Influence of Blood Inhomogeneity on Doppler OCT Signals in Retinal Capillary System

Abstract

Purpose: : To demonstrate applicability of Doppler Optical Tomography (OCT) technique for 3-D analysis of blood flow in retinal vascular system. An influence of optical inhomogeneity of human blood on Doppler measurement in retinal capillaries will be addressed.

Methods: : Ultra high resolution and high speed (230,000 Ascans/sec) Spectral OCT system was used for in vivo imaging of retinal vasculature in 3-D. In vivo studies were performed in healthy and pathologic eyes enabling to visualize net capillaries distributed densely in macular region (outside from avascular zone). Specialized scanning protocols were applied to measure low frequency Doppler signals caused by almost perpendicular orientation of vessles in the central retina. Measurement protocols and signal processing were based on the joint spectral and time domain OCT. To test applicability of the method additional in vitro studies of the human blood flow were conducted.

Results: : Detalied analysis of velocity data demostrates that we are able to see Doppler signal coming from the steady scattering medium under capillary while the blood is flowing. This effect is present in both in-vivo and in-vitro studies. Such signal is present due to optical inhomogenity of blood causing time dependent phase variation similar to Doppler signal. It also affects the velocity readout from blood vessles introducing additional broadening of velocity profles. We will demonstrate our calculations and experimental data showing how strong this effect can influence the velocity measurements in retinal blood vasculature especially in the case of small retinal capillaries.

Conclusions: : Measurements of Doppler data from retinal vasculature in 2-D and 3-D obtained with high resolution and high speed Spectral OCT system are demonstrated. We have shown that is possible to enhance visualization of retinal caplillaries using joint spectral and time domain OCT. However, the direct retrieval of blood flow velocity in small capillaries is limited due to orientation of capillaries and strong influence of optical inhomogenity of the medium. In this contribution we will demonstrate optimal conditions for measuring blood flow in small retinal vessles to enhance their visibilty and calculate the flow rate.

Keywords: imaging/image analysis: non-clinical • retina • imaging/image analysis: clinical