reproducibility of optical measurements in pseudophakic eyes using double-pass measurement system st...
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Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass
Measurement System
St Thomas Hospital, London
Anish Dhital, David Spalton, Kavita Gala
ASCRS 2010No Relevant Financial Disclosure
Purpose• To measure the reproducibility of optical measurements in
pseudophakic patients using a double pass measurement system.
Light scatter• Increasing interest in measurements of light scatter and impact on
visual function• Methods of measuring scatter
• Double-pass• Psychophysical• Hartmann-Shack
Variables that affect light scatter
• Tear film• Cornea• Anterior Chamber• Intra-ocular lens• Vitreous clarity
• Measurement error inherent in the system may also affect the repeatability of measurements
Montes -Mico et al IOVS 46:12:4468-44742005Nanavaty et al Ophthalmogica in press
Optical Quality Analysis System II (OQAS)
• Uses double-pass measurement system
• Images point spread function• Derives MTF, Strehl ratio, light
scatter (objective scatter index - OSI)
(Visionmetrics, Spain)
MTF - the ratio between contrast in the retinal image and contrast of the original object.OSI - objective parameter calculated by evaluating the amount of light on the outside of the double-pass image in relation to the amount of light on the centre. Higher OSI value relates to more intraocular scattering.Strehl ratio - measurement of an optical system (ratio of the MTF to that of a diffraction limited system). Thus, Strehl ratio will be a figure between 0 and 1, the higher its value, the better
Methods • Prospective reproducibility study
Population • Normal pseudophakic eyes,• At least three months post-cataract surgery
Measures• Refraction• OQAS measurements used customised OQAS II
machine
Methods• Refraction - to obtain the cylindrical correction to optimise
measurement.
• OQAS II - Pupils were dilated and measurements made for a 4mm exit pupil diameter.
• 3 sets of measurements of OSI and Strehl ratio from a single
eye at one visit were taken and the average calculated. This was repeated one to two weeks later.
• Correlation plots and Bland-Altman analyses were used to assess and compare the 2 sets of measurements.
Results
• 51 eyes from 29 patients were assessed, with a heterogenous group of intraocular lenses:• 20 eyes with an Alcon SA60AT• 5 eyes with an Akreos MI60,• 6 eyes with an Acrismart 36a IOL,• 6 eyes with an Alcon SA60WF• 6 eyes with the Morcher 89A ‘Bag in the Lens’• 2 eyes with Crystalens HD intra-ocular lens.• 6 eyes with unknown IOL type
Results
0
0.05
0.1
0.15
0.2
0.25
0 0.05 0.1 0.15 0.2 0.25
R2 = 0.78
Average of 2nd reading
Average of 1st reading
Correlation of Strehl ratio values
Results
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8 10 12 14 16
Correlation of OSI values
R2 = 0.95
Average of 1st reading
Average of 2nd reading
Results Bland-Altman Plot of Strehl ratio
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3
Coefficient of Repeatability = 0.033
Strehl ratio
Difference
Standard Deviation = 0.017
Results Bland-Altman Plot of OSI
Difference
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18
Ocular Scatter Index
Coefficient of Repeatability = 1.56
Standard Deviation = 0.79
Conclusion
• Optical measurements using a double-pass system are reproducible despites concerns of changes in the anterior segment between visits.