Stratus OCT User Manual PN 2660021134133 A
Analysis Protocols
6-39
Normalize
Select Normalize to eliminate background noise and to use the whole color scale in the
processed scan image. This function normalizes scan images with respect to noise and
signal strength. In other words, when you apply this function to scan images made with
different noise or signal strength, the resulting images appear equally “bright,” i.e., have
the same range of color.
The scan image false color scale operates in a signal value range of 0—255. Normalize
displays as zero (black) data points with values less than or equal to the average noise
level. It displays as 255 (saturated or white) data points with values greater than or equal
to the maximum signal value minus a fixed constant. It adjusts intervening signal values to
maintain their relative position in the new range. The resulting scan image uses the entire
color scale to express the relative reflectivity of the retinal structures between the noise and
saturation signal levels.
Align
Select Align to correct the data for effects due to patient motion in the axial direction.
Slight movements of the head toward and away from the instrument cause the scan image
to shift vertically, resulting in low-frequency “wiggles.” (This also happens if the scan beam
is not perpendicular to the retina over the whole scan—see
Aligning Large Scan
Patterns on page 4-5.) To correct for this movement, this algorithm compares each of the
longitudinal samples (A-scans) in the data set with its neighbor in a process called
correlation. In effect, it slides A-scan 2 in relation to A-scan 1 until the data align. Then it
slides A-scan 3 in relation to the now-aligned A-scan 2, and so on until all A-scans are
aligned.
Note: The Align function may introduce artifacts in the scan image, since it cannot
distinguish true retinal height changes from apparent changes due to patient
motion. Furthermore, the process does not work equally well on all types of images.
The operator must use judgment in interpreting the results.
Normalize + Align
Select Normalize + Align to perform both the above functions.
Smoothing
The two smoothing functions average out noise and blend the colors of the scan image.
Smoothing may be useful to appreciate more fully the large-scale features in the data. The
drawback, of course, is that some small details may be lost.
Gaussian Smoothing works by calculating a moving average of signal values in a 3 × 3
region. It weights the signal values according to a Gaussian function, such that the outer
points in the region are weighted less than the center point.
Median Smoothing is similar to Gaussian smoothing, except that it uses the median value
of the 3 × 3 region (i.e., the middle value when ordered by size) instead of the moving
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