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Clinical Mammographic and Tomosynthesis Units
Published in Paolo Russo, Handbook of X-ray Imaging, 2017
The intensity of the X-rays emitted is not uniform in the cathode-anode direction, but the number of emitted photons decreases on the anode side compared to the cathode side, because of the heel effect (Andolina and Lille 2010). Thereby, the X-ray tube is positioned with the cathode corresponding to the chest wall of the patient and the anode to the nipple area, to match the shape of the breast; in fact, despite the breast compression, the posterior part of the breast, close to the chest wall, remains thicker, requiring more photons, while the anterior part, close to the nipple, is thinner, and, consequently, a lower X-ray intensity is sufficient to produce a good image.
Diagnostic X-ray sources from the inside
Published in Rolf Behling, Modern Diagnostic X-Ray Sources, 2021
In the past, all-tungsten or all-molybdenum targets often suffered from reduced anode angles, reduced photon flux due to the heel effect, and reduced radiation field. Sintered and forged titanium–zirconium–molybdenum (TZM) anodes, coated or sintered with a tungsten–rhenium top layer, have turned out much more resistant to creeping under high temperatures. The segmented anode in Figure 6.60 is basically free of geometrical distortion.
Radiation Sources
Published in Harry E. Martz, Clint M. Logan, Daniel J. Schneberk, Peter J. Shull, X-Ray Imaging, 2016
Harry E. Martz, Clint M. Logan, Daniel J. Schneberk, Peter J. Shull
Transmission-anode geometry has a second attribute that can be useful. Because of the axial symmetry, the spectral and irradiance variation (heel effect) are very much less than in a common tube (see Figure 8.13 and Section 8.4.1.1.8, Heel Effect) in which the x-ray beam is not normal to the anode surface.
Estimation of the sugar content of fruit by energy-resolved computed tomography using a material decomposition method
Published in Journal of Nuclear Science and Technology, 2021
Here, is the X-ray energy spectrum calculated using the formula developed by Birch et al., and are the linear attenuation coefficients of Al and W, respectively, and and are the thicknesses of Al and W, respectively, used to correct to reproduce measured electric current. The W thickness correction is for taking into account the Heel effect, which is influenced by the X-ray path length inside the W target [19]. The response function can be written as,
Filter-based energy-resolved X-ray computed tomography with a clinical imager
Published in Journal of Nuclear Science and Technology, 2019
Tien-Hsiu Tsai, Takumi Hamaguchi, Hiraku Iramina, Mitsuhiro Nakamura, Ikuo Kanno
In Figures 5, 7, and 8, the spatial distribution of noise in each image is not uniform, and the lower part is noisier than the upper part. This can be explained by the projection data, i.e. the sinogram, after material decomposition. Figure 9 is the iodine sinogram of the case of ‘Sn/none’ and iodine tincture (A). Due to the iodine content in the contrast agent, a bright part appears at the center of the sinogram. On the right and the left, where iodine should not exist, the values are not completely zero because of detection noise. In addition, the non-zero values on the left are much more than those on the right, which results in nonuniform noise distribution in the CT image after a half-scan image reconstruction. The reason for this is considered to be the spatial variance of the X-ray beam quality. In two-channel imaging, the iodine result will be overestimated if the output of the high-energy channel is higher than expected. In other words, the noisy left part in Figure 9 may imply that the real spectrum there is harder (i.e. the energy is higher) than expected. In Sections 2.1 and 2.2, we assumed that the X-ray spectrum is spatially uniform as was obtained at the center of the detector. However, the beam quality of a real X-ray beam may be not completely uniform due to, for example, the heel effect of the X-ray tube [1,2]. For verification, we divided the detector into three parts (left, center, and right), estimated the correction factors for each part, and then performed the material decomposition separately. The resulting virtual monochromatic images are shown in Figure 10. After dividing the detector into three parts, the distribution of noise became more uniform (Figure 10(b)). Also, during the analysis, the corrected spectrum of the left part was slightly harder than that of the central part, which matches the expectation. Although the image noise in Figure 10(b) is not completely eliminated, this result demonstrates the influence of the spatially variant beam quality on the material decomposition. In practical use, dividing the detector into three parts may be insufficient. A spatially fine spectrum correction would be necessary, especially when imaging with a bowtie filter [1,2,15], which can drastically change the spatial uniformity of the beam quality. With the proposed beam-quality correction method described in Section 2.1, obtaining such fine data should be achievable within a reasonable length of time because of the reduction in required measurements.