I. Introduction
Nearly all gamma cameras presently used for single-photon radioisotope imaging in nuclear medicine are based on the detector system proposed by Anger [1] which consists of a large NaI(Tl) crystal optically coupled to an array of photomultiplier tubes. Several new detector designs have been proposed, investigated and developed in view of overcoming the limitations of the conventional gamma camera. Two main lines of technical developments have been followed: one based on bundles of small, optically isolated scintillators coupled to an array of photodetectors (position sensitive photomultipliers or silicon photodiodes) [2]–[4]; the other relies on pixellated cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) detectors with direct conversion of the -rays into electrical charge [5]. As for the scintillator devices, submillimeter spatial resolution has been achieved with CsI(Tl) and YAIO3: Ce but the uniformity, energy resolution and cost are problems. Concerning the CdTe and CdZnTe detectors, they have very good position and energy resolution but they are expensive and only small size cameras have been built so far.