Development of a Small-Animal SPECT System with a High-Purity Germanium Detector

Abstract

Advantages of high-purity germanium (HPGe) detectors over traditional scintillator detectors include excellent energy resolution, approximately 1% at 140 keV, which allows for better scatter rejection and simple multi-isotope acquisitions, and depth of interaction estimation, which can help reduce parallax error when incorporated into the reconstruction process. An HPGe system that is mechanically cooled, 90 mm in diameter, 10 mm thick, and comprised of two sets of 16 orthogonal strips that are each 4.75 mm wide with a 5 mm strip pitch was characterized. The detector has ~1.5 mm spatial resolution, 0.92% energy resolution at 140 keV, 55.4% intrinsic efficiency at 122 keV and has a flood-corrected integral uniformity of 3%. The HPGe-SPECT system has a single-pinhole collimator with a 1-mm diameter and a 70 degree opening angle with a focal length variable between 4.5 and 9 cm. An MLEM reconstruction algorithm was developed that utilizes an analytical system matrix derived from the system’s calibration parameters and helical scanning was implemented to allow for an extended axial field of view. Images of a hot-rod phantom and NEMA NU 4-2008 phantom are used to quantify the system’s image quality and one dual-isotope experiment demonstrated the advantage of HPGe’s energy resolution over a commercial system. Simulations were performed to investigate whether a stacked-detector configuration with both HPGe and a silicon detector, used with ¹²³I (27-32, 159 keV), where little or no multiplexing occurs in the Si projections can help to compensate for the image degradation caused by the multiplexed HPGe projections. Multiple phantoms were simulated to investigate image quality quantitatively, while the differential point response function was used to qualitatively evaluate multiplexing artifacts. Results indicated an improvement in image quality using the stacked-system over HPGe alone, and therefore future work will involve building an HPGe-Si stacked system.