Angle independent flow assessment with bidirectional Doppler optical coherence tomography

Bidirectional Doppler optical coherence tomography is a stable and accurate method to extract the absolute velocity of vessels close to perpendicular to the optical axis by illuminating the sample under two different angles. However it requires knowledge of the vessel angle in the en face plane. In this Letter, we demonstrate that a direct calculation of the flow out of bidirectional Doppler cross sections perpendicular to the illumination plane is independent of that angle and of the Doppler angle, thereby improving the accuracy and flexibility of that technique. We validate our approach with an in vitro experiment and in vivo measurements of a human retinal vessel and discuss the practical limitations of this approach. The method yields accurate flow values for most vascular plexuses without precise knowledge of the vessel orientation. The precision gradually decreases for larger en face angles.