An adiabatic approximation to the tissue homogeneity model for water exchange in the brain: II. Experimental validation.

A frequently reported limitation to using water as a tracer for measuring CBF has been the dependence of the CBF estimate on the experimental time (referred to as the falling flow phenomenon, FFP). To eliminate the FFP, we have developed the adiabatic solution of the tissue homogeneity model to replace the solution of the single-compartment Kety model. In Part I, the derivation of the adiabatic solution was presented. In this second part, the adiabatic solution was applied to measure CBF in rabbits using nuclear magnetic resonance spectroscopy and the tracer deuterium oxide. It was shown that the FFP, observable when the 2H clearance data were analyzed with the Kety equation, was significantly reduced when the same data were analyzed with the adiabatic solution of the tissue homogeneity model. By concurrently measuring CBF with radioactive microspheres, it was determined that the CBF estimates from the adiabatic solution were accurate for true blood flow values less than 60 mL x 100 g(-1) x min(-1). Above this value the CBF estimate was progressively underestimated, which was attributed to the diffusion limitation of water in the brain.