10.1 Diagnostic Ultrasound Overview
Key Takeaways
- Ophthalmic ultrasound uses high-frequency sound (A-scan ~8 MHz, B-scan ~10 MHz) to measure and image structures light cannot reach.
- A-scan biometry produces axial length for intraocular lens (IOL) power; B-scan produces a two-dimensional cross-section of the globe and orbit.
- Sound travels at 1641 m/s through cornea and lens, 1532 m/s through aqueous and vitreous, averaging 1555 m/s in a normal phakic eye.
- On the COA exam, Diagnostic Ultrasound is a small slice of a 200-question, 180-minute test, but biometry numbers and probe safety are high-yield.
10.1 Diagnostic Ultrasound Overview
Diagnostic ultrasound is the use of high-frequency sound waves to measure and image ocular structures. It is indispensable when the view is blocked — a dense cataract, hyphema (blood in the anterior chamber), or vitreous hemorrhage — because sound passes through opaque media that light cannot. The certified ophthalmic assistant (COA) is expected to understand the two main modes, the velocities of sound in the eye, and how patient preparation affects accuracy.
The two core modes
A-scan (amplitude scan) sends a single sound beam down the visual axis and displays returning echoes as vertical spikes on a one-dimensional graph. Each spike marks an acoustic interface (cornea, anterior lens, posterior lens, retina). The distance between the first corneal spike and the retinal spike is the axial length — the single most important measurement for choosing an intraocular lens (IOL) before cataract surgery. A-scan probes operate near 8 MHz.
B-scan (brightness scan) sweeps the beam to build a two-dimensional cross-sectional picture of the globe and orbit, displaying echo strength as dots of varying brightness. B-scan answers "what does it look like" — retinal detachment, tumor, foreign body, optic-disc drusen — while A-scan answers "how long is it." B-scan probes operate near 10 MHz; higher frequency means better resolution but less penetration.
Speed of sound in ocular media
Ultrasound machines convert echo travel time into distance, so the assumed velocity must be correct. Memorize these values:
| Tissue | Sound velocity |
|---|---|
| Cornea and crystalline lens (solid) | 1641 m/s |
| Aqueous and vitreous (fluid) | 1532 m/s |
| Average normal phakic eye | 1555 m/s |
| Aphakic eye (no lens) | 1532 m/s |
| Silicone-oil-filled eye | ~1040 m/s (much slower) |
If the wrong velocity is set — for example, leaving the phakic setting on a silicone-oil eye — the axial length and IOL power will be badly wrong. The assistant selects the velocity that matches the patient's actual lens status. A dense cataract changes lens density, and pseudophakic eyes contain an implant of known material, so most machines offer dedicated phakic, aphakic, pseudophakic (PMMA, silicone, or acrylic), and dense-cataract settings. Choosing the right mode is part of the assistant's job, not an afterthought.
What each spike represents
Reading the A-scan trace is a core skill. From left to right, the expected echoes are: the corneal spike (the entry surface), the anterior lens spike, the posterior lens spike, and finally the tall retinal spike followed by lower scleral and orbital echoes. The axial length is the distance from the cornea to the retina. A trace missing the lens spikes suggests an aphakic eye; a trace with extra spikes between lens and retina suggests vitreous opacity such as hemorrhage or a detached membrane.
Indications you should recognize
| Clinical situation | Why ultrasound is ordered |
|---|---|
| Cataract surgery planning | A-scan axial length feeds the IOL power calculation |
| Dense cataract or vitreous hemorrhage | B-scan images the retina the examiner cannot see |
| Suspected retinal detachment | B-scan confirms and maps the detachment |
| Suspected intraocular tumor (melanoma) | A-scan and B-scan assess internal reflectivity and size |
| Suspected foreign body after trauma | B-scan localizes a high-reflectivity object |
Why precision matters
A worked example shows the stakes: a corneal compression (indentation) error of just 0.4 mm during contact A-scan produces roughly a 1.00 diopter error in the calculated IOL power. Because the eye is only about 23–24 mm long on average, even sub-millimeter errors are clinically large. That is the difference between a patient seeing well and needing glasses or a lens exchange. The COA learns to take readings with the lightest possible corneal touch, or to use the immersion technique that avoids touching the cornea at all, then to repeat the measurement and compare it against the fellow eye before trusting any single number.
Patient preparation and comfort
Good data also depends on the patient. The assistant explains that a numbing drop will be used, that the probe or shell may feel like a light touch, and that the test is painless and brief. For A-scan and B-scan alike, topical anesthetic is instilled; the patient is asked to fixate steadily so the beam stays on the visual axis. Contact lenses are removed first because they alter the corneal surface and coupling. A calm, well-positioned, fixating patient produces far cleaner echoes than a squeezing, anxious one.
How this appears on the exam
COA questions in this area favor concrete facts: which mode gives axial length, what velocity applies to a given eye, what causes a falsely short axial length, what each spike on the trace represents, and how to prepare and reassure the patient. Treat distractors that confuse A-scan and B-scan roles, or that misstate a velocity, as traps. The exam rewards knowing the numbers and the indication, not vague descriptions of what ultrasound is. A final orientation point: ultrasound is one of several biometry pathways.
Optical biometry (partial coherence interferometry, the IOLMaster and similar devices) measures axial length with a laser and is now first-line for most clear-media eyes, but it fails through dense cataracts and media opacities — exactly when ultrasound A-scan remains essential. Knowing when optical biometry cannot be used, and that ultrasound is the fallback, is itself an exam-relevant judgment.
An A-scan and a B-scan are both ordered for a patient with a dense cataract. Which statement correctly distinguishes their roles?
A technician must perform A-scan biometry on a patient whose eye is filled with silicone oil after retinal surgery. Why must the velocity setting be changed from the normal phakic value?