Retinal flow velocity measurement using optical coherence tomography angiography

Abstract

Purpose : Detecting abnormalities in retinal blood flow using optical coherence tomography angiography (OCTA) may provide an early biomarker for diseases such as vein occlusions. However, OCTA signal detection based on repeated raster scanning suffers from low dynamic range due to the relatively large time interval between repeated scans. Here we report on our clinical results using M-mode scanning OCTA, which reduces the time interval between repeated A-scans and increases the dynamic range.

Methods : Normal subjects were scanned under an IRB-approved study using a commercial angiography 3x3mm scan and a prototype M-mode scan pattern. The prototype scan was designed to span a field of view of 2.14x1mm² with 175 A-scans per B-scan and 82 B-scans. Each A-scan was repeated 10x before moving to the next adjacent location. The scan was implemented on CIRRUS^TM 5000 HD-OCT with AngioPlex® OCT Angiography (ZEISS, Dublin, CA), operating at 67kHz A-scan rate (~15µs time interval between A-scans of the same location). OCT datasets were processed using Optical MicroAngiography among frames with different time intervals. Flow datasets from different time intervals were summed together to create one flow volume per scan. Subsequently, vasculature en face images were generated from inner limiting membrane to retinal pigment epithelium. We further validated our method using a microfluidic channel (40µm height, 60µm width) while injecting a mixture of milk and distilled water at different flow speeds (from 0 to 30mm/s).

Results : Fig. 1 shows OCTA en face images (Gray scale: Angiography 3x3mm, hot-color: M-mode) from three normal subjects. As is evident, small capillaries and branch vessels exhibit lower flow signal compared with main arteries. The OCTA signal at the center of the microfluidic channel was measured and plotted versus flow velocity (Fig. 2), confirming the correlation between OCTA signal and flow velocity.

Conclusions : Our results suggest that our M-mode OCTA can provide quantitative information about blood flow speed (from 2 to 15mm/s) in the retina.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Fig 1. OCTA images from normal subjects. Gray scale: Angiography 3x3mm, Hot-color: M-mode scan.

 

Fig 2. OCTA signal vs flow velocity in a microfluidic channel. Error bar indicates the variability between repeated measurements.