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+PET scanner, and have evaluated its effect on the noise-effective count (NEC) rate during bolus water activation, and FDG studies. This removable shielding system, known as the "NeuroShield" consists of a "U" shaped lead plate attached to the removable head-rest with a plastic coupling piece. Bolus water activation studies, with injected activities ranging from 170 to 750 MBq were performed before the NeuroShield was installed. Based on the known improvements at 370 Bq, and the way the dead time and random count rates change with activity the NEC was calculated with and without additional shielding. The glucose utilization studies were done on the same subjects first without, then with NeuroShield in place. Forty minutes after injection of 110 MBq ¹⁸F-FDG, subjects were scanned for one minute with the scanner\´s normal headrest. The subject then sat up to allow the NeuroShield to be installed, and a ten-minute emission followed by a ten-minute transmission scan were done in the same position. During the entire scan, a "rates" file recorded the prompt and delayed count-rates and live time every five seconds. Since the NeuroShield is not fixed in the gantry, its effectiveness at different axial positions was also measured. Count-rates from four subjects undergoing serial Raclopride studies were also evaluated as a function of shield position. The shielding was slightly more effective when placed forward of the field of view. The NEC improves by 24% in the early frames of the se studies. The percentage of random counts improved for the FDG studies from 17 to 12%/100 MBq of injected activity. The NEC in bolus water studies improves by up to 45% with additional shielding. Most of this improvement comes from a reduction in random counts