Retinal and choroidal oximetry using snapshot multispectral imaging
Choudhary, Tushar R.
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The principal aim of the research was to validate and develop a non-invasive multispectral imaging technique of measuring oxygen saturation in retinal and choroidal vasculature. The Image replicating imaging spectrometer (IRIS) snapshot multispectral fundus camera used for retinal imaging, and image analysis protocols used to perform oximetry are described. In vitro and in vivo validation of oximetry techniques was performed. For in vitro validation an artificial eye containing blood at varying oxygenation was used. For in vivo validation the pig was used as an animal model. The calculated oxygen saturation was compared to blood gas analysis (gold standard) results and was found to be in close agreement. Retinal oximetry was performed on healthy human subjects. The average oxygen saturation value (± SD) for retinal arterioles and venules were 96.08% ± 1.9% and 68.04% ± 2.1%, respectively. The application of retinal oximetry technique was explored by conducting human hypoxia trials, in which the effect of acute mild hypoxia on retinal oxygenation and autoregulation was assessed on healthy human subjects. Hypoxic exposure resulted in a decline in both retinal arterial and venous saturation, as well as a significant increase in retinal vessel calibre, suggesting an autoregulatory response. This thesis also explored the possibility of exploiting fundus reflections to measure the choroidal oxygenation non-invasively. Fundus reflection intensity at two wavelengths, 780 nm and 800 nm (oxygen sensitive and isosbestic) were used to calculate intensity ratio (R), which is directly proportional to the blood oxygenation. A pilot study on 10 healthy humans was conducted. Fundus reflection was recorded at room air (normoxia) and 15% inspired oxygen (mild hypoxia). A significant reduction (P < 0.001) in intensity ratio was observed during hypoxia.