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

New Device for Imaging and Quantifying Ocular Optical Parameters Required for

Exact Ray Tracing.

IntroductionFor 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

1. Calculation of intraocular lens power: a review. Olsen T. Acta Ophthalmol Scand. 2007 Aug;85(5):472-85. Epub 2007 Apr 2.

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.

3. 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)

IntroductionThe “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).

CorneaIOL

RetinaCornea

LensRetina

Full eye pseudophakic optical reconstruction Phakic eye: Anterior segment imaging

PurposeTo 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.

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)

Percentage change in the parameters below required for a 0.25D change in spectacle prescription.

Designing a ray tracing platform for IOL power calculation

Ocular parameter Short Eye Average Eye Long Eye

Anterior cornea radius 0.56 0.49 0.52

Posterior cornea radius-7.8 -6.3 -6.9

Cornea refractive index0.99 0.75 1.0

ACD phakic eye 6.2 7.0 8.6

ACD pseudophakic acrylic 3.1 4.1 5.4

Pupil size (2mm)-27 -26 -36

Pupil size (3mm)-12 -11 -16

Pupil size (4mm)-6.05 -5.7 -8.3

Anterior natural lens radius6.5 3.6 1.4

Posterior natural lens radius 3.4 2.9 3.0

Natural lens refractive index -0.11 -0.11 -0.11

Acrylic IOL radius 0.88 1.6 1.8

Acrylic IOL refractive index -0.15 -0.22 -0.25

Axial length -0.34 -0.35 -0.39

ResultsOur 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.

ResultsRIGHTEYE

Modified Purkinje Imager

Lenstar

Pentacam

LEFTEYE

Modified Purkinje Imager

Lenstar

Pentacam

Front Cornea Curvature

8.34 8.08 8.17 Front Cornea Curvature

8.34 8.12 8.17

Back Cornea Curvature

6.90 Not possible

6.80 Back Cornea Curvature

6.95 Not possible

6.89

Front Lens Curvature

9.00 Not possible

Not possible

Front Lens Curvature

8.37 Not possible

Not possible

Back Lens Curvature

-7.20 Not possible

Not possible

Back Lens Curvature

-6.85 Not posssible

Not possible

Anterior Chamber Depth

2.66 2,82 2.79 Anterior Chamber Depth

2.61 2.68 2.82

Lens Thickness

4.61 4.00 Not possible

Lens Thickness

4.70 4.64 Not possible

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

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.