Regions of interest in the venous sinuses as input functions for quantitative PET

As clinical PET becomes increasingly available, quantitative methods that a re feasible in busy clinical settings are becoming necessary. We investigat ed the use of intracranial blood pools as sources of an input function for quantitative PET. Methods: We studied 25 patients after the intravenous inj ection of [F-18]6-fluoro-L-m-tyrosine and compared sampled blood time-activ ity curves with those obtained in small regions of interest (ROIs) defined in the blood pools visible in the PET images. Because of the comparatively large dimensions of the blood pool at the confluence of the superior sagitt al, straight and transverse sinuses, a venous ROI input function was chosen for further analysis, We applied simple corrections to the ROI-derived tim e-activity curves, deriving expressions for partial volume, spillover and p artition of tracer between plasma and red blood cells. The results of graph ic and compartmental analysis using both sampled [C-s(t)] and ROI [C-r(t)] venous input functions for each patient were compared. We also used an anal ytic approach to examine possible differences between venous and arterial i nput functions in the cerebral circulation. Results: C-r(t) peaked signific antly earlier and higher than C-s(t) in this patient population, although t he total integral under the curves did not differ significantly. We report some apparent differences in the results of modeling using the two input fu nctions; however, neither the graphically determined influx constant, K-i, nor the model parameter that reflects presynaptic dopaminergic metabolism, k(3), differed significantly between the two methods. The analytic results suggest that the venous ROI input function may be closer to the arterial su pply of radiotracer to the brain than arterialized venous blood, at least i n some patient populations. Conclusion: We present a simple method of obtai ning an input function for PET that is applicable to a wide range of tracer s and quantitative methods and is feasible for diagnostic PET imaging.