Diagnostic Ultrasound

Diagnostic Ultrasound is 2% of the COA exam. It lets the physician look inside an eye that light cannot get through.

Why sound — when cataract, corneal opacity, or vitreous hemorrhage block the view, ultrasound images what light can't reach. Sound needs a continuous medium: coupling gel or immersion fluid, never an air gap. Air reflects nearly all of the sound back before it reaches the eye.

How it works — the probe sends short pulses of sound into the eye and listens for the echoes. Every boundary between tissues of different density sends part of the sound back. The machine times each echo to place it at its depth and shows how strong it was. Strong reflectors, such as the retina, the sclera, and foreign bodies, return tall spikes or bright dots; weak ones, such as fine vitreous opacities, return small spikes or faint dots.

Frequency trades depth for detail — posterior-segment probes run at about 10 MHz, which reaches the back of the eye and the orbit. Ultrasound biomicroscopy (UBM) runs at about 35–50 MHz: much finer detail, but only a few millimeters deep, so it is used for the front of the eye.

A-scan — one-dimensional amplitude spikes along a single beam; each spike is an interface (cornea, lens surfaces, retina). Used for axial length and for characterizing lesion reflectivity.

B-scan — the probe sweeps to build a two-dimensional cross-section of the globe and orbit, performed through closed lids with gel. Screens for retinal detachment, tumors, and posterior foreign bodies.

Standardized diagnostic A-scan — a calibrated A-scan aimed through a lesion measures how strongly its interior reflects sound. Melanomas classically show low-to-medium internal reflectivity, one of the clues the physician uses to tell one kind of lesion from another.

Measuring axial length before cataract surgery is its own content area, Biometry. This area is about the diagnostic survey.

Before the probe touches anything — confirm the patient, the eye, and what the physician wants answered. Ask about injury. A suspected open globe gets no pressure of any kind; imaging waits for the physician's direction.

Explain the test — it is painless, the gel feels cool, and the patient will be asked to look in different directions.

Through the lids — most B-scans are done with the patient seated or reclined and the probe on the closed lids with plenty of gel. If the probe is to touch the eye itself, the eye is anesthetized first.

Orient the probe — the probe has a marker, and the marker's position decides which part of the globe appears at the top of the screen. Place it deliberately and record it, or nobody can say later which part of the eye an image shows.

Sweep systematically — scan in transverse, longitudinal, and axial orientations so no part of the globe is missed. To examine an area, the patient looks toward it while the probe sits on the opposite side of the globe.

Scan dynamically — ask the patient to move the eye, then hold still, while the image runs. How a membrane moves is often more telling than how it looks.

Finish the study — label every saved image with the patient, eye, probe position, gain, and date. Wipe the gel from the lids, and clean and disinfect the probe per protocol before the next patient. Interpretation belongs to the physician.

Gain — gain is the scan's volume knob. Survey at high gain to reveal weak echoes, such as vitreous hemorrhage and fine opacities, accepting that everything brightens and blurs. Then lower the gain until only strong reflectors remain: retina, sclera, calcium, and foreign bodies. Comparing the two settings shows how strongly a structure reflects.

Aftermovement — when the eye stops moving, a detached vitreous keeps swirling and drifting. A detached retina moves more stiffly, because it is still anchored to the wall of the eye.

Shadowing — a very strong reflector, such as metal, glass, or calcium, blocks the sound behind it and leaves a dark shadow on the scan. A bright echo with a shadow behind it is a classic foreign-body sign.

The assistant does not diagnose from the screen, but recognizing these patterns makes for a better study, because it tells you which views and gain settings to capture before the patient leaves.

What makes a complete study — both the high-gain and low-gain views of any finding, the probe position and gaze for each image, and a note or video of how the finding moved.

Be ready to say when a B-scan is ordered, explain why gel is needed, compare A-scan, B-scan, and UBM, including their frequencies, use gain to judge reflectivity, tell a retinal detachment from a vitreous detachment by attachment and movement, recognize a foreign-body echo, and state the open-globe rule.