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Cross-Sectional and En Face Visualization of Normal Anterior Eye Circulations

Yan Li, PhD; Yali Jia, PhD; David Huang, MD, PhD; and Alison H. Skalet, MD, PhD

Optical coherence tomography (OCT) provides higher resolution than other non-contact anterior segment imaging modalities. Commercially available OCT systems, either dedicated to anterior eye imaging (such as Visante [Carl Zeiss Meditec Inc] or Casia [Tomey Corporation]) or hybrid retina/cornea platforms (such as Avanti [Optovue Inc] or Cirrus [Carl Zeiss Meditec Inc]), have been widely used in managing corneal disease, monitoring anterior angle structure, and planning for anterior eye surgeries.

ANTERIOR SEGMENT OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY

Conventional OCT images only tissue structure. As OCT technology advances, the new development of optical coherence tomography angiography (OCTA) offers a precise visualization of intravascular flow without the injection of contrast agents (such as fluorescein or indocyanine green).^1,2 OCTA was initially applied to evaluate posterior segment eye conditions such as retinopathies or choroidal neovascularization,^3,4 but the same technology can also be applied to angiography of the anterior eye.

In this chapter, we demonstrate OCTA of the anterior eye segment using a high-speed commercial spectral domain OCT (AngioVue Imaging System, Optovue Inc) with a corneal adapter module. The spectral domain OCT engine of the AngioVue Imaging System operates at 840 nm wavelength and generates 70,000 axial-scans/second. An Angio Cornea scan pattern was used to image the anterior segment of normal eyes. The scan composition is the same as the Angio Retina scan patterns (2 repeated B-scans at 304 raster positions, each B-scan consisting of 304 axial-scans, 1 horizontal priority plus 1 vertical priority raster scan volume). The Angio Cornea scan pattern is newly available (AngioVue software version 2015.1.0.58 and up) and has a scan size of 6 × 6 mm on the anterior segment of the eye.

The commercial version of the split-spectrum amplitude-decorrelation angiography technique and orthogonal registration algorithm integrated in the AngioVue software was used to detect blood flow and merge horizontal and vertical raster scan volumes.^5,6

Corneal Optical Coherence Tomography Angiography

A healthy cornea should have no blood vessel growth. Using OCTA to evaluate the pathological blood vessel infiltration into the cornea will be discussed in Chapter 34.

The blood supply of the limbus area originates from the anterior ciliary artery, which divides to form the conjunctival plexus, the episcleral plexus, and the intrascleral plexus.7 While slit-lamp photos record mostly superficial limbal vessels, OCTA is capable of delineating the deeper episcleral and scleral vessels, as well as the shallower conjunctival vessels.^8,9 A full-thickness en face limbal OCTA of a normal volunteer is shown in Figure 4-1B. The OCTA detected a much denser vascular network including vessels not visible on the slit-lamp photo (Figure 4-1A) or the en face OCT structure image (Figure 4-1C).

The axial vessel depth information was flattened out on en face OCTAs when the angiographic signal was projected to a 2-dimensional plane. However, the depth of the vessels within the tissue can be appreciated on cross-sectional OCTA images (Figure 4-1D) overlaid with OCT structure images (Figure 4-1E). The top of the angiographic signal indicates where the vessel actually inhabits. The elongated red flow signals (tails) shown below blood vessels were due to projection artifacts. The projection artifact is one of the most important artifacts affecting OCTA.¹⁰ It can be resolved by software algorithms such as that described by Zhang et al.¹¹ The depth-resolved conjunctival and scleral OCTAs are demonstrated in Figure 4-2. To separate the conjunctiva blood vessels from the episcleral and scleral vasculature, the posterior conjunctival boundary needs to be segmented. The bulbar conjunctiva includes an avascular conjunctival epithelium layer and the conjunctival stroma. The conjunctival stroma is composed of a network of irregularly arranged fibers, perfused blood vessels, and cystic spaces that are often in close association with the vessels. The conjunctival stroma highly scatters incident light and appears hyper-reflective in OCT images. There is a narrow demarcation line (marked by white arrowheads in Figure 4-2A) separating the conjunctival stroma from the Tenon’s capsule and the sclera underneath.¹² Custom software was used to identify the posterior conjunctival boundary in structural OCT images and generate the depth-resolved conjunctival (Figures 4-2B and 4-2C) and scleral (Figures 4-2B and 4-2D) angiograms.

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