Immersion is preferred over applanation.
With the immersion A-scan technique, the probe tip does not come into contact with the cornea. Instead, the ultrasound beam is coupled to the eye through fluid. Because there is no corneal compression, the displayed result more closely represents the true axial length.
Note: Be sure to set your ultrasound machine to immersion mode if it doesn’t automatically do so or you will get meaningless readings that are several mm too long.
Figure C – Breakdown of phakic axial length measurements using the immersion technique.
- Probe tip.
Echo from the tip of the probe, now moved away from the cornea, has become visible. - Cornea.
A double-peaked echo will show both the anterior and posterior surfaces. - Anterior lens capsule.
- Posterior lens capsule.
- Retina.
This echo needs to have a sharp 90-degree take-off from the baseline. - Sclera.
- Orbital fat.
The immersion technique requires the use of a Prager Scleral Shell, or a set of Ossoinig or Hansen Scleral Shells.
The patient lies supine, looking up at the ceiling, and the scleral shell is placed between the eyelids and centered over the cornea. The scleral shell is then filled with a 40-60 mixture of Goniosol and Dacriose, and the probe tip is placed into the solution. Align the ultrasound beam with the macula by having the patient look at the probe tip fixation light, then take your readings as usual.
In our office, we have found that the Prager Scleral Shell is easiest to use and gives very consistent readings.
Prager Scleral Shells can be obtained directly from:
- ESI, Inc. (http://www.eyesurgin.com/)
2915 Everest Lane N.
Plymouth, MN 55447
(763) 473-2533
(763) 208-8105 Fax
Click to email ESI, Inc.
Ossoinig Scleral Shells are lighter in weight, easing probe manipulation, and can be obtained from:
- Hansen Ophthalmic Development Laboratories
(319) 338-1285
Figure D – Note the typical immersion measurement consistency.
When the ultrasound beam is correctly aligned with the center of the macula, all five spikes (cornea, anterior and posterior lens capsule, retina, and sclera) will be steeply rising and of maximum height.
Adopting this technique is a crucial first step in enhancing the overall accuracy of your A-scans. Measurement consistency from one measurement to the next is often outstanding, due to the absence of corneal compression and the fixed position of the ultrasound probe over the corneal surface.
For further reading, 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:
