Directional property of the retinal reflection measured with optical coherence tomography and wavefront sensing
The last thirty years have experienced tremendous advancement in our understanding of light-tissue interactions in the human retina. Nevertheless, major gaps remain, and our modeling of light return from the back of the eye continues to evolve. The objective of this thesis is to investigate one of these gaps, specifically that related to the directional property (angular dependence) of the retinal reflection and in particular that of cone photoreceptors. Directionality of cones is commonly referred to as the optical Stiles-Crawford effect (SCE). While cone directionality is well known to originate from their waveguide properties, considerable uncertainty remains as to which reflections are waveguided. Since normal directionality of the photoreceptor requires normal morphology, the optical SCE has significant clinical interest.
The research presented in this thesis contains three main objectives. First, I evaluated the potential of spectral-domain optical coherence tomography (SD-OCT) to study the optical SCE. Second, motivated by these first results, I developed a custom high-resolution SD-OCT that was designed specifically for directional reflectance measurements. This allowed a more complete study to be performed and extended the analysis from photoreceptors to several other major layers of the retina. Directional properties were measured for the retinal pigment epithelium (RPE), two principle reflections of the photoreceptor layer (inner/outer segment (IS/OS) and posterior tips of outer segment (PTOS), Henle's fiber layer (HFL), retinal nerve fiber layer (RNFL), and finally the sum of all the layers considered (overall directionality). Reflectance of the IS/OS and PTOS were found highly sensitive to illumination angle regardless of retinal eccentricity. In contrast, the reflectance of the RPE showed little directionality. The reflectance of HFL and RNFL showed directional dependence, but unlike that of the photoreceptors, depended strongly on pupil meridian and orientation of the photoreceptor and ganglion axons that compose the layers, respectively. The reflectance of HFL and RNFL were consistent with scattering from cylindrical structures. Apparent thickness and brightness of HFL varied significantly with pupil entry position. Brightness of RNFL also varied significantly with entry position, but its apparent thickness did not. The overall retinal directionality was found consistent with the optical SCE reported in the literature.
The third objective evaluated a second optical method, based on Shack-Hartman wavefront sensing (SHWS), for measuring the optical SCE. Using a modified research-grade SHWS with custom algorithm, I demonstrated that the retinal reflectance can be readily extracted from the SHWS measurement and the spatial distribution of which is consistent with the optical SCE. This new method represents an attractive alternative to the conventional, highly customized instruments traditionally used for measuring the optical SCE and provides a more complete description of the eye's optical performance than currently implemented with SHWS technology.
0541: Biomedical engineering