Transcript Repeatability with the OQAS II

Reproducibility of Optical Measurements in
Pseudophakic eyes Using Double-Pass
Measurement System
St Thomas Hospital, London
Anish Dhital, David Spalton,
Kavita Gala
ASCRS 2010
No 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
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•
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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 46:12:4468-44742005
Nanavaty et al Ophthalmogica in press
Optical Quality Analysis System II
(OQAS)
(Visionmetrics, Spain)
• Uses double-pass
measurement system
• Images point spread function
• Derives MTF, Strehl ratio, light
scatter (objective scatter index
- OSI)
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:
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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
Correlation of Strehl ratio values
0.25
0.2
Average of
1st reading
R2 = 0.78
0.15
0.1
0.05
0
0
0.05
0.1
0.15
0.2
Average of 2nd reading
0.25
Results
Correlation of OSI values
20
18
R2 = 0.95
16
14
Average of
1st reading
12
10
8
6
4
2
0
0
2
4
6
8
10
12
Average of 2nd reading
14
16
Results
Bland-Altman Plot of Strehl ratio
Difference
0.06
0.05
0.04
0.03
0.02
0.01
0
-0.1
-0.05 -0.01 0
0.05
0.1
0.15
0.2
-0.02
0.25
0.3
Strehl ratio
-0.03
-0.04
-0.05
Standard Deviation = 0.017
Coefficient of Repeatability = 0.033
Results
Bland-Altman Plot of OSI
3
2.5
Difference
2
1.5
1
0.5
0
-0.5 0
2
4
6
8
10
12
14
16
18
Ocular Scatter Index
-1
-1.5
-2
Standard Deviation = 0.79
Coefficient of Repeatability = 1.56
Conclusion
• Optical measurements using a double-pass
system are reproducible despites concerns of
changes in the anterior segment between
visits.