Section 3
A-Scan Operation and Use
3
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EXAMINATION MODES
Before performing an axial length measurement
the user should always verify the operating
mode (cataract, dense cataract, aphakic,
pseudophakic or manual mode). Since this mode
governs the manner in which the instrument
evaluates an A-Scan pattern and the parameters
the instrument will utilize in its calculations,
mistakes and/or large numerical errors will be
avoided (even though the A-scan pattern appears
satisfactory).
IMMERSION TECHNIQUE
Although use of the water immersion technique
completely eliminates corneal compression as a
complicating factor, and can greatly assist with
the alignment of the probe along the visual axis,
it is still necessary to review and analyze
waveforms to ensure an acceptable reading.
The probe must be properly positioned within
the Immersion shell in order for the EZ-Scan
to automatically capture the waveform. If a
seemingly proper waveform is not automatically
captured, check to see whether the leading edge
of the corneal echo is either inside (as shown in
Figure 3-15) or outside the limits of the corneal
window. In either instance a warning message
“POOR CORNEA” will be displayed.
Only scans with a steeply rising retinal spike
should be accepted. A minor “stair-stepping” is
inherent in a digital waveform, but a reading
which is registered on a third or higher step
should not be accepted. In other words, the
horizontal threshold line (which is the “reading”
line) should cross on the first or second vertical
step of the retinal spike. Additionally, the retinal
spike should be “stepped” only by the thickness
of a single line – scans with additional lines
should be rejected.
There should also be a strong scleral spike,
immediately posterior to the retinal spike. The
scleral spike amplitude should be close to that of
the retinal spike, but can be slightly lower or
higher.
Figure 3-15 Poor Position in Immersion Mode
(Corneal Echo “left” of the Gate)
