Abstract
In this paper, we demonstrate the use of optical coherence tomography/optical microangiography (OCT/OMAG) to image and measure the effects of acute intraocular pressure (IOP) elevation on retinal, choroidal and optic nerve head (ONH) perfusion in the rat eye. In the experiments, IOP was elevated from 10 to 100 mmHg in 10 mmHg increments. At each IOP level, three-dimensional data volumes were captured using an ultrahigh sensitive (UHS) OMAG scanning protocol for 3D volumetric perfusion imaging, followed by repeated B-scans for Doppler OMAG analysis to determine blood flow velocity. Velocity and vessel diameter measurements were used to calculate blood flow in selected retinal blood vessels. Choroidal perfusion was calculated by determining the peripapillary choroidal filling at each pressure level and calculating this as a percentage of area filling at baseline (10 mmHg). ONH blood perfusion was calculated as the percentage of blood flow area over a segmented ONH area to a depth 150 microns posterior to the choroidal opening. We show that volumetric blood flow reconstructions revealed detailed 3D maps, to the capillary level, of the retinal, choroidal and ONH microvasculature, revealing retinal arterioles, capillaries and veins, the choroidal opening and a consistent presence of the central retinal artery inferior to the ONH. While OCT structural images revealed a reversible compression of the ONH and vasculature with elevated IOP, OMAG successfully documented changes in retinal, choroidal and ONH blood perfusion and allowed quantitative measurements of these changes. Starting from 30 mm Hg, retinal blood flow (RBF) diminished linearly with increasing IOP and was nearly extinguished at 100 mm Hg, with full recovery after return of IOP to baseline. Choroidal filling was unaffected until IOP reached 60 mmHg, then decreased to 20% of baseline at IOP 100 mmHg, and normalized when IOP returned to baseline. A reduction in ONH blood perfusion at higher IOP’s was also observed, but shadow from overlying retinal vessels at lower IOP’s limited precise measurements of changes in ONH capillary perfusion compared to baseline. Therefore, OCT/OMAG can be a useful tool to image and measure blood flow in the retina, choroidal and ONH of the rat eye as well as document the effects of elevated IOP on blood flow in these vascular beds.
Highlights
Perfusion of the retina, choroid and optic nerve head (ONH) is critical to maintaining normal vision, and reduction in perfusion may contribute to the development and progression of various ocular diseases, including glaucoma [1,2,3]
The different layers we identified from the image are the nerve fiber layer (NFL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), inner/outer segment (IS/OS), retinal pigment epithelium (RPE), choroid (CH) and sclera (SC)
Viewed posteriorly (Fig. 1C), a typical ultrahigh sensitive (UHS)-Optical microangiography (OMAG) image focused approximately 150μm posterior to the opening of the choroicapillaris demonstrates that the rat central retinal artery (CRA) enters the globe inferior to the ONH, which has a horizontally oval or kidney bean shape
Summary
Choroid and optic nerve head (ONH) is critical to maintaining normal vision, and reduction in perfusion may contribute to the development and progression of various ocular diseases, including glaucoma [1,2,3]. Laser Doppler velocimetry (LDV) [4,5,6] is used to measure RBF velocity by using a laser beam directed at a specific blood vessel, and the Doppler shift caused by moving red blood cells allows blood speed to be measured This method only provides information on flow in a single vessel, but no information about perfusion in the rest of the eye. When combined with phase-resolved analysis methods developed for Doppler OCT [21], it produces Doppler OMAG (D-OMAG) [22] that can be used for quantitative and directional blood flow imaging, with improved low-flow detection as compared to phase-resolved Doppler OCT It has been refined into ultra-high sensitive OMAG (UHS-OMAG) [23,24] which has the capability of imaging capillary vessels with speed as slow as 4 μm/s. The combined use of D-OMAG and UHS-OMAG to provide simultaneous 3D angiography and quantitative measurement of blood flow in rat retinal blood vessels has recently been described [25]
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