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P. H. Kok, H. W. van Dijk, L. A. Engelbrecht, T. J. T. P. van den Berg, F. D. Verbraak; A Model for the Effect of Disturbances in the Optical Media, e.g. Cataract, on the OCT Image Quality. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1852.
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© ARVO (1962-2015); The Authors (2016-present)
To understand and model how the loss of quality of optical coherence tomography (OCT) images results from disturbances in the optical media, in particular cataract.
Optical disturbances were approached by means of three main effects and each was simulated with artificial filters. The three effects (filters) were: light attenuation (absorptive and reflective filters), refractive aberrations (defocusing lenses) and light scattering/straylight (scattering filters). A series of each type of filter was used to cover the range of disturbances as can be observed in the clinic.In two healthy volunteers (respectively 27 and 51 years old) OCT measurements were performed using the StratusOCT (Zeiss). After pupil dilatation b-scans of the macula were acquired with the 6 mm line scan protocol. In case of the StratusOCT the quality of the OCT image is reported as Signal Strenght (SS), a metric based on the signal-to-noise ratio and ranging from 0-10. The SS of each line-scan was collected. During scanning, the filters were placed in front of the eye. All measurements were done twice at two time points by the same experienced examiner.
A strict linear relationship was found between the attenuation of the absorptive and reflective filters and the SS of the corresponding OCT images. The attenuation of these filters was expressed in optical density, determined for the 830 nm central wavelength of the OCT (OD= 830). This linear relationship indicates that the SS value purely represents light attenuation of the OCT scanning spot. Assuming the reduction of light from the OCT scanning spot on the retina (20 micron in diameter) to be the critical factor, this attenuation was determined for the scattering filters using a model eye and was expressed in OD= 830 as well. For the defocusing lenses, this attenuation (again expressed in OD= 830) was estimated by calculation, in which the effective entrance pupil size was a fit parameter. The same straight line as found for the absorptive and reflective filters could be fitted in the data points of the scattering filters and defocusing lenses.
The loss of OCT image quality in patients with disturbances in the optical media, expressed in SS, is fully explained by attenuation of the OCT scanning spot on the retina. The SS value indicates the one pass logarithmic attenuation of the OCT signal, according to SS = constant - (10·OD= 830).
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