Accuracy of stable xenon/CT measurements of regional cerebral blood flow: effect of extrapolated estimates of brain-blood partition coefficients.

Stable xenon/computed tomographic (Xe/CT) measurements of regional brain-blood partition coefficients for xenon (lambda) and of regional cerebral blood flow (rCBF) have been reported for animal models of stroke and for patients with a variety of neurological diseases. We estimated the uncertainty in reported measurements of lambda and rCBF using computer simulations based on Kety's blood flow equation. Anesthetized baboons were scanned repeatedly for 30-60 min while inhaling an 80:20% xenon-oxygen mixture, and the resulting CT enhancement values for user-defined regions of interest were used to define a mathematical function with which to fit simulated CT enhancement data. In computer simulation studies, extrapolated and fit values of lambda for the same dataset were used to calculate rCBF, and the calculated flow values were compared. Computer simulations for heterogeneous (gray-white matter) regions indicate that better estimates of flow are obtained by fitting lambda than by using extrapolated values of lambda to calculate flow, although, paradoxically, better estimates of lambda are obtained by extrapolation than by fitting lambda.