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QUESTION

WHAT IS DYELESS ANGIOGRAPHY (OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY) AND WHAT DO I NEED I KNOW ABOUT IT? WILL IT REPLACE FLUORESCEIN ANGIOGRAPHY?

Thomas Hwang, MD
Yali Jia, PhD

Optical coherence tomography (OCT) has revolutionized the management of retinal diseases by providing high-resolution, 3-dimensional (3D) structural images of the retina rapidly and noninvasively. OCT angiography (OCT-A) visualizes blood flow by obtaining OCT images at the exact same place sequentially and analyzing the difference between the images caused by the moving blood cells. This difference is called decorrelation or variance in the amplitude or phase of the OCT wave signal. Dense, contiguous images are then stitched together to create a volume that contains structural and flow information in 3D. These volumes are then segmented according to laminar structures and displayed in en face orientation to display angiograms, creating images that are analogous to fluorescein angiography (FA). These segmented volumes are called slabs. OCT angiograms can be presented in cross-sectional orientation, same as OCT B-scans, with colors to show where the flow is to demonstrate its relationship to the anatomical structures.

Currently available commercial systems include AngioVue (Optovue) and AngioPlex (Zeiss), which are high-speed spectral domain OCT systems (~70,000 axials scans/second). Commercial swept source OCT systems with 100,000 axial scans or faster are planned. Without need for a dye injection, angiographic images can be produced in a few seconds. The available field of view include 3 x 3 mm, 4.5 x 4.5 mm, 6 x 6 mm, and 8 x 8 mm. Capillary details are best seen on 3 x 3 mm angiograms. The standard output presents 4 slabs: superficial and deep retinal plexuses; the outer retina, which is normally avascular but where choroidal neovascularization (CNV) would be found; and the choriocapillaris (Figure 31-1).

The retinal vasculature is known to have up to 4 distinct layers. Around the optic disc, the radial peripapillary capillaries are found in the nerve fiber layer. In the macula, 3 distinct vascular plexuses are seen. The superficial vascular complex resides mainly in the ganglion cell layer and consists of the branching vessels that we are familiar with from FA. Two deeper capillary networks that flank the inner nuclear layer are called intermediate and deep capillary plexuses. These are networks of anastomosing capillaries of uniform density and caliber. They merge into a single layer in the midperiphery. At the foveal avascular zone edge, the capillaries from all 3 retinal vascular plexuses merge into a single layer. A distinct vascular network called the radial peripapillary capillary plexus can be found in the nerve fiber layer of the region near the optic disc. The current commercial output treats the intermediate and deep capillary plexus as a single plexuses and refers to it as deep plexus. Generally, FA only visualizes the superficial vascular complex. OCT-A can visualize all layers of the retinal vasculature.1

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