Topography / Tomography 2010 update
www.athensvision.eu Anast Charonis
“Anterior Segment Topology-2010 ”
1.Placido based technology with arc step reconstruction τοπογραφία ανάκλασης
2. Scanning Slit / transverse and rotationalτοπογραφία προβολής
3. Wavefront analyzers/ Corneal Wavefrontαµπερροµετρική τοπογραφία
• 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
GARBAGE IN
GARBAGE OUT
ALWAYS CHECK THE RAW DATA FIRST!!
Quiz #1How do you diagnose irregular astigmatism without a topographer?
• Direct ophthalmoscope
• Retinoscope
• Lohne keratoscope
• Keratometer
The Scheiner Principle -1619
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
Scheiner > Helmholtz > Javal-Schotz
J Schwiegerling
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
Curvature measures bending.
A small radius circle has a large curvature.
• Small radius• Large
curvature• Fast bend
• Large radius• Small
curvature• Slow bend
R
1=κCurvature Radius of curvature
Turner 1999
Keratometry K-quiz#2 true or false
1. K Determines curvature by measuring the size of a reflected ‘mire’
2. Doubling of image avoids problems with eye movements
3. In the B&L keratometer the mire has constant size, the image doubling is variable
4. In the Javal-Schiotz ophthalmometer the image doubling is constant and the mire separation is variable.
5. The radius scale used in keratometers is not exact
yes
yes
yes
yes
NO
Principle of keratometryQuiz #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
Reflecting
rings
Viewing
Aperture
with lens
Measure Surface Slope directly using Specular Reflection
“Normal” eye
equally spaced
symmetrical rings
Corneal astigmatism
oval-shaped rings
Aspherical cornea
non-equidistant rings
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
Although there is no universally accepted color scale in corneal topography, spectral directions are standardized.
Color Scale: Surface Curvature
• sharp• fast bend• short radius
• flat• slow bend• long radius
(+ +)
(+)
Red
BlueMin
Max
In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).
Turner 1999
Astigmatism: Axial Curvature Map
The bow-tie is actually an axial artifact
Axial CurvatureTurner 1999
K-AXIS
Astigmatism: Meridional Curvature Map
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.
Meridional CurvatureTurner 1999
Keratoconus: Axis-Based MapsAxial Curvature Meridional Curvature
Turner 1999
Quiz #4Axial vs Tangential Curvature
K axis
A
B
C
Point A: tangential power>axial: increased steepness
PointB: tangential=axial evtl spheric cornea
Point C= tangential <axial evidence of increased prolateness
PLACIDO >>> Perspective ReflectionMeridional/ Axial > Principal- Mean
Turner 1999
Keratoconus: Anterior Quad Map
Elevation (sphere) Mean Curvature
Axial Curvature Meridional Curvature
Turner 1999
pre-cornealwavefront
post-lenticular
wavefront
pre-retinal
wavefront
Curvature Vs Optics
Surface Geometry Curvature or Slope
Material Properties Refractive index
Optical Incidence Light ray incoming angle
Curvature, slope, elevation, thickness, and depth are all geometric properties. They quantify the
size, shape, and location of the refracting surfaces. No optics is involved.
Optical properties quantify net changes in the optical wavefronts traversing one or more refracting
surfaces.
curvature
depth
thickness
Turner 1999
Raytrace Power MapAxial Curvature [K] Refractive(optical) Power
This eye has with-the-rule astigmatism. Axial curvature, although expressed in keratometric
diopters, is NOT an optical property.
Raytrace power maps areoptical. Notice that on every meridian, raytrace power increases peripherally. This is a
manifestation of the spherical aberration caused by insufficient corneal prolateness.
Turner 1999
•oblate shape factor S, prolate shape factor E, and asphericity Q. The relations between these shape factors
are
•S = 1 - E = 1 + Q
•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
• The curvature tangential 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 is trichromatic
• The axial optical power map of a perfect sphere of 7,5mm ROC in 0,5step scale is hexachromatic
no
no
yes
yes
quiz#6true or false
• The virgin normal cornea shape can be described as a prolate ellipsoid
• The median value of the Q factor of the virgin cornea is ~-0,2.
• The spherical aberration of the normal young eye is very close to zero.
• 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
yes
yes
yes
Quiz 7
Keratoconic patients benefit the most from aspheric lenses when they have cataract surgery True or False ??
Aligned Curvatures: Axial Curvature
Axial Curvature [D]Anterior Cornea
Axial Curvature [D]Posterior Cornea
PosteriorHorizontal
VerticalF = 49.7S = 52.1
AnteriorF = -5.8
S = - 6.8
CornealF+F = 43.9 S+S = 45.3
∆ = 2.4 ∆ = 1.0 ∆ = 1.4
Turner 1999
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 -= 1.3375 is an approximation
Ant power (1,376-1.00) 1000/r =376/rPosterior power (1,336-1,376) 1000/r = -40/r
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
Tear Stability Analysis System-TSAS
WOC 2006
Placido curvature may be ambiguous.
CF
C
F
dist
ant p
atte
rndi
stan
t pat
tern
convex surface
concave surface
R > 0
image is small
and erect
image is small
and inverted
R < 0
Placidoimages
Turner 1999
Quiz #10
Which patient may have
A/ warpageB/ keratoconus
C/ astigmatism
Curvature topography is
Reference Axis dependant
This increases the false
Positives for KC
M Belin
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
• Advanced Topography systems can be utilized to center the ablation zone during Excimer laser surgery
• Wavefront hyperopic corrections are very sensitive in proper centration on the correct axis so ‘angle kappa” considerations are extremely important.
wrong
correct
wrong
Placido technology: discussion
� 2 >>3 dimensions� Curvature >>> Height
“Realize that Placido disk topography represents twenty-first-century technology applied to eighteenth-
century 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
Triangulation maps complex surfaces.
Topographic map of a Lincoln head
penny.
Elevation scale is 5 microns
per step.
Spatial resolution is about 0.25 mm.
Turner 1999
The Orbscan Unit Paradigm
• 1993 exciting idea
• 1996 curiosity in university practices
• 1997-00 toy for graduate students• 2001 B&L acquisition, Orbscan II,
a very successful story
• 2004 Do not perform Refractive Surgery without an Orbscan
X Eykarpides
Scanning slits measure several surfaces.
Turner 1999
Triangulation locates points in space
triangulatedspace pointcomplex object
Video
Camera
calibrated slit-beamsurface diffusely
reflectedcamera ray
Curvature, Height and Power, Pachymetry
• ORBSHOT placido reflective technology
• ORBSCAN slit scan technology measures multiple surfaces
• ORBSCAN II Integrates both technologiesslit scan + reflective
Use of “Corneal Topography”in Refractive Surgery
• Determine Fruste Keratoconus
• Determine OZ centration• Measurement of the functional OZ
• Estimation of the pupillary size
• WW size – AC depth
• Pachymetrical maps
March 1997 Orbscan I Concept 55
Fundamentals of Measurement
• Beam and Camera Calibration• Specular vs Diffuse Reflection• Edge Detection • Raytrace Triangulation• Eye Tracking• Surface Fitting• Backscatter Measurement
Turner 1999
0,7 sec/ pass240 data points per slit
Overlapping central 5 mmIncreases accuracy &
ResolutionEye Tracker
March 1997 Orbscan I Concept 58
Eye Tracking• 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.
Resolution
4µ centrally
8µ periphery
8
Maps, 3-D Views, and Section Profiles
Surface topography can be viewed in several different ways: as conventional color contour maps, as 3-D renderings, and a section profiles.
Reading Corneal Elevation Maps
John A. Vukich, MD
Global Perspective
Relevant features at the local level can be lost due to scale
.John A. Vukich, MD
Global Perspective
12,000 mi 12,000 microns
The elevation topography of both globes is small in comparison to the entire surface
12,000 Kilometers
The average lasik ablation is less than 100 microns. On the scale of the entire cornea this represents only .01%.
John A. Vukich, MD
Close-Fitting Reference Surfaces
Corneal topography differs from terrestrial topography in that the reference surface is not some fixed “mean sea-
level”, but is movable.For the cornea, a reference surface (typically, a sphere) is constructed by fitting the reference
surface as close as possible to the data surface.
A best-fit minimizes the square difference (always a positive number) between the two surfaces, but only within
a specified region known as the fit-zone.
Fit-zoneReference surface (sphere)
Data surface(cornea)
Topographic maps of terrestrial landscapes are displayed in the form of constant-elevation contours, measured from the
“mean sea-level” of the earth.
John A. Vukich, MD
Elevation Topology: Central Hill
Sharp center
Flat periphery
John A. Vukich, MD
Elevation Topology: Central SaddleJohn A. Vukich, MD
Relative elevation measures height difference from a best-fitting reference sphere.
Color Scale: Relative Elevation
In all elevation maps, green is the reference surface or zero level. Red is high and positive. Blue is low and negative.
• high• anterior to the
reference surface
• low• posterior to the
reference surface
reference
(+)
(-)
anterior
posterior
Red
BlueMin
Max
Turner 1999
Elevation (sphere) Elevation (sphere)Turner 1999
Toric Patterns
Central Saddle
When toricity dominates, a central saddle will form.
John A. Vukich, MD
Elevation (sphere)10 mm fit-zone
Elevation (sphere)5 mm fit-zone
Both maps contain exactly the same information. They look different, because altering the fit-zone changes the size and alignment of the
reference sphere.
7.16 mm radius 6.88 mm radiusJohn A. Vukich, MD
Astigmatism: Elevation vs. Axial Map
Axial CurvatureElevation (sphere)
Elevation and axial curvature maps look different, almost reversed.
On the axial map, the steep axis follows the sharpest curvature (red line segments) and aligns
to the red-shifted bow-tie.On the relative elevation map, the steep axis dips
below the reference surface and runs from “sea” to “sea”.
Turner 1999
Που γίνεται η φωτοκερατεκτοµή στον αστιγµατισµό? (-/+ κύλινδρος) Quiz # 13
Όχι!
Crossed Curvatures: ElevationElevation (sphere)Anterior Cornea
Elevation (sphere)Posterior Cornea
In this case posterior astigmatism augments anterior astigmatism. This case is atypical and
occurrences are relatively rare.
Turner 1999
Orbscan technology quiz 14
• Orbscan II directly measures elevation data utilizing specular reflection technology
• Orbscan II measurements are accurate because a passive eye tracker is utilized
• The actual data-acquisition time per scan is 0,7 secs
wrong
wrong
wrong
• Anterior Symmetric, Posterior Symmetric• Anterior Symmetric, Posterior Asymmetric• Anterior Asymmetric, Posterior Symmetric• Anterior Asymmetric, Posterior Asymmetric• Decentered Apex Symmetric• Decentered Apex Asymmetric
Topographic Types of Astigmatism - New Concept
Daniel S. Durrie, MD
Example - ASPS
Daniel S. Durrie, MD
Example - ASPA
Daniel S. Durrie, MD
Example - DAS
Daniel S. Durrie, MD
Example - DAA
Daniel S. Durrie, MD
Posterior Cone: Recommended Quad
Elevation, Anterior Elevation, Posterior
Mean Curvature, Anterior Thickness, Cornea
Quiz #15
Do you think that this elevation topography set may hide a cone???
Posterior Cone: Recommended Quad
Elevation, Anterior Elevation, Posterior
Mean Curvature, Anterior Thickness, Cornea
Toric Keratoconus: Recommended Quad
Elevation, Anterior Elevation, Posterior
Mean Curvature, Anterior Thickness, Cornea
Quiz 16A/ 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)
Θα κάνατε laser σε αυτόν τον κερατοειδή? quiz #17
244µ
413µ
Study case
• 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)
Quiz22 # 66 y.o with cat, wants a multifocal lens
Is she eligible for an Alcon- Restor??
Old Rx: -3.00-2.25x180
New RX: -5,25-3.00x 005 7/10-
Elevation topography Quiz #23
• Posterior cornea astigmatism may counter balance anterior astigmatism so the overall refractive astigmatism is less
• What was previously known as “lenticularastigmatism” usually with the orbscan is proven to be posterior corneal
• Misaligned elevation topographies are generally considered safer in respect to postoperative ectasia
yes
yes
No
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 placidosystem is 20x compared to Scheimplug
• Motion artifact always exist with Scheimpflug(less in a dual system) but much less in placido
• Scheimpfug increased classification agreement between normal and abnormal among “experts”
yes
yes
yes
NO
Retinal image quality Quiz 25
• Pupil size is a major player and is defractionlimited
• 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)
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.
RAA
The shape of the aberrated WF is a fundamental description of the optical
quality of the eye.
Quiz 26
• What is the shape of the wavefront with a Z1
1, Z 2
0
Zernicke error ?