Optical coherence tomography (OCT) has affected the ophthalmology landscape as much as any other imaging device. OCT has changed not only the way we view anatomy and pathology, but also the treatment of eye disease. Aside from histological images, we never previously had the ability to view ocular anatomy at the cellular level, and certainly not in vivo, which allows us to view anatomical structures while they are functioning.

Spectral-domain OCT (SD-OCT)

SD-OCT measures the interference of light as different layers of anatomical structure affect it. This light, which is emitted from a diode laser, passes through anatomical layers and, depending on the density of those layers, is scattered and reflected back to the imaging system. This returning light is matched with a “control” beam of light and evaluated, producing a cross-sectional view of the structure. When first introduced to ophthalmology, OCT technology necessitated a moving spectrometer that measured the returning light. With SD-OCT, the spectrometer does not need to move, so it acquires more data, faster. SD-OCT also allows for significant noise reduction and, with eye-tracking software, reduces movement artifacts (see Figure 1, below).

OCT systems utilize multiple scans (a-scans) to produce a single line scan (b-scan), and most OCT systems can capture 70,000 “a-scans” per second. Scans are performed in the X-axis (horizontal) and Y-axis (vertical), which allows for 3-D rendering of images and quantifiable volumetric measurement. Newer systems allow for visualization of the Z-axis (depth), which allows for en face (“on the face”) viewing of structures (Figure 2, page 19).

As with any imaging device, media clarity is crucial. Therefore, as OCT utilizes light to produce images, anything that affects light affects the resulting image. This includes cataract, vitreous opacities, and dry cornea (Figure 3, page 19). Blinking during OCT acquisition is acceptable (even encouraged) as most systems capture multiple images to create a composite image.

OCT angiography (OCTA)

OCTA, while new, has quickly gained ground in becoming a clinical standard. Taking no more time than a standard OCT to perform — and typically alongside OCT — this technology gives the physician significantly more data to evaluate and determine a baseline diagnosis or progress of treatment. One of the most common reasons for a patient visit in ophthalmology is for injection of anti-VEGF, which treats diseases that tend to produce neovascularization. OCTA is a prime clinical tool for establishing baseline and follow-up for these patients, as it images neovascular complexes to the capillary level. In some cases, OCTA can reveal abnormal vasculature not seen on standard dye angiography or OCT (Figure 4, below).

OCTA utilizes Brownian Motion with X-, Y-, and Z-axis scanning (Figure 5a, page 20). Brownian Motion is the random motion, or movement, of particles suspended in fluid resulting from the collision of faster moving particles in the same fluid.

The only particle that continually moves in the posterior segment of the eye is blood — both in retinal vasculature and choroidal vasculature. The particles that we observe with this method are red blood cells, which are “suspended” in blood, and collides with the other particles that make up blood. By only targeting moving red blood cells and eliminating any non-moving structure, we can designate areas in which blood is “moving” by assigning white to movement and black to non-movement. The resulting image looks much like a fluorescein angiogram, with active vasculature in white and non-active areas in black (Figures 5b, 5c, page 20).

Software allows for color coding of vasculature, depending on the level they appear (Figure 5d, above). Acquisition of OCTA images is quick, averaging five seconds, and can be performed on patients without dilation and no injection of dye. The resulting image allows the physician to “scroll” through cross-sectional slabs and determine the level of vasculature.

How OCTA differs

Of course, OCTA and standard dye angiography have major image differences. OCTA only demonstrates movement, therefore it does not detect intra-tissue fluid that is labeled “pooling” or “staining” on standard dye angiography. OCTA is also not a dynamic test and cannot evaluate blood flow as a standard dye angiogram would; therefore, the designation as an “angiogram” is misleading. Because OCTA utilizes SD-OCT (and, in some cases, swept-source OCT [SS-OCT]) technology, that too can be confusing. The simplest, and most effective, way of differentiating between OCT and OCTA is that OCT reveals anatomical structure while OCTA reveals vascular structure.

From a patient standpoint, OCTA differs from OCT in that blinking is not encouraged during acquisition. As OCTA is designed to document movement in the eye, along with eye movement, blinking results in movement artifacts that can obscure data. As with any diagnostic test, explaining expectations to the patient prior to testing is crucial.

OCT angiography is still relatively new to ophthalmology, with only two FDA-approved systems at the time of this article (Zeiss AngioPlex and Optovue AngioVue). Therefore, it is imperative to establish two points:

1. Determination of what is “normal.” The only way to recognize abnormal patterns is to scan healthy eyes to establish normative patterns. This allows us to identify and label anatomy affected by disease and the pathological result of disease.
2. A vocabulary for anatomical and pathological findings. Visualization of vascular structure in vivo has not been performed at this level, and there are bound to be new findings or hypotheses proven. Creating agreed upon nomenclature to identify and describe findings will not only assist others, but will allow the technology to progress as well.

OCT tips

OCT may be the most utilized imaging device in ophthalmic practice, and for good reason. Not only can physicians diagnose disease based on OCT images, but they can also follow potential changes in patients between visits. Also, OCT is non-invasive, so it allows for frequent imaging of patients without too much disruption to a clinical schedule.

In addition, OCT is a relatively easy instrument to use. However, the ophthalmic staff member operating the device must understand the following:

1. Ocular disease and how it affects the patient’s vision. The patient must subjectively fixate on a target, and the device typically centers the scan on the patient’s fixation. If disease pathology affects the patient’s ability to fixate, scans may be performed outside the desired area of anatomy.
2. Artifacts. As with any imaging device, artifacts may appear on images, whether from the device or the patient. Recognizing these and the need to produce an artifact-free image is crucial to producing an optimal image.

Looking to the future

The future direction of both OCT and OCTA is to capture even more data, faster. One of the modalities being evaluated, SS-OCT, utilizes a swept laser at 1050 nm instead of the standard super luminescent diode at 880 nm. This allows for deeper penetration and a wider window of focus. SS-OCT also scans at 100,000 a-scans per second, allowing for faster acquisition of higher resolution images. However, cost and limited manufacturers may be a prohibitive factor for standard clinical practice. OP

Darrin Landry, CRA, OCT-C, has been an ophthalmic photographer since 1990, and has served on the Board of Education and has been an expert advisor in the development of certification for OCT for the Ophthalmic Photographers’ Society. He is the author of three books: “Retinal Imaging Simplified,” “Optical Coherence Tomography: A Clinical Atlas of Retinal Images,” and “Optical Coherence Tomography and OCT Angiography” (with Amir Kashani, MD, PhD), as well as multiple peer reviewed articles. Mr. Landry is the co-founder of Bryson Taylor Inc., an ophthalmic and management consulting firm that contracts with biotech, academic institutions, reading centers, private practices, and the pharmaceutical industry. Email him at Darrin@brysontaylor.com.

Financial disclosure: Mr. Landry is a consultant for Carl Zeiss Meditec.