A-scan Biometry Immersion Applanation Vector-A/B-scan Axial Length Measurements Ultrasound IOL Intraocular Lens Calculations Eye Cataract Surgery Eyes
2008-09-13 Sound is defined as a vibratory disturbance within a solid or liquid that travels in a wave pattern with a frequency from 20-20,000 hertz (Hz). To be considered ultrasound, sound waves must have a frequency greater than 20,000 Hz (20 KHz), rendering them inaudible to human ears. In ophthalmology, most A-scan and B-scan ultrasound probes use a frequency of approximately 10 million Hz (10 MHz) predesigned by the manufacturer. This extremely high frequency allows for restricted penetration of the sound into the body, but excellent resolution of small structures. This meets the unique needs, because 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 solids and liquids. 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 (see Image 1). 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. 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 along the visual axis. If the probe is held in a perpendicular manner to the visual axis, 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 nonperpendicular manner, part of the echo is diverted at an angle away from the probe tip, and, therefore, is not received by the machine. The more nonperpendicular the angle of incidence, the weaker the signal and the shorter the spike amplitude (see Image 2). 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 Image 3. 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.|
IOL Intraocular Lens Calculations 
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| Perfecting Biometry by Ultrasound. | |
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Three different A-scan techniques are presently in use: 1. Applanation A-scanA-scan biometry by immersion has better reproducibility, which leads to an overall increase in accuracy. The immersion technique can be performed rapidly and with greater confidence than the applanation method. Making the change from the applanation to immersion is well worth the small learning curve. However, all forms of ultrasound based biometry have basic two limitations. First, they use a rather large 10-MHz sound wave to measure a relatively small distance. Second, the area around the center of the macula is not flat, but thinnest at the fovea, with thicker shoulders. In order to overcome these two accuracy barriers, axial length must be measured by partial coherence interferometry using the Carl Zeiss Meditec IOLMaster. By the applanation biometry method, an ultrasound probe is placed directly on the cornea, which slightly indents the surface. More desirable, by the immersion technique the ultrasound probe does not come into direct contact with the cornea, but instead uses a coupling fluid between it and the probe, preventing compression. Using a 10-MHz ultrasound transducer, by the immersion method, the typical accuracy for axial length measurements is within 0.12 mm. This results in approximately 0.28 D of post-operative refractive error in an eye of average axial length. This error would be more for a shorter eye and less for a longer eye. However, the total refractive error involves all components of the measurement process and is closer to 0.36 D, taking into account keratometry, a 2-variable IOL power calculation formula and the configuration of the capsulorrhexis. A-scan biometry by immersion will display an axial length somewhat longer than applanation, because there is no corneal compression and the displayed axial length is closer to the true axial length. The difference between applanation and immersion can be anywhere between 0.14 mm to 0.28 mm depending on the degree of corneal compression. Upon implementing immersion A-scan ultrasonography, it is best to aim for a -0.75 D target refraction and start with the IOL manufacturer recommended ACD, A-constant, or Surgeon Factor, rather than the constants that were "personalized" in your Outcomes Database during the course of past applanation A-scans. Failure to target a higher degree of myopia in the beginning may result in unexpected post-operative hyperopia of approximately +0.50 diopters in eyes of normal axial length. This is continued until your new immersion biometry lens constants can be established. As described by Dr. Holladay, immersion A-scan ultrasonography can also be successfully carried out for the phakic eye with all gates set to the aphakic sound velocity of 1,532 m/sec. Using this Advanced A-scan technique, an extra +0.32 mm is added to the displayed axial length to correct for the thickness and different velocities of the lens and cornea. Measuring the axial length in this way avoids a number of common problems and has become our preferred method for immersion A-scan biometry.
1. Applanation A-scan biometry: Essential information: |
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A-scan biometry by applanation requires that the ultrasound probe be placed directly on the corneal surface. This can either be done at the slit lamp, or by holding the ultrasound probe by hand.
Even in the most experienced hands some compression of the cornea is unavoidable, this typically being 0.14 mm - 0.28 mm. The popularity of the applanation method is due to how quickly it appears to be accomplished. |
Figure A - Phakic axial length measurement using the applanation method.
a: Initial spike (probe tip and cornea)Note: When echoes b through d are high and steeply rising, the ultrasound beam is most likely on axis. The scleral echo should easily be identified and the orbital fat echoes should descend quickly and at a steep angle. If there are no scleral or orbital fat echoes visible, the ultrasound beam is most likely aligned with the optic nerve rather than the macula. |
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Measurements taken by applanation method will frequently show variability from one to the next, as a result of inconsistent corneal compression, and will be seen even under the most experienced guidance. The way to avoid this is to change to the immersion technique, as described below. |
Figure B - Note the typical applanation measurement
variations.
2. Immersion A-scan biometry: Preferred over applanation: With the immersion A-scan technique, the probe tip does not come into contact with the cornea. |
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Instead, the ultrasound beam is coupled to the eye through fluid. Because there is no corneal compression, the
displayed result more closely represents the true axial length.
Note:Be sure to set your ultrasound machine to immersion mode, if it doesn't automatically do so, or you will get meaningless readings that are several mm too long. |
Figure C - Break-down of phakic axial length measurements using the immersion
technique.
a: Probe tip. Echo from tip of probe, now moved away from the cornea and has become visible. |
In our office we have found that the Prager Scleral Shell is easy to use, and gives very consistent readings. The Prager Scleral Shell can be obtained directly from: ESI, Inc. (http://eyesurgin.com/)A set of Ossoinig Scleral Shells, which are lighter in weight, easing probe manipulation, can be obtained from Hansen Ophthalmic Development Laboratories at (319) 338-1285. |
Figure D - Note the typical immersion measurementconsistency.
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When the ultrasound beam is properly aligned with
the center of the macula, all five spikes (cornea, anterior and posterior lens
capsule, retina and sclera) will be steeply rising and of maximum height. Changing to this technique is an important first step in improving the overall accuracy of your A-scans. Measurement consistency from one measurement to the next is often outstanding, due to the lack of corneal compression and the fixed position of the ultrasound probe over the surface of the cornea. |
3. Immersion vector A / B-scan biometry: Another method for measuring the axial length is to use an immersion B-scan/vector A-scan technique. The immersion technique prevents corneal compression and the two-dimensional B-scan display helps guide the superimposed vector A-scan for measurements directly to the fovea. An immersion echogram through the posterior fundus is obtained using a horizontal axial B-scan approach. The goal is to center the cornea and lens echoes in the echogram while simultaneously displaying the optic nerve void near or slightly above the center. The A-scan vector is then adjusted so as to pass through the middle of the cornea as well as the anterior and posterior lens echoes. Such alignment assures that the vector will intersect the retina in the region of the fovea. This technique is particularly important when the macula lies on the sloping wall of the staphyloma. |
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Figure E - The B-scan is oriented horizontally with the vector A-scan going through the center of the cornea (C), anterior lens (L1) and posterior lens (L2). With this alignment, the vector A-scan will intersect the retina at
the approximate center of the macula, just below the void of the optic nerve.
This technique has the advantage of the operator being able to direct the
axial length measurement to the region of the fovea, giving the refractive,
rather than the anatomic axial length. For an eye with a mature cataract,
or a high axial myope, with a peripapillary posterior staphyloma, this is
the preferred biometry technique by ultrasound.
For further reading, we highly recommend the
book "A-scan Axial Length Measurements" by Sandra Frazier Byrne.
It is an excellent resource that you wouldn't want to miss. Also, there is an excellent, national certification program in Ophthalmic Biometry available for your technicians: American Registry of Diagnostic Medical Sonographers. |