Transillumination techniques in ophthalmic imaging
Weber, Timothy Daniel
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In vivo imaging of the human cornea and retina is typically performed in a reflection geometry. Images are formed from light that has backscattered off corneal microstructures or backreflected from the retina. In this configuration, artifacts caused by superficial surface reflections are often encountered. These unwanted reflections can either globally overwhelm the signal or cause local glare, complicating reliable image quantification. This thesis describes a pair of alternative ophthalmic imaging techniques based instead on transmitted light, which inherently avoids these artifacts. For retinal (i.e. fundus) imaging, we describe a mesoscopic transmission imaging method, which we call transcranial fundus imaging. The method uses deeply penetrating near-infrared light delivered transcranially from the side of the head, and exploits multiple scattering to redirect a portion of the light towards the posterior eye. This unique transmission geometry simplifies absorption measurements and enables flash-free, non-mydriatic imaging as deep as the choroid. We use multispectral image sets taken with this new transillumination approach to estimate oxygen saturation in retinal blood vessels. In the cornea, we describe a new technique for non-contact phase-contrast microscopic imaging. It is based on fundus retro-reflection and back-illumination of the crystalline lens and cornea. To enhance phase-gradient contrast, we apply asymmetric illumination by illuminating one side of the fundus. The technique produces micron-scale lateral resolution across a 1-mm diagonal field of view. We show representative images of the epithelium, the subbasal nerve plexus, large stromal nerves, dendritic immune cells, endothelial nuclei, and the anterior crystalline lens, demonstrating the potential of this instrument for clinical applications.
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