OCT-C cheat sheet

OCT is based on low-coherence interferometry: broadband light is split into sample and reference arms, and interference occurs only when the path-length difference is within the coherence length.

Axial resolution improves as source bandwidth increases and is approximated by Δz ≈ 0.44 λ²/Δλ in air; retinal spectral-domain OCT typically achieves about 5–7 µm axial resolution.

Transverse resolution is determined by the focused beam spot size, typically about 15–20 µm in retinal OCT, and is independent of axial resolution.

Time-domain OCT uses a moving reference mirror and a single photodetector; spectral-domain OCT uses a fixed reference and a spectrometer, with Fourier transformation of the spectrum yielding the A-scan.

Swept-source OCT uses a tunable laser and a single photodetector; longer wavelengths near 1050 nm improve choroidal penetration and reduce sensitivity roll-off compared with 840 nm spectral-domain OCT.

An A-scan is a one-dimensional depth profile of reflectivity; a B-scan is a two-dimensional cross-section assembled from adjacent A-scans; an en face image is a coronal slice parallel to the retinal surface.

Common retinal scan patterns include macular cube/raster scans, single line scans, radial scans through the optic nerve head, and circumpapillary circle scans for retinal nerve fiber layer thickness.

Deeper structures produce weaker signals, so signal-to-noise ratio and sensitivity roll-off determine how well the choroid and outer retina are visualized.

OCT detects backscattered light intensity and phase; it does not directly measure fluorescence or absorption, although OCT angiography derives flow information from motion contrast.

Inner retinal layers from inner to outer are the internal limiting membrane, retinal nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, and outer nuclear layer.

Outer retinal bands include the external limiting membrane, ellipsoid zone (formerly IS/OS junction), interdigitation zone (COST), retinal pigment epithelium, and Bruch's membrane.

At the foveola the retinal nerve fiber layer, ganglion cell layer, and inner plexiform layer are absent; the outer nuclear layer is thickest and Henle fiber layer is present, producing the foveal pit.

The vitreoretinal interface is the boundary between the posterior vitreous cortex and the internal limiting membrane; it is normally invisible or appears as a faint line.

The choroid lies between Bruch's membrane and the sclera and includes the choriocapillaris, Sattler's medium-vessel layer, and Haller's large-vessel layer.

The optic nerve head shows a central cup, neuroretinal rim, and peripapillary retinal nerve fiber layer; the lamina cribrosa appears as alternating hyperreflective and hyporeflective bands.

The retinal pigment epithelium is a highly hyperreflective band; the ellipsoid zone lies just external to the external limiting membrane and is a marker of photoreceptor integrity.

Peripapillary retinal nerve fiber layer thickness is greatest superiorly and inferiorly and least temporally, producing the double-hump pattern used in glaucoma analysis.

Dilate the pupil unless contraindicated; a pupil of at least 4 mm is generally needed for adequate retinal scan width and signal strength.

Ask the patient to fixate on the internal target for macular scans; use an external fixation target for optic nerve or peripheral scans.

Center the scan on the fovea by aligning the OCT beam with the foveal reflex and checking the en face or fundus image.

Optimize focus, polarization, and Z-offset; confirm signal strength is in the good or green range before capturing.

Select the correct protocol: macular cube for retinal thickness mapping, circumpapillary circle for retinal nerve fiber layer, radial scans for the optic nerve head, and anterior segment scan for cornea.

Averaging multiple B-scans improves signal-to-noise ratio but increases acquisition time and motion sensitivity.

Instruct the patient to blink before capture and hold still; use the chin rest and forehead strap for stability.

For anterior segment imaging, use a corneal adapter or wide-angle-to-angle scan and align the corneal vertex.

Signal strength below the manufacturer's threshold, often less than 6/10 or about 20 dB, reduces segmentation reliability and should prompt repeat acquisition.

Motion artifact appears as horizontal misalignment or stepped B-scans; it is reduced by faster scanning, eye tracking, and averaging.

Mirror artifact duplicates structures across the zero-delay line; it is identified by inverted or duplicated anatomy and corrected by adjusting Z-offset.

Blink or eyelid shadowing causes vertical dark bands; repeat after blinking and retracting the eyelid.

Segmentation errors occur at boundaries with low contrast, such as drusen, epiretinal membrane, or severe edema; automated lines must be verified manually.

Shadowing beneath dense media opacity, hemorrhage, or a thick vessel obscures deeper layers and can mimic retinal thinning.

Decentration causes the fovea to be off-center and invalidates thickness maps; recapture with the fovea centered.

Out-of-register artifacts occur when consecutive B-scans are misaligned; eye tracking and registration correct them.

Epiretinal membrane appears as a hyperreflective line on the internal limiting membrane with underlying retinal wrinkling; vitreomacular traction shows a partial posterior vitreous detachment with focal traction.

Full-thickness macular hole shows a full-thickness defect with elevated edges and intraretinal cysts; lamellar hole is partial-thickness.

Cystoid macular edema appears as hyporeflective intraretinal cystoid spaces; subretinal fluid is a hyporeflective space between the retina and retinal pigment epithelium.

Drusen are dome-shaped retinal pigment epithelium elevations with intact overlying ellipsoid zone; pigment epithelial detachment is a larger elevation of the retinal pigment epithelium.

Choroidal neovascularization appears as a hyperreflective subretinal or sub-retinal pigment epithelium lesion with fluid, often with disruption of the retinal pigment epithelium and outer retinal bands.

Diabetic macular edema shows cystoid spaces, hard exudates, and disruption of the ellipsoid zone; severity correlates with visual prognosis.

Retinal vein occlusion shows intraretinal edema and hemorrhage; central retinal artery occlusion shows inner retinal hyperreflectivity and later thinning.

Choroidal nevi and melanoma appear as choroidal masses; OCT shows compression of the overlying choriocapillaris and shadowing of deeper structures.

Anterior segment OCT uses longer wavelengths near 1310 nm for greater penetration through cornea and sclera.

Corneal layers on OCT are epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium; Bowman's and Descemet's are hyperreflective.

Pachymetry measures corneal thickness; anterior segment OCT pachymetry is used for LASIK screening and assessment of corneal edema.

Anterior segment OCT evaluates LASIK or PRK flap depth, interface, and buttonhole, and also measures anterior chamber depth and angle.

In keratoconus, anterior segment OCT shows corneal thinning, epithelial thinning over the cone, Bowman's layer disruption, and posterior surface elevation.

Corneal edema shows increased thickness and hyporeflective stroma; Descemet's folds appear as undulations.

Anterior segment OCT guides DALK or DSEK graft sizing and evaluates graft apposition and interface fluid.

OCT is non-ionizing but uses near-infrared laser light; output must comply with ANSI Z136.1 and manufacturer exposure limits.

Limit scanning time and avoid unnecessary repeated scans, especially in eyes with poor tear film or after dilation.

Disinfect the OCT chin rest, forehead rest, and any contact surfaces according to manufacturer instructions between patients.

Handheld or contact anterior segment probes require approved disinfection or sterile covers; never reuse a single-use cover.

Follow standard precautions and hand hygiene; use gloves when contact with tears or ocular secretions is likely.

Document patient identity, eye, scan protocol, signal strength, and any adverse events in the record.

Protect patient privacy under HIPAA; do not store or transmit identifiable images without authorization.

Explain the procedure and obtain informed consent when required; warn about bright light and the need to hold still.