Optical Coherence Tomography Angiography, also known as OCT-A, is a fairly new technology that allows doctors to better assess retinal and choroidal vascular disease and blood flow. OCT-A can be used to assess structural changes in diseases such as age-related macular degeneration, diabetic retinopathy, and vein occlusions.
A few things that I wondered when we were first learned about this technology were:
- How does it work?
- When would you use it?
- How does it compare to the gold standard in retinal blood flow, intravenous fluorescein angiography (IVFA)?
Let’s try to answer these three questions!
How OCT-A works: OCT-A basically takes multiple cross-section B-scan images rapidly from the same point on the retina and then compares the difference between the scans to calculate the blood flow between them. The basis of it is called motion contrast. The “decorrelation signal” determines how fast, or slow, the blood is moving (higher decorrelation signal=faster RBC movement) based on the differences in the B-scans(1-3). To simplify, in tissue that has no movement (no blood flow or flow that is too slow to be detected), the signals that are sent back will be identical and therefore no blood is imaged and the area will be dark. In tissues where there is blood flow, the signals will be different and the areas of flow will be bright.
When should I use OCT-A? A few main reasons this would be used is to determine if AMD is wet or dry, if there is neovascularization in a diabetic patient, and if there is perfusion after vein occlusions. Other uses may be considered such as in patients with glaucoma, to visualize the microvasculature of the optic nerve and peripapillary area to determine if the optic nerve head is being adequately perfused. OCT-A has the advantage of allowing both the structure and function of the retina to be viewed in patients with these diseases (3), allowing for more detailed assessment of microvascular pathology and aiding in clinical diagnosis.
The OCT-A produces images of the retina that are automatically segmented into 4 zones: 1) the superficial capillary plexus (ILM-IPL), 2) the deep capillary plexus (INL-OPL), 3) the outer retinal zone (ONL-BM), and 4) the choriocapillaris (3). Note that the segmentation lines can also be manipulated allowing visualization of any retinal layer including the pre-retinal space. We expect there to be perfused blood vessels in the two capillary networks of the inner retina and in the choriocapillaris. In the outer retina, absence of vasculature is expected, and therefore normally results in a black scan (1). In each of these scans, you are able to see neovascularization by looking for formation of vessels in areas you would not expect there to be any (for example, in the outer retinal layer or in the fovea). You are also able to look for areas of ischemia; this is especially important for patients with diabetes. While we look for microaneurysms, IRMA, or neo fundoscopically to determine the stage of diabetic retinopathy, with OCT-A we are able to see even earlier ischemic signs such as enlargement of the foveal avascular zone (FAZ) (4).
OCT-A not only allows you to view where there is neo (or classify IRMA versus true neo), but it also helps us better diagnose our patients with AMD. Previously, OCT was great in helping determine wet versus dry AMD. However, OCT-A can identify the presence of a CNVM as well as identify the size and type of CNVM and whether it is occult, classic, sub-retinal, or sub-RPE (1). The detailed image OCT-A gives of neovascular membranes (as in the picture to the right) will allow doctors to regularly, and non-invasively, evaluate their patients’ disease progression and therefore better guide referral and treatment.
How does OCT-A compare to IVFA? Intravenous Fluorescein Angiography (IVFA) remains the gold standard in monitoring the function of the retinal vasculature (2). There are advantages and disadvantages to both IVFA and OCT-A, and therefore reasons and times that each are indicated. IVFA is the standard due to its large field of view and ability to show exact filling times of retinal vessels including late stage pooling and leakage, something that OCT-A cannot do (1-4). IVFA is good at imaging very slowly flowing blood unlike OCT-A which compares the same spot over time, therefore capturing faster flowing blood more precisely. There are however advantages to using OCT-A. The first one being that OCT-A is non-invasive and can be quickly obtained. An IV does not have to be inserted and there is no risk of nausea, allergic reaction, or anaphylaxis as there is with fluorescein angiography (2). OCT-A can therefore be used when FA is contraindicated and more frequently without putting patients through the hassle of being hooked up to an IV. OCT-A allows for microvasculature to be imaged in high resolution due to not capturing leakage that may obscure the view of layers below it (1), as well as having a high resolution of 5μm (the tradeoff is a limited field of view). This is especially important in the FAZ, where leakage on FA can blur the images of this highly important area (4).
In conclusion, OCT-A is a very beneficial technology that allows doctors to better monitor and make appropriate referrals for the care of their patients. There are still many times traditional FA is indicated and needed, but the addition of OCT-A will hopefully reduce invasive scans and enhance the care of our patients.
A special thank you to the NSU Oklahoma College of Optometry for use of scans.
- Majcher, Carolyn, and Susan Ly Johnson. “Imaging Motion: a Review of OCT-A.” Review of Optometry, 15 Mar. 2017, reviewofoptometry.com/article/imaging-motion-a-review-of-octa.
- Spaide, Richard F, et al. “Imaging of Retinal Vascular Layers.” JAMA Ophthalmology, American Medical Association, 1 Jan. 2015, jamanetwork.com/journals/jamaophthalmology/fullarticle/1910581.
- Lighthizer, Nathan. “OCT Angiography: The Next Chapter in Posterior Imaging.” Oklahoma Association of Optometric Physicians, Sept. 2017, Tulsa, OK.
- Novais, Eduardo, and Caroline Baumal. “The Clinical Utility of OCT Angiography.” Review of Ophthalmology, 10 Jan. 2017, www.reviewofophthalmology.com/article/the-clinical-utility-of-oct-angiography.