Cerebral Transit of an Intravascular Tracer May Allow Measurement of Regional Blood Volume but Not Regional Blood Flow

To the Editor: Tracers remaining in the vascular bed have for many y ears been used to study the cerebral circulation. In particular, intravenously in­ jected radioactive tracers, allowing external moni­ toring of the transit over the head, have been widely used. More recently, the same approach has been applied to the conventional X-ray contrast medium combined with a sequence of computed tomog­ raphy (CT) scans. For each pixel, a transient en­ hancement is observed, Intuitively, one might, perhaps, think that such rapid transit data would give information about local blood flow. This is not correct. As will be shown, the only valid parameter that one can derive is the local blood volume, more precisely the local plasma volume. The kinetic analysis is based on the assumption that one has obtained a complete curve of the pas­ sage of the indicator bolus through the volume of brain tissue recorded from. This means that one assumes that recirculation has been eliminated by the conventional monoexponential downslope ex­ trapolation procedure. For every resolution element chosen, the tran­ sient signal enhancement, be it of radioactivity or of X-ray attenuation, is a measure of the residue function, ACJt), of tracer in the local plasma volume. If one were able to introduce all tracer as an infinitely short bolus, an impulse, at the arterial inflow to the brain, then the impulse residue func­ tion would be recorded, that is,