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A-scan Biometry
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Warren Hill, M.D.
IOL Intraocular Lens Power Calculations
A-scan Ultrasonography
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Perfecting Biometry by Ultrasound.
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Three different A-scan techniques are presently in use:
1.
Applanation A-scan
2.
Immersion A-scan
3.
Immersion vector-A / B-scan
A-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-scan
s. 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.
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Figure A - Phakic axial length measurement using the applanation method.
a: Initial spike (probe tip and cornea)
b: Anterior lens capsule
c: Posterior lens capsule
d: Retina
e: Sclera
f: Orbital fat
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.
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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.
b: Cornea. Double-peaked echo will show both the anterior and posterior surfaces.
c: Anterior lens capsule.
d: Posterior lens capsule.
e: Retina. This echo needs to have sharp 90 degree take-off from the baseline.
f: Sclera.
g: Orbital fat.
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Prager Scleral Shell
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The immersion technique requires the use of a Prager Scleral Shell, or
a set of Ossoinig Scleral Shells. The patient lies supine, looking up
at the ceiling and the scleral shell is placed between the eyelids and
centered over the cornea. The scleral shell is then filled with a 40-60
mixture of Goniosol and Dacriose and the probe tip is placed into the
solution. Align the ultrasound beam with the macula by having the
patient look at the probe tip fixation light, then simply take your
readings as usual.
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/)
2915 Everest Lane N.
Plymouth, MN 55447
(763) 473-2533
(763) 208-8105 Fax
tab@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 measurement
consistency.
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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.
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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.
The immersion B-scan/vector A-scan technique is still limited by the resolution
of a 10-MHz sound wave and variations in retinal thickness around the fovea.
However, it does allow for direct visualization of the area being measured
and, as a result, has even better consistency than that the immersion A-scan
technique.
The main drawbacks are that the equipment is more expensive, there is a greater
level of skill required and the measurements do take longer to carry out.
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.
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