In this study, we demonstrate a novel scanning pattern for improving flow quantification in optical coherence tomography angiography (OCTA) with a high scanning efficiency. A bidirectional interleaved scan pattern was introduced to adjust the adjacent inter-scan time in order to achieve OCTA sensitivity to different flow speeds. This bidirectional scanning protocol uses a triangular function on the fast scanning direction, meaning that it takes the same time in completing B-scans at adjacent lateral positions, acquired in opposite directions. By applying this scheme, the duty cycle is increased to almost 100%. To improve the linear velocity range represented by OCTA signals, different inter-B-scan intervals (at least two) are required to visualize flow at different speeds. In our scanning protocol, the time between the first and second repetition is different than the time between second and third repetition, allowing a total of 3 different inter-scan times (1-2, 2-3 and 1-3) to be computed to improve flow quantification. A retinal OCTA of a healthy subject was acquired using our 400-kHz swept source OCT system. The volumetric scan was acquired in less than two seconds, potentially minimizing the prevalence of motion artifacts, which are more predominant in the scanning intervals most sensitive to slow speed flow. By averaging the three different images generated by 3 different inter-scan times, flow with large linear range (up to 5.2 mm/sec according to our prior calibration) is apparent on en face OCTA.
Phase wrapping is a crucial issue in Doppler optical coherence tomography (OCT) and restricts its automatic implementation for clinical applications that quantify total retinal blood flow. We propose an automated phase-unwrapping technique that takes advantage of the parabolic profile of blood flow velocity in vessels. Instead of inspecting the phase shift manually, the algorithm calculates the gradient magnitude of the phase shift on the cross-sectional image and automatically detects the presence of phase wrapping. The voxels affected by phase wrapping are corrected according to the determined flow direction adjacent to the vessel walls. We validated this technique in the rodent retina using a prototype visible-light OCT and in the human retina with a commercial infrared OCT system. We believe this signal processing method may well accelerate clinical applications of Doppler OCT in ophthalmology.