Blood-flow magnetic resonance imaging of the retina

This study describes a novel MRI application to image basal blood flow, physiologically induced blood-flow changes, and the effects of isoflurane concentration on blood flow in the retina. Continuous arterial-spin-labeling technique with a separate neck coil for spin labeling was used to image blood flow of the rat retina at 90 x 90 x 1500-mu m resolution. The average blood flow of the whole retina was 6.3 +/- 1.0 ml/g/min under 1% isoflurane, consistent with the high blood flow in the retina reported using other techniques. Blood flow is relatively constant along the length of the retina, except it dipped slightly around the optic nerve head and dropped significantly at the distal edges where the retina terminates. Hyperoxia (100% O-2) decreased blood flow 25 +/- 6% relative to baseline (air) due to vasoconstriction. Hypercapnia (5% CO2 + 21% O-2) increased blood flow 16 +/- 6% due to vasodilation. Increasing isoflurane (a potent vasodilator) concentration to 1.5% increased blood flow to 9.3 +/- 2.7 ml/g/min. Blood-flow signals were confirmed to be genuine by repeating measurements after the animals were sacrificed in the MRI scanner. This study demonstrates a proof of concept that quantitative blood flow of the retina can be measured using MRI without depth limitation. Blood-flow MRI has the potential to provide unique insights into retinal physiology, serve as an early biomarker for some retinal diseases, and could complement optically based imaging techniques. (C) 2007 Elsevier Inc. All rights reserved.