While the system is internally calculating the axial length from the
interference signal, it automatically analyzes the SNR.
SNR > 2.0 The measured value is valid.
SNR in the range 1.6 … 2.0 The measured value is not reliable.
The signal-to-noise ratio may be low for the following reasons:
Dense medial opacity along the visual axis
Restless patients
Alignment of instrument to patient eye is not optimal
Very high ametropia (> 6.0), corneal scars
Retinal pathology
Note:
In this case, "Borderline SNR" does not mean a wrong result, but is there to remind you to verify this measurement.
Accurate signal peaks can be determined by comparison with other
values of the measurement series of this eye (and of the other eye, if
necessary). See also the sections Example signal curves of AL Measurements and "Shifting the measuring cursor" below.
SNR < 1.6 The measured value is unusable.
It is marked on the display as an erroneous result.
This implies that the true measuring signal does not clearly stand out
from noise. As a rule, the results of such measurements are not
usable and should be deleted. You may delete them from the list by
using the Del key once they are highlighted.
Note:
Erroneous results (display: Error) are not considered in mean value
calculation.
The measured value may be transferred to the list of measured values
by a click on the measuring cursor (white dot).
See to it that the measured values are consistent.
Shifting the measuring cursor:
The measuring cursor (white point) is automatically placed on the
center of the signal peak with the highest absolute amplitude. The
corresponding axial length value is displayed beside the graph and in
the display field. The SNR is calculated and displayed for this
signal peak. The measuring cursor is placed in the center between the
regions corresponding to half the maximal amplitude. If the signal
curve is symmetrical (Gaussian curve), the cursor is positioned
exactly above the maximum of the signal. For Interpretation of Axial
Length Measurements it maybe be necessary to shift the
measuring cursor to verify axial length agreement between values in a
measurement series.
There are two ways to move the measuring cursor to another peak. In
both cases, it is advisable to do this with the signal curve being
zoomed.
1. Automatic positioning on a "remote" peak:
Place the mouse (arrow) cursor in the measuring cursor (white point).
Holding the left mouse button depressed, move the measuring cursor over
the desired peak. A vertical blue line will appear below the
measuring cursor for easier orientation. This line will be moved
along with the cursor.
When you release the mouse button, the measuring cursor will
automatically "snap in" over the desired peak.
Then, the display will show the corresponding axial length value and
the corresponding SNR, which will always be smaller than the maximal
SNR found automatically. This recalculated measured value will appear
in the display field denoted by an asterisk (*).
Example:
Explanation:
This manipulation will work only if the measuring cursor is moved
fully across the (local) maximum of the desired target peak. This
procedure is necessary for the search algorithm to reliably find the
desired peak without returning and snapping-in to the original
(higher) peak. Closely adjacent peaks (double peaks) cannot be
separated by this automatic method unless the curve adjoining them
drops down below a value which is less than half the amplitude of
their maxima.
Note:
While dragging the measuring cursor, the original axial length value
and SNR remain displayed beside the signal curve. The new axial
length value and the corresponding SNR will be calculated and
displayed only when you release the mouse button.
2. Fine-shifting the measuring cursor:
Note:
This manipulation always requires previous zooming in.
Proceed as described above under Item 1, but use the right mouse
button for dragging the measuring cursor. This way the automatic peak
detection is deactivated and the white point may be positioned at any
place over the measuring curve.
When you release the mouse button, the current axial length and the new
SNR will be calculated and displayed.
This kind of manipulation is advisable with closely adjacent double or
triple peaks.
Also, in this case, the recalculated axial length is denoted in the
display field by an asterisk (*).
Explanation:
Even if you undo the manipulations with the measuring cursor (by
moving it back to the automatically found maximal peak) and the
measured value agrees with the original one, the asterisk after the
measured value will remain. This is to indicate that the curve has
been manipulated.
The described manipulations of the measuring cursor may be performed
both in axial length measurement mode (after the current individual
measurement) and in post measurement editing mode.
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Note:
In this case, "Borderline SNR" does not mean a wrong result, but is there to remind you to verify this measurement.
