a method for measuring organ blood flow first applied to the brain in 1944 by CF Schmidt and SS Kety. A chemically inert indicator gas is equilibrated with the tissue of the organ of interest, and the rate of disappearance from the organ is measured. Blood flow is calculated on the assumption that the tissue and venous blood concentrations of the indicator gas are in diffusion equilibrium at all blood flow rates and that the rate of disappearance of the indicator from the tissue is a function of how much of the indicator is in the tissue at any time, i.e., the rate of disappearance is assumed to be exponential.