Update on choroidal vascular imaging using optical coherence tomography

Abstract

The choroid being the most vascular tissue plays an important role in nutrition of outer retinal structures as well as the visual function. Recent literature provides information about the choroidal thickness and its change in various chorioretinal diseases. Detailed assessment of choroidal vasculature is yet to be explored. This review evaluates the undergoing research in choroidal vascular imaging using various optical coherence tomography techniques such as en-face, phase variance, and swept-source. The authors also discuss automated segmentation of choroidal vessels and its application in choroidal assessment as well as future directions.

Keywords::

• choroidal imaging
• choroids
• EDI
• enhanced depth imaging
• SD-OCT
• swept-source OCT

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

Key issues

• Haller’s layer can be measured manually as the thickness of the large choroidal vessels. The choriocapillaris layer and Sattler’s layer (medium choroidal vessel layer) thicknesses can be obtained by subtracting the large choroidal vessel layer (Haller’s layer) thickness from the total choroidal thickness.

• The mean thickness of the subfoveal large choroidal vessel layer (Haller’s layer) thickness was measured to be 204.3 ± 65.9 μ. The mean subfoveal medium choroidal vessel layer choriocapillaris layer was measured to be 52.86 ± 20.63 μ. The mean ratio of the large choroidal vessel layer to the total choroidal thickness was reported to be 0.7 ± 0.06.

• The choroid-equivalent region is defined as the upper and lower surfaces of the choroidal vasculature segmentation. The choriocapillaris-equivalent thickness is defined as the distance between Bruch’s membrane and the upper surface of the choroid-equivalent region.

• The light-to-dark ratio, ratio of bright pixels (stroma) and dark (vessel) in the subfoveal area can also be measured to assess the choroidal vasculature quantitatively. A high ratio can be interpreted as a higher proportion of choroidal vessel lumen (dark pixels) than choroidal stroma (bright pixels) in a subfoveal cross-sectional area.

• Using swept-source optical coherence tomography en-face images, the lateral and medial posterior ciliary arteries as well as major draining choroidal veins and their entrance sites, branching locations and coursing directions can be visualized noninvasively.

• Choroidal vessel density can be calculated using 6 × 6 en-face volumetric scans using swept-source optical coherence tomography.

• Vessel diameter can be calculated on each choroidal C scan, by calculating the distance between the coordinates of the edges of the vessels. Choriocapillaris vessel diameter measured in the horizontal axis in healthy controls ranged from 2.8 to 6.5 pixels (average 4.42), middle choroidal vessels 7.0 to 10.8 pixels (average 8.31) and deeper choroidal vessels 15.6 to 22.1 pixels (average 18.38).

• Phase-variance optical coherence tomography allows for the virtual segmentation of the retinal and choroidal layers, providing opportunities to observe subtle changes in the microvasculature during the progression and treatment of ocular diseases.

• The automated identification of choroidal layers as well as choroidal vessels using 2D-OCT and/or en-face scans is underway.

• Imaging the choriocapillaris in vivo is challenging because of its very small size, highly pigmented choroidal melanocytes and its location below the retinal pigment epithelium.