Steady State Radioisotopic Assessment of Brain Function

Abstract

The classical radioisotopic technique for assessing cerebral perfusion is based on the single carotid injection of a diffusible gas such as ¹³³Xe and external recording of the wash-out slopes over the brain; assuming the instantaneous distribution, following injection, of the tracer to the whole brain matter, flow can be calculated from wash-out rates (13). This is a dynamic approach, based on the external recording of time activity curves. However, blood flow (and eventually other functions) of an organ can also be investigated using steady state rather than dynamic approaches. All steady state methods for assessing flow are essentially based on the extraction principle of Sapirstein (19). This states that if a tracer is completely extracted from the circulation in its first pass through an organ the concentration in different parts of the organ is proportional to blood flow. The advantage of steady state over dynamic measurements would be the possibility of obtaining much higher counting statistics, which are required for the tomographic (and therefore tridimensional) assessment of the intracerebral distribution of radioactive isotopes.