A-Scan Operation and Clinical Use Section 3
scan. In such a case, a scan may have to be
accepted based on its meeting the remaining
criteria.
EXAMINATION MODES
Before performing an axial length measurement
the user should always verify the operating
mode (cataract, dense cataract, aphakic, or
pseudophakic). 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).
WATER 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 with the macula, it is
still necessary to review and analyze waveforms
to ensure an acceptable reading.
The probe must be properly positioned within
the Prager shell in order for the PacScan™ 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 3-14) 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,
about 1.5 to 2.0 mm posterior to the retinal
spike. The scleral spike amplitude should be
close to that of the retinal spike, and can be
slightly lower or higher.
Figure 3-14. Poor Position in Immersion Mode
3 - 12 Doc # 0300-A-1901-3D
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