Simultaneous PET-MRI Confirms That Cerebral Blood Flow Does Not Confound PET Neuroreceptor Activation Studies.

Positron emission tomography (PET) and endogenous competition paradigms are widely used for studying neuroreceptor activation in humans in vivo. Changes in cerebral blood triggered by the experimentation, such as amphetamine administration, could influence both tracer delivery and washout, thus biasing the results. A recent study tested this assumption in baboons by measuring radiotracer binding with PET while measuring simultaneously cerebral blood flow with arterial spin labeled functional magnetic resonance imaging. Cerebral blood flow was modulated by CO2 inhalation. Hypercapnia led to substantial alterations in blood flow with no detectable alteration in binding of the reversibly binding radiotracers [11C]raclopride and [18F]fallypride. These results rule out a serious confound for the endogenous competition paradigm, and demonstrate the importance of simultaneous PET and MRI measurements when studying brain function.