PET molecular imaging of 15O-labeled water is the gold standard for measuring blood flow in humans. However, this requires an on-site cyclotron to produce the short-lived 15O tracer, which is cost-prohibitive for most clinical PET centers. The purpose of this study was to determine if the early uptake of 18F-FDG could be used to measure regional blood flow in tumors in the absence of 15O-water.
Methods: PET scans were obtained in patients being evaluated for tumor perfusion and glucose metabolism in a phase I dose-escalating protocol for endostatin, a novel antiangiogenic agent. A 2-min perfusion scan was performed with a bolus injection of 2,220 MBq (60 mCi) of 15O-water, which was followed by a 370-MBq (10 mCi) dose of 18F-FDG. Four sequential scans of 18F-FDG uptake were acquired, consisting of an early 2-min uptake scan-or first-pass scan-and 3 sequential 15-min late 18F-FDG uptake scans. Regions of interest (ROIs) were drawn on 2 or more tumor sites and on back muscle, as a control ROI, for each patient. Arterial blood concentration was derived from the PET scans by drawing an ROI over a large artery in the field of view. Blood flow was computed with a simple 1-compartment blood flow model using the first 2 min of data after injection.
Results: Blood flow estimated from the early uptake of 18F-FDG was linearly correlated with 15O-measured blood flow, with an intercept of 0.01, a slope of 0.86, and an R2 regression coefficient of 0.74 (r = 0.86). The 18F-FDG tumor extraction fraction relative to 15O-water averaged 0.86. A preliminary case study of a patient with prostate cancer confirms the utility of the first-pass 18F-FDG blood flow analysis in tumor diagnosis.
Conclusion: These results suggest that the first-pass uptake of 18F-FDG may provide an estimate of perfusion in a tumor within the limitations of incomplete extraction of 18F-FDG compared with 15O-water.