Digital Imaging of Umbelliferone Clearance: A Method for Repeated Measurements of Cerebral Cortical Blood Flow with High Temporal and Spatial Resolution

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Abstract
We have developed a procedure for digital imaging of the exposed cerebral cortex during elution of a fluorescent dye. This avoids disturbing the cortex and has provided a method for the repeated estimation of regional CBF (rCBF) with a high topographical resolution. Under varying conditions of MABP and arterial blood gases, grey-level images of the exposed cortex irradiated with ultraviolet light (340 or 370 nm) were digitised (8 bits) at 15-s intervals after the injection of 1–2 ml of saturated umbelliferone solution into the lingual or external carotid artery of anaesthetised cats and rabbits. Specifically designed software allowed (a) regions of interest (ROIs) in the exposed cortex to be defined that were automatically applied to the sequence of images in a selected clearance and (b) solution of the initial slope equation for rCBF from the decay in grey-level fluorescence by exponential regression. Separate software that solved the equation at the level of a single pixel allowed a pseudocolour map of cortical rCBF to be generated. The factors affecting the resolution of this technique have been identified and quantified. Thus consistent and reproducible results were obtained provided that the fluorescence enhancement exceeded 20 grey levels and the r2 coefficient for regression was 90% or above. Mean rCBF values of 99.5 [95% confidence interval (CI), 89.4–110] ml 100 g−1 min−1 were obtained for rabbits ( N = 12; mean MABP = 75.2; mean Paco2 = 32.9; Pao2 = 111.8; pH 7.38) and 65.1 (95% CI, 55.1–75.1) ml 100 g−1 min−1 for cats ( N = 8; mean MABP = 92.8; Paco2 = 31.5; Pao2 = 114.6, pH 7.40). The method registers the cerebrovascular responses of autoregulation and CO2 reactivity and can detect transient changes in rCBF induced by direct cortical stimulation. Early experience suggests potential use of the method for the monitoring of changes in regional rCBF during ischaemia and for sequential measurement of changes in cortical perfusion in any study where a high topographical resolution is needed.