6.3 OCT & fundus photography

OCT and Fundus Photography

Imaging documents the structure and circulation of the retina and optic nerve, providing objective baselines and progression tracking that complement the visual field. The COT usually operates these instruments, so image quality depends directly on technician skill.

Optical Coherence Tomography (OCT)

Optical coherence tomography (OCT) is a non-contact, non-invasive imaging technique that uses low-coherence near-infrared light to produce high-resolution cross-sectional images of the retina and optic nerve — often called an 'optical biopsy.' It is analogous to ultrasound but uses light instead of sound, resolving the individual retinal layers to just a few microns. No dye and no touching of the eye are required.

Clinical uses:

• Glaucoma — OCT measures the retinal nerve fiber layer (RNFL) thickness around the optic disc and the ganglion cell complex in the macula, comparing each against a normative database. Structural thinning often precedes detectable visual-field loss, making OCT valuable for early diagnosis and monitoring.
• Macular disease — OCT quantifies macular thickness and reveals fluid, edema, drusen, and membranes in diabetic macular edema, age-related macular degeneration (AMD), macular holes, and epiretinal membranes. It is routinely used to time and assess response to anti-VEGF injections.
• OCT angiography (OCTA) — a dyeless extension that images retinal and choroidal capillary flow.

Technician tips: dilate the pupil when the protocol calls for it, ensure a clear ocular media and a smooth tear film (lubricate dry eyes and ask for a blink before capture), center and focus the scan on the target (disc or macula), and check the signal-strength / quality score. Reject scans with segmentation errors, blink artifact, or motion streaks, and repeat rather than pass along a poor image.

Fundus Photography

Fundus photography creates a permanent record of the retina, optic disc, and vessels for baseline documentation and side-by-side comparison over time.

• Color fundus photography — standard documentation of the disc, macula, vessels, and any lesions such as hemorrhages, exudates, or pigmentary changes.
• Red-free (green filter) photography — removes red light so that structures reflecting green stand out. It enhances contrast of the nerve fiber layer, retinal hemorrhages, microaneurysms, and blood vessels.

Fluorescein Angiography

Fluorescein angiography (FA) evaluates the retinal and choroidal circulation dynamically.

• The dye: sodium fluorescein is injected into an arm vein; a rapid series of photographs taken through a blue excitation filter and a yellow-green barrier filter captures the dye as it transits the eye.
• Phases: choroidal (pre-arterial) filling, arterial, arteriovenous/capillary, venous, and finally the late (recirculation and staining) phase, unfolding over roughly ten minutes.
• Uses: diabetic retinopathy, neovascularization, vein and artery occlusions, and areas of leakage or choroidal neovascular membrane.
• Safety and counseling: warn the patient that the skin turns yellow and the urine turns bright orange for 24–48 hours, and that transient nausea is common shortly after injection. Emergency equipment and drugs must be ready because rare anaphylaxis can occur. (Indocyanine green, ICG, is a related dye used to image the deeper choroidal circulation.)

The Technician's Role in Image Quality

High-quality images are a technician responsibility, not an accident of the machine. Key duties:

• Explain the procedure, set expectations, and obtain cooperation and consent for dye studies.
• Optimize dilation, focus, alignment, and fixation before capturing.
• Recognize and re-capture common artifacts — blink, media opacity, eyelash shadow, and small-pupil vignetting.
• Label every image with the correct patient identity, eye (OD/OS), and date so the physician compares the right images over time.

Scan Types and Related Imaging

OCT instruments offer several scan protocols the technician selects by clinical question. A macular cube or raster scan builds a thickness map plus detailed line scans through the fovea for macular disease, while an optic-disc (RNFL) circle scan samples the peripapillary nerve fiber layer for glaucoma. Modern spectral-domain and swept-source systems capture these in seconds and overlay the results on a color-coded deviation map — green for within normal limits, yellow for borderline, and red for outside normal limits. Teach the patient to fixate the internal target, blink fully just before capture, and hold still, because a single blink or micro-saccade can drop the signal-strength score below the acceptable threshold and corrupt the layer segmentation.

Two related photographic studies round out the toolkit. Fundus autofluorescence images the natural fluorescence of lipofuscin in the retinal pigment epithelium with no injected dye, which is useful in AMD and inherited retinal disease. Indocyanine green (ICG) angiography uses an infrared-absorbing dye that stays within choroidal vessels, so it complements fluorescein by imaging the deeper choroidal circulation in conditions such as occult choroidal neovascularization and central serous retinopathy. Across all of these studies the technician verifies patient identity and allergy history, confirms the correct eye, maximizes pupil dilation and media clarity, and reviews each capture on screen before ending the session so a repeat can be taken while the patient is still in the chair.

Imaging Modalities at a Glance