What does OCT stand for and what does it measure?

OCT stands for Optical Coherence Tomography. It uses near-infrared light interference to create high-resolution cross-sectional images of retinal and optic nerve tissue. It measures retinal layer thicknesses, identifies fluid accumulation, drusen, and other structural changes in the retina, and measures the retinal nerve fiber layer (RNFL) thickness for glaucoma assessment.

What is the RNFL and why is it important in glaucoma?

The retinal nerve fiber layer (RNFL) is the innermost layer of the retina, composed of the axons of retinal ganglion cells -- the cells whose axons bundle together to form the optic nerve. In glaucoma, ganglion cells die and their axons disappear, causing the RNFL to thin. RNFL thinning is a structural sign of glaucoma damage that often precedes detectable visual field loss. Serial OCT RNFL measurements track whether the nerve fiber layer is progressively thinning over time.

What signal strength is acceptable for an OCT scan?

Acceptable signal strength depends on the instrument: Zeiss Cirrus OCT requires a signal strength of 6/10 or higher (green range); Heidelberg Spectralis requires a quality score of 15 dB or higher. Below these thresholds, the image is too noisy for reliable layer segmentation and measurement. A cataract, small pupil, or poor fixation are common causes of low signal strength.

What is a segmentation error in OCT and why does it matter?

OCT instruments automatically identify (segment) the individual retinal layers and measure their thicknesses. A segmentation error occurs when the software misidentifies a layer boundary, often in eyes with dense cataracts, unusual anatomy, or severe retinal disease. Even a high-quality scan can produce inaccurate thickness measurements if the segmentation is wrong. Segmentation errors can make normal eyes appear abnormal or mask real disease, so they must be detected and corrected or flagged.

Does OCT require eye drops?

Standard posterior segment OCT (macula and RNFL scanning) is noninvasive and noncontact -- no anesthetic or dilating drops are needed for routine scans. Some doctors order dilation before OCT to improve image quality (larger pupil = more light entering = better signal) or for specific protocols that require wider retinal imaging. OCT angiography (OCTA) may also benefit from dilation. The CPOA should confirm whether dilation is ordered before beginning the scan.

OCT (Optical Coherence Tomography) for the CPOA Exam

What Is OCT?

Optical Coherence Tomography (OCT) is a noninvasive imaging technology that uses near-infrared light to produce high-resolution cross-sectional images of the retina, optic nerve, and other ocular structures. It is analogous to ultrasound, but uses light instead of sound, achieving much higher resolution (approximately 5-10 microns axially, compared to 150 microns for ultrasound).

OCT has revolutionized the diagnosis and monitoring of retinal diseases (macular degeneration, diabetic macular edema, macular holes) and glaucoma (retinal nerve fiber layer analysis, optic disc parameters). It is one of the most commonly performed diagnostic tests in modern optometry and ophthalmology practices.

What OCT Measures

Macula OCT

A macular OCT scan produces a detailed cross-sectional image through the center of the retina, showing all the individual retinal layers from the inner limiting membrane to the retinal pigment epithelium (RPE) and choroid. Key measurements include:

Central macular thickness (CMT): The thickness of the retina at the foveal center. Increased CMT indicates macular edema; decreased CMT may indicate advanced atrophy or macular hole.
Retinal layer integrity: Each of the 10 retinal layers can be visualized and assessed for disruption, fluid, or pathological deposits (drusen, fluid beneath the retina in AMD).

Optic Nerve / RNFL OCT

An optic nerve OCT scan circles the optic disc and measures the thickness of the retinal nerve fiber layer (RNFL) -- the layer composed of axons of the ganglion cells that form the optic nerve. RNFL thinning is the structural signature of glaucoma damage.

Results are compared to a large normative database by age and displayed in a color-coded map (green = normal, yellow = borderline, red = outside normal range).
Serial RNFL scans over time reveal progressive thinning in glaucoma patients.

CPOA Role in OCT

Before the Scan

Determine which OCT protocol is ordered (macula, optic disc, RNFL circle, anterior segment, wide-field, angiography).
No anesthetic or drops are typically required for standard posterior segment OCT (noninvasive, noncontact).
If dilation is ordered (for better scan quality or a specific protocol), instill dilating drops and wait for adequate dilation.
Enter patient data into the OCT instrument and select the correct scan protocol and eye.

Capturing the Scan

Position the patient at the OCT with chin in chin rest and forehead against bar.
Align the scan window with the patient's pupil.
Ask the patient to fixate on the internal fixation target -- this centers the scan on the correct anatomical location (fovea for macular OCT, optic disc for RNFL scan).
Focus the instrument using the refraction adjustment until the retinal image appears sharp and detailed on the preview monitor.
Capture the scan when the signal strength is adequate (most instruments display a signal strength indicator -- aim for the green zone).
Review the captured scan immediately for image quality.

Quality Indicators

Signal strength / quality index: Most OCTs display a signal strength (Cirrus OCT: 6/10 or higher is acceptable; Heidelberg Spectralis: quality score 15 dB or higher). Low signal strength produces noisy images with poor layer delineation.
Segmentation errors: The OCT software automatically segments (identifies and measures) retinal layers. Occasionally, the software segments incorrectly -- especially with dense cataracts or very abnormal retinas. Segmentation errors produce inaccurate measurements even with a good-quality scan.

💡 Clinical Tip: After capturing a scan, check that the RNFL thickness profile shows a normal double-hump pattern (superior and inferior peaks, temporal and nasal troughs) for glaucoma monitoring scans. An unusually flat or asymmetric RNFL profile may indicate a segmentation error or genuine pathology -- flag it for the doctor.

⚠️ Common Mistake: Capturing an OCT without ensuring the fixation light is centered correctly. If the patient is not fixating on the correct target, the scan may be off-center, missing the fovea for macular scans or failing to circle the disc properly for RNFL scans. Centered scans are critical for reproducibility across visits.

Common OCT Findings the CPOA Should Recognize

The CPOA does not interpret OCT clinically, but knowing what requires flagging is important:

Subretinal or intraretinal fluid: Dark (hyporeflective) spaces between or beneath the retinal layers -- common in wet AMD and diabetic macular edema. Flag immediately.
RNFL thinning in the red zone: RNFL sectors colored red on the deviation map -- significant glaucoma suspect finding.
Disrupted inner segment/outer segment (IS/OS) junction: Loss of the bright reflective line deep in the outer retina, suggesting photoreceptor damage.

Key Takeaways

OCT produces cross-sectional images of the retina and optic nerve at micrometer resolution using near-infrared light.
Macular OCT measures retinal layer thickness and integrity; RNFL OCT measures optic nerve fiber layer thickness for glaucoma monitoring.
The CPOA captures OCT scans, assesses signal strength and centering, and flags quality issues or obvious abnormalities.
Minimum acceptable signal strength: Cirrus 6/10 or greater; Spectralis 15 dB or greater.
Segmentation errors can produce inaccurate measurements even with an otherwise good scan.
Ensure fixation is centered on the correct target before capturing macula or RNFL scans.