Title

Polarization properties of the retinal nerve fiber layer investigated with multispectral imaging micropolarimetry

Date of Award

2000

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Biomedical Engineering

First Committee Member

Robert W. Knighton, Committee Chair

Abstract

Evaluation of the retinal nerve fiber layer (RNFL) is valuable in diagnosing glaucoma and other ocular neuropathic diseases. Several optical methods have been developed to assess the RNFL quantitatively. Knowledge about the optical properties of the RNFL and the underlying mechanisms is essential to understand these techniques and improve their assessment ability.Experimental studies show that the RNFL reflectance arises from the scattering of light by cylindrical structures. The measured reflectance spectra of the RNFL suggest that both thin and thick cylinders contribute to the RNFL reflection and experiments with colchicine suggest that microtubules are a likely candidate for the thin cylinders.The RNFL was modeled mathematically as an ensemble of uniformly distributed cylinders assumed to be form birefringent. The Mueller matrix of the RNFL model was derived and decomposed to reveal its polarization properties. The diattenuation of the model was due to the cylindrical scattering properties. The retardance exhibited in the model included the phase changes due to cylindrical scattering and to the RNFL birefringence. Reflection by the RNFL model preserved polarization.A multispectral imaging micropolarimeter was designed and calibrated. It was used to investigate the reflectance and polarization properties of the RNFL of isolated rat retinas. The RNFL retardance measured in transmission was constant over visible wavelengths, which suggested that only one mechanism was involved in the RNFL birefringence and agreed with the assumption of the RNFL being form birefringent. Measurements of diattenuation spectra of the RNFL reflectance revealed that intrinsic diattenuation is small at all wavelengths. The results led to two possible conclusions (1) if the RNFL reflection arises from scattering by spatially separated cylinders, the refractive index of these cylinders is close to that of the surrounding medium and (2) cylindrical structures other than spatially-separated cylinders may dominate the RNFL reflection; axonal membranes are a possible candidate. The reflected light from the RNFL preserved its polarization.Knowledge of the polarization properties of the RNFL reflectance could help to understand the measured signals in some optical techniques, such as optical coherence tomography (OCT) and scanning laser polarimetry (SLP), and therefore improve their assessment ability of the RNFL.

Keywords

Health Sciences, Ophthalmology; Engineering, Biomedical; Physics, Optics

Link to Full Text

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