When optical biometry cannot measure the eye accurately, usually because the cataract is too dense, the established backup test is ultrasound A-scan biometry.
Ultrasound A-scan has been in routine clinical use for decades. It is the recognised fallback measurement technique for cataract surgery worldwide, and it remains the test of choice when light-based methods fail.³
This page explains what ultrasound A-scan is, how it works, and why interpretation by an experienced surgeon matters at least as much as the test itself.
What Ultrasound A-Scan Biometry Is
Ultrasound A-scan (the “A” stands for amplitude) uses high-frequency sound waves rather than light to measure the inside of the eye.
A small probe sends pulses of ultrasound into the eye. These sound waves travel through the cornea, the lens, and the vitreous, and bounce off the back wall of the eye. The probe detects the returning echoes and converts the timing of those echoes into distances.³
The primary measurement is the axial length, the same value optical biometry tries to obtain, just by a different physical principle.
Why Sound Succeeds Where Light Fails
Light is scattered and absorbed by a dense cataract. Sound is not.
Ultrasound passes through cloudy lens material with relatively little signal loss, which is why it has remained the gold-standard backup test in advanced cataract cases for more than four decades.² The denser the cataract, the greater the advantage of ultrasound over light-based methods.
Two Techniques, Contact and Immersion
There are two main ways to perform A-scan biometry:
Contact A-scan. The probe is placed directly against the anaesthetised cornea. It is quicker to perform and does not require additional equipment. The main limitation is the risk of corneal indentation, pressing the probe too firmly compresses the eye and can shorten the measured axial length, which directly translates into IOL power error if not corrected.²
Immersion A-scan. A small cup of saline is placed against the eye and the probe is held in the saline without touching the cornea. This removes the compression artefact and is widely considered the more accurate of the two techniques. It is slower and more operator-dependent.¹
Published comparisons consistently show that immersion biometry yields slightly longer and more reproducible axial length measurements than contact technique, by an average of around 0.2 mm, a difference that is clinically significant for IOL power calculation.¹ ²
What a Good A-Scan Looks Like
A reliable ultrasound A-scan produces:
- clean, sharp spikes at each major interface (cornea, anterior and posterior lens surfaces, retina, sclera),
- a tall, well-defined retinal spike,
- consistent values across repeated acquisitions,
- a low standard deviation between repeats (ideally well under 0.1 mm),
- physiologically plausible component dimensions,
- and reasonable inter-eye symmetry.
What a Poor A-Scan Looks Like
Poor scans are recognisable to an experienced operator. They show:
- fragmented, noisy waveforms,
- wide variation between repeated readings,
- anatomically impossible component values, for example, a measured lens thickness of 20 mm or a vitreous chamber less than 1 mm in an adult eye,
- the probe gating onto the wrong internal interface,
- and isolated outliers that contradict the rest of the cluster.
These artefacts can occur even in skilled hands. They are part of the test, not a sign of a faulty machine.
Why Interpretation Matters More Than the Test Itself
A machine cannot reliably tell the difference between a real eye and a misgated artefact. A trained surgeon can.
Interpretation involves:
- recognising waveform morphology,
- comparing readings across repeated acquisitions,
- identifying the consistent cluster and the outliers,
- cross-checking against the fellow eye and any historical refraction,
- and discarding values that cannot anatomically be true.
The test is only as good as the person reading it.
Where Ultrasound A-Scan Fits in the Blue Fin Vision® Pathway
At Blue Fin Vision®, ultrasound A-scan is used whenever optical biometry has failed or produced unreliable readings. Every scan is reviewed by the operating consultant before IOL calculation and surgical planning are finalised.
Where any residual uncertainty remains, multiple IOL formula strategies are used to bracket the final lens choice, and intra-operative refinement is available as an additional safety layer.
This sequence, optical biometry first, ultrasound biometry as the defined fallback, consultant interpretation at every stage, is how modern complex cataract care is delivered safely.
References
- Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis. Graefe’s Archive for Clinical and Experimental Ophthalmology. 2000;238(9):765-773.
- Shammas HJ. A comparison of immersion and contact techniques for axial length measurement. Journal of the American Intraocular Implant Society. 1984;10(4):444-447.
- Hoffer KJ. Ultrasound velocities for axial eye length measurement. Journal of Cataract and Refractive Surgery. 1994;20(5):554-562.
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