Encyclopedia - Ultrasound: A-Scan

An A-scan instrument is an ultrasound device that is used to identify normal and abnormal eye tissue, called a diagnostic A-scan, or for measuring the length of the eye from front to back, called a biometrics A-scan. The A-scan emits very high frequency radar-like waves that are reflected by ocular structures, converted into electrical impulses, and displayed on a screen as a series of echo spikes.

The A-scan procedure is a painless, noninvasive technique usually performed in the doctor's office. A topical anesthetic (numbing eye drop) is administered, and then the ultrasound probe is touched to the surface of the anesthetized eye. The probe emits sound waves that bounce back from various structures within the eye, producing the image on a video screen.

The biometric A-scan is used for measuring the length of the eyeball, from cornea to retina. Combined with values from other sources, this measurement is used to determine the power of an artificial intraocular lens to be implanted during cataract surgery.

The diagnostic A-scan is used to differentiate tissues within the eye, most commonly tumors. For instance, echoes produced from a malignant melanoma are different from those reflected from a benign or noncancerous hemangioma. This highly specialized exam requires special training.