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Module 1 Section 2 |
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Module 1: |
Basic A-scan Biometry | ||
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Section 2: |
Scanning Procedure | ||
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There are three basic methods which are used to obtain an axial length measurement using an A-scan ultrasound instrument. Each method has its pros and cons. No matter which method is used, there are techniques to be used and scan characteristics to watch for that can improve the accuracy of the scan. Instrument set-up: The A-scan instrument is prepared for measurement. Some instruments require a calibration procedure before a measurement is performed. Mode selection: Most instruments require you to select a mode of measurement. This insures that the instrument is using the correct velocity-of-sound for the particular eye you are measuring. The choices are usually:
Patient instruction: An informed patient will be more cooperative. Tell the patient that you will be performing some measurements to determine the power of the implant that will replace his cataractous lens. Tell the patient that you may be measuring both eyes for comparison. Some patients get upset if you start by measuring the eye that is not going to have surgery. Keratometry: Perform keratometry before you perform A-scan biometry, and before any drops are instilled. This will eliminate any disruption of the corneal surface. Lubricating drops can be used during keratometry if the mires are not clear. Disinfection: A solid probe tip should be disinfected with an alcohol wipe before use. Wipe the tip or let it air dry before it is used. If there is some question about contamination, a 10 minute soak in 3% hydrogen peroxide can be used. A solution of 1 part household bleach to 10 parts distilled water is also an effective disinfectant after a 10 minute soak. Membrane type tips should be rinsed with saline before use. If there is some question about contamination, a new tip should be used. Membrane type tips are not used on new instruments, so they are becoming obsolete. The probe is hand held by the examiner and is applied to the cornea without any other means of support. The patient can be instructed to fixate on the light inside the probe tip. A fixation target for the fellow eye, such as an X on the wall, can also be used. If necessary, the patient can be simple instructed to stare straight ahead. Advantages: No additional set-up is required. There is good control over patient positioning. There is less dependence on steady patient fixation. The patient does not have to maintain positioning in a headrest, and the examiner can easily hold the eyelids open if necessary. Disadvantages: The examiner has less precise control over the positioning of the probe relative to the eye. There is a danger of putting too much pressure on the cornea, causing corneal compression and a shortening of the axial eye length. This is a significant potential error source. A .4mm compression error can result in a 1 diopter error in the calculated IOL power. The A-scan probe is mounted in place of the prism tip on the applanation tonometer. The patient is positioned at the slit-lamp and the probe is applied to the cornea in the same fashion that applanation tonometry is performed. The patient is instructed to fixate on the light in the probe, or the slit-lamp fixation light can be used with the fellow eye.
Insert the A-scan probe into the tonometer prism holder with the tip pointing toward the patient. Loop the cord of the probe in such a manner as there is no tension on the cord. Decrease the reading on the pressure drum until the probe tips backwards. Now increase the drum reading until the probe just tips forward. This will insure that there is minimal pressure on the cornea.
Advantages: A cooperative patient can be held in a steady position. The slit lamp allows more precise control of the probe. The forward pressure of the probe tip is somewhat limited by the tonometer mount, thus minimizing the possibility of corneal compression by the probe tip. Disadvantages: Additional set-up is required. Good patient fixation is critical because the probe cannot be adjusted on the vertical axis (the tonometer mount does not allow the probe to be tipped up or down). The patient must maintain position in the head rest. It is more difficult to assist patients who cannot keep the eye open.
For an A-scan to be an acceptable measurement of the axial length of a phakic eye, it must meet certain criteria:
What's wrong with this scan? 1) A lens spike is missing 2) There is a small spike in front of the retina spike 3) The orbital fat spikes are poorly defined.
See Module 2: Advanced A-scan Biometry for a discussion of the pseudophakic scan. A-scan instruments calculate measurements by means of electronic measuring "gates" or "lights". To be sure that the instrument is obtaining a proper measurement, check the position of the gates on the display.
The instrument to the right has measuring lights that attach to the cornea, lens, and retina spikes.
Corneal compression occurs when too much pressure is exerted onto the cornea by the probe. This can result in an erroneous axial eye length measurement. Corneal compression can be monitored by observing the anterior chamber depth (AC, or ACD) read-out that most A-scan instruments provide. Typically, an instrument will give a read-out for anterior chamber depth, lens thickness, and total eye length. Most eyes will have an ACD reading between 2.5 and 4 mm. Pay attention to the ACD reading on successive measurements. If the ACD reads 3.1, 3.2, 3.1 on three measurements, and then reads 2.7 on the fourth, assume that the 2.7 results from a compressed cornea and throw out the measurement. The corneal compression error factor can be avoided by using the immersion technique. For more information see Module 2: Advanced A-scan Biometry. Reproducibility is an indicator of accuracy. If you are to have confidence in the accuracy of your measurement, you must be able to reproduce the measurement. This means being able to take four or five consecutive measurements with consistent appearance, consistent ACD read-outs, and consistent total length results. A commonly accepted level of reproducibility in terms of total length is to have several measurements within .20 mm of each other.
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Set-up:
Some probes will not fit into tonometer prism holder if the screw is in
place. You can remove the small screw from the prism
holder if it is still in place. This screw is meant to prevent the tonometer prism
from being inserted backwards, but it may also prevent you from mounting the A-scan
probe.
The immersion method is employed to eliminate corneal
compression as an error factor in axial length measurements. A "shell" is
placed on the eye to provide a "water bath" over the cornea. The water
bath provides a medium through which the probe can measure the eye without touching the
cornea. For a detailed discussion of the technique, see Module
3, Section 2.
The probe lightly touches the cornea and is
positioned such that the barrel of the probe is aligned with the optical axis of the
eye. The operator aims the probe toward the macula of the eye. Alignment is
guided by the appearance of the scan or by some other signal from the A-scan
instrument. Alignment with the optical axis will be indicated by high lens spikes
and a high retina spike on the scan graph.
Spike height will vary as small adjustments are made in the
alignment of the probe relative to the cornea. In the manual mode the operator will
have to freeze the scan with a foot-pedal when an acceptable scan is obtained. The 
The anterior lens spike (B), the posterior
lens spike (C), and the retina spike (E) must be
present. The probe tip / cornea spike is represented by (A).
The scan of an 
This instrument has two gates: one for the cornea spike and one that
the retina spike must come within. The gates are depicted in yellow at the bottom of
the scan.
Extra lens
spikes