Abstract
Purpose :
To measure absolute blood velocity and flow rates in human retinal vessels in three dimensions (3D).
Methods :
Five healthy volunteers were imaged with the FDA 3.4 MHz Fourier domain mode-locked swept source-based adaptive optics (AO) system. The 6.54 kHz optical coherence tomography (OCT) B-scan rate together with micron-level 3D resolution of the AO-OCT channel were used to trace individual retinal blood cells (BC) in both space and time. A standard OCT angiography (OCTA) imaging protocol (2°×2° and 8 repeat B-scans) was adopted from which both the OCT intensity volume and the corresponding OCTA volume were generated simultaneously. The OCTA volume was generated using a standard amplitude decorrelation method and used to quantify morphological parameters including vessel diameter and 3D vessel orientation. Individual streaks caused by BC motion in the intensity volume were analyzed by a modified 3D Radon approach to extract depth-resolved velocity. Flow rates were then computed and visualized voxel-wise by combining the vessel morphology and the velocity measures.
Results :
Flow velocity from 20 retinal vessels with caliber between 10 and 100 µm were quantified with our quantitative AO-OCTA method. A wide range of blood velocities was captured from ~3 to 30 mm/s, including low flow rates in retinal capillaries surrounding the foveal avascular zone. Accuracy was demonstrated in a branched arteriole where conservation of flow rates was measured to have <4% difference between the parent and sum of the branches. The results show a moderate linear correlation between mean velocity and vessel diameter with a slope of 0.18 ± 0.04 (r2 = 0.53). The regression analysis between the mean blood flow rate and the vessel diameter (log-log) showed a slope of 2.55 ± 0.15 (r2 = 0.94). The velocity profile in depth in retinal vessels showed a blunt parabolic shape.
Conclusions :
Our methodology achieves quantitative 3D mapping of blood velocities and flow rates with a single-volume AO-OCT acquisition. Unlike Doppler OCT, which has poor sensitivity to perpendicular flow predominant in the retinal vasculature, the proposed method offers the ability to visualize and map fine variations in blood cell flow within the retinal microvasculature in 3D. AO-enhanced ultra-fast OCT swept source-based systems provide a promising platform for quantitative and objective blood flow measurement.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.