 |
Module 1 Section 1 |
||
|
Module 1: |
Basic A-scan Biometry | ||
|
Section 1: |
Basic Concepts | ||
|
A-scan biometry is also called an axial length measurement, or simply an "A-scan" or "A's". This measurement is combined with keratometry ("A's and K's") in a formula to determine the power of the intraocular lens that replaces the natural lens that is removed from the eye during cataract surgery. Accuracy is important. A .4mm error in the measurement may result in a one diopter error in calculated IOL power. This module covers concepts and techniques necessary to arrive at an accurate measurement.
Strong reflections also occur as the sound beam encounters the posterior lens surface and the retina. Spikes representing these reflections appear at their corresponding positions along the baseline. The first spike represents the probe tip as it comes into contact with the cornea.
If the sound beam is aimed off axis, the beam will hit a highly reflective surface at an angle and sound energy is reflected away from the transducer, producing low spikes on the baseline. The highest spikes occur when the beam strikes a highly reflective object in a perpendicular orientation.
For a given eye and a given probe orientation, the higher the gain level is, the higher the spikes will be above the baseline.
Because of these factors, spike height becomes an important indicator as to the accuracy of the A-scan. Tall lens and retina spikes indicate that the ultrasound beam is striking these structures in a perpendicular fashion, giving an indication that the beam is aligned with the optical axis of the eye.
This is a significant potential error source. A .4mm compression error can result in a 1 diopter error in the calculated IOL power. Ultrasound travels at different speeds through materials of different densities. Ultrasound is conducted very well through water and materials containing a large percentage of water. Ultrasound waves do not travel through air. The denser the material is, the faster sound travels through it. A-scan instruments compute distances by measuring the time it takes ultrasound to travel through the structures of the eye. More specifically, the time it takes an ultrasound beam to travel from the probe, bounce off of an object, and return to the probe. The following are the speeds of ultrasound through the structures of the eye:
Many A-scan instruments do not measure each individual segment in the eye. Instead they use an average velocity of sound. Many use 1550 for a phakic eye and 1532 for an aphakic eye. |
|||
The A-scan probe
projects a thin sound beam that travels through liquid or tissue. When the sound
beam encounters the interface of a substance that is dissimilar from the substance it is
traveling through, part of the sound beam energy is reflected, and part of the sound
energy projects through the new substance. The more dissimilar the substances are,
the stronger the reflection, or echo, is.
When the A-scan beam is projected into the phakic (natural lens) eye it
travels through the aqueous humor and encounters a dissimilar substance in the anterior
lens surface. Energy is reflected back to the transducer in the probe tip. The
instrument displays the intensity of the reflection as a spike above the baseline (which
represents distance) on a graph.
Even a highly reflective structure such as the retina will produce a
relatively low spike if the sound beam strikes the retinal surface at an angle.
Gain is the electronic amplification of the signal coming back to
the probe. There is usually a manual control that can be changed by the
operator. Some instruments adjust the gain automatically.
The optical axis of the eye is the distance from the corneal apex to the
fovea. This is also the longest cornea-to-retina distance in the normal eye.
A light beam, or ultrasound beam, passing along the optical
axis of the eye will encounter the anterier lens capsule, the posterior lens capsule, and
the retina in a perpendicular orientation.
Since ultrasound does not travel through air, the
A-scan probe must come into contact with the cornea, either directly or through a liquid.
If a corneal-contact method is used, there is a danger of the probe putting too
much pressure on the cornea, causing the cornea to compress, which results in an
artificial shortening of the axial eye length.