Slow Blood Flow Measurement in Retinal Vessels with Ultrahigh Speed OCT System

Abstract

Purpose: : Flow measurement has been one of the main research interests in Optical Coherence Tomography since the invention of this technique. Development of different Doppler OCT methods enabled flow detection in the retinal and choroidal vessels. Introduction of high speed OCT systems allowed for 3-D imaging and measurement of blood flow in the range of several to tens of mm/s. However, the possibility of OCT imaging of slow flows (a fraction of mm/s), typically encountered in small vessels and vessels nearly perpendicular to the imaging beam remains an open question. The purpose of this study is to introduce a method of posterior eye segment Doppler OCT imaging which enables slow flow measurement. Application of this method to detection of vascular network in the macula and optic disc area will be demonstrated. Discussion of the possibilities and limitations of slow flow measurement in the eye will be provided.

Methods: : The study was performed with ultrahigh speed (200 000 axial scans/s) spectral / Fourier domain OCT instrument with a CMOS camera. A resonant scanner was introduced to the system to enable slow flow measurements. Data analysis was performed using a method of joint Spectral and Time domain OCT allowing for simultaneous generation of structural and flow images.

Results: : We will show slow flow imaging and measurement results obtained in the eyes of healthy volunteers. Two- and three dimensional OCT tomograms of the vascular network up to the capillary level will be presented. Comparison with results obtained with previously reported OCT methods for slow flow detection and vasculature visualization will be demonstrated.

Conclusions: : Recent reports on ultrahigh speed OCT show that this technique is well suited for fast flow imaging and measurement. However, the axial flow velocities in the most critical retinal areas where sight threatening diseases develop are small. This provides a challenge: acquisition of subsequent data for flow analysis should be slowed down to enable detection of slow flows. However, slowing down the OCT imaging to "standard" speeds is not recommended (introduces motion artifacts and limits the imaging to two dimensional tomograms). We have developed a method achieving the goal without sacrificing the imaging speed. 3-D visualization of slow flows is possible with ultrahigh speed OCT. The method may be used for further study to answer an important question if flow imaging can be useful for ophthalmic diagnostics.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • retina • imaging/image analysis: non-clinical