A single CdZnTe detector for simultaneous CT/SPECT imaging

Clinical CT/SPECT systems acquire CT and SPECT data sequentially using different detectors in close proximity to minimise patient movement and interscan delay. We have developed a prototype simultaneous CT/SPECT imager, using a single CdZnTe detector, with the goal of improving image coregistration and decreasing scan time. A 16-pixel CdZnTe detector was operated in pulse-counting mode with 50 ns shaping time. Energy discrimination is used to separate the CT and SPECT data. Simultaneous SPECT and CT images were obtained for a phantom with the X-ray flux limited to reduce pulse pile-up in the radionuclide energy window. At 140 keV, the efficiency and energy resolution are 70% and 10%, respectively, and were constant for fluence rates up to 103 cps per detector element for 140 keV gamma rays, but degrade rapidly at higher fluence rates. In pulse-counting mode, the maximum count rate of 103 cps per element from the CdZnTe detector is sufficient for SPECT imaging, but is considerably lower than the fluence rates encountered in CT. The smallest lesion visually detectable in SPECT is 9 mm and the CT spatial resolution is smaller than 4.5 mm. Image registration is intrinsic because the data can be acquired simultaneously with a single detector with the same reconstruction geometry.