Concentration recovery for SPECT CNS dopamine-receptor studies for monkeys

Most commercial SPECT scanners do not properly compensate for scattering and partial volume effects, which severely impacts the quantitation of clinical and research SPECT studies, especially in CNS dopamine studies. Hence, in this paper, a brain phantom containing a monkey skull was used to calibrate a Picker Prism 3000 SPECT camera for quantitative studies of 123I and 99mTc-labeled radiopharmaceuticals. In the calibration study, two small vials representing basal ganglia were inserted into a skull phantom. A series of images were taken with different concentration ratios between the basal ganglia and the other brain compartments to simulate the dynamic imaging study of the dopamine D2 receptor in a monkey brain. Based on this calibration, a simple method is proposed for count correction in the region of interest (ROI). This technique is superior to using a single calibration factor from a cylindrical uniform activity phantom since it can compensate for both the partial volume effect and the dynamically changing effect of scattering. A significant difference between the true and measured values was observed in the correction factors (by a factor as much as 3). Furthermore, a cylindrical phantom, approximately the size of a monkey's head and containing the same basal ganglia inserts, was used to investigate the feasibility of using a cylindrical phantom with the proposed correction method. The results indicate that it is feasible to use a cylindrical phantom for the case where the activity inside the brain cavity and the activity in the surrounding tissue are similar.