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Aberrometry for the Common Joe

Aberrometry for the Common Joe. Thomas O. Salmon, OD, PhD, FAAO. 8/19/11 Revision. Basic teaching objectives. What are aberrations? What are aberrometers? How do they work? How do we interpret the data? How can you diagnose abnormal values?. 1. What are aberrations?.

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Aberrometry for the Common Joe

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  1. Aberrometry for the Common Joe • Thomas O. Salmon, OD, PhD, FAAO 8/19/11 Revision

  2. Basic teaching objectives • What are aberrations? • What are aberrometers? • How do they work? • How do we interpret the data? • How can you diagnose abnormal values?

  3. 1. What are aberrations? • Aberrations = refractive errors • Myopia, hyperopia, astigmatism = LOA • But there are others … • Higher-order aberrations (HOA) • Coma • Spherical aberration • Trefoil • Others

  4. Clarification … which aberrations? • Monochromatic • Not chromatic • Not Seidel • Coma • Spherical aberration • Oblique astigmatism • Petzval • Curvature of field

  5. Pupil size, pupil size, pupil size, … 20/200 sized letter Courtesy of Dr. Donald Miller, Indiana University Retinal Imaging and Vision at the Frontiers of AdaptiveOptics, Physics Today (Jan 2000)

  6. Summary 1 • What are aberrations? A. Refractive errors • What are higher-order aberrations? A. Refractive errors beyond sphere and cyl.

  7. 2. What are aberrometers? • Instruments that measure … • Refractive errors • (super auto-refractors) • Sphere, cylinder + HOAs • (Higher-order aberrations) • ≠ corneal topography! COAS

  8. K topography vs aberrometry High-resolution keratometry High-resolution auto-refraction

  9. History of HO aberrometry Reagan • 1970s and 80s • Strategic defense • Refraction through the atmosphere • Shack-Hartman wavefront sensor • Astronomy • Adaptive optics (AO) Shack

  10. Ocular HO aberrometry research • 1960 MS Smirnov • 1977 Howard Howland • 1990 Josef Bille and … • Junzhong Liang • J Opt Soc Am A, July 1994 • Mid 1990s - other labs Howland Bille Liang

  11. IU Shack-Harmann Wavefront Sensor (catch a wave!)

  12. Clinical HO aberrometry • Laser refractive surgery • Large HOAs • Clinical aberrometry • Wavefront-guided LASIK COAS

  13. 3. How do they work? • Light is projected in. • Reflect off the retina • Light passes through the eye’s optics. • Catch the light. • Analyze it. • Reconstruct the optical wavefront’s shape

  14. Courtesy of Alcon

  15. Shack-Hartmann aberrometry x } + } y Light exiting the eye Analyze each dot’s position Reconstruct wavefront shape

  16. What does the wavefront tell you? • Flat wavefront = perfects optics (no aberrations) • Elevations = wavefront errors • Wavefront distortions reveal refractive errors. Topographic map Surface plot

  17. Higher-order wavefront maps Post LASIK Normal eye

  18. Optical quality metrics Modulation transfer function (MTF) Point spread function (PSF) Strehl ratio Visual performance metrics Simulate the patient’s vision! Design the ideal optical correction for the eye W-guided RS, CL, spectacles, IOL, etc. Besides refractive errors …

  19. Simulated retinal image emmetropia

  20. Simulated retinal image for AI -1.00 -0.50 x 100 HO RMS 0.51 µm (6.0-mm pupil)

  21. Simulated retinal image emmetropia

  22. Simulated vision for TO -3.00 -2.00 x 170 HO RMS 0.38 µm 6.0-mm pupil

  23. Summary 3 Q. How do aberrometers work? A. Measure light that has passed through the eye's optics. Q. Why? (Who cares?) A. The wavefront provides a wealth of info about the eye's optics and vision.

  24. Does this eye have good or bad optics? How good or bad is the optical correction? Are HO aberrations the problem? Specifically: Which aberrations does this eye have? How bad are they? 4. How do we interpret wavefront data?

  25. Which aberrations are present? • Wavefront - distorted by all the aberrations combined • Specifically, which ones are present? • Need Zernike analysis to break it down. • Zernike system = hierarchy of aberrations • Each aberration causes a particular shape of wavefront distortion.

  26. -3 Z 7 Some example wavefronts sphere astigmatism trefoil

  27. + . . . + = + + + + + Zernike analysis breaks it down sphere astigmatism trefoil coma spherical aberration Z44 Z42

  28. m Z or Z(n,m) n order (n) Zernike modes 0 0 1 -1 1 2 -2 2 0 3 HOA -3 -1 3 1 4 -4 4 -2 0 2 5 -5 -3 -1 3 5 1

  29. Z1-1 Z11 Combined Zernike modes 1 Z11 Z2-2 Z20 Z22 2 Z22 Z3-3 Z33 Z31 3 Z31 Z3-1 order (n) Z33

  30. Magnitude & axis form piston 0 Z0 prism 1 Z11 sphere astigmatism 2 Z20 Z22 coma trefoil 3 Z31 Z33 spherical aberration quadrafoil 4 Z40 Z44 Z42 order (n) secondary astigmatism

  31. Which aberrations are present. Breaks the wavefront down into the Standard Zernike modes or … Magnitude & axis form Each Zernike mode = one aberration But, … how bad are the aberrations? Z analysis also provides a value for each. Zernike analysis tells us …

  32. One for each Zernike mode Units in microns ± sign Must specify pupil size Absolute Zernike coefficient = magnitude Zernike coefficients

  33. Unit = µm + or - values Conventional Rx: +0.25 -0.75 x 111 Zernike coefficients 2nd order 3rd order 4th order Mode: Z2-2 Z20 Z22 Z3-3 Z3-1 Z31 Z33 Z4-4 Z4-2 Z40 Z42 Z44 Coefficient (µm): .56 .27 .64 -.03 .07 -.05 .06 .03 .04 .11 0 -.08 Rx: +0.19 - 0.67 x 111 Pupil diameter: 5.6 mm Total RMS: 0.76 µm Higher-order RMS: 0.51 µm

  34. The basic data - individual Zernike coefficients How bad are combined aberrations? Total aberrations (LOAs + HOAs) Just higher-orders (HOA RMS) Or, just third-order aberrations, etc. RMS wavefront error

  35. Summary 4. Interpreting the data • Aberrometers measure wavefronts • Wavefront - distorted by aberrations • Zernike analysis - which aberrations are present • Zernike coefficients - how bad they are • Data in microns, with ± signs • RMS - magnitude of grouped aberrations • Pupil size, pupil size, pupil size !

  36. 5. Diagnosis - what’s normal? • Aberrometry - diagnoses abnormal optics • Ideal eye = zero aberrations, but … • every eye has some aberrations. • So, are those Zernike or RMS values good or bad? • Need reference norms

  37. OCO Norms • JCRS Dec 2006 • 2,560 normal eyes • 9 sites • Zernike & RMS norms • Data on www • Google “Dr. Salmon”

  38. Downloadable info • Full article in PDF • Norms table - PDF & Excel • Signed Zernike coefficients • Absolute values • Combined (polar) Zernike modes • RMS for HOA and orders 3, 4, 5, 6 • http://arapaho.nsuok.edu/~salmonto

  39. HOA results • Prominent individual HOAs (6.0-mm pupil) • Z3-1 (vertical coma) = 0.14 • Z40 (spherical aberration) = 0.13 • Z3-3 (oblique trefoil) = 0.11

  40. Summary 5 Q. How can you know what’s normal? A. Compare the data to norms If more than double the norms … (for that pupil size) suspect abnormal optics.

  41. Summary – the basics • Aberrations = refractive errors • Aberrometers measure wavefronts. • Zernike analysis tells which aberrations • Zernike coefficients & RMS tell how bad the aberrations are. • Compare values to norms • Mean HO RMS (6 mm) = 0.33 µm

  42. And ... Dont’ forget pupil diameter!

  43. Case example • 34 yof, complained of shadows, diplopia, glare, eye strain, especially at night • RK OU at age 21 • Spec Rx = plano, 20/20- OU • Aberrometry confirmed large HOAs

  44. HOA wavefront maps (5.0 mm) RMS=0.89 µm RMS=0.92 µm

  45. Zernike coefficients

  46. HO RMS & pupil size

  47. Aberrometry & refractive surgery • Replace conventional auto-refractor • Verify pre-op refraction • Guide choice of laser procedure • Data sent to laser for custom correction • Evaluate quality of the correction

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