Photoelectric method for estimating hemodynamic changes in regional cerebral tissue.

A new photoelectric method for measuring hemodynamic changes in a thin layer of cerebral tissue in situ is described. The apparatus consists of a miniature lamp of 0.7 mm OD and a photodiode with a color filter having maximum light transmission at 548 nm. The lamp was chronically or acutely implanted into the brain tissue of animals as a light source, and the intensity of the light transmitted through the cerebral tissue was monitored with the photodiode from the brain surface. The intensity of the transmitted light was quantified for cerebral blood volume (CBV). The mean transit time of blood (t) was obtained from tissue hemodilution curves produced by saline injection into the carotid artery. The regional blood flow was calculated from the values of CBV and t according to the Stewart-Hamilton equation. The apparatus was shown to be stable, rapid in response, easy to operate, and to give reproducible results. Possible sources of error are discussed.