Progress in developing a combined x-ray CT and SPECT system using CdZnTe detector

A medical imaging system providing both x-ray transmission and radionuclide measurements would allow correlation of structural and functional information. We therefore are evaluating a pixellated CdZnTe detector for combined x-ray CT and SPECT imaging with various readout electronics. Gamma-ray spectra of 57Co measured using NIM electronics (2-microsecond(s) shaping time) and multichannel fast photon-counting electronics (50-ns shaping time) produced energy resolutions of 6.5 keV FWHM and 17 keV FWHM respectively at 122 keV. Fast photon-counting electronics achieved linear x-ray count-rate response up to 4 X 105 cps. Dual-mode digital readout electronics are described, which promise to improve SPECT and x-ray CT performance in comparison to the fast-counting electronics. The leakage current and x-ray response with the dual-mode electronics are studied. The leakage current as small as tens of pA is measured, while detector current over 5 orders of magnitude is measured with linearity over 4 orders of magnitude. Results suggest that the CdZnTe detector is capable of performing both x-ray CT and SPECT with the fast photon-counting electronics, and the digital readout electronics can improve the x-ray CT performance.