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A-Scan Biometry

Ultrasound Principles

Sound is defined as a vibratory disturbance within a solid or liquid that travels in a wave pattern. When the sound frequency is between 20 hertz (Hz) and 20,000 Hz, the sound is audible to the human ear. To be considered ultrasound, sound waves must have a frequency of greater than 20,000 Hz (20 KHz), rendering them too high in frequency to be audible to the human ear. [1] In ophthalmology, most A-scan and B-scan ultrasound probes use a frequency of approximately 10 million Hz (10 MHz) that is predesigned by the manufacturer. This extremely high frequency allows for not only restricted depth of penetration of the sound into the body but also excellent resolution of small structures. This meets unique needs, because, at times, the probe is placed directly on the organ to be examined, and its structures are quite small, requiring excellent resolution.

The velocity of sound is determined completely by the density of the medium through which it passes. Sound travels faster through solids than through liquids, an important principle to understand because the eye is composed of both. In A-scan biometry, the sound travels through the solid cornea, the liquid aqueous, the solid lens, the liquid vitreous, the solid retina, choroid, sclera, and then orbital tissue; therefore, it continually changes velocity.

The known sound velocity through the cornea and the lens (average lens velocity for the cataract age group, ie, approximately 50-65 y) is 1641 meters/second (m/s), and the velocity through the aqueous and vitreous is 1532 m/s. The average sound velocity through the phakic eye is 1550 m/s. The sound velocity through the aphakic eye is 1532 m/s, and the velocity through the pseudophakic eye is 1532 m/s plus the correction factor for the intraocular lens (IOL) material. [2] The cornea is not routinely factored in because of its thinness. If one were to consider 1641 m/s at about 0.5 mm, only 0.04 mm would need to be added to the total eye length, which in no way alters the IOL calculation.

The echoes received back into the probe from each of these interfaces are converted by the biometer to spikes arising from baseline. The greater the difference in the two media at each interface, the stronger the echo and the higher the spike. [2] If the difference at an interface is not great, the echo is weak and the displayed spike is short (eg, vitreous floaters, posterior vitreous detachments). No echoes are produced if the sound travels through media of identical densities and velocities, eg, young, normal vitreous or the nucleus of a noncataractous lens, in which the A-scan display goes down to baseline.

source: medscape.com