Experimental feasibility of multi-material decomposition imaging in small animal SPECT/CT system

The photon counting detectors such as cadmium zinc telluride (CZT) and cadmium telluride (CdTe) have powerful advantages compared to energy integrating detectors. CZT or CdTe can detect individual gamma-ray or x-ray photon with energy discrimination. In recent years, energy-resolving and material decomposition x-ray imaging based on photon counting detectors has attracted attention from biomedical imaging researchers. We evaluated a large-area (20 cm × 20 cm) CZT detector originally built as a radionuclide detector for a small animal SPECT/CT system in combination of a microfocus x-ray source with a general goal of developing a material-decomposition imaging method. In this paper, we present experimental results from a feasibility study of multi-material decomposition imaging using the developed small animal SPECT/CT system. Our small animal SPECT/CT system has the unique capability to arrange detectors and sources flexibly. For the multi-material decomposition scan, the CZT detector and the x-ray tube were re-arranged in line. List-mode data of a phantom containing 7 different materials, with a range of densities and atomic numbers, illuminated by x-ray were acquired. The x-ray exposure conditions were: 50 kVp and 0.5 mA with 1.5-mm Al filtration. The acquired image was corrected for bad and hot pixels, gain, and offset. The attenuation profiles of each material were calculated against the x-ray energy. Measured attenuation profiles of each material were in a good agreement with the reference data of the NIST physics laboratory. In this study, we demonstrated that multi-material decomposition imaging is experimentally feasible using the photon-counting CZT detector and polychromatic x-ray. Since our system allows rotation and a large active area of CZT, we will acquire material-decomposition data tomographically in the near future.