SPECT instrumentation: performance, lesion detection, and recent innovations

The use of a gamma camera for single photon emission computed tomography (SPECT) imaging places greater demand on camera performance than does planar imaging, since camera specifications that are adequate for planar imaging may produce serious image artifacts in SPECT. Manufacturers have responded to the challenge with improvements in both hardware and software, but the user must pay careful attention to quality control procedures. Field nonuniformity is caused mainly by spatial distortion, which may vary during rotation; some cameras incorporate a means of automatically correcting for angular changes. The purpose of performing SPECT is to improve lesion detection, ie, to improve contrast, which is influenced by many factors. Attenuation corrections may be applied either before or after reconstruction. For Tc-99m an attenuation coefficient less than that for water, ie, either 0.11 or 0.12, would appear to give better results; the visual appearance is improved but the effect on contrast is minimal. Scattered photons are the major cause of loss of contrast in SPECT images. Recent developments in on-line energy corrections allow the use of narrower photopeak windows as well as asymmetric energy windows. Offpeak energy windows have demonstrated significant improvements in contrast; however, the increase in nonuniformity can cause artifacts which may limit their use. Simultaneous collection of separate images, one in the photopeak region and the other in the scatter region (92 to 125 keV) may enable a transaxial scatter image to be subtracted from the transaxial photopeak image. This may permit quantification of the true radioactivity distribution. New collimator designs introduced to improve SPECT resolution include cast collimators which can be made with more uniform hole construction than the lead-foil type; for brain imaging, long bore parallel hole and converging fan beam collimators as well as astigmatic collimators which converge in both planes with different lines of focus. Some of these improve both sensitivity and resolution compared to parallel hole collimators. Noncircular orbits have also been introduced in order to improve resolution, but their use presents many problems which require careful monitoring. Transaxial multicrystal systems have been developed over many years, but have not been widely used, primarily because of their inherent complexity, high costs, and limited applicability. Some of these drawbacks may be overcome by a new type of SPECT camera currently under development by several different groups.(ABSTRACT TRUNCATED AT 400 WORDS)