Ophthalmic Imaging

Ophthalmic Imaging is 5% of the exam. It covers optical coherence tomography (OCT), fundus photography, and angiography: each test's purpose and how to prepare the patient. The physician orders and interprets; the assistant prepares the patient and captures clean, correctly labeled images.

When a dense cataract or vitreous hemorrhage blocks light, B-scan ultrasound takes over (see the Diagnostic Ultrasound guide); measuring the eye for cataract surgery is in the Biometry guide.

How it works — OCT builds its picture from light, not sound or X-rays: a near-infrared beam scans the eye, and the light reflected from each layer (low-coherence interferometry) builds micron-resolution cross-sections of the retina and optic nerve. No contact, dye, or needles: just a chin rest and steady fixation.

Macular OCT — a thickness map plus line scans through the macula show fluid within or under the retina, macular holes, and epiretinal membranes, and track treatment, such as eye injections, visit by visit: the injection clinic's scorecard.

Glaucoma OCT — a circle scan around the disc measures the peripapillary retinal nerve fiber layer (RNFL); a macular scan measures the ganglion cell complex (GCC). Glaucoma thins both, and OCT can measure the loss before fields change.

Anterior segment OCT — images the cornea, angle, and anterior chamber depth without contact. Light cannot pass the pigmented iris, so what lies behind it needs ultrasound biomicroscopy.

OCT angiography (OCT-A) — repeated scans of the same spot: still tissue looks the same each time, moving blood does not, and the difference maps retinal blood flow with no dye. The costs are a smaller field, motion sensitivity, and no view of leakage, so it does not replace fluorescein angiography.

Prepare the patient — explain that the test is quick, painless, and light-based; the patient's job is to keep looking at the target. Many scans work through an undilated pupil; small pupils and dense cataract may still force dilation. Scan before gonioscopy or anything that leaves gel on the cornea.

Align and capture — center the image on the pupil and focus. The patient blinks fully just before each capture, then holds still on the target. Center macular scans on the fovea and nerve fiber scans on the disc; an off-center scan gives misleading numbers.

Signal strength — check it before saving. Low signal and blurred layers usually mean media opacity, a dried-out tear film, or movement: blink, artificial tears, re-align, rescan. A weak scan can measure layers falsely thin; quality problems caught at the chair never get misread as disease later.

Artifacts — a blink leaves a dark band; movement breaks or doubles vessels; a floater casts a shadow down through the layers; a droopy lid or lashes can block the beam. Rescan, and note what cannot be fixed.

Why dilate — a larger pupil admits more light and widens the camera's view. Check acuity and pupils first, screen the angles per protocol, and warn about blurred near vision and light sensitivity for roughly 4–6 hours.

Color photographs — record the disc, macula, vessels, and lesions. Baseline disc photos are the glaucoma reference for every later visit, so new cupping, a disc hemorrhage, or notching can be recognized; a stereo pair (two slightly offset images) adds cup depth.

Red-free (green) filter — blood absorbs green light, so vessels and hemorrhages turn dark and sharp and nerve fiber striations show.

Fundus autofluorescence (FAF) — the RPE's own glow, from lipofuscin, photographed without dye. Bright and dark patterns map stressed and lost RPE in macular degeneration and inherited retinal disease.

Ultra-widefield — a standard camera covers about 30–50°; ultra-widefield systems capture far into the periphery in one shot, often without full dilation.

Technique — on a camera with an eyepiece, focus its crosshair for your eye first, then center on the pupil at the right working distance and focus on the vessels. Lift a droopy lid with a finger braced on the orbital rim, never pressing on the eye. Warn before each flash (a brief afterimage is normal). Poor focus, misalignment (haze, glare, dark edge crescents), and lid, lash, and blink shadows cause most avoidable retakes.

External and slit-lamp photography — external photos record the lids, face, and orbit; slit-lamp photos record the anterior segment, with diffuse light for an overview and a slit beam for depth. Dated, consistent photos make change measurable and document the eye before surgery.

Fluorescein angiography (FA) — fluorescein injected into an arm or hand vein is photographed as it passes through the eye's vessels, glowing yellow-green under blue light. FA shows leakage, closed (occluded) vessels, and areas without blood flow in diabetic retinopathy, vein occlusions, and macular degeneration.

Before the injection — the physician orders and explains the study. The assistant confirms the consent form is signed, asks about earlier dye reactions and allergies, dilates per the order, and usually takes color and red-free photos first. Only staff trained and permitted under state law and office policy place the IV. Explain the dark room, flashes, and sequence; coach fixation; keep an emesis basin handy.

Timing — start the timer with the injection, because every frame is timed from it; dye usually reaches the eye about 10–15 seconds later.

Missed early phases cannot be re-shot without another injection.

What to expect — brief nausea is common and passes quickly; vomiting is occasional. The skin may look yellowish for several hours, sometimes up to a day, and the urine bright yellow for about a day. Both are harmless; say so beforehand.

Reactions — stop the study, call the physician at once, and stay with the patient; the physician decides which reaction it is. Pallor, sweating, nausea, and lightheadedness without hives, swelling, or breathing trouble are most often a simple faint (vasovagal): recline the patient, raise the legs. Hives or itching are not expected effects; watch closely, as they can be the first sign of anaphylaxis. Wheezing, facial or throat swelling, or hives with lightheadedness is anaphylaxis, rare but real: activate the emergency protocol immediately (epinephrine per standing orders, oxygen, EMS). Burning and swelling at the injection site mean dye has leaked under the skin: stop injecting, do not massage, follow office protocol. Keep the emergency kit within reach.

ICG angiography — indocyanine green glows in infrared light, which passes through the RPE pigment that hides the choroid from fluorescein, so ICG shows the choroidal circulation. The dye contains iodide, so allergy screening classically includes iodine sensitivity.

Label every image — before saving, verify patient, eye (OD or OS), and date on every image. A wrong-eye or wrong-patient label can steer treatment to the wrong target.

Images are health information — they are part of the medical record and protected health information; a face photo identifies the patient on sight. Keep them in the practice's system, never on a personal phone or social media, even without a name.

Be ready to match each test to what it shows and whether it needs injected dye, choose OCT scans for macular disease and glaucoma, say why dilation helps fundus photos, tell expected fluorescein effects from a reaction, and explain the iodine question before ICG.