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In the name of Godthe most kind and merciful

Basic of orbscan

H.Movahedan MD

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What is Topography?

Topographic Technologies• Placido disk –based topography

• AstraMax : ( three-dimensional topography )

• Elevation-based topography:– Slit-scanning topography (orbscan)–– Scheimpflug imaging (PentacamScheimpflug imaging (Pentacam--GalileiGalilei--Precisio)Precisio)

• Artemis : (ultrasound digital topography)

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Elevation Based Topography

• Orbscan:– Placido disc & slit scanning

• Pentacam:– Scheimpflug imaging

• Galilei :– Dual scheimpflug imaging & placido disc

• Precisio: – Scheimpflug imaging

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Schematic picture of the Scheimpflug PrinciplePicture / Focus Plane

Objective Plane

Film PlaneObjective Plane

Film Plane

Picture / Focus Plane

Point of Intersection

Schematic picture of a Camera

Problem of a normal Camera:

limited depth of focus

Advantage of the Scheimpflug Camera:

•higher depth of focus,

•sharp picture, but distorted

The Scheimpflug law says: To get a higher depth of focus, move the three planes, provided that the picture plane, the objective plane and the film plane has to cut each other in one line or one point of intersection.

Scheimpflug Principle

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October 1997 Orbscan II Concept 9

The ORBSCAN II Concept

• Slit-scan triangulation and perspective reflection are complementary technologies: where one is weak the other is strong.

• ORBSCAN II exploits a synergistic combination of the two technologies.

• The result is unequaled accuracy in the elevation and curvature of clinically-relevant, complex surfaces.

October 1997 Orbscan II Concept 10

Hybrid Technology of ORBSCAN II

1. Measure surface elevation directly by slit-scan triangulation.

2. Measure surface normal directions directly using perspective reflection, supplemented with triangulated elevation.

3. Unify triangulated and reflective data to obtain accurate surfaces in elevation, slope, and curvature.

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October 1997 Orbscan II Concept 11

Slit-Scan Triangulation locates surface points individually.

Triangulate an edge point in camera object space (x, y, z)by mathematically intersecting the diffuse reflectedcamera edge raywith the calibrated slit-beam surface.

complex object

VideoCamera

October 1997 Orbscan II Concept 12

Deriving Elevation

Reflection• Requires integration.• Accuracy degrades

peripherally.• Can not traverse

discontinuities.• Placido skew ray

error is significant in clinically relevant abnormalities (e.g., keratoconus).

Triangulation• Primary

measurement.• Uniformly accurate

over the measured surface.

• Measures complex surfaces.

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Reflective and Slit-scan Technologies

• One image, one surface.

• Angle-dependent specular reflection.

• Measures slope (as a function of distance).

• Multiple images, multiple surfaces.

• Omni-direction diffuse backscatter.

• Triangulates elevation.

Two prevailing technologies have been used for corneal topography. Both have advantages and disadvantages. ORBSHOT employs Placido reflective technology. ORBSCANemploys slit-scan technology. ORBSCAN II employs a hybrid combination encompassing the best of both worlds.The overwhelming advantage of slit-scan systems is that they measure multiple ocular surfaces.

Integrates multiple disparate technologies: slit-scan, reflective, ultrasound.

How ORBSCAN is Multidimensional

Measures multiple ocular surfaces: anterior cornea, posterior cornea, anterior iris, anterior lens.

Displays multiple and complete mathematical surfaces: curvature, power, elevation, thickness.

Multidimensionality makes ORBSCAN powerful. The wide variety of maps can make ORBSCAN bewildering.The first purpose of this basic map reading course is to introduce, both technically and clinically, the most powerful ORBSCAN maps and measures.

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Color Problem• Contours are evenly spaced (the normal band being

exceptional).• Unfortunately, adjacent colors are NOT perceived as

uniformly equidistant.• An interpretation should never be based on color.

– Instead perceive the topography behind the colors, contours, and reference objects.

• The third purpose of this course is to warn you about numerous interpretation pitfalls:– contour and color problems– axial artifacts– elevation distortion and relativity– power and curvature confusion

• high power• short focal length

Red

BlueMin

Max

• low power• long focal length

(+)

(+ +)

A high power lens has a short focal length. Red is high power, and blue is low power.

Optical Power

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Elevation can be positive or negative. Red is high and positive (anterior to the reference surface).

• high• anterior to the

reference surface

• low• posterior to the

reference surface

Red

BlueMin

Max

reference level

(+)

(-)

anterior

posterior

Elevation (from a reference surface)

Corneal thickness equals posterior corneal depth. Red is thinner (the posterior cornea is higher).

• thin

Red

BlueMax

Min

• thick

(+)

(+ +)

Thickness

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Global Perspective

12,000 mi12,000 microns

The elevation topography of both globes is small in comparison to the entire surface

12,000 miles

Topographical Elevation is Relative

5.6 mi 5.6 miles

12,000 miles

On a local scale these features are significant

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Although they appear large up close, terrestrial mountains are very small (1000 times smaller) when compared to the size of the earth.Similarly, local clinical features (microns high) are a thousand times smaller than the cornea (millimeters in radius).This disparity in scale makes it IMPOSSIBLE to simultaneously map both the global curvature and clinically significant local elevation features of the cornea.To see clinically relevant features, corneal global curvature must first be removed. This is done by measuring elevation relative to some close-fitting reference surface, a process that inevitably introduces distortion.

Removing Global Curvature

Local clinical features (microns high) are a thousand times smaller than the cornea (millimeters in radius).This disparity in scale makes it IMPOSSIBLE to simultaneously map both the global curvature and clinically significant local elevation features of the cornea.To see clinically relevant features, corneal global curvature must first be removed. This is done by measuring elevation relative to some close-fitting reference surface, a process that inevitably introduces distortion.

Removing Global Curvature

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Close-Fitting Reference Surfaces

Topographic maps of terrestrial landscapes are displayed in the form of constant-elevation contours, measured from the “mean sea-level” of the earth.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)

Default settings:• shape = sphere

Elevation Relativity

Click for more on reference surface shapesClick for more on reference sphere alignments

Changing the shape, size, or alignment of the reference surface changes the relative elevation, just like changing the mean sea level would change the heights of mountains.The default reference surface is the best-fit sphere. Other reference surface shapes are available for special applications.The sphere is especially useful, because it is only reference surface with no unique symmetry axis (i.e., every diameter is a symmetry axis).Two parameters define the best-fit sphere. Radius specifies size. Center location specifies alignment.The default sphere alignment is floating, which means its center location is unconstrained. Other alignments may force the reference sphere to lie on the map axis.

• alignment = floating

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Myopic Pre-Post Elevation Maps

Elevation (sphere) Elevation (sphere)

These are pre-op (left) and post-op (right) elevation maps of a myopic with-the-rule astigmatic eye corrected with Lasik.The post-operative central “sea” is not a concavity but a central flattening.The ring of relatively highest terrain is not absolutely higher (more anterior) than the “sea” bottom near the map center.

Different Fit-Zone Elevation Maps

Elevation (sphere)10 mm fit-zone

Elevation (sphere)5 mm fit-zone

These elevation maps are the same exam of a normal eye having with-the-rule astigmatism. They differ only by the size of the fit-zone used to construct the reference sphere.Both maps contain exactly the same information. They look different, because altering the fit-zone changes the size and alignment of the reference sphere.For a normal cornea, a smaller fit-zone results in a smaller reference sphere that better fits the central cornea.

7.16 mm radius 6.88 mm radius

A small fit-zone bring outs small central anomalies, like central islands.A large fit-zone emphasizes global shape. By default, the largest 10 mm diameter fit-zone is used.

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Spherical reference surface

Elevation DistortionData surface profile

As an example of distortion, consider the corneal surface (i.e., data surface) following refractive surgery for myopic correction. It is centrally flattened by the surgery.To see surface features (which are highly exaggerated in this drawing), elevation must be measured with respect to some reference surface.When relative elevation is mapped, the reference surface is effectively flattened. This distorts the relative elevation profile.This relative elevation peak is NOT the highest point on the cornea.This apparent central "concavity" does NOT exist.

Relative elevation profile

Elevation Topology: Central Hill

The normal cornea is prolate, meaning that meridional curvature decreases from center to periphery.Prolateness of the normal cornea causes it to rise centrally above the reference sphere. The result is a central hill.

Sharp center

Flat periphery

Immediately surrounding the central hill is an annular seawhere the cornea dips below the reference surface.In the far periphery, the prolate cornea again rises above the reference surface, producing peripheral highlands.

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Elevation Topology: Central Saddle

The regular astigmatic cornea is toric, meaning that meridional curvature has maxima and minima in different directions, 90 degrees apart.The sharp (or steep) profile falls below the reference surface.The flat profile rises above the reference surface.The resulting elevation topology is a central saddle.A saddle is shaped like a riding saddle, rising peripherally in one direction, and falling peripherally in a mutually perpendicular direction.

Prolateness and Toricity Balance

Fit-zone sizeCorneal radius

T, ToricAmplitude

E, ProlateShape Factor

Central Hill Central Saddle

Central elevation topology is determined by the balance of corneal prolateness and toricity.

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Prolate Patterns

Central Hill

When prolateness dominates, a central hill will appear.

Toric Patterns

Central Saddle

When toricity dominates, a central saddle will appear.

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Central SaddleCentral Hill

Prolateness-Toricity Fulcrum

The position of the prolateness-toricity fulcrum is determined by the size of the fit-zone with respect to the mean corneal radius.

Large fit-zones favor prolate shapes, preferentially producing central hills.

Small fit-zones favor toric shapes, preferentially producing central saddles.

Elevation topology is only invariant when fit-zone size and reference sphere alignment are both fixed.

T, ToricAmplitude

Fit-zone sizeCorneal radius

E, ProlateShape Factor

Basic Map Reading 10:Optical Diagnosis

Point spread functionImage convolutionRaytrace analyses

Case report: Lasik ODCase report: Bilateral Lasik

29 points

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Point Spread Function

Raytrace analysis through one or both corneal surfaces can be used to calculate the Point Spread Function (PSF), which is the retinal image of a single point of light. This PSF is aberrated.

The main focus (pink and red) is spread horizontally and irregularly. A lower intensity blur pattern (green and blue) spreads far beyond the main focus. Extra foci appear at the vertices of the blur pattern.

Image Convolution

Original PictureSimulatedRetinal Image

The easiest way to interpret a PSF is to directly see how is affects the simulated retinal image.The original picture is disassembled into points, each point is spread out into the PSF pattern, and the patterns are reassembled to form the simulated image.

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Lasik OD: Raytrace OD

This is the aberrated total corneal PSF of a patient that underwent Lasik OD. The procedure did not correct the original myopia and resulted in severe subjective symptoms (blurred vision, glare, etc.).

The simulated retinal image lets us see what the patient sees.

Original PictureSimulatedRetinal Image

Summary

• Corneal Elevation Topography is viewed relative to a reference surface

• Standardization of the reference surface is necessary to allow meaningful comparisons

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Thank you for your attention