Using Higher Energy Radioisotopes for Crystal Identification in a SPECT System

A cost-effective solution for high-resolution clinical single-photon emission computed tomography (SPECT) is a set of modular pixelated scintillation crystal detector readout with an array of medium-sized, conventional single-anode photomultiplier tubes (PMTs). This is the approach of C-SPECT, a dedicated cardiac SPECT and X-ray transmission computed tomography (TCT) system, which uses NaI(Tl) scintillation crystals and 51 mm in diameter circular PMTs arranged in a hexagonal pattern. While not a unique difficulty, the process of identification of each crystal pixel using a flood generated by a distant calibration source without collimation is exacerbated by the small pixel size and a small number of PMTs used. As a result, the peak and valley ratios of the raw flood map are too low to be adequate for pixel identification. We investigated the improved performance of using a higher energy radioisotope (i.e., Sodium-22 or Germainum-68) for our crystal identification flood. Improved pixel sharpness with the higher energy radiation source (511 keV photons) was measured and compared to a flood at 140.5 keV. This improvement results in a better outline of pixel boundaries that is crucial near the centers of PMT positions and the edges of the detector module. Finally, we confirmed the suitability and reliability of using this procedure for pixel identification for our nominal use of Technetium-99m and estimate the fraction of events that are misidentified to neighboring pixels.