Laser scanning tomography will be employed to quantitatively assess the thickness of the nerve fiber layer. A pinhole is located at a plane conjugate to the focal plane of the scanning laser beam. This confocal arrangement assures that only light originating from the illuminated focal plane on the retina passes through the pinhole and is detected by the photomultiplier. Consequently, images with high spatial resolution in all directions are obtained. By capturing and processing 32 images spaced between 10 and 30 microns in depth, a pseudo three dimensional image of the area being observed can be generated. In the polarization dependent imaging mode, the contrast of nerve fiber layer images is enhanced. In order to evaluate the thickness of the nerve fiber layer from polarization dependent measurements, the nerve fibers are approximated by an array of parallel thin cylindrical rods immersed in a medium of different refractive index (Muller-cell cytoplasm). The obtained data will yield point by point thickness measurements of the nerve fiber layer. To assess the accuracy of this technique, unfixed and fixed post-mortem human eyes will be studied. Fixed tissue will be examined with light microscopy to determine the accuracy of the methods and calibrate the instrument for clinical measurements in patient eyes. Next, nerve fiber layer thickness will be measured, and the reproducibility determined for the eyes of normal subjects and glaucoma patients. These data will be compared with standard photographic techniques for assessing the presence of the nerve fiber layer.
