How is OCT different from fundus photography?

Fundus photography captures a two-dimensional surface image of the retina, showing color and surface detail but not cross-sectional depth. OCT creates a cross-sectional (B-scan) image that shows the individual layers of the retina in depth, similar to looking at a slice rather than the top of a cake. OCT provides structural thickness measurements that photography cannot.

What does RNFL thinning on OCT indicate?

RNFL thinning indicates loss of retinal nerve fiber axons. This is the primary structural change in glaucoma and typically occurs before visual field defects become detectable on standard perimetry. RNFL thinning in the superior and inferior quadrants of the optic nerve scan is particularly significant, as these regions carry the arcuate nerve fibers serving the central visual field.

What does the color coding on OCT printouts mean?

OCT devices compare patient measurements against a normative database matched by age. Results are color-coded: green means the value is within normal limits (above the 5th percentile), yellow (borderline) means the value falls between the 5th and 1st percentile, and red (outside normal limits) means the value is below the 1st percentile for the patient's age group.

Why is patient fixation important during OCT scanning?

OCT builds its images by combining hundreds to thousands of individual A-scans into a B-scan slice. If the patient's eye moves during the capture, the scans do not align properly, creating motion artifacts. These artifacts can appear as apparent thickness changes or missing data that may be misinterpreted as real pathology. Steady fixation throughout the scan ensures accurate segmentation of retinal layers.

OCT (Optical Coherence Tomography): A CPO Exam Guide

What Is OCT?

Optical coherence tomography (OCT) is a non-invasive imaging technology that produces high-resolution, cross-sectional images of the retina and other ocular structures. It is often described as the optical equivalent of ultrasound: instead of sound waves, OCT uses near-infrared light to create detailed tissue images based on the reflectivity of different layers.

Since its introduction in clinical practice in the 1990s, OCT has transformed ophthalmic diagnosis and monitoring. It is now one of the most commonly performed diagnostic tests in optometry and ophthalmology, used in almost every examination of the posterior segment and in many anterior segment evaluations as well.

How OCT Works

OCT is based on low-coherence interferometry. The device splits a near-infrared light beam into two paths: one directed into the patient's eye and one directed at a reference mirror. Light reflecting back from retinal tissue interferes with the reference beam. By analyzing the interference patterns across different time delays (depths), the machine constructs a cross-sectional slice of the tissue.

Modern spectral-domain OCT (SD-OCT) captures thousands of A-scans per second by analyzing the full spectrum of returned light simultaneously, rather than mechanically scanning a reference mirror. This produces the high-resolution, high-speed imaging that clinicians rely on today. Even newer swept-source OCT (SS-OCT) uses a longer wavelength laser, allowing deeper tissue penetration useful for imaging the choroid and structures behind the retina.

When setting up a patient for OCT, make sure they are fixating steadily on the internal target. Even small eye movements during the scan create segmentation artifacts that can make normal retina look diseased on the thickness maps.

Retinal Layers Visible on OCT

A high-quality OCT scan reveals all major retinal layers as distinct bands of light and dark. From the innermost (vitreal) surface to the outer aspect, the main visible layers include:

Internal limiting membrane (ILM)
Nerve fiber layer (NFL)
Ganglion cell layer (GCL)
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
External limiting membrane
Ellipsoid zone (IS/OS junction, photoreceptor inner/outer segment junction)
Retinal pigment epithelium (RPE)
Bruch's membrane and choroid

The ability to see individual layers allows detection of pathology at a cellular level, long before it would be visible by indirect ophthalmoscopy.

Key Measurements: RNFL and Macular Thickness

Retinal Nerve Fiber Layer (RNFL) Thickness

Scanning the optic nerve head with OCT generates a circular scan around the disc that measures the thickness of the retinal nerve fiber layer at multiple clock-hour positions. This is compared against a normative database stratified by age, and color-coded:

Green: Within normal limits
Yellow: Borderline (between 5th and 1st percentile)
Red: Outside normal limits (below 1st percentile)

RNFL thinning in the superior and inferior quadrants (where the arcuate fibers are densest) is a hallmark of early glaucomatous damage. OCT often detects these changes before the patient notices any visual field defect.

A yellow or borderline RNFL result does not automatically mean glaucoma. Small optic disc size, image quality issues, and ethnic variation in normative data all affect results. Always correlate with clinical exam and visual field.

Macular Thickness and Volume

Macular OCT generates a topographic map of retinal thickness across the central 6 mm of the macula. It displays average thickness by sector (using the ETDRS 9-zone grid) and flags values outside normative limits. This is essential for monitoring:

AMD: Detecting subretinal or intraretinal fluid, drusen topography, and geographic atrophy.
Diabetic macular edema: Quantifying macular thickening due to fluid accumulation from leaking vessels.
Epiretinal membranes: Identifying distortion of the inner retinal surface.
Vitreomacular traction: Seeing the vitreous pulling on the macula before a hole forms.

Enhanced Depth Imaging (EDI-OCT) and Choroidal Thickness

By adjusting the reference arm position, some OCT devices can image deeper into the choroid with enhanced depth imaging (EDI-OCT). Choroidal thickness is a research and clinical parameter of interest in conditions like central serous chorioretinopathy and myopia progression.

Anterior Segment OCT

Modified OCT systems can image the anterior segment (cornea, angle, lens) with high resolution. This is used to measure corneal thickness, evaluate the anterior chamber angle for narrow-angle detection, and assess corneal conditions like keratoconus or post-surgical changes.

Key Takeaways

OCT uses low-coherence interferometry to create cross-sectional images of retinal layers without touching the eye.
RNFL thickness measured by OCT is a key tool for detecting and monitoring glaucoma.
Macular OCT measures retinal thickness and volume, essential for AMD, diabetic macular edema, and other maculopathies.
Color coding (green/yellow/red) compares patient values against age-matched normative data.
Eye movement artifacts during scanning cause errors in segmentation and thickness maps.