Measurement of the change in noise-effective count rate during PET brain studies with additional shielding

We have recently installed removable shielding on a CTI HR/sup +/ positron emission tomography (PET) scanner, and have evaluated its effect on the noise-effective count (NEC) rate during typical brain studies. This removable shielding system, known as the "NeuroShield", consists of a U-shaped lead plate attached to the removable headrest with a plastic coupling piece. The NEC was calculated from information obtained in the "head curves" collected during three different types of study: 1) from subjects undergoing 60-min 250-MBq /sup 11/C-Raclopride studies in different bed positions with the NeuroShield in place; 2) from subjects undergoing bolus /sup 15/O-water activation studies, with injected activities ranging from 170 to 750 MBq.; and 3) from subjects undergoing glucose utilization studies first without, then with the NeuroShield in place. During the Raclopride studies, the NeuroShield was slightly more effective when placed forward of the field of view (FOV). NEC improvements of up to 24% in the early frames were obtained with the NeuroShield in place. The NEC values during bolus water studies improved by up to 45% with additional shielding. The percentage of random counts diminished during the fluoro-deoxyglucose (FDG) studies from 17% to 12%/100 MBq of injected activity and the NEC value improved by 5% for a 100-MBq dose.