Studies of CZT for PET applications

CdZnTe (CZT) has been investigated by several researchers as a detector material for positron emission tomography (PET) applications. CZT detectors can be manufactured into 1 cm³ or larger detectors with pixelated anodes, providing high spatial resolution at the 1 mm level or smaller. Indeed the spatial and energy resolution of CZT can be far superior to those of current state-of-the-art PET detectors, most of which are scintillator-based. On the other hand, at 511 keV its timing performance and photopeak detection efficiency are generally inferior, which pose challenges that must be surmounted. In order to obtain sufficient efficiency with a practical number of electronics channels and interconnections in a realistic full-scale system, the focus is on thick detectors (∼10 mm). However, the timing becomes more challenging with increasing thickness due to the low charge mobility. We evaluated planar and coplanar grid CZT detectors, as well as position-sensitive pixel-anode detectors to assess methods of improving timing performance. For a 7.5-mm thick coplanar grid detector at 1000 V bias, we obtained a preliminary time resolution vs. BaF₂ detector of 21 ns FWHM by fitting the digitally sampled rising edge of the cathode signal. Optimization of the front-end electronics and data-processing methods is expected to further improve these results. We are currently characterizing the performance of a pixel cube detector of ∼1 cm³ in size with a single cathode and a 4 × 4 array of anodes with 2.5-mm pitch on the opposing side, for which a data acquisition system has been designed and fabricated.