Transcript Ray tracing “perfect” eyes: The limits of accuracy in IOL power

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
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
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)
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).
IOL
Cornea
Retina
Full eye pseudophakic optical reconstruction
Lens
Cornea
Retina
Phakic eye: Anterior segment imaging
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.
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, HaagStreit)
Designing a ray tracing
platform for IOL power calculation
 Percentage change in the parameters below required for a
0.25D change in spectacle prescription.
Ocular parameter
Short Eye
Average Eye
Long Eye
Anterior cornea radius
0.56
0.49
0.52
-7.8
-6.3
-6.9
Posterior cornea radius
0.99
0.75
1.0
Cornea refractive index
ACD phakic eye
6.2
7.0
8.6
ACD pseudophakic acrylic
3.1
4.1
5.4
-27
-26
-36
-12
-11
-16
-6.05
-5.7
-8.3
Pupil size (2mm)
Pupil size (3mm)
Pupil size (4mm)
6.5
3.6
1.4
Anterior natural lens radius
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
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.
Results
RIGHT
EYE
Modified
Purkinje
Imager
Lenstar
Pentacam
LEFT
EYE
Modified Lenstar
Purkinje
Imager
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
2.61
Chamber Depth
2.68
2.82
Lens Thickness
4.61
4.00
Not
possible
Lens Thickness
4.64
Not
possible
4.70
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.