Construction and calibration of detectors for high-resolution metabolic breast cancer imaging

Each of two detectors used in our Positron Emission Mammography (PEM) system consists of four 36 mm x 36 mm x 20 mm bismuth germanate (EGO) crystal detector blocks coupled to a crossed-wire anode position-sensitive photomultiplier tube (PS-PMT). To facilitate high spatial-resolution imaging, the crystal blocks have been finely pixelated using a diamond saw. In each detector, 36 x 36 1.9 mm x 1.9 mm crystal elements are coupled directly to the PMT window and, on the opposite face of the blocks, 35 x 35 elements are offset by 1.0 mm along both the x-and y-axis of the PS-PMT. As part of a system calibration routine, a novel method for crystal element identification has been developed. This algorithm successfully identifies 59 x 49 crystal elements on each detector face. These results are used to generate a Look-Up-Table (LUT) that is accessed during image formation for the effective correction of spatial distortion inherent in the detectors. Crystal identification also facilitates the capability for accurate energy discrimination, since the detector gain is considered on an element-by-element basis by accessing an energy LUT. Employing a third LUT, which contains the relative efficiencies of individual crystal elements results in improvement in image uniformity from 50% to 13%.