Topography and Tomography of cornea in detail

Easy to read learn and understand the difference of topography and Tomography

1. Corneal Topography and Tomography Presenter: Dr. Deepshikha Moderator: Dr. Jyoti Gadhwal Maam

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

3. 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.

4. 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

5. Orbscan II Placido disc added in orbscan 1

6. 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.

7. 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.

8. 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.

9. 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

10. 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.

11. 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 .

12. Abnormal shapes include • a. Horizontal displacement of the TL . • b. Dome shape. The TL is vertically displaced . • 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 .

13. 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

14. 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.

15. 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 .

16. • 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.

17. • 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 .

18. • 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 .

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