Evaluation of the ratio method compared with graphical analyses for estimating nigrostriatal function in human 18F-dopa PET studies with or without carbidopa.

The striatal to cerebellar (S/C) activity ratio for estimating nigrostriatal function was compared with the Patlak analyses in 18F-dopa (FD) positron emission tomography (PET) with pretreatment with 100 mg carbidopa (CD). Two different time-activity curves of plasma FD or cerebellar 18F were used for the Patlak analyses. The S/C ratio increased linearly with time for 120 min and the ratio at 120 min correlated closely with the uptake constants by the two Patlak analyses (r = 0.94, 0.96) in six normal volunteers and six parkinsonism patients. The S/C ratio and the uptake constant by the cerebellar Patlak analysis without CD also showed a fairly good correlation (n = 30, r = 0.85). Then, the S/C ratios with and without CD were compared. Since CD increased both the striatal and the cerebellar radioactivities proportionally within each subject, CD did not change the S/C ratios. The S/C ratios both with and without CD were simple and comparable to the uptake constants in the FD PET studies.