Imaging Functional Hyperemia in Response to Flicker Stimulus in the Rat Retina Using Ultrahigh-Speed Spectral / Fourier Domain OCT

Abstract

 

Purpose

 

Assessing ocular hemodynamics in vivo is an important research area because it is thought that the reduction in functional hyperemia may be one of the earliest markers for retinal diseases such as diabetic retinopathy. This study investigates the feasibility of measuring total retinal blood flow and capillary hyperemic response in the rat retina using OCT.

 

Methods

 

An ultrahigh-speed spectral OCT system at 840nm with a 244kHz A-scan rate was developed. At this scan rate, the maximum axial velocity measurable with Doppler OCT was 75.4mm/s. More than 30 Sprague-Dawley rats were anesthetized and imaged to date under an approved protocol by the Committee on Animal Care at MIT. A diffuse 12.5Hz flicker stimulus at 532nm was applied to the rat retina during OCT imaging. Total retinal blood flow was measured by raster scanning the central retinal artery (as indicated by a square in Fig. 1(A)) to acquire repeated volumes at 55Hz, calculating Doppler axial velocity components in the en face plane and integrating over the vessel cross section. 3D capillary angiography images were generated by scanning multiple B-scans at the same location and calculating speckle decorrelation between consecutive B-scans as shown in Fig. 1(B).

 

Results

 

Fig. 1(C) shows a representative functional hyperemic response to a continuous flicker stimulus started at t = 2s. An acute transient increase in total retinal blood flow can be clearly observed. Figs. 1(D, E) show representative capillary images of the same retina acquired before and during flicker stimulus, respectively, where brighter pixels correspond to faster speeds. An increase in the brightness of some capillaries as well as dilation of larger retinal vessels can be observed. These capillaries become brighter because the degree of speckle decorrelation increases with the speed of moving particles, an effect that cannot be detected by simple intensity imaging or fluorescein angiography.

 

Conclusions

 

These results demonstrate the ability of ultrahigh-speed OCT to image functional hyperemia in the rat retina. Functional hyperemia in small animal models of diabetes is currently under investigation.

 

 

Fig. 1. (A) OCT en face projection. (B) En face Doppler images at different time points. (C) Flicker-induced hemodynamic response on total retinal flow. (D, E) A capillary image acquired before and during flicker stimulus. Scale bars: 100µm.

 

Keywords: 436 blood supply • 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 688 retina