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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.
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.
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:
Free CPO and CPOA exam prep on Opterio—including diagnostic imaging and instrument use.
Fundus photography—capturing high-quality retinal images.
Understanding glaucoma—where OCT RNFL analysis is critical.
AMD—where macular OCT drives treatment decisions.
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Optical Coherence Tomography (OCT) is a non-invasive imaging technology that uses near-infrared light waves to create high-resolution, cross-sectional images of ocular tissue. It works similarly to ultrasound but uses light instead of sound waves. The OCT device measures the time and intensity of reflected light from different tissue layers—a technique called interferometry. Modern spectral-domain OCT (SD-OCT) can resolve tissue structures as thin as 3–5 micrometers, allowing visualization of individual retinal layers. Because it is non-contact and uses harmless infrared light, patients experience no discomfort during the scan.
OCT can image: (1) The macula—detecting macular holes, epiretinal membranes, central serous retinopathy, and diabetic macular edema (DME). It quantifies retinal thickness and maps fluid. (2) The optic nerve head—measuring retinal nerve fiber layer (RNFL) thickness, optic disc cupping, and rim tissue. Critical for glaucoma detection and monitoring. (3) The retinal layers in detail—identifying drusen in AMD, geographic atrophy, and choroidal neovascularization. (4) The anterior segment—some OCT devices image the cornea, anterior chamber angle, and crystalline lens. OCT is essential in glaucoma monitoring, AMD management, and diabetic retinopathy follow-up.
Macular OCT scans are centered on the fovea and macula, measuring retinal thickness across the central 6–9 mm of the posterior pole. Results are displayed as a thickness map (often color-coded) showing areas of thickening (fluid, edema) or thinning (atrophy, cell loss). Optic nerve OCT (also called RNFL OCT) scans are centered on the optic disc and measure the thickness of the retinal nerve fiber layer in a circular pattern around the disc. RNFL thinning in superior and inferior sectors is the earliest structural sign of glaucomatous damage. Many practices perform both scans at the same visit for comprehensive posterior segment evaluation.
Paraoptometrics are often responsible for performing OCT scans under the doctor's supervision. This involves: (1) Positioning the patient at the OCT instrument with chin in the rest and forehead against the bar. (2) Explaining the procedure to the patient—look at the fixation light, try not to blink during the scan. (3) Aligning the instrument to the patient's pupil using the joystick (the instrument typically shows a live fundus image for alignment). (4) Initiating the scan and confirming image quality—good signal strength (typically >6/10) and adequate centration. (5) Saving and labeling the scan correctly in the patient's record. (6) Flagging poor quality scans for repeat (excessive blinking, eye movement, small pupil artifact).
Poor OCT image quality can result from: (1) Small or poorly dilated pupils—dim lighting, pharmacologic dilation, or a larger-beam instrument setting may help. (2) Excessive blinking—coach the patient to blink between scans and hold still during the scan; use a topical lubricating drop to improve comfort. (3) Cataracts or corneal opacities—media opacities reduce signal transmission, lowering image quality; document media status. (4) Unsteady fixation or nystagmus—use an internal fixation target, ask the patient to focus on the dot, and take multiple scans. (5) Tear film instability—have the patient blink once, then hold the eye open during the scan. A signal strength below 6/10 should prompt a repeat attempt.
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