New Device for Imaging and Quantifying Ocular Optical Parameters Required for Exact Ray Tracing.

1. New Device for Imaging and Quantifying Ocular Optical Parameters Required for Exact Ray Tracing. Eugene Ng, Arthur B. Cummings, Patrick P. Collins, Alexander V. Goncharov, Diana Bogusevschi, Chris Dainty, Michael C. Mrochen. The authors of this paper have received research funding from the National Digital Research Center, Ireland

2. Introduction • For current intraocular lens (IOL) power calculation formulae to work, current optical biometric devices have to convert optically measured parameters (such as axial length) into ultrasound equivalents.1 • Systematic and random errors occur when “conversion/fudge” factors and techniques to correct image distortion are used to modify optical raw data (Scheimpflug or optical coherence tomographers).2,3 • Calculation of intraocular lens power: a review. Olsen T. Acta Ophthalmol Scand. 2007 Aug;85(5):472-85. Epub 2007 Apr 2. • The thickness of the aging human lens obtained from corrected Scheimpflug images.Dubbelman M, van der Heijde GL, Weeber HA.Optom Vis Sci. 2001 Jun;78(6):411-6. • Optical distortion correction in Optical Coherence Tomography for quantitative ocular anterior segment by three-dimensional imaging. Ortiz S, Siedlecki D, Grulkowski I, Remon L, Pascual D, Wojtkowski M, and Marcos S. Optics Express, Vol. 18, Issue 3, pp. 2782-2796 (2010)

3. Introduction The “perfect” eye: If corneal and IOL / lens curvatures, distances and refractive indices are known, strict laws of physics should govern the prediction of refractive status of the eye in a Ray Tracing environment (Zemax, ZDC). Lens IOL Cornea Retina Cornea Retina Full eye pseudophakic optical reconstruction Phakic eye: Anterior segment imaging

4. Purpose • To realise the goal of precribing 3 dimensionally accurate individualised IOL, a history-free technique of ray tracing using the true physical parameters of Snell’s Law will be necessary. • This involves the simultaneous recovery of curvature, distance and refractive index of each ocular interface. • To this end, we have developed a new device to image and quantify the physical parameters required by exact ray tracing.

5. Method • The accuracy required for each parameter was calculated using Zemax to the tolerance of 0.25D total ocular refraction at the spectacle plane. • These requirements were used to design a Modified Purkinje Imaging (MPI) device capable of simultaneously recovering the above-mentioned parameters. • One bench-top prototype was used for inorganic calibration and a second unit was deployed to capture images from eyes prior to cataract surgery. • Parameters obtained using MPI were compared to those obtained using a Scheimpflug camera (Pentacam, Oculus) and optical low coherence reflectometry (Lenstar, Haag-Streit)

6. Designing a ray tracing platform for IOL power calculation • Percentage change in the parameters below required for a 0.25D change in spectacle prescription.

7. Results • Our current technique is still evolving and the process of recovering unadulterated data from raw images is at present, laborious. As a result, insufficient eyes were imaged using a standardized protocol to allow meaningful statistical analysis. • However, a complete set of data from one patient (right and left eye) demonstrates the potential of this technology.

8. Results Table comparing parameters obtained by Modified Purkinje Imager with those obtained by Scheimpflug and optical low coherence reflectometry (all units in mm).

9. Discussion / Conclusion • A novel technique is currently in development to recover true physical ocular parameters within tolerances that are suitable for use with ray tracing. • Exact ray tracing may enhance the results of optical calculations for IOL power determination and ablation profiles of refractive lasers.