaging. The condition of "thermal confinement" requires the laser pulse duration τ<sup>p</sup> to be shorter than temporal duration τth of thermal diffusion from the heated volume. This condition implies that there is negligible heat diffusion during the laser pulse so that the PA image essentially reveals the initial absorbed energy distribution in the sample. The second condition of "stress confinement" requires that time τ<sup>s</sup> for stress to transit the heated volume should be longer than the laser

It can be shown that a nanosecond laser pulse impacting a biological tissue sample satisfies the conditions for PA imaging. Thermal diffusion length during the

<sup>2</sup> where DT is the thermal diffusivity of the

 <sup>1</sup>*=*

but both are addictive and identified as illegal drugs.

*Photoacoustic Imaging - Principles, Advances and Applications*

and heroin in the wavelength range 9.6 to 10.6 micron [28].

**5. Photoacoustic imaging**

**Figure 14.**

pulse duration τp.

**26**

laser pulse is given by δ<sup>T</sup> ¼ 2 τpDT

absorption and ultrasonic wave propagation.

*Schematic illustration of the principle and processes involved in photoacoustic image formation.*

sample. A typical value of DT <sup>¼</sup> <sup>1</sup>*:*<sup>4</sup> � <sup>10</sup>�3cm2*=*s for most tissue [29] gives <sup>δ</sup><sup>T</sup> <sup>¼</sup> 0*:*05μm for a laser pulse of τ<sup>p</sup> ¼ 5 ns. Duration of thermal diffusion is given by <sup>τ</sup>th <sup>¼</sup> L2 *=*4DT where L is the radius of the spherical shell of heat propagation in the sample. Thus for L ¼ 15μm, we get τth ¼ 0*:*4 ms. If "v" is the velocity of sound in tissue, the time taken by the stress to transit a sample length L in the heated region is given by τ<sup>s</sup> ¼ L*=*v. Assuming v ¼ 1*:*5mm*=*μs we get τ<sup>s</sup> ¼ 10 ns. This simple calculation shows that we would achieve a spatial resolution of 15μm in the PA image if τ<sup>s</sup> ¼ 10 ns and τth ¼ 0*:*4 ms. Thus we find that the process of heat generation by a 5 ns laser pulse would satisfy the two conditions for generating PA signals efficiently.

#### **5.1 Photoacoustic microscopy and photoacoustic tomography**

Photoacoustic tomography (PAT) and photoacoustic microscopy (PAM) are the two methods of PA imaging. In the PAT mode, an expanded laser beam illuminates the whole sample, and laser photons are absorbed at various points in the sample generating ultrasonic waves. PAT acquires depth-dependent information by timeof-flight measurements of the acoustic waves. An ultrasonic transducer placed outside the sample detects the PA signal, which is measured either by moving a single transducer around the sample or by using an array of transducers. PA image is obtained from the data set of PA signals by using appropriate reconstruction algorithms in the computer. In the case of PAM, the laser beam is focused into a tiny volume, and ultrasonic waves from this localized region are imaged by the detector. To obtain a 3D image, transverse sections at different depths in the sample are scanned in two dimensions. The axial resolution of the image can be as good as the optical resolution ð Þ <1μm where the depth information is determined by the time of flight of ultrasonic waves.

#### **5.2 Photoacoustic microscopy of zebra fish**

Fluorescence microscopy is an effective tool in thin biological samples like single-celled organisms, but with slightly thicker samples, it becomes difficult to know where exactly the fluorescence originates. In a complex organism, like zebra fish, it is crucial to image deeper and deeper while the organism is kept alive. Fluorescent light emerging from the point of absorption suffers multiple scattering in the tissue on its way to the optical detector. This leads to loss of information on the origin and propagation path of the fluorescent light, giving rise to a blurred

image and destruction of the spatial resolution. PA detection of optical absorption circumvents these limitations, because the sound waves travel through the diffuse biological media with much less distortion than light.

prostate tissue, benign prostatic hyperplasia (BPH), and normal human prostate tissue. The preliminary results of investigations carried out by Dogra et al. [32] show that there was a significant difference in the mean PA intensity of dehydroxy hemoglobin (dHb) and lipid between malignant and normal prostate. There was also a significant difference in the mean intensity of dHb between malignant prostate and BPH. There was, however, no significant difference in HbO2, dHb, and lipid between normal prostate tissue and BPH. Laser radiation at 1064 nm and 1197 nm has been used to obtain PAT images, corresponding to optical absorption of hemoglobin and lipid, to determine the clustering prostate cancer tissue at each wavelength [33]. It was found that 1064 nm PAT in conjunction with ultrasound image is more effective in identifying prostate cancer biopsy targets than the PAT at

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

*DOI: http://dx.doi.org/10.5772/intechopen.88840*

This chapter starts with a brief history of photoacoustic effect and photoacoustic spectroscopy. A simple mathematical derivation for the generation of PA signal in gaseous and solid samples is followed by experimental methods. The design and construction of a variety of PA cells and detectors have been described along with their use in the investigation of gaseous, solid, and liquid samples. Some illustrative

I am grateful to all the authors and scientists whose results have been cited to make the presentation meaningful. I am thankful to Dr. Punam Rai for taking care of my health, Sudheer for the help with the computer, and to my grandchildren, Leo and Mia, for their innocent inquiries during the course of my writing.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

examples of trace detection of explosives and harmful chemicals have been discussed. A brief account of the principle and application of the emerging tech-

nique of PA imaging is discussed at the end of the chapter.

1197 nm.

**6. Conclusion**

**Acknowledgements**

**Author details**

**29**

Surya Narayan Thakur

Banaras Hindu University, Varanasi, India

provided the original work is properly cited.

\*Address all correspondence to: snthakur@gmail.com

The experimental components used in PAM by Harrison et al. [30] are shown in **Figure 16a**. The laser beam, from a tunable source (L), is diverted down the 45° reflecting cone R, by the prism P. The curved surface of the cone directs the laser beam in a horizontal plane which is reflected downward by the corner polished surfaces inclined at 45° with the horizontal plane. The laser beam then travels downward along the longer sides of the ultrasound probe (T) to be focused about 10.5 mm below the bottom of the probe (see **Figure 16a**). The position of the ultrasound transducer (T) is adjusted vertically to match its focus with the laser focus by maximizing the PA signal from carbon fiber sample [30]. The combined ultrasound and laser light probe is mounted on a voice coil (VC) stage which is driven by a programmed motor controller to achieve oscillations at 10 Hz over a length of about 10 mm. The imaging system can be run in three different modes, (i) ultrasound mode, (ii) PA mode, and (iii) interlaced PA-ultrasound mode, and it can produce images of microvascular structures to depths of 2–3 mm in vivo as illustrated in **Figure 16b** for zebra fish.

In the recording of images of **Figure 16b**, the zebra fish was held on a rotating platform immersed in water. The position of the laser focus was fixed at a particular depth inside the body of the sample, and the platform was rotated through 360° to record the two-dimensional sections [31]. The location of fluorescent protein mCherry (in red) is clearly seen in the image of the zebra fish brain at the top, and transverse image slices of the zebra fish hindbrain are shown in the lower half of **Figure 16b**, where each slice is separated by a depth of 0.5 mm inside the tissue.

### **5.3 Photoacoustic imaging of prostate cancer**

PA imaging is emerging as a new diagnosis technique with specificity, high resolution, and enough imaging depth for early detection of prostate cancer. Ex vivo multispectral PA imaging has been carried out to differentiate between malignant

#### **Figure 16.**

*(a) Experimental setup for combined photoacoustic (PA) and ultrasound (US) imaging (with permission from Ref. [30]). (b) In vivo image of a section of the zebra fish brain and five transverse image slices through the hindbrain (with permission from Ref. [31]).*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter DOI: http://dx.doi.org/10.5772/intechopen.88840*

prostate tissue, benign prostatic hyperplasia (BPH), and normal human prostate tissue. The preliminary results of investigations carried out by Dogra et al. [32] show that there was a significant difference in the mean PA intensity of dehydroxy hemoglobin (dHb) and lipid between malignant and normal prostate. There was also a significant difference in the mean intensity of dHb between malignant prostate and BPH. There was, however, no significant difference in HbO2, dHb, and lipid between normal prostate tissue and BPH. Laser radiation at 1064 nm and 1197 nm has been used to obtain PAT images, corresponding to optical absorption of hemoglobin and lipid, to determine the clustering prostate cancer tissue at each wavelength [33]. It was found that 1064 nm PAT in conjunction with ultrasound image is more effective in identifying prostate cancer biopsy targets than the PAT at 1197 nm.

## **6. Conclusion**

image and destruction of the spatial resolution. PA detection of optical absorption circumvents these limitations, because the sound waves travel through the diffuse

The experimental components used in PAM by Harrison et al. [30] are shown in **Figure 16a**. The laser beam, from a tunable source (L), is diverted down the 45° reflecting cone R, by the prism P. The curved surface of the cone directs the laser beam in a horizontal plane which is reflected downward by the corner polished surfaces inclined at 45° with the horizontal plane. The laser beam then travels downward along the longer sides of the ultrasound probe (T) to be focused about 10.5 mm below the bottom of the probe (see **Figure 16a**). The position of the ultrasound transducer (T) is adjusted vertically to match its focus with the laser focus by maximizing the PA signal from carbon fiber sample [30]. The combined ultrasound and laser light probe is mounted on a voice coil (VC) stage which is driven by a programmed motor controller to achieve oscillations at 10 Hz over a length of about 10 mm. The imaging system can be run in three different modes, (i) ultrasound mode, (ii) PA mode, and (iii) interlaced PA-ultrasound mode, and it can produce images of microvascular structures to depths of 2–3 mm in vivo as illus-

In the recording of images of **Figure 16b**, the zebra fish was held on a rotating platform immersed in water. The position of the laser focus was fixed at a particular depth inside the body of the sample, and the platform was rotated through 360° to record the two-dimensional sections [31]. The location of fluorescent protein mCherry (in red) is clearly seen in the image of the zebra fish brain at the top, and transverse image slices of the zebra fish hindbrain are shown in the lower half of **Figure 16b**, where each slice is separated by a depth of 0.5 mm inside the tissue.

PA imaging is emerging as a new diagnosis technique with specificity, high resolution, and enough imaging depth for early detection of prostate cancer. Ex vivo multispectral PA imaging has been carried out to differentiate between malignant

*(a) Experimental setup for combined photoacoustic (PA) and ultrasound (US) imaging (with permission from Ref. [30]). (b) In vivo image of a section of the zebra fish brain and five transverse image slices through the*

biological media with much less distortion than light.

*Photoacoustic Imaging - Principles, Advances and Applications*

trated in **Figure 16b** for zebra fish.

**Figure 16.**

**28**

*hindbrain (with permission from Ref. [31]).*

**5.3 Photoacoustic imaging of prostate cancer**

This chapter starts with a brief history of photoacoustic effect and photoacoustic spectroscopy. A simple mathematical derivation for the generation of PA signal in gaseous and solid samples is followed by experimental methods. The design and construction of a variety of PA cells and detectors have been described along with their use in the investigation of gaseous, solid, and liquid samples. Some illustrative examples of trace detection of explosives and harmful chemicals have been discussed. A brief account of the principle and application of the emerging technique of PA imaging is discussed at the end of the chapter.

## **Acknowledgements**

I am grateful to all the authors and scientists whose results have been cited to make the presentation meaningful. I am thankful to Dr. Punam Rai for taking care of my health, Sudheer for the help with the computer, and to my grandchildren, Leo and Mia, for their innocent inquiries during the course of my writing.

### **Author details**

Surya Narayan Thakur Banaras Hindu University, Varanasi, India

\*Address all correspondence to: snthakur@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[3] Tyndall JG. Action of an intermittent beam of radiant heat upon gaseous matter. Royal Society of London. 1881; **31**:307

[4] Bell AG. Upon the production of sound by radiant energy. Philosophical Magazine. 1881;**11**:510

[5] Kasha M. Characterization of electronic transitions in complex molecules. Discussions of the Faraday Society. 1950;**9**:14

[6] Lewis GN, Kasha M. Phosphorescence and the triplet state. Journal of the American Chemical Society. 1944;**66**:2100

[7] Rosencwaig A, Gersho A. Theory of the photoacoustic effect with solids. Journal of Applied Physics. 1976;**47**:64

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[9] Narayanan K, Thakur SN. Photoacoustic spectrum of iodine in the 490–680 nm (20402–14700 cm<sup>1</sup> ) region. Applied Optics. 1992;**31**:4987

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*DOI: http://dx.doi.org/10.5772/intechopen.88840*

*Photoacoustic Spectroscopy of Gaseous and Condensed Matter*

In: Meyers RA, editor. Encyclopedia of Analytical Chemistry. Chichester: John

[28] Prasad RL, Thakur SN, Bhar GC. CO2 laser photoacoustic spectra and vibrational modes of heroin, morphine and narcotine. Pramana. 2002;**59**:487

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**Chapter 3**

*Chin Hsia*

**Abstract**

systems.

**33**

**1. Introduction**

A Balanced Slew-Rate

High-Voltage Integrated Bipolar

Ultrasonic Imaging Applications

This chapter describes the use of silicon-on-insulator (SOI) technology to develop balanced slew-rate pulse generators for medical ultrasound scanners, especially for multi-channel portable systems. Since ultrasonic transducers are usually composed of piezoelectric materials, most of which are capacitive, and the resonant frequency is usually in the order of tens of MHz, it is preferred to convert the high-frequency excited signals into high-voltage pulses to efficiently drive the transducers. In addition, the second harmonic leakage of the high-voltage pulse signal output by the pulse generator needs to be controlled such that the pulse generator can be applied to tissue harmonic imaging. Based on these considerations, the pulse generator architecture with balanced rising and falling edges proposed in this chapter is designed by synthesizing low-power, high-speed level shifters and a high-voltage H-bridge output stage to output high-voltage pulse signals with low harmonic distortion. The entire circuit integrates an 8-channel pulse generator, producing pulse signals >100 Vpp. The rise and fall times of the pulses are within 18.6 and 18.5 ns, respectively. The overall quiescent current is <60 uA (including the on-chip power supply). The output current drive capability is >2 A and the second harmonic distortion is as low as 40 dBc, indicating that the integrated pulse generator can be used in advanced, portable ultrasonic harmonic imaging

**Keywords:** high-voltage pulse generator, ultrasound transmitter, medical imaging,

Compared with computed tomography (CT), X-ray and magnetic resonance imaging (MRI), medical ultrasound imaging is relatively fast, inexpensive, portable and radiation-free, and has become one of the most popular modalities in clinical examination. Currently, besides conventional B-mode and Doppler ultrasonic modality, advanced imaging formation method, such as photo-acoustic (PA) imaging, elasto-graphic imaging, as well as harmonic imaging is gradually becoming an important medical assessment tool for these echo-graphic systems. PA imaging offers unique capabilities in studying biological tissue based on optical absorption

harmonic distortion, low-power on-chip supply

Pulse Generator for Medical

## **Chapter 3**
