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.
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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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Scleral Shells
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The immersion technique requires the use of a Prager Scleral
Shell (pictured
at the top of this page), or a set of Ossoinig or Hansen 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 easiest to use,
and gives very consistent readings.
The Prager Scleral Shell can be obtained directly from:
ESI, Inc. (http://www.eyesurgin.com/)
2915 Everest Lane N.
Plymouth, MN 55447
(763) 473-2533
(763) 208-8105 Fax
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.
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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.
For further reading, we highly recommend the
book A-scan Axial Length Measurements by Sandra Frazier Byrne.
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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