Immersion A-scan Technique
For those who are already comfortable with immersion A-scan biometry, here are some simple refinements you can make to increase the overall accuracy of your measurements. The information below is based on the original work of Holladay, and has been modified to include a small correction for the central corneal thickness, as described by Hoffer.
The human eye is mostly composed of aqueous and vitreous, both
of which have an ultrasound velocity of 1,532 m/sec. Only the cornea
and crystalline lens have different ultrasound velocities. If the
eye is measured at an ultrasound velocity of 1,532 m/sec., a corrected
axial length factor (CALF) of +0.32 mm is added to the apparent
axial length (AAL 1532) to obtain
the true axial length (TAL).
As these differences represent a relatively small percentage
of the total axial length measurement, a single CALF of +0.32 mm
can be universally applied for phakic eyes of all axial lengths.
This method is more accurate than using an average ultrasound velocity,
such as 1,548 m/sec., and makes the measurement independent of
axial length. Below is a description of how this works.
Setting the Ultrasound Velocity:
The ultrasound velocity for all gates is set to 1,532 m/sec. and measurements are made by the immersion technique.
Adjustments to Ultrasound Velocity Settings:
The cornea has a typical ultrasound velocity of 1,641 m/sec. At the slower ultrasound velocity of 1,532 m/sec., it's central thickness will be slightly underestimated. With an average central corneal thickness of 0.55 mm, the underestimation would be approximately +0.04 mm.