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/s for phakic eyes. Such a one-size-fits-all approach can introduce errors because 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 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, even with the most advanced techniques.
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. Performing your immersion A-scans at an ultrasound velocity of 1,532 m/s avoids corneal compression and renders the measurement immune to subtle errors that result from differences in axial length.
1,532 m/sec. Immersion A-scan using the Alcon UltraScan.
Note: all gates above are set to 1,532 m/s, and the technique is by immersion. As is typical, the consistency between individual scans is excellent. This technique is not for use with 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 calculating the intraocular lens power.
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
How to learn more about A-scan techniques:
We highly recommend the book A-scan Axial Length Measurements by Sandra Frazier Byrne.
There is an excellent, national certification program in Ophthalmic Biometry available for your technicians:
