THE EFFECT OF ACTIVITY OUTSIDE THE DIRECT FIELD-OF-VIEW IN A 3D-ONLY WHOLE-BODY POSITRON TOMOGRAPH

The ECAT EXACT3D (CTI/Siemens 966) 3D-only PET tomograph has unprecede nted sensitivity due to the large EGO (bismuth germanate) detector vol ume. However, the consequences of a large (23.4 cm) axial field-of-vie w (FOV) and the need for a patient port diameter to accommodate body s canning make the device more sensitive to photons arising from activit y outside the direct (coincidence) FOV. This leads to relatively highe r deadtime and an increased registration of random and scatter (true) coincidences. The purpose of this study is to determine the influence Df activity outside the FOV on (i) noise-equivalent counts (NEC) and ( ii) the performance of a 'model-based' scatter correction algorithm, a nd to investigate the effect of side shielding additional to that supp lied with the tomograph. Annular shielding designed for brain scanning increased the NEC for blood flow ((H2O)-O-15) measurement (integrated over 120 s) by up to 25%. For C-11 tracer studies, the increase is le ss than 5% over 120 min. Purpose-built additional body shielding made to conform to the shape of a volunteer, reduced the randoms count rate in a heart blood Bow measurement ((H2O)-O-15) by about 30%. After sca tter correction the discrepancy between ROI count ratios for compartme nts within the 20 cm diameter 'Utah' phantom differed by less than 5% from true (sampled) activity concentration ratios. This was so with or without activity outside the FOV and with or without additional side shielding. Count rate performance is thus improved by extra shielding but more improvement is seen in head than in body scanning. Measuremen t of heart blood flow using bolus injections of (H2O)-O-15 would benef it from the use of detectors with lower deadtime and superior timing r esolution such as LSO (lutetium oxyorthosilicate).