The measurement of cerebral blood flow by means of inert diffusible tracers

The author presents his development of the techniques for the measurement of total and regional cerebral blood flow, based on the principles that describe the exchange of inert diffusible tracers between blood and tissue. From the recognition that the uptake and distribution of such tracers would be independent of metabolism and neural function, but dependent instead on blood flow and purely physical factors such as diffusion and solubility, it was possible to transform equations of Fick, Bohr, and Krogh developed to describe steady states of oxygen distribution, to the dynamic processes involved in the distribution and equilibration of inert gases and other diffusible and nonmetabolized substances. The equations thus developed made possible the quantitative measurement of total and local blood flow in the brains of animals and man, and ultimately, the imaging of functional activity in the human brain through the coupling between activity and perfusion described 100 years ago by Roy and Sherrington.