Emission computerized axial tomography, ECAT, gamma camera systems are now available commercially from several manufacturers. To be cost effective the gamma camera must also be used for standard radionuclide imaging. Physical evaluation of these systems in tomographic mode is still somewhat of an art rather than a science since the performance of the equipment in tomographic mode depends to a large extent on the skill of the operator and his knowledge of the entire tomographic reconstruction process. This paper attempts to describe and explain the procedures involved in producing ECAT images using the gamma camera. The interelationships of the relevant parameters are indicated. Physical evaluation of the process is illustrated using a commercially available system, in particular the ability to produce quantitatively accurate reconstructions of clinically relevant data. The gamma camera is the 40 cm field of view Maxicamera/61 400T (IGE Ltd.) and the image processor if the IMAC 7310 (ADAC/CGR Ltd.). Keyes,-W.I. (Cumberland Infirmary, Carlisle (UK))