Physical attributes of single-photon tomography.

Physical properties important for assessment of the potentials of emission computed tomography implemented by collimated detector systems include sensitivity, statistical and angular sampling requirements, attenuation compensation, resolution uniformity, and multisection design constraints. The limited angular range and sampling interval of coded aperture methods for longitudinal tomography impose severe limitations on quantitative imaging capabilities. These methods are discussed. Disadvantages of limited angular range are avoided by transverse section devices that have lower sensitivity than comparable positron devices. It is shown here, however, that the sensitivity for a single section device for head transverse section 2 cm thick can be congruent to 200 events sec-1 microCi-1 per axial cm for 2 x 2 cm resolution. This is 40% of that for a well-designed positron system of similar resolution. The problem of attenuation compensation for constant attenuation, as in brain imaging, is well understood and a fast algorithm of the convolution type gives excellent results if angular sampling is over 360 degrees. Though there is a need to move the single-photon detector array over wide angular distances for adequate sampling, dynamic ECT is possible for the measurement of biological washout kinetics of clinical importance, such as clearance studies in brain. Based on physical principles, experiences with low sensitivity devices, and the prospects for devices with sensitivity comparable to positron tomographs, single photon tomography has sound potentials for research and clinical studies of the adult brain and whole body in small subjects. Practical whole-body tomography in adults is limited to nonquantitative lesion detection.