Investigation of photon cross-talk in simultaneous SPECT-CT imaging

The goal of this study was to investigate scatter photon cross-talk in two SPECT/CT imaging systems when performing SPECT and CT scans for the same axial field of view (FOV) simultaneously. The Monte Carlo simulation software, Geant4 Applications for Emission Tomography(GATE), was employed in the investigation. The first system was a clinical SPECT/CT with similar geometrical configurations as the Philips BrightView XCT. We compared the projections and energy spectrums of the X-ray photons first scattered by different objects and then detected by the SPECT camera with parallel-hole and pinhole collimators respectively. The second system investigated was a newly designed small animal SPECT/CT system of which the SPECT and CT modules had been previously developed and tested separately in our lab. In this case, phantoms of both mouse and rat sizes were modeled in simulation to study the scatter cross-talk of the X-ray photons for the SPECT camera with pinhole collimators. The results showed that, in the clinical SPECT/CT system, the probability of the X-ray photons being scattered and then detected by the SPECT camera was in the order of 3.9-6.8*10^-5 and 1.47.5*10^-6 with the parallel hole and pinhole collimators correspondingly. So the estimated count rates of detecting scattered X-ray photons by the SPECT detector were 4070 and 1.4-7.5 MCts/second per rnA X-ray tube current respectively. In the small animal system, the probability dropped to 2*10^-7 and the count-rate estimate was lowered to 250KCts/second per rnA. In conclusion, considering the count-rate capabilities of most current SPECT cameras, scatter photon cross-talk is not a major issue for simultaneous small animal SPECT/CT imaging. It is, however, a major hurdle for simultaneous clinical SPECT/CT imaging.