Optical Coherence Tomography (OCT) has become one of the most important diagnostic tools in modern optometry. Paraoptometric staff are increasingly responsible for performing OCT scans, ensuring image quality, and appropriately labeling and filing results. Understanding how OCT works, what it detects, and how to troubleshoot image quality issues is tested on both CPO and CPOA certification exams.
How OCT Works: The Basics
OCT uses near-infrared light (approximately 840 nm wavelength) and a principle called low-coherence interferometry to measure the depth of reflections from different tissue layers. Think of it like "light-based ultrasound"—instead of timing the echo of sound waves, the instrument times the echo of light waves reflected from tissue interfaces.
- Time-Domain OCT (TD-OCT) — 400 A-scans/second — 8–10 µm axial — Older generation; largely superseded. Reference speed.
- Spectral-Domain OCT (SD-OCT) — 20,000–70,000 A-scans/second — 3–7 µm axial — Current standard. Faster, higher resolution, less motion artifact.
- Swept-Source OCT (SS-OCT) — 100,000+ A-scans/second — 5–8 µm axial — Longer wavelength (1050 nm) penetrates deeper—better choroidal imaging.
- Widefield OCT — Variable — Variable — Extended scan width; useful for peripheral retina assessment.
Clinical Applications of OCT
Macular OCT
- Diabetic Macular Edema (DME) — Intraretinal cysts, subretinal fluid, diffuse thickening—primary indicator for anti-VEGF treatment decisions
- Age-Related Macular Degeneration (AMD) — Drusen deposits, subretinal fluid, RPE elevations, geographic atrophy—distinguishes dry from wet AMD
- Macular Hole — Full-thickness defect in the fovea—staged by size and shape
- Epiretinal Membrane (ERM) — Hyper-reflective layer on inner retinal surface causing wrinkling/distortion
- Central Serous Retinopathy — Subretinal fluid accumulation beneath detached sensory retina
Optic Nerve / RNFL OCT (for Glaucoma)
The RNFL (Retinal Nerve Fiber Layer) scan measures nerve fiber thickness around the optic disc in four quadrants: superior, inferior, nasal, temporal (SINIT). Results are compared to a normative database and color-coded:
- Green — Within normal limits (5th–95th percentile of normative database)
- Yellow — Borderline (1st–5th percentile)—warrants monitoring
- Red — Outside normal limits (<1st percentile)—significant RNFL thinning, suspicious for glaucoma
Performing the OCT Scan: Step by Step
Identify the Correct Scan Protocol
Know whether the doctor ordered macular OCT, optic nerve OCT, or both. Select the correct protocol on the instrument before starting.
N: 1
Position the Patient
Chin in chin rest, forehead against bar. Eyes level with the instrument. Adjust height so the pupil aligns with the instrument aperture.
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Instruct the Patient
"Look at the fixation light (the blinking dot or star). Try not to blink during the scan, which only takes a few seconds. I'll tell you when to blink."
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Align the Instrument
Use the joystick to center the pupil in the live camera view. Achieve a clear fundus image and appropriate OCT signal alignment.
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Check Signal Strength
Most instruments display a signal strength value (e.g., 1–10). Aim for ≥7. Below 6, image quality is compromised—attempt repositioning or pupil dilation before retrying.
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Initiate and Capture the Scan
Press the capture button. Hold the instrument steady during acquisition. Review the B-scan for obvious artifacts (blink, eye movement bands).
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Verify and Save
Confirm the scan is correctly labeled (patient name, eye, date, scan type). Save to the electronic record. Note any quality issues in the chart.
N: 7