Stratus OCT User Manual PN 2660021134133 A
Scan Acquisition Protocols
4-5
• For scans in a related series, only scan and fixation LED placement are adjustable
after the first scan. Several scan protocols consist of a series of related scans you
acquire one by one. To preserve the design integrity of these scan protocols, you
cannot adjust the size, number of lines, number of A-scans or placement of the
landmark after you save the first scan in the series. In this way, the analyses designed
to apply to these scan protocols will be meaningful.
• Exceptions: The only exceptions are the RNFL Thickness (3.4) and the RNFL Map
protocols. The former permits you to resize the aiming circle for all 3 scans; the
latter permits you to adjust scan size for all 6 scans, and both permit you to adjust
placement of all features for every scan.
• For scans in a related series, the landmark and scan are locked in their relative
position after the first scan. This is so you can use the landmark as a siting feature,
enabling you to keep each scan in a series in its preset pattern of relative placement.
Thus, you can preserve the design integrity of the scan pattern and the
meaningfulness of applicable analyses.
Number of A-Scans
By default, the number of A-scans is 512 for all protocols except the Fast Scan protocols
(fixed at 768 A-scans total). However, you can adjust this number using the
OCT Image
Tab while scanning (see page 3-17), or through the Scan Variables option on the Scan
menu before scanning (see Number of A-Scans, page 3-15). Note that in scan
alignment mode, the aiming pattern always consists of 64 A-scans.
Size vs. Resolution
Note that scan resolution decreases as you increase scan size (length or radius). (To adjust
scan size, see
Size, page 3-14.) This is because the scanner takes a certain number of
A-scans along each scan irrespective of size. (For most scans, you can adjust the number of
A-scans—see
Adjust Scan Variables, page 3-15.) Conversely, scan resolution increases
as you decrease scan size.
Aligning Large Scan Patterns
If the alignment is not correct for large scan patterns, part of the scan beam may be
blocked at the pupil edge. This causes parts of the scan image to be very weak or absent
entirely. In this case, reposition the patient module a few millimeters laterally or vertically
to accommodate the edge of the scan pattern. If this fails, try moving the patient module
1-2 mm towards or away from the eye. When the alignment is correct, the scan image
appears equally strong over its whole length.
Measuring Nerve Fiber Layer Thickness
When you measure nerve layer fiber thickness using any of the circle scan types, it is
desirable that the scan beam be as perpendicular to the retina as possible. As the scan
proceeds around the circle, if the beam is not perpendicular, there may be significant
variation in axial distance to the retinal surface. When this variation is minimized, the scan
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