System calibration and statistical image reconstruction for sub-mm stationary pinhole SPECT

Iterative SPECT reconstruction requires position-dependent point spread functions (PSFs) specifying the response of the detectors to a point source at every position in the instrument. To obtain accurately reconstructed images, the PSFs should incorporate all important effects of photon transport and imaging geometry. The PSFs can be measured directly, but measuring in each voxel of the object space can be impractical for sub mm-resolution instruments. This study describes a method for generating complete PSF lookup tables from a limited number of point source measurements for a stationary small-animal SPECT system with 75 pinholes (U-SPECT-I, J. Nucl. Med., 2005, pp. 1194-1200). The method is based on modeling the shape of the PSFs, measured at a limited number of positions. Subsequently the model parameters are generalized to estimate and shape PSFs at missing positions. The method enables us to obtain 0.45 mm reconstructed resolution with 0.6 mm pinholes and 0.35 mm resolution with 0.3 mm pinholes with a SPECT system which has 3.2 mm intrinsic detector resolution.