Abstract
Based on wavefront sensor images an objective and quantitative method is presented for characterising cataract. By separating direct and scattered light in the focal plane of the microlenses, the new procedure is able to make two-dimensional maps of the spatial variation of scattering properties in the crystalline lens, and also provides a single figure descriptive for the whole eye. The developed evaluation algorithm successfully quantifies cataract, especially that of nuclear type. To demonstrate its operation, a custom-built measurement setup was constructed using a Shack-Hartmann wavefront sensor with 40 times 32 microlenses to capture 12-bit images of the pupil plane, and a superluminescent diode of 830 nm wavelength as a light source. Slit-lamp clinical measurements served as reference for calibration and to estimate the accuracy of the new method. The tests were carried out on 78 eyes with cataract in different progression state ranging from healthy to above 5 on the LOCS III scale. The residual error of the calibration (i.e. the standard deviation of difference between clinical reference and our algorithmic characterisation) turned out to be pm 0.29 category on the LOCS III N scale, which approximates the pm 0.33 precision of classic cataract measurements carried out with the greatest care.
Highlights
Based on wavefront sensor images an objective and quantitative method is presented for characterising cataract
For the evaluation of raw wavefront sensor images we developed a novel algorithm that quantifies cataract by decimal values interpreted on the LOCS III N scale
Examples of raw wavefront sensor images are shown in Fig. 4: it can be seen as the focal spots are getting more blurred in case of eyes with more pronounced cataract
Summary
Based on wavefront sensor images an objective and quantitative method is presented for characterising cataract. Visual observation is compared to a standard reference image sequence to determine the grade of progression; e.g. in the case of nuclear cataract there are six categories (plus the scatter-free case) In addition to this coarse figure, the method is strongly subjective, since the diagnosis is affected by the personal judgement and specific experience of the examiner. Further drawbacks are that by using a slit-lamp only backward scattering can be examined instead of the physically relevant effect, and the provided metric is not directly related to the visual loss of the patient[5] The latter problems are eliminated in case of the C-Quant[6] equipment, which investigates how a disturbing source affects the visibility of a target viewed by the patient. The study could reveal some correlation between the two measurements, though the lack of exposure control possibility strongly reduced the interpretability of the captured images
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