The type of ultrasonography used to determine the length of the eyeball is

• A. A-scan
• B. B-scan
• C. C-scan
• D. D-scan

Answer: (A)

Measuring the eye’s length along the visual axis is done with one-dimensional ultrasound biometry, called A-scan. It sends a short ultrasonic pulse down the visual axis and records the time it takes for echoes from successive ocular interfaces (cornea, lens, retina) to return. That time is converted into distance, giving the axial length in millimeters. This precise measurement is crucial for accurate intraocular lens power calculations in cataract surgery.

In contrast, B-scan ultrasound produces two-dimensional cross-sectional images of the eye and is used to evaluate the posterior segment when media opacities prevent direct visualization; it does not provide the single, precise axial length needed for IOL calculations. The other terms aren’t standard methods for this measurement.