OCT Retinal Bootcamp | Eyes On Eyecare

OCT Retinal Bootcamp

by Daniel Epshtein, OD, FAAO and April M Lewis, OD, FAAO

This course will review the basics of OCT analysis with regards to retinal pathology, highlighting the identifying characteristics of retinal lesions as viewed by OCT.

4.7 (17 ratings)
Updated Oct 6, 2021
45m
1 quiz
What You'll Learn
  • The unique characteristics that distinguish various retinal lesions on OCT
  • How to use the OCT diagnostic report to aid in the management of retinal pathologies

Introduction and Quiz

Optical coherence tomography (OCT) is a non-invasive ocular imaging method that uses light waves to create cross-sectional images of ocular tissues at a micron level resolution. The OCT images are constructed according to the amount of light that is scattered and reflected from the individual ocular tissues. Scans may be viewed in grayscale or pseudo-color scale, both based on tissue reflectivity. OCT imaging can help identify a pathology, determine the severity, and guide management decisions.

OCT Retinal Bootcamp Quiz

10 Questions

Take this quiz on OCT and retinal pathology to complete the course.

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OCT Terminology

An A-scan consists of a single one-dimensional axial scan of the eye, imaging the retina in the anterior-posterior axis. A collection of many adjacent A-scans can be merged to create a two-dimensional cross-section of the retina. This cross-section comprises the B-scan component of OCT imaging. Many adjacent B-scans can be merged to create a volumetric three-dimensional representation of the retina, called a cube scan. Alternatively, a raster scan can provide higher resolution than a cube scan, but is limited to only 5 parallel cross-sections through a given area of the retina. A radial scan is another higher resolution option that images several cross-sections arranged in a radial pattern centered on a particular area of interest, often the fovea.

A B-scan is made up of many adjacent A-scans to create a “slice” of the retina

Figure 1: A B-scan is made up of many adjacent A-scans to create a “slice” of the retina.

A cube scan is made up of many adjacent B-scans to create a 3D representation of the retina

Figure 2: A cube scan is made up of many adjacent B-scans to create a 3D representation of the retina.

OCT Basics

Homogenous structures allow light to pass through the material relatively undisturbed and therefore reflect little light back to the OCT. These structures appear as either dark grey or black, or as cooler colors. Homogenous ocular tissues include the outer nuclear layer, where the nuclei are tightly packed into a regular structure, and the vitreous. Areas of serous fluid will also appear dark on an OCT B-scan.

Heterogenous structures reflect significantly more light back to the OCT, so they appear as light grey or white, or warmer colors. In a healthy eye, the retinal nerve fiber layer (RNFL) appears nearly white in an OCT B-scan because the layer is very reflective. The outer plexiform layer and any areas of fibrous scarring also appear bright on an OCT cross-section.

The Normal OCT

When viewing an OCT of a healthy retina, layers that correlate to the anatomical and histological structures can be identified and evaluated. For example, the retinal pigment epithelium (RPE), ellipsoid zone, and external limiting membrane can be seen, should always be scrutinized because they correlate very well to visual acuity.

In this healthy macula, each layer of the retina can be distinctly identified due to differences in their reflectivities

Figure 3: In this healthy macula, each layer of the retina can be distinctly identified due to differences in their reflectivities.

When viewing the OCT diagnostic report, begin by verifying the identifying patient information and date. Ensure the signal strength is acceptable: a strength of 9 or 10 is preferred, but 7 or 8 may be acceptable in certain circumstances. Next, review the retinal thickness map to gain a topographical overview of the area of retina being evaluated, then the quantitative map to ascertain numerical retinal thickness values. Review the B-scans included on the report to ensure the scan placement is accurate and centered over the area of interest. When evaluating a B-scan, it may be helpful to first locate the RPE. This is a useful landmark in most cases because it is relatively thick and bright on the scan, making it easy to identify. Then, depending on the pathology, focus anteriorly or posteriorly to the RPE to locate the layer of interest.

In this OCT diagnostic report, the top box indicates where identifying patient information and signal strength scores are located.  The second box identifies the topographical retinal thickness map for each eye.  The next box highlights the quantitative or numerical thickness map for each eye.  The bottom box indicates where the B-scan image for each eye can be found.  This image depicts the use of a pseudo-color scale

Figure 4: In this OCT diagnostic report, the top box indicates where identifying patient information and signal strength scores are located. The second box identifies the topographical retinal thickness map for each eye. The next box highlights the quantitative or numerical thickness map for each eye. The bottom box indicates where the B-scan image for each eye can be found. This image depicts the use of a pseudo-color scale.


Vitreomacular Interface Disorders

Vitreomacular interface disorders can be difficult to identify solely with fundoscopy. Though visual acuity may be a helpful indicator, it is not a reliable differentiator in these cases. Under such circumstances, OCT imaging is a valuable tool and will identify the proper diagnosis 100% of the time.

OCT imaging can identify specific characteristics of vitreomacular interface disorders that are crucial to differential diagnosis

Figure 5: OCT imaging can identify specific characteristics of vitreomacular interface disorders that are crucial to differential diagnosis.

Vitreomacular Adhesion

Vitreomacular adhesion (VMA) is non-pathological. The vitreous and the retina are inherently attached, and at some point during a patient’s lifetime, the vitreous will likely pull away from the retina, leading to a posterior vitreous detachment (PVD). If the vitreous and macula are attached and there is no compromise to the macular contour during this process, this stage of PVD formation is labelled VMA and considered a benign finding.

Vitreomacular Traction

When the posterior cortical vitreous actively pulls on the retina, distorting retinal architecture, it is termed vitreomacular traction (VMT). VMT can easily be identified at the vitreoretinal interface on the B-scan. This traction can disrupt the retinal contour and cause thickening. When viewed on the retinal thickness maps, any area of central thickening will correlate with the area of traction on the B-scan.

Here a case of VMT has been imaged with a raster scan, allowing multiple views of the entire affected area.  Anterior to the RPE, retinal cysts may develop secondary to the traction.  Vitreomacular traction can resolve spontaneously without intervention, and in such cases, any remaining cysts may be observed for resolution

Figure 6: Here a case of VMT has been imaged with a raster scan, allowing multiple views of the entire affected area. Anterior to the RPE, retinal cysts may develop secondary to the traction. Vitreomacular traction can resolve spontaneously without intervention, and in such cases, any remaining cysts may be observed for resolution.

Epiretinal Membrane

In the case of an epiretinal membrane (ERM), the fibrous sheet at the vitreomacular interface can be identified on the scan. This membrane may cause no traction, and therefore no impact on visual acuity. In these cases the ERM may be monitored. As progression occurs, some thickening may be seen on both the B-scan and the retinal map. OCT imaging can be helpful to diagnose the severity of the ERM and the Macular Change Analysis can be used to guide management.

The traction of this progressed ERM has caused the fovea to begin to detach.  Thickening may be seen on both the retinal map and the B-scan image

Figure 7: The traction of this progressed ERM has caused the fovea to begin to detach. Thickening may be seen on both the retinal map and the B-scan image.