Three different A-scan biometry techniques are presently in use:
Prager Shell used for
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.
all forms of ultrasound based biometry have two basic 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,
Quantel Aviso Ultrasound
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
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 Biometry 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
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