Vectorial retinal blood flow and optical vessel vivisection for 3D angiography with high resolution resonant Doppler OCT

PURPOSE: To perform high speed and completely non-invasive 3D angiography at the optic nerve head region and to reconstruct vectorial 3D blood flow from optically segmented blood vessel structure as a pre-requisite for testing retinal function and physiology. METHODS: A novel Doppler Fourier domain OCT systems, so-called resonant Doppler OCT, allows circumventing the limitations of standard FDOCT to record fast blood flow signals. Standard Doppler FDOCT suffers from fringe blurring due to sample motion which leads to a small velocity detection range. The novel resonant scheme tunes a Doppler shift in the reference arm to the sample Doppler shift resulting in enhanced signal for moving structures whereas static structure signals are suppressed. As a result one obtains already optically a segmentation or optical vivisection of retinal blood flow in 3D. This data is used for extracting the true flow vectors along the retinal vessels (any single beam Doppler method yields only the axial projection of the flow velocity). The realized resonant FDOCT system works at 20.000 scans/sec with an axial resolution of 8µm in air. A full 3D volume of 2240(X) x 88(Y) x 1024(Z) pixels takes 10sec. RESULTS: 3D volumes of retinal structure at the optic nerve head region are recorded with different reference Doppler offsets. The images yield a clear segmentation of 3D vessel structure with suppressed static structure signals. Sets of 3D images with opposite reference Doppler shift are recorded. Differential analysis of the images yields the quantitative bidirectional axial velocity components in 3D. Axial flow components of up to +/-10mm/s with a velocity precision of 400µm/s were measured enhancing standard Doppler FDOCT velocity ranges by a factor of four. Advanced image processing algorithms allow in addition reconstruction of the true vectorial flow along the blood vessels assuming laminar flow. CONCLUSIONS: Resonant Doppler OCT yields completely non-invasive 3D angiography. It allows assessing depth resolved and quantitative blood flow at the optical nerve head with high precision and large velocity range providing directly the 3D vascularization structure. Any physiologic changes at the retina will result in changes of blood flow especially in large vessels close to the optic nerve head that provide the inner retinal blood supply. The method might therefore have high clinical value for the assessment of retinal function.