An A-scan shows its measurements as vertical spikes, or peaks, on an oscilloscope screen. An oscilloscope is an instrument that draws the shape of a wave. The probe sends high-frequency sound waves into the eye. Each time a wave hits a new surface, part of it echoes back to the probe. Every strong echo draws one peak on the screen. The result is a row of spikes, not a photograph of the eye.
The order of the peaks matches the order of the structures the sound passes through. The space between two peaks stands for the distance between those two structures. The machine turns those spacings into millimeters, so a clean tracing gives the axial length and the depths inside the eye. This is why A-scan is called an A-mode or one-line study: the sound waves travel in a straight line. A B-scan sends radiating sound waves instead and builds a two-dimensional picture of eye and orbit. Reading the spikes is the technician's job, because poorly shaped peaks mean the probe was not lined up and the value must be repeated. Good tracings show tall, sharp, evenly spaced spikes, and the machine prints several readings so the average can be checked.
Remember the shapes: A-scan gives spikes, B-scan gives a picture. If the peaks look low or rounded, take the measurement again before the surgeon uses it. Knowing how each ultrasound mode displays its data belongs to the diagnostic ultrasound content area of the COA exam.
Incorrect Answers
- B. A two-dimensional image of the globe is the product of B-scan ultrasonography. That mode is used to find and measure tumors, not to give a length.
- C. A color-coded map of the front of the cornea comes from corneal topography, which uses projected light rings rather than sound.
- D. Magnified pictures of the innermost corneal cells come from specular microscopy, a slit-lamp or video technique. No sound waves are involved in that test at all.
Testing Pearls
- A-scan results appear as vertical peaks on an oscilloscope; the gaps between peaks are distances.
- A-scan sound waves travel in a straight line; B-scan waves radiate outward.
- A-scan measures, B-scan pictures: length and depth versus a two-dimensional view.
- Both modes rely on the echo of high-frequency sound waves.
- Poor peaks mean poor alignment, so repeat the reading before it is used for lens power.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
