Angular-Dependent Axial-Shift Correction for Pinhole SPECT

Compensation for pinhole-SPECT calibration parameters has been previously shown to be important for achieving good resolution. It has also been shown that angular-dependent (AD) compensations for the variation of radius of rotation can reduce reconstruction artifacts. In this study, we evaluate the effect of AD compensation of axial mechanical shifts (AMS) on the axial resolution of reconstructions. Measuring the AD AMS was accomplished using point-source scans. The centroids of the scans were fitted to their expected values in the transverse and axial directions. The differences in the axial direction between the data and fit were used to calculate the AD AMS under the assumption that the differences were due only to this shift. The effect on reconstruction was evaluated using a cold-disk phantom with 1.6 mm thick disks. Triple-head acquisitions using circular and helical paths were performed. The data were reconstructed with and without the calculated correction. The results show that the amplitude of the axial shift is approximately 0.5-1.0 mm for our system. The angular dependence of the heads is similar and suggests that the paths of the heads are distorted by gravitational effects. The circular and helical reconstructions show substantial improvement with compensation. In conclusion, AD compensation for AMS has a large impact on reconstruction quality. It is likely that the further development of other AD compensations may also have a large impact on resolution