View of the brain through a flow-coloured time window

Radioactive tracer substances administered to study cerebral functions open a time-dependent window to the brain. We can study e.g. regional cerebral blood flow (rCBF) or chemical properties such as receptors for transmitter substances. Since the tracer substance is carried by the blood flow into the brain its initial distribution will be proportional to the rCBF. The possibility to study more specific brain functions depends on the available time for redistribution of the tracer substance according to its specific chemical affinity. I have studied with the aid of a simple mathematic model the time needed for distribution and redistribution of freely diffusable tracer substances in the brain. The results indicate that SPECT measurements of brain receptor properties in theory give at least as good recording conditions as corresponding PET studies. Clinical SPECT receptor studies will presumably soon contribute to our knowledge of the epileptic pathogenic mechanisms.