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.
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.
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.
In A-scan biometry, one thin, parallel sound beam is emitted from the probe tip at its given frequency of approximately 10 MHz, with an echo bouncing back into the probe tip as the sound beam strikes each interface. An interface is the junction between any two media of different densities and velocities, which, in the eye, include the anterior corneal surface, the aqueous/anterior lens surface, the posterior lens capsule/anterior vitreous, the posterior vitreous/retinal surface, and the choroid/anterior scleral surface.
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.
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. See the image below.
High quality contact A-scan of the phakic eye. Note the 5 high-amplitude spikes and the steeply rising retinal spike, as well as the good resolution of the separate retinal and scleral spikes.
In the case of a cataractous lens, multiple spikes occur within the central lens area as the sound beam strikes the differing densities within the lens nucleus. This spike height, or amplitude, is therefore what gives the information on which to base the quality of the measurements. In fact, the “A” in A-scan is from the word “amplitude.”
Spike height is not only affected by the difference in density as it travels through the eye but also by the angle of incidence, which is determined by the probe orientation to the visual axis. If the probe is held so that the emitted sound beam strikes the corneal vertex, anterior lens, posterior lens, and retina in a perpendicular manner, it is in the proper position to receive the echoes back into the probe tip so they can be converted to spikes. Because sound waves can be reflected and refracted the same as light rays, if the probe is held in a nonparallel manner, part of the echo is diverted at an angle away from the probe tip, and, therefore, is not received by the machine.
The greater the angle of incidence, the weaker the signal and the shorter the spike amplitude. See the image below.
When the sound beam incidence is parallel and coaxial to the visual axis (upper image), most returning echoes are received back into the probe tip to be interpreted on the display as high-amplitude spikes. When the sound beam incidence is oblique to the visual axis (lower image), part of the returning echo is reflected away from the probe tip, with only a portion received by the probe. As a result, the spikes will be compromised.
The shape and smoothness of each interface also affects spike quality. An irregularity in the surface of an interface causes reflection and refraction of the returning sound waves away from the probe tip and, therefore, weaker echoes.
That is why it is important to know whether macular pathology is present that could adversely affect spike quality. A perfect high, steeply rising retinal spike may be impossible when macular pathology is present (eg, macular edema, macular degeneration, epiretinal membranes, posterior staphylomas). See the image below.
If the macular surface is smooth (upper image) more of the echoes are received back into the probe to be displayed as high-amplitude spikes. If the macular surface is convex (center image), as with macular edema or pigment epithelial detachments, part of the echoes is reflected away from the probe tip. If the macular surface is irregular (lower image), as in macular degeneration or epiretinal membranes, reflection of the echoes away from the probe tip also will occur.
In addition, sound is absorbed by everything through which it passes before it travels on to the next interface. The greater the density of the structure it is passing through, the greater the amount of absorption. This principle explains why retinal spike quality is reduced in the case of an extremely dense cataract; the lens absorbs much of the sound and less sound actually reaches the retinal surface.