Computer tomography (CT) is a radiological imaging method which yields transverse tomographic images reflecting with high accuracy the spatial distribution of x-ray attenuation in the part examined. The contrast resolution achieved by CT permits the visualization of soft tissue structure heretofore invisible with conventional radiographic procedure. The CT image is reconstructed by a computer applied algorithm, from a series of x-ray attenuation measurements obtained at different angles around the subject, by means of radiation detectors; The signal to noise ratio for these measurements is optimized by reducing the contribution of scattered radiation and of system noise. The CT principle of image reconstruction is also applicable in nuclear medicine imaging, with the goal of achieving contrast improvement and quantitative assessment of radionuclide distribution over conventional "projection" techniques. In this application, however, the attenuation of the radiation in the tissue interposed between the radionuclide and the detector must be taken into account to yield quantitatively accurate images. Furthermore, the variation of the field of view of the collimator as a function of distance contributes another unwanted variable to the reconstruction process. A mathematically rigorous tomographic reconstruction which would account for these variables is unavailable for gamma ray-emitting radionuclides, although approximate solutions of that problem have led to the satisfactory images of the head. Images of larger inhomogeneous organs are less quantitative. The detection of coincidence annihilation counting of positron-emitting radionuclides allows the accurate correction for the attenuation of that radiation in tissues and permits depth independent collimation which renders these radionuclide particularly suitable for their imaging by CT. A number of CT devices designed for the imaging of positron emitting radionuclides are yielding nuclear medicine images of quality and potential clinical usefulness unequaled by more conventional projection techniques.