OCT vs. OCT-A: Assess retinal structure and function with advanced imaging technology
Interest in optical coherence tomography angiography (OCT-A) surged with the advent of the Current Procedural Terminology (CPT) code for this technology. Since January 1, 2025, eyecare providers have been able to bill to 92137 with an average reimbursement of $56.93. (This code specifies that OCT-A must be performed and interpreted on the same day.)
Looking back
Before OCT-A, time-domain optical coherence tomography (TD-OCT) was available, which provided a noninvasive cross-sectional look at the retinal and choroidal architecture. TD-OCT was a huge development in eye care because it allowed for a structural, non-invasive rendering of the retina. However, the images were often pixelated and hard to interpret.
Thus, a few years later, spectral domain (SD) OCT became available. SD-OCT captured more scans per second, which resulted in higher resolution and higher quality images than TD-OCT.
However, SD-OCT allows only for visualization of the retinal anatomy. To assess retinal vasculature and perfusion, patients needed to be referred to a retinal specialist for fluorescein angiography (FA), which provided information about the retinal vasculature. FA necessitates an injection and is not free from risks and side effects.
Technology advancements such as OCT-A have made it possible to view blood flow without an injection, enabling optometrists, who cannot perform injections, to access retinal and choroidal blood flow. OCT-A is safe and can be performed on any patient without adverse effect.
Image 1: Top Left: Time-domain OCT. Top Right and Bottom: Spectral Domain OCT
OCT vs OCT-A
A standard OCT image is called a B-scan or a structural cross-section of the retina. OCT B-scans are used to generate the data for OCT-A, which is based on sequential B-images.
Benefits to OCT imaging include:
- Excellent visualization of macular architecture
- Noninvasive and easy to perform
- Useful for identifying drusen, fluid, and retinal layer disruptions
Standard OCT provides a stationary image of the retinal and choroidal anatomy. However, OCT does not allow for visualization of blood flow. It is a good choice to view structural changes such as drusen, fluid, and elevation or disruptions in retinal layers.
Image 2. Standard OCT image.
Conversely, OCT-A reveals retinal function, specifically blood flow within vessels. This enables clinicians to visualize choroidal neovascular membrane, ischemia, or poor perfusion in conditions like diabetic retinopathy or vascular occlusion.
OCT-A uses a motion contrast system which employs the use of an algorithm to identify retinal blood flow within a blood vessel. The algorithm assesses difference in reflectivity or signal in sequential B-scan images to create angiograms of retinal and choroidal blood flow at various layers.
Image 3. OCT Angiography (OCT-A) image.
Note that hemorrhages won’t show up on OCT-A because the technology images only what is moving within blood vessels. Fluid coming out of a vessel will not be imaged.
Benefits to OCT-A imaging include:
- Better patient care
- Better patient experience
- Faster diagnosis or rule out
- Fewer patients referred out
- Additional practice revenue
Interpreting the flow overlay or pixelation
Understanding how to interpret the OCT-A flow overlay image is crucial to success. Blood flow will be highlighted on OCT B-scans as “pixelation” or red coloration. Pixelation will be seen anywhere in the retina and choroid where blood flow exists. Thus, pixelation should be seen within the inner retina because capillary beds run within the inner retinal network. Pixelation will be seen in the choroid as well because the choroid is all blood flow. Pixelation or red should not be seen in areas that are avascular, such as the vitreous or the retinal pigment epithelium (RPE).
When trying to determine if a patient has an abnormal area of vascularity, looking at pixelation—and knowing where it should appear—is important.
Image 4. Blood flow highlighted on OCT B-scans as “pixelation” or red coloration.
En face imaging: Slicing through the retina
En face cuts are horizontal cuts through the retinal anatomy (and thus the various capillary networks) that are used to create angiograms. Angiograms can be obtained from any area of the retina and choroid to visualize what's happening at the vascular level.
The most superficial capillary network of the retina, the superficial capillary plexus (SCP), looks at the inner limiting membrane (ILM) through the inner plexiform layer (IPL). The capillary bed innervates the superficial retina. The blood vessels are white, the black area in the center is the foveal avascular zone, where there's no blood flow. Remember, where you see black on an angiogram, there is no blood flow; white shows blood vessels.
Going further posterior through the retina, next is the deep capillary plexus (DCP). The caliber and look of the blood vessels or capillaries are very different than in the superficial plexus. Many retinal diseases, such as macular telangiectasia and retinal angiomatous proliferation, originate in the deep plexus. The deep capillary plexus is not well visualized with FA, thus OCT-A has been instrumental in understanding the pathophysiology behind these diseases.
Next seen is the avascular outer retinal zone, which correlates with the RPE. It should be black because the RPE is an avascular tissue.
Finally, you see the chorio capillaris, which should be a white spongy homogeneous angiogram because it consists of blood.
Image 5: 4-up OCT-A images showing the superficial plexus, the deep plexus, the outer retina, and the chorio capillaris.
Using OCT-A clinically
In order to become proficient at OCT-A, it is important to familiarize yourself with normal OCT-A printouts. This will make it much easier to identify abnormal areas on the angiograms.
When used in tandem, OCT and OCT-A provide a complete clinical picture of the health and integrity of the retina. It allows for outstanding visualization of both structure and blood flow. This results in early diagnosis of disease and outstanding patient care.
VISIONIX has been a leader in oct technology
- First FDA-cleared spectral domain OCT
- First anterior/posterior segment OCT
- First OCT-A (AngioVue)
- First OCT-A quantification metrics (AngioAnalytics)
- Still the only FDA-cleared OCT-A metrics
- First Full Range retina SD-OCT (Solix)
- First OCT with meibomian gland imaging (Solix)
Julie Rodman, OD, MSc, FAAO, FORS
Dr. Julie Rodman is the Chief of the Broward Eye Care Institute in Fort Lauderdale, FL and a Professor of Optometry at Nova Southeastern University. Her research interests include OCT/OCT-A and Vitreoretinal Disease. Dr. Rodman has authored over thirty publications with an emphasis on retinal disease. She recently published “Optical Coherence Tomography Atlas: A Case Study Approach,” the first reference book on this topic written by an optometrist. Dr. Rodman is a member of the AOA, AAO, FOA, and ORS. She has been the recipient of numerous teaching awards and was recognized as a Primary Care Optometry News “Top 300” Optometrists and “Newsweek Best Optometrists of 2021.”
This article originally appeared on ODWire.org in December 2025: https://www.odwire.org/community/threads/oct-vs-oct-a.173190/
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