Some ultrasound machines employ a single average ultrasound velocity of
1,555, 1,553, 1,550, or 1,548 m/sec for phakic eyes. Such a
one-size-fits-all approach can induce errors due to the fact that the
most appropriate average ultrasound velocity is not the same for all
axial lengths. For example, an axial myope of 29.00 mm is best measured
at an average velocity of 1,550 m/sec, while an axial hyperope of 20.00
mm mm is best measured at an average velocity of 1,560 m/sec. This is
one of several reasons why measurements for long and short eyes tend to
be less accurate, in spite of the very best technique.
Instead, try setting the velocity of your ultrasound machine to 1,532
m/sec (the aphakic velocity) for all gates and add +0.32 mm to the
displayed result. Doing your immersion A-scans at an ultrasound
velocity of 1,532 m/sec avoids corneal compression and makes the
measurement immune to subtle errors that result from differences in
axial length.
To read more about how this technique was derived, click on this link: Refined
Immersion A-scan.
1,532 m/sec. Immersion
A-scan using the
highly accurate Alcon UltraScan — Fig. 1 above.
Note that all gates (above) are set to 1,532 m/sec. and that the
technique is by immersion. As is typical, the consistency between
individual scans is excellent. This technique is not for use by the
applanation method.
The true axial length is obtained by adding +0.32 mm to the displayed
axial length. In this case, it would be: 21.74 mm +0.32 mm = 22.06 mm.
This new number is then used for intraocular lens power calculations.
By adopting the above changes in the approach to A-scan
ultrasonography, you and your staff can significantly tighten the
accuracy of these measurements and increase the satisfaction of your
cataract surgery patients. If you have any questions, you are welcome
to contact us.
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