A High-Purity Germanium Imaging System for Limited-Angle Nuclear Breast Tomography
Campbell, Desmond LeOtis
Nuclear Breast Imaging (NBI) addresses the need for improved screening and diagnostic imaging to better identify disease in women with mammographically dense breast tissue. High-Purity Germanium (HPGe) cameras are an appealing technology for single-photon NBI due to their 3D position-sensitivity and superb energy resolution (1% FWHM at 140 keV). Compact HPGe systems have been explored in other disciplines, however extensive work is required to determine its potential benefit to breast cancer imaging. To carry out the objective of exploring HPGe-detector potential performance for NBI, Monte Carlo computational methods are employed to accomplish three specific aims. First, the planar imaging performance of a dedicated HPGe breast system is compared to a commercial Cadmium Zinc Telluride (CZT) imaging system. HPGe simulations are modeled and validated against the intrinsic and extrinsic properties of an existing compact HPGe detector. Second, various parallel-hole collimators are evaluated for incorporation into a limited-angle tomographic (LAT) acquisition scheme for a single camera. A pre-computed system matrix mapping the field of view to detector space is determined and applied for iterative reconstruction using projections with depth-of-interaction estimation. Third, a dual-head imaging system is investigated by acquiring two sets of projection data from opposing-view HPGe cameras and testing different image processing methods for combining collected projections. These simulation studies reveal several advantages to HPGe detectors in NBI. Compared to CZT, HPGe detectors provide a 23% higher count sensitivity, less contributions from out-of-view sources and better scatter rejection. Equivalent to better tumor contrast and SNR for 1-cm diameter tumors are observed with HPGe detectors over CZT. Wide-bore, registered collimation provided further enhancements to count sensitivity without sacrificing total (combined intrinsic and collimator) spatial resolution or image quality. Utilizing the LAT acquisition with a single or two-camera imaging system grants higher SNR and detection capability for tumors than planar or conjugate counting methods, respectively.