Abstract
Imaging the retinal vasculature offers a surrogate view of systemic vascular health, allowing noninvasive and longitudinal assessment of vascular pathology. The earliest anomalies in vascular disease arise in the microvasculature, however current imaging methods lack the spatiotemporal resolution to track blood flow at the capillary level. We report here on novel imaging technology that allows direct, noninvasive optical imaging of erythrocyte flow in human retinal capillaries. This was made possible using adaptive optics for high spatial resolution (1.5 μm), sCMOS camera technology for high temporal resolution (460 fps), and tunable wavebands from a broadband laser for maximal erythrocyte contrast. Particle image velocimetry on our data sequences was used to quantify flow. We observed marked spatiotemporal variability in velocity, which ranged from 0.3 to 3.3 mm/s, and changed by up to a factor of 4 in a given capillary during the 130 ms imaging period. Both mean and standard deviation across the imaged capillary network varied markedly with time, yet their ratio remained a relatively constant parameter (0.50 ± 0.056). Our observations concur with previous work using less direct methods, validating this as an investigative tool for the study of microvascular disease in humans.
Highlights
The eye is the only organ in the body where the deep internal vasculature can be observed directly, noninvasively and in high resolution
Spatial variability was high even over the small area shown (~0.8 x 0.2°), with erythrocyte velocities ranging from ~0.5-3.1 mm/s across space in this sequence
It is instructive to divide the capillaries into segments; we defined a length of vessel as a capillary segment if it consisted of at least 5 data points along the vessel that were unbroken by missing data or a branch/crossing, and in which single file erythrocyte flow was subjectively apparent
Summary
The eye is the only organ in the body where the deep internal vasculature can be observed directly, noninvasively and in high resolution. Correlation between abnormalities in the retinal vasculature and abnormalities elsewhere in the body have been demonstrated in a diverse array of conditions including diabetes [1], hypertension [2], stroke [3,4], Alzheimer’s disease [3,5], migraine [6] and glaucoma [7] Each of these conditions manifests pathology in the microvasculature [8,9,10,11,12,13], which is the primary site of exchange between blood and tissue. These include laser Doppler flowmetry [14], laser speckle contrast imaging [15] and Doppler OCT [16] These methods all rely on the relationship between the velocity of red blood cells and the changes in frequency imparted to light that is scattered from them [17]. These issues render the above techniques ineffective for study of the microvasculature, some approximations have been made [18,21]
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