Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 22-5. SPEeT images created by filtered backprojection. The projection images were filteredusing the filter kernels shown in Fig. 22-4. Left: An image produced using filter kernel A,which exhibits a significant loss of spatial resolution. Center: An image produced using filter kernelB, which provides a proper amount of smoothing. Right: An image produced using the rampfilter, which shows good spatial resolution but excessive statistical noise.The calculated projection images are compared with the actual projection imagesand, based on this comparison, the assumed activity distribution is adjusted. Thisprocess is repeated several times, with successive adjustments to the assumed activitydistribution, until the calculated projection images approximate the actual projectionimages.fu was stated above, in each iteration projection images are calculated from theassumed activity distribution. The calculation of projection images uses the pointspread function of the scintillation camera, which takes into account the decreasingspatial resolution with distance from the camera face. The point spread function canbe modified to incorporate the effect of photon scattering in the patient. Furthermore,if a map of the attenuation characteristics of the patient is available, the calculationof the projection images can include the effects of attenuation. If this isdone, iterative methods will partially compensate for the effects of decreasing spatialresolution with distance, photon scattering in the patient, and attenuation inthe patient.Iterative methods are computationally less efficient than filtered backprojection.However, the increasing speed of computers, the small image matrix sizes usedin nuclear imaging, and development of computationally efficient algorithms havemade iterative reconstruction feasible for SPECT.x- or gamma rays that must traverse long paths through the patient produce fewercounts, due to attenuation, than do those from activity closer to the near surface ofthe patient. For this reason, transverse image slices of a phantom with a uniformactivity distribution, such as a jug filled with a well-mixed solution of radionuclide,will show a gradual decrease in activity toward the center (Fig. 22-6 left). Attenuationeffects are more severe in body SPECT than in brain SPECT.Approximate methods are available for attenuation correction. One of the mostcommon, the Chang method, assumes a constant attenuation coefficient throughoutthe patient. Approximate attenuation corrections can over- or undercompensate