IOL Power Calculations
Advanced A-scan Setting ~ 1,532 m/sec
Changing the ultrasound velocity to 1,532 m/sec:
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.
Fig. 1 — 1,532 m/sec. Immersion A-scan using the Alcon UltraScan
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.
Read more about this technique: Immersion A-scan Technique
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: