**Abstract**

Topical review of recent trends in Modeling and Regularization methods of Diffuse Optical Tomography (DOT) system promotes the optimization of the forward and inverse modeling methods which provides a 3D cauterization at a faster rate of 40frames/second with the help of a laser torch as a hand-held device. Analytical, Numerical and Statistical methods are reviewed for forward and inverse models in an optical imaging modality. The advancement in computational methods is discussed for forward and inverse models along with Optimization techniques using Artificial Neural Networks (ANN), Genetic Algorithm (GA) and Artificial Neuro Fuzzy Inference System (ANFIS). The studies carried on optimization techniques offers better spatial resolution which improves quality and quantity of optical images used for morphological tissues comparable to breast and brain in Near Infrared (NIR) light. Forward problem is based on the location of sources and detectors solved statistically by Monte Carlo simulations. Inverse problem or closeness in optical image reconstruction is moderated by different regularization techniques to improve the spatial and temporal resolution. Compared to conventional methods the ANFIS structure of optimization for forward and inverse modeling provides early detection of Malignant and Benign tumor thus saves the patient from the mortality of the disease. The ANFIS technique integrated with hardware provides the dynamic 3D image acquisition with the help of NIR light at a rapid rate. Thereby the DOT system is used to continuously monitor the Oxy and Deoxyhemoglobin changes on the tissue oncology.

**Keywords:** Diffuse Optical Tomography (DOT), Near Infrared (NIR), Forward model, Inverse model, Regularization, Artificial Neural Networks (ANN), Genetic Algorithm (GA) and Artificial Neuro Fuzzy Inference System (ANFIS)

### **1. Introduction**

A recent survey was taken in the UK, reported 4,884 deaths from a brain tumor and about 11,633 deaths from breast cancer. The tumor detection is complicated and earlier detection leads to better chances of effective treatment, thereby increasing the survival rate. In the last decade, the concept of imaging has raised by the discovery of the X- ray radiography technique. The imaging techniques are highly meant for diagnostic applications in medical field. Different parts of a body have

different range of absorption level hence the penetration of propagating light photon level varies for each and every organ, whereas this is the major concept considered for imaging. The imaging trend started with the X- ray radiography [1], it provides a one-dimensional image of the bony structures in a photographic film which could give the visualization of bony defects and the soft tissue tracks are identified only after the administration of contrast agents or dyes. The advanced version of the X- ray radiography is the Digital Radiography system which also provides a single plane image and it has the additional features such as data collection system, processing, display and storage system. Here the data obtained can be stored in a memory for future use. The limitation is even after the dye usage only the large variations in soft tissues can be identified.

In X - ray computed tomography the imaging of the organ is done in various angles and the reconstruction is demonstrated mathematically over the computer and displayed on the monitor. For the soft tissue examination, the dye fluids are pumped into the ventricles for providing the variation or contrast in the image. Here the noise increases inherently over the square root of the dose as the dose must be increased to preserve the same amount of noise. Therefore, over dosage leads to the side effects such as skin allergy, then came the existence the nuclear imaging.

Nuclear Medical Imaging (NMI) [1] systems utilize the radioisotopes for imaging. The small amount of radioactive chemicals is injected into the arm vein or inhaled through, and then the amount of radioactivity of the organ is examined using the radiation detectors. NMI includes Emission Computed Tomography which displays the single plane slice of the object with radioactivity, insisting same as.

X - ray computed tomography. In Single Positron Emission Tomography, gamma camera is used to create a three-dimensional representation of the radioisotope injected organ. Positron Emission Tomography (PET) imaging provides the cross- sectional images of positron emitting isotopes, which demonstrate the biological function and even physiological and pathological characteristics. The injected radioisotope may create allergic reactions and it takes hours to get clear from the blood and it's a time-consuming process.

Magnetic Resonance Imaging (MRI) uses a magnetic field and high radio frequency signals to obtain anatomical information about the human body as crosssectional images. The imaging technique needs the subject to be still while imaging, when there occurs a move and it blurs the output image. Radiations utilized here are highly ionized which causes harm and it is a tremendous time consuming and cost inefficient process for early tumor detection. The Ultrasonic imaging system is used for obtaining images of an almost entire range of internal organs in the abdomen. While it is completely reflected at boundaries with gas and there is a serious restriction in investigation of and through gas containing structures. The ultrasonic waves could not penetrate the bony structures hence imaging the brain is impossible.

Diffuse Optical Tomography (DOT) [2, 3] employs near infra-red light of range 700-1000 nm [4] which is non-invasive and non-ionizing radiation, therefore causes no harm or side effects. It has its main application of imaging the soft tissue organs such as the brain and breast for diagnosing tumor using the biological parameters [5, 6] such as oxygenation etc. The brain and breast tumor or lesion can be detected by examining the oxygenated, deoxygenated hemoglobin, water and lipids (proteins). DOT imaging [7] provides a number of advantages, such as reduced size setup in turn lead to portability, real-time imaging, low instrumental cost and less time consumption when compared to the other imaging techniques but is generally known to have a low image resolution which limits its further clinical application. **Table 1**: Compares Biomedical Imaging Modalities- Diffuse optical


*Diffuse Optical Tomography System in Soft Tissue Tumor Detection DOI: http://dx.doi.org/10.5772/intechopen.98708*

#### **Table 1.**

*Comparison of biomedical imaging modalities.*

**Figure 1.**

*Absorption spectra of deoxy-hemoglobin (Hb), oxy-hemoglobin (HbO2), lipids and water.*

tomography evaluated with Computer Tomography (CT), Magnetic Resonance Imaging (MRI), and Positron Emission Tomography (PET). The parameters namely cost, imaging time, size, sensitivity and specificity are compared.

The main absorbers of near-infrared (NIR) light in blood-perfused tissues are Oxy-hemoglobin, deoxyhemoglobin, Lipids (Bulk proteins) and water. NIR Spectral Window absorption spectra are between 650 and 1000 NM are shown in **Figure 1** is obtained from compiled absorption data for water [8] and hemoglobin [9]. Hence, light in this spectral window penetrates deeply into tissues, thus allowing for non-invasive investigations. The NIR light penetration depth into tissues is limited, by the hemoglobin absorption at shorter wavelengths and by the water absorption at longer wavelengths.

Different systems in DOT are Continuous Wave (CW) imaging [6], Time Domain (TD) and Frequency Domain (FD). Continuous imaging is the study of hemodynamic and oxygenation changes in superficial tissues. It requires a source of constant intensity modulated at low frequency. Measuring the intensity of light transmitted between two points on the surface of the tissue is economical. Optimum sensitivity is achieved by a number of distinct sources and detectors. Intensity measurements are sensitive and are unable to distinguish between the absorption

and scattering effects. Time Domain (TD) system uses photon counting detectors, slow but highly sensitive. The temporal distribution of photons is produced in short duration. Short pulses of light are transmitted through a highly scattering medium known as a Temporal Point Spread Function (TPSF). Frequency Domain (FD) [9] system is relatively inexpensive, easy to develop and provides fast temporal sampling up to 50HZ.The system acquires quick measurements regarding the amplitude and phase of scattering and absorption in the frequency domain at high detected intensities.

#### **2. Methods**

#### **2.1 Forward model**

The NIR light propagates within the biological tissue in a turbid medium [5]. Light particles scatters with cell particles and the medium either absorbs or scatter the light. The positions and orientations of scatters are described by mesoscopic and macroscopic. In mesoscopic the particles in turbid media of dense concentration and light transport are modeled by Radiative Transport Equation (RTE) [3]. In macroscopic photon transport on mean free path, diffusion approximation holds good for turbid media. Therefore, the isotropic scattering effect and light transport within the tissues is described by the diffusion Equation.

#### *2.1.1 Radiative transport equation*

Light transport in tissues derived using RTE, assumes the energy particles do not change in collisions hence refractive index is constant with the medium [8]. RTE is used to describe anisotropic field and the photon propagation in tissue, is given by

$$\left(\hat{s}, \nabla I(r, \alpha, \hat{s})\right) + \left(\mu\_a + \mu\_s + \frac{i\alpha}{c}\right) I(r, \alpha, \hat{s}) = \mu\_s \left[f(\hat{s}, \hat{s}')I(r, \alpha, \hat{s})d^2\hat{s}' + q(r, \alpha, \hat{s})\right] \tag{1}$$

I(r, **ω**, ŝ) is radiance with modulation frequency **ω** at point r, in the direction ŝ. μa, μ<sup>s</sup> are absorption and scattering coefficients respectively and c is the speed of light. The scattering phase function.

f (^**s**, ^**s** 0 Þ is used to characterize the intensity of a beam, that is scattered from the direction ^**s** <sup>0</sup> into the direction ŝ. The scattering phase function commonly used Henyey - Greenstein scattering function.

$$f(\cos \theta) = \frac{1}{4\pi} \left[ \frac{1 - \text{g}^2}{\left(1 + \text{g}^2 - 2\text{g}\cos\theta\right)^{3/2}} \right] \tag{2}$$

where θ is the angle between the two directions ŝ and ŝ', and g is the anisotropy factor which is used to characterize the angular distribution of tissue scattering.

The fluence at point r modulation frequency ω and in the direction ŝ is defined by

$$\rho(r,\alpha) = \int\_{4\pi} I(r,\alpha,\mathbf{\hat{s}}) \tag{3}$$

The Monte Carlo Method is used to solve the radiative transfer Equation.

#### *2.1.2 Diffusion approximation*

The directional flux magnitude is less compared to isotropic fluence magnitude within the tissue. The light field 'diffuses' means the scattering interaction dominates over absorption. The diffusion equation [10] approximation is given as

$$-\nabla K(r)\nabla \rho(r,\alpha) + \left(\mu\_a(r) + \frac{i\alpha}{\varepsilon(r)}\right)\rho(r,\alpha) = q\_0(r,\alpha) \tag{4}$$

*μa r*ð Þ absorption coefficient, *q*0ð Þ *r*,*ω* is isotropic source, *Φ*(r, *ω*Þ is photon influence rate with modulation frequency *ω* at position r. The velocity of light in medium c(r) at any point r is defined as *c*0*=n r*ð Þ where c0 is the speed of light in vacuum, n(r) is the index of refraction.

Diffusion coefficient is described as

$$K(r) = \frac{1}{\Im\left[\mu\_a(r) + \mu\_s'(r)\right]} \tag{5}$$

Where the reduced scattering coefficient is μs<sup>0</sup> ð Þ¼ r μs rð Þ ½ � 1 � g rð Þ , g(r) is anisotropy factor. The refractive index mismatch at the tissue boundary is eluded by applying Robin boundary condition (type III). Eq. (5) solved using Finite Element Method (FEM) which provides stable solution [11].
