Emission computed tomograph (ECT) consists of detector system, mechanism for scanning, electronics for data access and control, and computer system. ECT images have to be reconstructed carefully to correct for tissue absorption, to reduce scattered radiation and so on. These were described in detail. Also here mentioned basic design and performance of HEADTOME system developed in our institute as a first dual purpose emission computed tomograph for positron and single photon emission imaging. The system has shown the resolution of 10 and 11 mm FWHM at center (of field of view) with ^<99m>Tc and ^<68>Ga, respectively, and the sensitivity of 20 Kcps/μCi/ml (with 20 mm slice spacing). The HEADTOME is routinely used in clinical study.
