Improving Position Estimation in Double-Sided Strip (DSS) High-Purity Germanium HPGe Detectors for Gamma-ray Imaging

Abstract

Double-sided strip (DSS) high-purity germanium (HPGe) detectors are potentially useful for nuclear medicine imaging, particularly in Single Photon Computed Emission Tomography (SPECT), where high energy and high spatial resolution are required for an accurate reconstructed SPECT image. The DSS electrode configuration allows for sub-strip spatial resolution, reducing the number of readout channels required. Reconstructed images in our first prototype small-animal SPECT system exhibited artifacts, which we attributed to mis-positioning of events near the strip edges. To study the detector response and acquire data to facilitate the development of an alternative position estimation method, we scanned a detector with a focused beam (~25x25 $\mu m^{2}$ at 131 keV, Beamline 6-ID-D) at the Advanced Photon Source at Argonne National Laboratory. Maximum-likelihood position estimation was performed at each of nine depths separately using 1-D (independent x and y estimation) and 2-D multivariate Gaussian models. Results show that we can reduce the distortion of mis-placed events at the edge, with a trade-off in spatial resolution. We have also used the Fisher Information Matrix to calculate the Cram\'{e}r-Rao lower bound to probe the expected limiting spatial resolution as a function of interaction location. The variation in our spatial resolution did not reach the Cram\'{e}r-Rao Lower Bound limit; which could be due to the following detector limitations: photo-electron range, the size of the charge cloud, and Compton scatters.