  Laser Doppler holography is a planar blood flow imaging technique recently
introduced in ophthalmology to image human retinal and choroidal blood flow
non-invasively. Here we present a digital method based on the Doppler spectrum
asymmetry that reveals the local direction of blood flow with respect to the
optical axis in out-of-plane vessels. This directional information is overlaid
on standard grayscale blood flow images to depict flow moving towards the
camera in red and flow moving away from the camera in blue, as in ultrasound
color Doppler imaging. We show that thanks to the strong contribution of
backscattering to the Doppler spectrum in out-of-plane vessels, the local axial
direction of blood flow can be revealed with a high temporal resolution, which
enables us to evidence pathological blood flow reversals. We also demonstrate
the use of optical Doppler spectrograms to quantitatively monitor retinal blood
flow reversals.

Medical Physics;Image and Video Processing