Corrections for detection efciency variations, noise component, random coinci­dences, scatter coincidences, and photon attenuation are made prior to reconstruc­tion in the FBP method. In the MLEM or OSEM method, these factors are incorporated a priori in the estimated image and need not be applied separately. In general, iterative reconstruction methods do not produce artifacts that are observed with the FBP method and provide a better signal-to-noise ratio in regions of low tracer uptake (Fig.12.16). Overall, iterative methods provide high-quality images and are routinely used in image reconstruction in PET and SPECT.Despite many improvements in the OSEM method, it is a big challenge to have good-quality images in obese patients.

The detectors in CT scanners are made of materials such as ceramics, gadolin­ium oxysulde, and gemstone, and in some units, xenon gas. X-rays interact with these detector materials and produce visible light that is processed by a photodiode, producing a signal. Normally, a large number of such detectors are arranged in a full ring or in a partial ring in the form of an arc around the patient. In the full ring sys­tem, the detectors are xed in 360° around the patient and the x-ray tube rotates around (Fig.12.17a), whereas in the partial ring, both the detectors and the x-ray tube are mechanically tied together in 180° opposition in the gantry and the two together rotate around the patient (Fig.12.17b). The detected data are acquired in seconds and stored in a matrix of choice (64×64, 128×128, etc.) in a computer. The stored data are then processed to reconstruct subject images in different projec­tions. Currently, multislice CT scanners are available, providing 6, 16, 64, or 128 slices. In helical or spiral CT scanners, the patient table moves along the body while the x-ray tube rotates around the body, resulting in a spiral pattern of motion of the x-ray tube around the subject. This technique reduces the time of scanning to a few seconds.

Either CT or SPECT imaging can be performed rst, followed by the other. For example, CT images are taken rst with the organ of interest in the CT eld of view. Next, the scan table with the patient in the same position is moved to the center of

the SPECT FOV, and images are taken. Both CT (anatomical) and SPECT (func­tional) images are reconstructed and then fused together by applying appropriate alignment algorithms. Various vendors provide commercially available fusion soft­ware, namely, Syngo of Siemens Healthcare, Extended Brilliance Workspace of Philips Healthcare, MIM of MIMVISTA, and Centricity of GE Healthcare. Because the position of the patient on the table does not change, both CT and SPECT images are aligned very accurately, and the overall accuracy is improved by 20–25% com­pared to either modality alone.

methods of attenuation correction in SPECT imaging, with particular attention to estimate the values of I and x, and given below is a brief description of the com­monly used ones.

(12.8)

(12.9)

Considering Fig.12.19 and applying Eqs. (12.7 and 12.9) becomes