PET Measurement of rCBF in the presence of a neurochemical tracer.

Functional neurochemical imaging can indicate neurotransmitter release by detecting changes in receptor occupancy. A dual tracer positron emission tomography (PET) technique is presented here to extend such studies by simultaneously measuring changes in regional cerebral blood flow (rCBF). This would permit correlations of task or drug induced changes in rCBF and neurochemical function. In this proposed method, the rapidly varying signal from a blood flow tracer is distinguished from the slowly changing signal due to a long-lived neurochemical tracer. As a proof of principle, baseline studies were carried out in rhesus monkeys. Two monkeys were anesthetized with isoflurane, and [18F]fallypride (t1/2=110 min), a dopamine D2 receptor antagonist, was injected. Starting 99-137 min after injection, PET images were acquired every 10 s while the blood flow tracer [17F]fluoromethane (t1/2=65 s) was administered by inhalation in a repeating pattern of 45 s on/45 s off. The observed time-activity curves for 2 ml brain regions were fit with a three compartment lung-body-brain model of fluoromethane kinetics with whole brain perfusion fixed. Comparing consecutive 6 min scans, reproducibility of relative rCBF and striatal [18F]fallypride concentration were 9 and 8%, respectively.