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:

Cornea

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.

Anterior Chamber

As aqueous has an ultrasound velocity of 1,532 m/sec., this portion of the measurement requires no correction.

Lens Thickness

Here there are two dependent variables: age and lens velocity. For a hypothetical 72-year old, the correction for lens thickness and lens velocity would be determined as follows:

1.  Lens velocity calculation:

      1,659 m/sec. - (72-10)/2 = 1,628 m/sec

2.  Lens thickness calculation:

      LT = 4.00 mm + 72/100 = 4.72 mm

3.  Under-estimation calculation:

      4.72 mm x (1 - (1532/1628)) = +0.28 mm

Vitreous

As vitreous has an ultrasound velocity of 1,532 m/sec., this portion of the measurement requires no correction.

True Axial Length

The true axial length (TAL), is the underestimation of the corneal thickness, plus the underestimation of the lens thickness added to the apparent axial length measured at 1,532 m/sec. (AAL 1532)

TAL = AAL 1532 + 0.04 mm + 0.28 mm

TAL = AAL 1532 + 0.32 mm

Simply adding +0.32 mm to the apparent 1,532 m/sec. axial length for all eyes will give the true axial length, which is then entered into the IOL calculation formula.

The greatest error by this technique for any axial length, and any age patient, is 0.043 mm, which is less than 0.25 D. This is more than twice the resolution of the surgical exercise, given the fact that intraocular lenses typically come in 0.50 D steps.

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 Sonographers.