Abstract
Evaluation of the retinal nerve fiber layer (RNFL) is used in the diagnosis of glaucoma and other optic nerve diseases which cause visual loss by damaging optic nerve fibers. Various optical methods are being developed to assess the RNFL directly as aids to diagnosis, and interpreting them requires a means to predict reflectance of the RNFL from a knowledge of its structure.Experimental studies show that light reflected by the RNFL has the geometry of light scattered by cylinders. Anatomical data were obtained from electron micrographs of cross-sections of amphibian RNFL and possible cylindrical scattering structures (axonal membranes, microtubules, and neurofilaments) were modeled as arrays of parallel thin fibers. Microtubules and neurofilaments are already thin fibers, and axonal membranes were decomposed into arrays of adjacent parallel fibers. A numerical method was developed to estimate the scattered field of the RNFL based on the simplified model. For an incident wave polarized parallel to the cylinder axis (TM mode), the internal field of each fiber was chosen equal to the incident field based on the Born approximation. For an incident wave polarized perpendicular to the cylinder axis (TE mode), the internal fields of microtubules and neurofilaments were also determined by the Born approximation, but the internal fields were assigned to each axonal membrane fiber by means of the modified Born approximation that depended on membrane orientation. Far field scattering was then calculated by field summation.Calculated scattering was much larger from axonal membranes than from microtubules or neurofilaments. Calculated spectra showed increased scattering at shorter wavelengths, which was consistent with the general tendency of measured spectra. Calculated backscattering from axonal membranes was approximately equal for the TM mode and the TE mode, but from microtubules TE mode scattering was less than TM mode. Calculated backscattering from the RNFL was proportional to its volume. Form birefringence of the RNFL was estimated statistically with the structural data obtained and the approximations for the internal field.A limitation of the model is that it regards the RNFL as a matrix of parallel cylinders. The existing misalignment in the actual RNFL should be considered in the future.