Correction of spatial distortion, gain nonuniformity and efficiency variation in detectors for positron emission mammography

Each of two detectors used in our positron emission mammography (PEM) system consists of four 36 mm×36 mm×20 mm bismuth germanate (BGO) 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 pixellated into small elements on two opposite faces. In total, there are 71×71 elements on each detector. In order to improve the quality of images obtained using this system, we have identified three required aspects of detector calibration. First, in order to correct for spatial distortion inherent in the detectors, an accurate method for crystal element identification has been developed. This algorithm successfully identifies 59×49 crystal elements, and these results are used to generate a distortion Look-Up-Table (LUT). Second, corrections for a marked spatial variation of detector gain are required in order to facilitate accurate energy discrimination. This variation is quantitated by generating energy spectra specific to each of the identified crystal elements, and recording the relative gain of each element in an energy LUT. Finally, by using a third LUT containing the relative efficiencies of crystal elements, the flood-image nonuniformity has been reduced from 50% to 13%