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Z. Zhong, T. Chui, H. Song, B. Petrig, S. Burns; Blood Velocity Profile Measurements in Retinal Vessels Using an Adaptive Optics Scanning Laser Ophthalmoscope. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1202.
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© ARVO (1962-2015); The Authors (2016-present)
To measure blood velocity profiles in retinal vessels at different cardiac phases noninvasively.
A supercontinuum laser (Fianium) with 840nm filter was used as the light source. AO control was maintained using two deformable mirrors (BMC MEMS DM and Mirao 52d DM) and a Shack Hartmann sensor. The laser beam steering was performed by a 10 kHz line scanner and a programmable vertical scanner. Both directions of each line scan were captured, producing a line scan frequency of 20 kHz. Pixels were sampled at 25 MHz. A rotatable stage rotated the line scan to cross blood vessels at a low angle, increasing the width of the intersection area, and thus the resolution of the velocity profiles. The vertical scan was programmed to briefly pause while the line scan crossed the target blood vessel, allowing 20 kHz sampling of the same intersectional area. During this pause, the image frame consists of a direct measure of the intensity change over time for each pixel. The motion of cells passing the scanning line forms diagonal straight lines. The slopes of these lines indicate the velocity components of the cells in the scan direction, given the constant time interval between line scans. From the position of the cells within the lumen, the velocity profile across the vessel lumen was computed. The cardiac cycle was determined using a separate cardiac pulse monitor synchronized to the AOSLO system.
Velocity profiles as a function of cardiac cycle were recorded in retinal blood vessels of all orders in size, except in capillaries and slightly bigger vesssels where the concept of velocity profile does not apply. Arteries and veins were readily differentiated by the direction of flow. As expected, the blood velocity profiles fluctuate with the subject’s cardiac cycle, with more pulsatility in arteries than in veins. The velocity profiles in retinal blood vessels have the shape of blunted parabolas, whose bluntness increase with decreasing vessel size. For the first order arteries and veins, the ratio of V_max (Centerline blood velocity) over V_mean (Average blood velocity across lumen) is around 1.6. This ratio decreases to 1.35 in second order vessels and 1.2 in third order vessels.
The blood velocity profiles in all retinal blood vessels are measurable with the assistance of adaptive optics and high speed imaging. This technique is sensitive to the cardiac cycle of the subject. It is now possible to better compute average volumetric blood flow. This technique also has the potential to monitor hemodynamics in retinal blood vessels, which may be altered in retinal diseases such as diabetic retinopathy.
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