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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 2010 No Relevant Financial Disclosure. Purpose.

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slide1

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
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
slide3

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 46:12:4468-44742005

Nanavaty et al Ophthalmogica in press

optical quality analysis system ii oqas
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
Methods
  • Prospective reproducibility study

Population

  • Normal pseudophakic eyes,
  • At least three months post-cataract surgery

Measures

  • Refraction
  • OQAS measurements used customised OQAS II machine
methods1
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
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
results1
Results

Correlation of Strehl ratio values

R2 = 0.78

Average of 1st reading

Average of 2nd reading

results2
Results

Correlation of OSI values

R2 = 0.95

Average of 1st reading

Average of 2nd reading

results3
Results

Bland-Altman Plot of Strehl ratio

Difference

Strehl ratio

Standard Deviation = 0.017

Coefficient of Repeatability = 0.033

results4
Results

Bland-Altman Plot of OSI

Difference

Ocular Scatter Index

Standard Deviation = 0.79

Coefficient of Repeatability = 1.56

conclusion
Conclusion
  • Optical measurements using a double-pass system are reproducible despites concerns of changes in the anterior segment between visits.