**3. Dual energy imaging**

134 Imaging of the Breast – Technical Aspects and Clinical Implication

after having passed through simulated tissue and bone (150mm soft tissue + 50 mm water + 30 mm bone). This exit beam now shows that only X-ray photons with >37 keV are available to contribute to the image. The very low energy input photons are completely absorbed in

Fig. 1. Comparison of the ratio of the incident photons (original beam) to the exit photons after going through different body tissues including water. Only the exit photons contribute

Figure 1 implies that the spectrum of the input x-ray beam has to be optimized for each imaging modality for minimum patient dose. For instance, a thicker body part such as the pelvis, which contains lots of bony structures, absorbs most of the low energy X-ray beams below 30 keV so we need to use higher X-ray beam energies to get a reasonable number of exiting x-ray photons for imaging while avoiding overdosing the patient with low energy X-

By contrast, imaging of the breast, which is smaller (thinner) relative to the pelvis and does not contain any bony structure, requires much lower X-ray energy to get sufficient X-ray

1 Note that some low energy X-rays are removed by the inherent filtration of the X-ray tube. In many cases the exit window of the X-ray tube is made of 2mm Aluminum which removes most of the X-ray

So, how can we optimize contrast of the x-ray image using the minimum dose?

to the imaging.

radiation below 15 keV.

**2. Using the right x-ray energy range** 

rays which would be completely absorbed in the thick pelvis.

the body, adding extra patient dose, because there is no exit photon for imaging1.

In many cases X-ray images must capture both hard (bony) structures and soft tissues, which may be overlapping, in the same picture. A method to separate two different tissues, and optimize the contrast between them, is to acquire two images at different x-ray energies and do a weighted subtraction. This is called dual energy imaging. Taking images at low and high energies and doing a subtraction can remove either the soft or the bony structure depending on the techniques used.

## **3.1 Brief theory of dual energy imaging**

The data processing methods that make use of dual energy data are referred to as decomposition techniques and are divided into three classes based on the type of information returned by the technique. The three types of information about the object being imaged that may be returned by decomposition techniques are: (1) effective atomic number and density; (2) attenuation due to Compton scattering and electron density; or (3) the physical densities of two known components (Walter, Wu et al. 2004).

<sup>2</sup> This range will vary depending on the size and density of the breast.

Contrast Enhancement in Mammography Imaging Including K Edge Filtering 137

greatly improved the visibility of malignant cancerous tissues (Chen, Jing et al. 2006; Arvanitis, Royle et al. 2007; Puong, Bouchevreau et al. 2007), the combination of dual energy CT and Iodine contrast together offers yet more improvements to our ability to separate different tissues from one another in an image (Carton, Lindman et al. 2007), (Puong,

Dual energy imaging is used in many fields of medical imaging, such as for lung imaging when we need to separate (or suppress) the overlapping rib shadows from the soft lung tissues to enhance visualization of lung nodules (Kashani, Varon et al. 2010), or, similarly, for digital angiography to visualize the heart behind the ribs (Ersahin, Molloi et al. 1994). In many cases dual energy angiography is used in combination with Iodine (I) or Gadolinium

Dual energy imaging is an important technique for performing bone densitometry tests (Shimura, Nakajima et al. 1993) and specialized systems have been developed and sold by

Generally dual energy imaging is implemented by use of X-ray tube voltage switching to obtain two images; one at low and one at high kVp values. To avoid motion artifacts

Different methods and detectors have also been developed for dual energy imaging. Coello et al. (Coello, Dinten et al. 2007) built a system where, instead of switching voltages, a filter wheel containing multiple filter materials is rotated in front of the X-ray tube, providing different X-ray spectra. This paper also provides an excellent overview of the theory of dual

Beam hardening has a significantly deleterious effect on the precision of CT reconstruction. It can be avoided by using monochromatic X-ray beams. Furthermore, for each imaging object there is an ideal energy at which enough X-ray photons make it through the object to allow for low noise imaging while still using a low enough X-ray energy to provide the best contrast ratio. Synchrotrons are ideal sources of monochromatic beams of high flux. Some experiments on breast imaging with synchrotrons were made and compared to normal mammographic images (Fiedler and et al. 2004). A further advantage of monochromatic imaging is that it allows for phase contrast imaging, which greatly improves the contrast even between different soft tissues. However, the size and price of a synchrotron make it impractical for general radiographic use. X-ray tubes with monochromators can provide monochromatic x-ray beams but the flux is too low for practical applications (Donath,

Different methods have been developed to approximate monochromatic beams i.e. to provide quasi monochromatic beams, which consist of not a single X-ray energy but a

energy imaging and its optimization for best signal to noise ratio and image contrast.

between the two images quick changes in tube voltage are necessary.

Patoureaux et al. 2007; Saunders, Samei et al. 2008).

(Gd) contrast agents (Fiedler, Elleaume et al. 2000).

GE and Hologic (GE ; Hologic).

**3.3 Dual energy imaging methods** 

**4. Monochromatic X-ray imaging** 

**5. Quasi monochromatic X-ray beam imaging** 

**5.1 Generation of quasi monochromatic beams using diffraction** 

Pfeiffer et al. 2008).

narrow range of X-ray energies.

The ability to determine material composition from dual energy techniques fundamentally results from the fact that the linear attenuation coefficient of X-ray absorption, ((E)), for an element has a unique functional dependence on photon energy. Over the range of x-ray photon energies used, the attenuation coefficient is dominated by two major attenuation processes: the photoelectric effect and Compton scattering. A major simplifying factor of the analysis is that each of these two processes has a fixed and unique functional dependence on energy that can serve as a linear basis set for any material. That is,

$$
\mu(\mathbf{E}) = \mathbf{x}\mu\_\mathbf{p}(\mathbf{E}) + \mathbf{y}\mu\_\mathbf{c}(\mathbf{E}) \tag{1}
$$

where p(E) is the linear attenuation as a function of energy due to the photoelectric effect and c(E) is the linear attenuation as a function of energy due to the Compton scattering effect and x and y are material specific constants.

A consequence of this relationship is that the energy dependence of any material's attenuation coefficient can be expressed as the linear combination of any two other materials. Therefore, each material can be characterized by two density values, pA p*B*, which are derived from the attenuation measured at two different kVp spectra, A(E), B(E). As in,

$$
\mu(\mathbf{E}) = \mathbf{x}\mu\_{\mathrm{p}}(\mathbf{E}) + \mathbf{y}\mu\_{\mathrm{c}}(\mathbf{E}) = \mathbf{p}\_{\mathrm{A}}\mu\_{\mathrm{A}}(\mathbf{E}) + \mathbf{p}\_{\mathrm{B}}\mu\_{\mathrm{BC}}(\mathbf{E})\tag{2}
$$

The important consequence for image formation is that these two material density values are available to encode the pixel values apart from or in combination with Hounsfield units in the case of CT imaging. Materials with similar density can now be differentiated based on average atomic number. Of particular interest is the use of contrast agents with atomic numbers significantly different from the usual materials present in the body as these agents will show up on images in high contrast to the normal materials of the body. Moreover, because the dual energy analysis makes explicit use of the energy dependence of the attenuation, the beam hardening artifacts are absent here, a distinct advantage over conventional CT. Also, in the dual energy technique as opposed to the conventional approach, the accuracy of the CT number associated with pixels is not affected by the beam hardening corrections which must approximate the energy dependent attenuation with data from a single energy value.

#### **3.2 Dual energy imaging applications**

Dual energy imaging was used in CT scanning as long ago as 1976 (Alvarez and Macovski 1976) when it was also being used for the exact determination of the atomic number of elements (Rutherford, Pullan et al. 1976). It has been used for mammography as well. Johns et al. described the first applications of dual energy imaging to mammography (Johns, Drost et al. 1983). Later work optimized the method to get the best SNR with minimum dose (Johns and Yaffe 1985). Since that time, several articles were published about optimization of parameters in dual energy breast imaging. Boone at al. (Boone, Shaber et al. 1990) analyzed detector parameters, effects of X-ray parameters and filtrations and the effect of scatter on the quality of the dual energy images. Kappadath et al. (Kappadath, Shaw et al. 2004) used digital subtraction techniques and a method they developed called DEDM (Dual Energy Digital Mammography) to improve the visibility of micro calcifications.

Dual energy techniques have also been used to improve Computed Tomography (CT) imaging. While 3D CT scans and the use of Iodine (I) as a contrast agent have, on their own, greatly improved the visibility of malignant cancerous tissues (Chen, Jing et al. 2006; Arvanitis, Royle et al. 2007; Puong, Bouchevreau et al. 2007), the combination of dual energy CT and Iodine contrast together offers yet more improvements to our ability to separate different tissues from one another in an image (Carton, Lindman et al. 2007), (Puong, Patoureaux et al. 2007; Saunders, Samei et al. 2008).

Dual energy imaging is used in many fields of medical imaging, such as for lung imaging when we need to separate (or suppress) the overlapping rib shadows from the soft lung tissues to enhance visualization of lung nodules (Kashani, Varon et al. 2010), or, similarly, for digital angiography to visualize the heart behind the ribs (Ersahin, Molloi et al. 1994). In many cases dual energy angiography is used in combination with Iodine (I) or Gadolinium (Gd) contrast agents (Fiedler, Elleaume et al. 2000).

Dual energy imaging is an important technique for performing bone densitometry tests (Shimura, Nakajima et al. 1993) and specialized systems have been developed and sold by GE and Hologic (GE ; Hologic).
