Blood-flow magnetic resonance imaging of the retina

Abstract

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 90×90×1500-μm 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% CO 2 +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. Bloodflow 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.

Prognostic MRI biomarkers of treatment efficacy for retinopathy

“…If the spatial resolution is somewhat less than 10 pixels across the retina, then changes in oxygenation and/or retinal and choroidal perfusion, such as during a hyperoxic provocation, can produce variations in signal or in-flow artifacts, making interpretation of intraretinal signal intensities uncertain (11,14). Alternatively, different groups are currently investigating arterial spin labeling methods that measure some combination of retinal and choroidal perfusion more directly (97,98,15). It is not yet clear whether these methods will be able to unambiguously resolve retinal circulatory from choroidal contributions.…”

Evaluating anesthetic protocols for functional blood flow imaging in the rat eye

“…This interest is attributable to the importance of ocular blood flow as a parameter in studies of ocular pathophysiology and its potential to serve as an early marker of a variety of ocular diseases, including age-related macular degeneration, diabetic retinopathy, and glaucoma. The importance of ocular blood flow has driven the development of multiple measurement technologies including retinal/dynamic vessel analysis, 1 microsphere methods, [2][3][4] laser Doppler-based velocimetry [5][6][7][8] and flowmetry, [9][10][11] laser speckle imaging, [11][12][13] magnetic resonance imaging (MRI), [14][15][16] color Doppler ultrasound imaging, [17][18][19] and optical coherence tomography (OCT). [20][21][22][23][24][25][26][27][28][29][30][31] En face Doppler OCT, the modality used in this study, is a noninvasive, OCT-based technique that measures the Doppler velocities in the en face plane.…”

“…Using arterial spin labeling (ASL) MRI they found that ketamine-xylazine anesthesia (100 mg∕kg ketamine and 10 mg∕kg xylazine) had associated blood flow values that were 29% lower (0.88 AE 0.22 mL∕g∕ min versus 1.3 AE 0.44 mL∕g∕ min) in the retinal tissues (p < 0.01) and 42% lower (4.3 AE 1.9 mL∕g∕ min versus 7.7 AE 2.1 mL∕g∕ min) in the choroidal tissues (p < 0.01) compared to flows during isoflurane anesthesia (1.1%). In another study using ASL MRI, Li et al 15 measured total ocular blood flow in rats anesthetized under two different isoflurane dosages. Under 1.0% isoflurane anesthesia the basal blood flow was 6.3 AE 1.0 mL∕g∕ min, while under 1.5% isoflurane the basal blood flow was 9.3 AE 2.7 mL∕g∕ min.…”

Laser speckle contrast imaging of blood flow in rat retinas using an endoscope

“…Blood flow in the retina can be noninvasively imaged using Doppler optical coherent tomography, 1 scanning laserDoppler flowmetry, [2][3][4] laser speckle contrast imaging (LSCI), 5 and magnetic resonance imaging (MRI), [6][7][8] among others. LSCI, in particular, offers a cost-effective means to measure instantaneous blood flow with high-spatiotemporal resolution.…”

“…These results are consistent with blood-flow changes reported by using laser Doppler flowmetry 20 and MRI. 21 LSCI was then used to investigate visual stimulation in the rat retina. Time-course data from several regions of interest showed increased blood flow during visual stimulation (Fig.…”