Comparison of radio-labeled butanol and iodoantipyrine as cerebral blood flow markers.

Cerebral blood flow (CBF) techniques based on the principle of indicator fractionation rely upon free diffusibility of the blood flow indicator into brain (i.e. complete cerebral extraction). Extraction of two commonly-used indicators, iodoantipyrine and n-butanol, was evaluated in rats by measuring torcular venous efflux after systemic injection of the indicator under conditions of normal and high CBF. The extraction of n-butanol was found to be virtually complete at all blood flows examined; iodoantipyrine, on the other hand, was completely extracted only at flows under 180 ml/100 g/min, despite the fact that the oil: water partition coefficient for iodoantipyrine exceeds that for n-butanol. Brain uptake indices for the two indicators were also measured: brain uptake of n-butanol was greater than that of iodoantipyrine, and the difference was more marked if the indicator entered brain mixed with blood than if it entered as a bloodless bolus. Blood components may thus interact with iodoantipyrine to retard its movement across the blood-brain barrier and thereby limit extraction of this lipid-soluble substance. Inasmuch as iodoantipyrine is diffusion-limited at blood flows above 180 ml/100 g/min, butanol is a more accurate CBF indicator above the normal flow range in the rat.