A-scan ultrasound biometry, commonly referred to as an A-scan (short for Amplitude scan), is a routine type of diagnostic test used in
optometry
Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. Optometrists are health care professionals who typically provide comprehensive primary eye care.
In the Un ...
or
ophthalmology
Ophthalmology ( ) is a surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a me ...
. The A-scan provides data on the length of the
eye, which is a major determinant in
common sight disorders. The most common use of the A-scan is to determine eye length for calculation of intraocular lens power. Briefly, the total refractive power of the
emmetropic
Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the a ...
eye is approximately 60. Of this power, the cornea provides roughly 40 diopters, and the crystalline lens 20 diopters. When a cataract is removed, the lens is replaced by an artificial lens implant. By measuring both the length of the eye (A-scan) and the power of the cornea (
keratometry
A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the German physiologi ...
), a simple formula can be used to calculate the power of the intraocular lens needed. There are several different formulas that can be used depending on the actual characteristics of the eye. All the information here is not valid for medical purposes.
The other major use of the A-scan is to determine the size and ultrasound characteristics of masses in the eye, in order to determine the type of mass. This is often termed quantitative A-scan.
Instruments used in this type of test require direct contact with the
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
, however a non-contact instrument has been reported. Disposable covers, which come in actual contact with the eye, have also been evaluated.
See also
*
B-scan ultrasonography
Medical ultrasound includes diagnostic techniques (mainly imaging techniques) using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscle ...
*
Ultrasonography
References
* Santodomingo-Rubido, J.; Mallen, E.A.H.; Gilmartin, B.; and Wolffsohn, J.S. (2002)
A new non-contact optical device for ocular biometry.''British Journal of Ophthalmology''. 86, 458–462.
* Cass, K.; Thompson, C.M.; Tromans, C.; and Wood, I.C.J. (2002). BALA
Evaluation of the validity and reliability of A-scan ultrasound biometry with a single use disposable cover.''British Journal of Ophthalmology''. 86, 344–349.
External links
Medscape: A-scan
Diagnostic ophthalmology
Medical ultrasonography