Animal SPECT imaging on a shared PET/SPECT ring detector with elliptical-pinhole collimator

In this study, animal SPECT imaging functionality was developed in Nucmed Inliview 3000 PET/SPECT/CT system with an elliptical-pinhole collimator in the shared PET/SPECT ring detector. The collimator was designed to allow simultaneous PET/SPECT dual-tracer imaging. It contains 5 pinhole rings with 10 pinholes per ring. The transverse plane of pinhole was designed to be elliptical to reduce the axial overlap of the SPECT projection. A modified sensitivity formulae for elliptical pinhole was derived and penetration effect was considered in this formulae. Helical SPECT data were acquired by rotating the collimator about the body axis and translating the animal bed along the same axis. Geometrical calibration was implemented to determine the actual parameters of multi-pinhole collimator. OS-EM algorithm was developed and the system matrix was obtained by analytical calculation with the modified sensitivity formulae for elliptical pinhole involved. The penumbra field of elliptical pinhole was defined. Significant artifact reduction was observed after incorporating the penumbra field effect in the reconstruction process. An ultra-micro hot-rod phantom study was performed. The 1.35 mm hot rods can be clearly identified from the reconstructed image. A whole-body mouse bone scan image was presented and shows that the whole skeleton and the vertebra can be clearly seen. A reasonable SPECT/CT fusion image was provided. We conclude that reliable SPECT images can be achieved with this PET/SPECT/CT system with the proposed collimator design and methodology development.