Geometrical calibration for an animal PET converted SPECT

This work is part of our continued effort on using a collimator inside an animal PET scanner to make a cost-effective animal SPECT. As compared to previous design, it employed two new components for SPECT imaging: 1) two replaceable collimators for different applications: a slit-slat insert for large field of view and a pinhole insert for high resolution, and 2) the collimator is rotated for tomographic imaging. The SPECT system´s geometry parameters include center of rotation (COR) and slit aperture center (SAC) positions for the slit, septa-to-crystal position (SCP) in axial direction for the slat, and center of rotation (COR) and pinhole aperture center (PAC) positions for the pinhole. To calibrate these parameters, we first derived a transaxial and an axial projection model based on the centroid location and the full profile of the point spread functions (PSF), respectively. The models were then used in a least-square based calibration method. We demonstrated the accuracy of calibration method with Monte-Carlo simulated point source studies. The calibrated geometrical parameters were used in both analytical and simulated system matrix derivation. Image reconstruction of phantom and animal studies was performed to demonstrate imaging performance. We showed that 1) the proposed calibration method reaches a unique and accurate solution, 2) the estimation error is less than 0.10 mm for COR/SAC/PAC and 0.05 mm for SCP calibration, and 3) high resolution images and volume imaging capability were achieved with both slit-slat and pinhole collimator inserts. We conclude that the calibration methods developed was effective.