Induced hemodilution detected by reflectometry for measuring microregional blood flow and blood volume in cat brain cortex

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Abstract
A non-invasive optical method was developed to measure microregional cerebrocortical blood flow (mrCBF) and blood volume (mrCBV) in the superficial areas of the brain with dimension of .apprx. 1 mm3. Through a glass window firmly fixed in the parietal bone of anesthetized cats, the intensity of light diffusely scattered from the exposed brain cortex was continuously measured at 366 nm by a microscopic-reflectometric system linked to a PDP 11/10 laboratory computer. The optical density (OD) of the tissue was calculated from the ratio of diffusely scattered light intensity for a given instant to that of the blood-free cerebrocortical tissue. For measuring mrCBF, an isotonic dextran-saline solution (Rheomacrodex 10%) was injected as a nondiffusible indicator into the cerebral circulation and the subsequent cerebrocortical hemodilution was monitored as an OD change. In model experiments the OD of blood samples was a linear function of the blood content of the samples. The dilution curve for a finite injection and for a step injection was computed from the reflectance optical density curves applying the equator established in in vitro model studies. Comparing the hemodilution monitored from small pial arteries to those from capillary fields, the dispersion of the bolus in the supplying arteries was negligible relative to that in the capillaries. mrCBV monitored by the reflectance base line is an accurate measure of distribution of the bolus traversing the illuminated tissue volume. The microregional mean transit time (.hivin.t) was evaluated as the integral of the dilution function for a step injection from the appearance time to the disappearance time of the indicator. mrCBF was computed as the ratio of mrCBV to .hivin.t. The microreflectometric indicator dilution method in conjunction with NADH microfluorometry may correlate tissue microhemodynamics and metabolism in the same minute volume of the brain cortex.