Monoexponential analysis of 133Xe clearance curves for regional cerebral blood flow measurements.

The theoretical properties of a monoexponential flow index, analogous to the one used earlier by other investigators for regional CBF (rCBF) measured after intraarterial injection, were investigated after the administration of 133Xe intraarterially, intravenously, and by inhalation under high and low flow conditions. The sensitivity of the flow index to changes in fast flow components or changes in the weight ratio between the fast and the slow flow compartments was found to be dependent on whichever part of the 133Xe clearance curve was used for the flow calculation and on the shape of the input function for 133Xe. Since biexponential analysis of the clearance curves includes a monoexponential approximation for each of the two components of the clearance curve corresponding to the high and the low flow "families" in the brain, the limitations of the monoexponential flow index observed are in principle also valid for the results of biexponential analysis of the clearance curves.