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Impressum
Measurement of retinal nerve fiber layer thickness by means of a new electrooptical ellipsometer in consideration of corneal aberration
Schmidt E., Scheuerle A. F., Gölz S., Bille J., Burk R. O. W.
Universitätsaugenklinik Heidelberg, INF 400, 69121 Heidelberg; Physik. Institut der Univ. Heidelberg, A Überle Str., 69120 Heidelberg
Objective: Based on the principles of a confocal scanning laser
ophthalmoscope (SLO) this new ellipsometer was designed to provide a quantitative
and three dimensional profile of the RNFLT by detection of phaseshift.
The disturbing influence of double refraction, mostly produced by an aspherical
cornea, was minimized by the Mueller formula.
Methods: The ellipsometer corresponds to a confocal SLO. Polarization
and analyzer units contain Pokelzellen with a phase shift of l/4 and l/2
and allow for images of 16 different intensity levels. The scan is performed
at an angle of 25° and has a resolution of 256*128 pixel. The 16 images
of different intensities are used to calculate a mean image and the phase-shift
in every single point scanned. As there is a linear correlation between
phase-shift and RNFLT the data can now be converted into a color coded
map. We examined 14 eyes of 14 healthy volunteers. The RNFLT profile and
the mean height were measured along a 360° circle with a 1,45-fold
diameter centered around the optic disc. Nine parameters of the contour
line including height and position of a) maxima, and b) minima, and c)
max. height variation of contour line were measured and analyzed.
Results: Retinal structures were clearly visible in the intensity
image as well as in the RNFLT-profile. The course of the contour line
demonstrated a "double-hump" configuration. The mean RNFLT along
the circle was 89±12µm, the height of the maxima was 160
± 20µm at 84 ± 20° (superior) and 170 ±
20µm at 278 ± 26° (inferior). The height of the minima
was 40 ± 10µm at 0 ± 13° (nasal) and 25 ±
5µm at 195 ± 16° (temporal). Max. height variation of
contour line 145 ± 20µm.
Conclusion: These measurements deliver medically relevant data
/ images which are easy to interpret. The large standard deviation of
measured values correspond to the expected deviation in interindividual
retinal variability. Measurements in glaucomatous eyes will show if there
is a significant difference between both groups. In order to use the ellipsometer
in clinical routine the usage and momentarily difficult data evaluation
must be automated and simplified.