Eye Doctor Arizona
Eye Surgery Arizona

  IOL Power Calculations
Immersion A-scan Biometry
printable version
IOL Power Calculations
Alcon AcrySof Toric IOL Cataract Surgery Arizona
A-scan Biometry
Biometry Methods Explained
Applanation A-scan
Immersion A-scan
Immersion Vector A/B-scan
Immersion A-scan Technique
Advanced A-scan Setting
Bag vs. Sulcus
Corneal Transplantation
Effective Lens Position Cataract Surgery Arizona
Haigis Formula
High-Extreme Axial Myopia
IOL Calculation Formulas
Limbal Relaxing Incisions
Outcomes Database
Post Keratorefractive Surgery Cataract Surgery Arizona
Pseudophakic CFs
Refractive Vergence Formula
Silicone Oil
Validation Guidelines
Optical Biometry Optical Biometry
SIA Calculator
Physician Downloads SIA Calculator
Special Thanks

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.

0.55 mm x (1- (1532/1641)) = 0.0365 mm = +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


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.

A-scan Biometry 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.


IOLMaster Help

A-scan Biometry

Intraocular Lens Power Calculations

East Valley Ophthalmology
5620 East Broadway Road
Mesa, Arizona 85206

Tel: +1-480-981-6111
FAX: +1-480-985-2426

Arizona's Top Eye Doctors - East Valley Ophthalmology provides this online information for educational and communication purposes only and it should not be construed as personal medical advice. Information published on this website is not intended to replace, supplant, or augment a consultation with an eye care professional regarding the viewer/user's own medical care. East Valley Ophthalmology's disclaims any and all liability for injury or other damages that could result from use of the information obtained from this site. Please read our full Terms, Privacy, Infringement

Eye Doctors for Mesa, Gilbert, Chandler, AZ and surrounding areas.

A few of our featured topics:


Copyright East Valley Ophthalmology. All rights reserved.