Topography / Tomography 2010 update

Transcription

1 Topography / Tomography 2010 update Anterior Segment Topology Placido based technology with arc step reconstruction τοπογραφία ανάκλασης 2. Scanning Slit / transverse and rotational τοπογραφία προβολής 3. Wavefront analyzers/ Corneal Wavefront αµπερροµετρικήτοπογραφία Anast Charonis

2 Dynamic Corneal Imaging (Grabner et al) Indentation Topography- Elasticity Ocular Response Analyzer (Reichert) Ocular hysteresis and IOP- Viscoelasticity Electronic Speckle Pattern Interferometry (ESPI) Measures out of Plane corneal displacement Manual Keratometry Classical Corneal Topography Placido based principles Keratometry, Axial, Tangential, Power Maps Keratometric / physiological corneal indices Corneal asphericity/ related clinical relevance Corneal Tomography (Elevation Topography) Transverse Slit Scanning/ Rotational Scheimpflung Wavefront basics Corneal Wavefront

3 GARBAGE IN Quiz #1How do you diagnose irregular astigmatism without a topographer? Direct ophthalmoscope GARBAGE OUT ALWAYS CHECK THE RAW DATA FIRST!! Retinoscope Lohne keratoscope Keratometer

4 The Scheiner Principle keratometry Measures the curvature of the cornea along the 2 principal meridiens at two points 3-4 mm apart on the paracentral cornea Assumes the rest of the cornea is perfectly spherocyndrical

5 Scheiner > Helmholtz > Javal-Schotz Corneal topography Placido disc technology Large target Eyesys, Keratograph, Dicon CT 200, Eyemap-Alcon, atlas topographer- (H-Z) Medium target Eyesys Vista, Shin-Nippon- Reichert CTS Small Target Tomey TMS-3/4, Optikon Scout Portable,Magellan-Nidek J Schwiegerling

6 Turner 1999 Curvature measures bending. Small radius Large curvature Fast bend Large radius Small curvature Slow bend 1 Curvature κ= R A small radius circle has a large curvature. Radius of curvature Keratometry K-quiz#2 true or false 1. K Determines curvature by measuring the size of a reflected mire yes 2. Doubling of image avoids problems with eye movements yes 3. In the B&L keratometer the mire has constant size, the image doubling is variable yes 4. In the Javal-Schiotz ophthalmometer the image doubling is constant and the mire separation is variable. yes 5. The radius scale used in keratometers is not exact NO

7 Principle of keratometry Quiz #3 If you examine a 55d cornea the specular reflected image of the cornea is going to be different from that of a 40d cornea. Is it going to be larger or smaller? The mire image of the B&L keratometer is always constant and measures the curvature at specifically 3mm around the cornea apex. Yes or No Keratoscope: Placido's disc Disc with white reflecting rings(also called mires) and convex lens in the center Held in front of the eye and illuminated Examine the shape of the reflected rings off the pre-corneal tear film handle Viewing Aperture with lens Reflecting rings

8 Normal eye equally spaced symmetrical rings Corneal astigmatism oval-shaped rings Aspherical cornea non-equidistant rings Measure Surface Slope directly using Specular Reflection

9 Corneal topography With a keratometer one typically measures up to 4 points of the central corneal surface Computer-analysis of a picture from Placido's disc can measure the curvature at thousands of points on the whole cornea Turner 1999 Color Scale: Surface Curvature Although there is no universally accepted color scale in corneal topography, spectral directions are standardized. (+ +) Max Red sharp fast bend short radius (+) Min Blue flat slow bend long radius In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).

10 Turner 1999 Axial Curvature Astigmatism: Axial Curvature Map K-AXIS The bow-tie is actually an axial artifact

11 Turner 1999 Meridional Curvature Astigmatism: Meridional Curvature Map Axial Curvature Meridional Curvature Keratoconus: Axis-Based Maps You can get axial curvature from a meridional map. The axial value at any point is the average meridional curvature along a radial, from the map center to the point of interest. Turner 1999

12 Quiz #4Axial vs Tangential Curvature A PLACIDO >>> Perspective Reflection Meridional/ Axial > Principal- Mean B K axis C Point A: tangential power>axial: increased steepness PointB: tangential=axial evtl spheric cornea Point C= tangential <axial evidence of increased prolateness Turner 1999

13 Elevation (sphere) Mean Curvature Keratoconus: Anterior Quad Map Turner 1999 Curvature Vs Optics depth Axial Curvature Meridional Curvatu Turner 1999 thickness pre-corneal wavefront curvature Surface Geometry Material Properties Optical Incidence post-lenticular wavefront Curvature or Slope Refractive index pre-retinal wavefront Light ray incoming angle Curvature, Optical properties slope, elevation, quantify thickness, net changes and in depth the optical are all wavefronts geometric traversing properties. one They or more quantify refracting the size, shape, and location surfaces. of the refracting surfaces. No optics is involved.

14 Turner 1999 Axial Curvature [K] Refractive(optical) Power Raytrace Power Map oblate This eye shape has factor with-the-rule S, prolateastigmatism. shape factor E, Axial and asphericity Raytrace power Q. The maps relations areoptical. between Notice these that shape on factors every meridian, curvature, raytrace although power increases expressed are peripherally. in keratometric This is a manifestation diopters, of the is NOT spherical an optical aberration property. S = 1 - E = 1 + Q caused by insufficient corneal prolateness. Asphericity is the negative of the prolate shape factor. Keratometry K-quiz#5 true or false The axial map of a perfect sphere of 7,5mm ROC in 0,5step scale is monochromatic yes The curvature tangential map of a perfect sphere of 7.5mm ROC in 0,5step is no trichromatic The axial optical power map of a perfect sphere of 7,5mm ROC in 0,5step is trichromatic The axial optical power map of a perfect sphere of 7,5mm ROC in 0,5step scale is hexachromatic no yes

15 quiz#6 true or false The virgin normal cornea shape can be described as a prolate ellipsoid yes The median value of the Q factor of the virgin cornea is ~-0,2. yes The spherical aberration of the normal young eye is very close to zero. yes The corneal spherical aberration of the normal young eye is not zero, it would theoretically be zero if the q factor was ~-0,52 yes Quiz 7 Keratoconic patients benefit the most from aspheric lenses when they have cataract surgery True or False??

16 Axial Curvature [D] Axial Curvature [D] Aligned Curvatures: Axial Anterior Cornea Curvature Horizontal Vertical Anterior Posterior F = 49.7 S = 52.1 F = -5.8 S = Posterior Cornea Corneal F+F = 43.9 S+S = 45.3 = 2.4 = 1.0 = 1.4 True corneal power, Quiz # 8 The Physiological index (PI)of the cornea is 1,337 and is used in modern topography systems FALSE: The Keratometric Index is used because the corneal power Is estimated only by measurements of the front corneal surface The KI however is 1,337 KI is not the refractive index of anything N -= is an approximation Ant power (1, ) 1000/r =376/r Posterior power (1,336-1,376) 1000/r = -40/r Turner 1999

17 True corneal power, Quiz # 9 The ratio of ROC between anterior and posterior corneal surface is stable and is ~0,882 (that s why KI is valid in IOL calculations) The ratio of ROC changes after RxSurgery because the ROC of the posterior cornea remains unchanged and the compensating Keratometric index is not valid. Nevertheless all Scheimpflung posterior power images are (in 2010) still invalid! Placido based assumptions The precorneal tear is perfect The corneal apex, line of sight and the VK normal are all the same There are no curvature discontinuities The central cornea curvature is actually not measured

18 Tear Stability Analysis System -TSAS WOC 2006 distant pattern distant pattern Placido curvature may be ambiguous. image is small and erect image is small and inverted convex surface C F C concave surface F R > 0 R < 0 Placido images Turner 1999

19 Quiz #10 Which patient may have A/ warpage B/ keratoconus C/ astigmatism Curvature topography is Reference Axis dependant This increases the false Positives for KC M Belin

20 Curvature limitations pseudokeratoconus curvature abnormalities occur when the line of sight, apex and VK normal do not line up keratoconus maps have been created in normal aspherical surfaces with angular decentrations as small as 5 degrees Quiz#11 Centering topography maps Proper centration is not an issue anymore with advanced reconstruction algorithms wrong Advanced Topography systems can be utilized to center the ablation zone during Excimer laser surgery correct Wavefront hyperopic corrections are very sensitive in proper centration on the correct axis so angle kappa considerations are extremely important. wrong

21 Placido technology: discussion 2 >>3 dimensions Curvature >>> Height Realize that Placido disk topography represents twenty-first-century technology applied to eighteenthcentury science, in that it digitizes a Placido disk measurement, similar to the idea of installing GPS positioning on a horse and buggy WCC 2005 Michael W Belin

22 Triangulation maps complex surfaces. The Orbscan Unit Paradigm Topographic map of a Lincoln head penny. Elevation scale is 5 microns per step. Spatial resolution is about 0.25 mm. Turner exciting idea curiosity in university practices toy for graduate students B&L acquisition, Orbscan II, a very successful story Do not perform Refractive Surgery without an Orbscan X Eykarpides

23 Scanning slits measure several surfaces. Triangulation locates points in space calibrated slit-beam surface complex object triangulated space point diffusely reflected camera ray Video Camera Turner 1999

24 Curvature, Height and Power, Pachymetry Use of Corneal Topography in Refractive Surgery ORBSHOT ORBSCAN ORBSCAN II placido reflective technology slit scan technology measures multiple surfaces Integrates both technologies slit scan + reflective Determine Fruste Keratoconus Determine OZ centration Measurement of the functional OZ Estimation of the pupillary size WW size AC depth Pachymetrical maps

25 Fundamentals of Measurement Beam and Camera Calibration Specular vs Diffuse Reflection Edge Detection Raytrace Triangulation Eye Tracking Surface Fitting Backscatter Measurement 0,7 sec/ pass 240 data points per slit Overlapping central 5 mm Increases accuracy & Resolution Eye Tracker Turner 1999 March 1997 Orbscan I Concept 55

26 Eye Tracking Resolution 4µ centrally 8µ periphery 40 slit images are acquired in two 0.7 second periods. During acquisition, involuntary saccades typically move the eye about 50 microns. Eye movement is measured from anterior reflections of the stationary slit-beam and other light sources. Eye tracking data permits saccadic movements to be subtracted from the final topographic surface. March 1997 Orbscan I Concept 58

27 Maps, 3-D Views, and Section Profiles Reading Corneal Elevation Maps John A. Vukich, MD Surface topography can be viewed in several different ways: as conventional color contour maps, as 3-D renderings, and a section profiles. 8

28 Global Perspective Global Perspective Relevant features at the local level can be lost due to scale. John A. Vukich, MD 12,000 mi Kilometers 12,000 microns The elevation topography of both globes is small in comparison to the entire surface The average lasik ablation is less than 100 microns. On the scale of the entire cornea this represents only.01%. John A. Vukich, MD

29 Close-Fitting Reference Surfaces Topographic maps of terrestrial landscapes are displayed in the form of constant-elevation contours, measured Data surface from the mean sea-level of the earth. (cornea) Elevation Topology: Central Hill Sharp center Flat periphery John A. Vukich, MD Fit-zone Corneal topography differs from terrestrial topography in that the reference surface Reference is not surface some fixed (sphere) mean sealevel, but is movable. For A best-fit the cornea, minimizes a reference the square surface difference (typically, (always a positive sphere) number) is constructed between the by two fitting surfaces, the but reference only within a specified region known as the fit-zone. surface as close as possible to the data surface. John A. Vukich, MD

30 Elevation Topology: Central Saddle John A. Vukich, MD Turner 1999 Color Scale: Relative Elevation Relative elevation measures height difference from a bestfitting reference sphere. anterior (+) reference Max Red high anterior to the reference surface (-) posterior Min Blue low posterior to the reference surface In all elevation maps, green is the reference surface or zero level. Red is high and positive. Blue is low and negative.

31 Turner 1999 Elevation (sphere) Elevation (sphere) Toric Patterns Central Saddle When toricity dominates, a central saddle will form. John A. Vukich, MD

32 Elevation (sphere) 10 mm fit-zone Elevation (sphere) 5 mm fit-zone Turner 1999 Elevation (sphere) Axial Curvature Astigmatism: Elevation vs. Axial Map Both maps contain exactly the same information. They look different, because altering the fit-zone changes the size and alignment of the reference sphere. On the axial map, the steep axis follows the sharpest On the relative curvature elevation (red line map, segments) the steep and axis aligns dips below the reference to the red-shifted surface and bow-tie. runs from sea to Elevation and axial curvature sea. maps look different, almost reversed mm radius 6.88 mm radius John A. Vukich, MD

33 Που γίνεται η φωτοκερατεκτοµήστον αστιγµατισµό? (-/+ κύλινδρος) Quiz # 13 Elevation (sphere) Elevation (sphere) Anterior Cornea Posterior Cornea Crossed Curvatures: Elevation Όχι! In this case posterior astigmatism augments anterior astigmatism. This case is atypical and occurrences are relatively rare. Turner 1999

34 Orbscan technology quiz 14 Orbscan II directly measures elevation data utilizing specular reflection technology wrong Orbscan II measurements are accurate because a passive eye tracker is utilized wrong The actual data-acquisition time per scan is 0,7 secs wrong Topographic Types of Astigmatism - New Concept Anterior Symmetric, Posterior Symmetric Anterior Symmetric, Posterior Asymmetric Anterior Asymmetric, Posterior Symmetric Anterior Asymmetric, Posterior Asymmetric Decentered Apex Symmetric Decentered Apex Asymmetric Daniel S. Durrie, MD

35 Example - ASPS Example - ASPA Daniel S. Durr Daniel S. Durrie, MD

36 Example - DAS Example - DAA Daniel S. Durrie, MD Daniel S. Durrie, M

37 Elevation, Anterior Elevation, Posterior Posterior Cone: Recommended Quad Elevation, Anterior Elevation, Posterior Posterior Cone: Recommended Quad Mean Curvature, Anterior Thickness, Cornea Mean Curvature, Anterior Thickness, Cornea Quiz #15 Do you think that this elevation topography set may hide a cone???

38 Elevation, Anterior Elevation, Posterio Toric Keratoconus: Recommended Quad Θα κάνατε laser σε αυτόν τον κερατοειδή? quiz #17 Mean Curvature, Anterior Thickness, Cornea Quiz 16 A/ This is not keratoconus B/ Elevation topography can t show the cone because of extreme toricity C/ once the astigmatism is filtered out The cone presents itself (mean c) 244µ

39 Study case 413µ 46 y.o presents 33 months postop lasik with «sudden» loss of vision in her left eye. 24 months ago she was sc 10/10(BSCVA +0,25 sph) 17 months ago she was sc 9/10 (BSCVA -0,50-050x110) Today she was 2/10- (BSCVA 9/10 with 3,25-0,75 x115)

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41 Quiz22 # 66 y.o with cat, wants a multifocal lens Is she eligible for an Alcon- Restor?? Old Rx: x180 New RX: -5, x 005 7/10- Elevation topography Quiz #23 Posterior cornea astigmatism may counter balance anterior astigmatism so the overall refractive astigmatism is less yes What was previously known as lenticular astigmatism usually with the orbscan is proven to be posterior corneal yes Misaligned elevation topographies are generally considered safer in respect to postoperative ectasia No

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43 Scheimpflug Tomography Quiz 24 A substantial advantage of the rotating slit (compared to the Scanning slit) is the acquisition of a common central reference point in each scan Curvature measuring sensitivity in placido system is 20x compared to Scheimplug yes Motion artifact always exist with Scheimpflug (less in a dual system) but much less in placido yes Scheimpfug increased classification agreement between normal and abnormal among experts NO yes

44 Retinal image quality Quiz 25 Pupil size is a major player and is defraction limited WFE is also strongly pupil diameter dependant Scatter is the main reason why cataracts degrade visual performance The optical quality of the cornea can also be expressed in Zernicke terms (Corneal wavefront)

45 Quiz 26 The wavefront error (WFE) is the error between the actual wavefront (red) and the ideal wavefront (yellow) as a function of location within the pupil. What is the shape of the wavefront with a Z 1 1, Z 2 0 Zernicke error? The shape of the aberrated WF is a fundamental description of the optical quality of the eye. RAA