2. TOPOGRAPHY VS TOMOGRAPHY
• the term topography refers to a detailed representation or
description of the surface characteristics of a structure
• tomography is the process of generating a two
dimensional cross sectional image of a slice through a
three dimensional structure
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15. Disadvantages
• absence of information about posterior corneal surface
• limited corneal surface coverage obviating data from the
paracentral and peripheral cornea
• small degrees of abnormalities are not identifiable
• curvature is calculated not measured
• Because the image is generated off the tear film, irregularities in
tear film can significantly impact the quality and fidelity of a
Placido disk topography.
• lack of patient fixation may affect the quality of the topographic
image.
16. Slit scanning system (orbscan)
• Scanning Slit is one of the elevation based methods for assessment of
corneal curvature and power.
• Multiple complimentary slits are used to perform an assessment of the
corneal surface.
• In the Orbscan , 40 slits (20 each from nasal and temporal side) are
projected on the cornea to assess 240 points on each slit.
• The triangulation between the reference slit beam surface and the reflected
beam captured by the camera can be used to analyse the anterior and
posterior corneal curvature and corneal thickness
20. Scheimpflug principle
• The non planar shape of cornea can potentially lead to spurious results and
therefore the use of schiempflug principle in corneal imaging is a welcome new
change.
• Theodre Scheimpflug , an Austrian army man worked extensively on a method
for correcting arial skew distortion in perspective photographs.Even though the
technique was described before him, his development of the principle led his
name to be associated with the principle.
• In an ideal scenario , the lens plane and the image plane are parallel. Therefore
a linear object will form a plane of focus parallel to the lens plane and thus can
be focused totally on the image plane (Figure 3a).
• Consider a situation , when the object is not parallel to the prospective image
plane. It will not be possible to focus all of the image on a plane parallel to
image plane (Figure 3b). Thus this may lead to image distortion.
• According to the Scheimpflug principle, when a planar subject is not parallel to
the image plane , an oblique tangent can be drawn from the image, object, and
lens planes. This point of intersection is called the Scheimpflug Intersection
(Figure 3c). A careful manipulation of the image plane and lens plane can create
a focused, sharp image on the non parallel object.
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24. Description of the Unit
• The OCULUS Pentacam is a rotating Scheimpflug camera . The rotational measuring procedure generates
Scheimpflug images in three dimensions. It takes a maximum of 2 seconds to generate a complete image of the
anterior eye segment. Any eye movement is detected by a second camera and corrected for in the process to
some extent.
• 25000 true elevation points.
• Topography and pachymetry of entire anterior and posterior surfaces of cornea from limbus to limbus are
calculated and depicted. The analysis of the anterior eye segment includes a calculation of the chamber angle,
chamber volume and chamber depth .
• In a moveable virtual eye, images of the anterior and posterior surface of the cornea, the iris and the anterior
and posterior surfaces of the lens are generated. The densitometry of the lens is automatically quantified.
25. • The Scheimpflug images taken during the examination are digitalized in the main unit and all image data are
transferred to PC. When examination is finished, the PC calculates a 3D virtual model of the anterior eye
segment, from which all additional information is derived.
• The Scheimpflug law states: To get a higher depth of focus, move the three planes, provided that the picture
plane, the objective plane and the film plane have to cut each others in one line or one point of intersection.
Advantages of the Scheimpflug camera include higher depth of focus and sharp picture.
26. Pentacam vs Galilei vs Sirius
• The Pentacam combines a rotating Scheimpflug camera with a static camera to
acquire multiple photographs of the anterior eye segment.
• The Galilei Dual Scheimpflug Analyzer integrates a Placido disc and a dual rotating
Scheimpflug system for corneal topography and three dimensional analysis of the
anterior eye segment.
• The Sirius Scheimpflug Analyzer integrates a Placido disc and a mono rotating
Scheimpflug system for corneal topography and three-dimensional analysis of the
anterior eye segment.
• The aim of integration of Placido disc into Galilei and Sirius is to enhance the
analysis of the anterior corneal surface
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64. The computer displays the thickness map in two patterns:
1. Five values: A central value representing the central
thickness, and four values around at the 5 mm central
circle .
2. Distributed values all over the cornea : The distributed
pattern is more important and valuable.
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66. The pachymetry map has three main landmarks :
cornea apex (orange arrow), Thinnest location TL
(red arrow), and the two opposing points on the
vertical meridian at the central 5-mm circle (white
dotted arrows).
The normal difference between the
superior (S) and inferior (I) points is ≤ 30 μm.
• Shape: The normal pachymetry map has a
concentric shape .
67. Abnormal shapes include
• a. Horizontal displacement of the TL .
• b. Dome shape. The TL is vertically displaced
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• c. Bell shape. There is a thin band in the
inferior
part of the cornea . It is a hallmark for Pellucid
Marginal Degeneration (PMD).
• d. Keratoglobus. A generalized thinning
reaching
the limbus .
68. Thickness Profiles
• These profiles are only displayed in the Pentacam.
There are two pachymetry profiles:
• Corneal Thickness Spatial Profile (CTSP) and
• Percentage Thickness Increase (PTI).
• The former describes the average progression of thickness starting from the
TL to corneal periphery in relation to zones concentric with the TL. The latter
describes the percentage of progression of the same
69. The normal profile is a curved line plotted in red, following (but not necessarily
within) the course of the normative black dotted curves, with an average of 0.8–
1.1 .
When there is a fast transition of thickness between the TL and corneal
periphery, the average will be high, and vice versa e.g. in an oedematous
cornea, the average will be low and the curve will be flat.
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71. Abnormal profiles include:
• a. Quick Slope . The red curve leaves its course
before
6-mm zone. It is encountered in FFKC & ectatic
disorders. The average is usually high > 1.1 .
72. • d. Inverted . The red curve follows an
upward course. It
is encountered in some cases of PMD. The
average is
very low < 0.8 and may take a minus value.
73. • c. Flat shape . The red curve takes a
straight course. It
is encountered in diseased thickened
(oedematous)
corneas such as Fuch’s dystrophy & cornea
Guttata.
The average is low < 0.8 .
74. • b. S-shape . The red curve has
a shape of an “S”. It is
encountered in FFKC and ectatic
disorders. The
average is usually high > 1.1 .
