Modeling of Pixelated Detector in SPECT Pinhole Reconstruction

A challenge for the pixelated SPECT detector is that the detector response of a gamma-ray photon varies with the incident angle and the incident location within a crystal. The normalization map obtained by measuring the flood of a point-source at a large distance can lead to artifacts in reconstructed images. In this work, we investigated a method of generating normalization maps by ray-tracing through the pixelated detector based on the imaging geometry and the photo-peak energy for the specific isotope. The normalization was defined for each pinhole as the normalized detector response for a point-source placed at the focal point of the pinhole. Ray-tracing was used to generate the ideal flood images for a point-source at 360 mm distance and at pinhole focal point (with pinhole collimator off). To calculate crystal efficiencies, the 360 mm ideal flood image was divided pixel-by-pixel by the conventional point-source flood measurement at the same distance. The normalization for the pinhole was obtained by multiplying the ideal flood image at the pinhole focal point by the crystal efficiencies and being scaled by the incoming photon flux image. The normalizations were incorporated in the iterative OSEM reconstruction as a component of the projection matrix. Applications to single-pinhole and multi-pinhole imaging showed that this method greatly reduced the reconstruction artifacts.