CA1296804C

CA1296804C - Ocular topology system

Info

Publication number: CA1296804C
Application number: CA000536320A
Authority: CA
Inventors: Vladimir Portnoy
Original assignee: Allergan Inc
Current assignee: Allergan Inc
Priority date: 1986-05-05
Filing date: 1987-05-04
Publication date: 1992-03-03
Anticipated expiration: 2009-03-03
Also published as: DE3714896A1; JPS62277937A; FR2598072A1

Abstract

ABSTRACT

Topological information of the cornea and anterior chamber of an eye is optically developed by projecting toward said eye a target image sharply focused in a movable flat plane. As the plane is moved through the eye, the reflection of the target is in focus first at the center and then at increasingly radially outward locations. The reflection is detected and clipped to delete all image portions which are out of focus. As the image plane is moved through the eye, the clipped reflection traces the topology of the eye. This information can be fed into a computer and used to compute appropriate topological displays. Such computations can readily be made insensitive to any eye movement during the measuring process.

Description

9123.021P D-1742 3~2~ 3C)4 OCULAR TOPOLOGY SYSTEM

Field of the Invention This invention relates to a method and apparatus for accurately measuring the topography of a cornea, and more particularly, to a method in which topographi-cal information is obtained by moving the image posi-tion of a target projected towards the cornea and detect-ing the in-focus portions of the image as its position is moved.
Background of the Invention .
In preparation for corneal surgery such as is commonly used for vision correction, it is necessary for the surgeon to have highly accurate information concerning the shape and thickness of the cornea through-out its surface. It has been conventional in the pastto measure the contour and thickness o~ the cornea by optically or ultrasonically examining the cornea at a number of distinct points on its surace. The problem with this approach is that since the cornea is often quite irregular in shape and thickness, unappreciated variations in the measured parameters may occur between the measuring locations, and it is consequently possi-ble that damage to the cornea may occur in surgery due to cuts of excessive or insufficient depth. Another problem arises from the fact that the cornea can move during the examination, and it is therefore difficult to correlate the various locations at which measure-ments are being taken in sequence.
It is therefore desirable to provide a method by which the contour and thickness of the cornea can be measùred on a substantially continuous basis and in a manner in which eye movement during the measuring process is of little or no consequence.

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Summary of the Invention Generally speaking, the invention solves the above-stated problems by provision of a method for optically determining the topology of an eye, comprising the steps of:
(a) projecting toward the eye a target image sharply focused in an image surface substantially perpendicular to the direction of projection;
(b~ moving the image surface through at least part of said eye;
(c) detecting the in-focus portions of the reflection of the target image by surfaces of the eye; and (d) producing a representation of the location of the in-focus portions of the reflected target image with respect to a reference point ~or a plurality of positions of the image surface.
Preferably, the invention comprises projecting onto the cornea a target image which is focused in an image plane (or, more generally, an image surface of known curvature) with a very shallow depth of field and'ltl1e reflection of the taryet image from the cornea is observed by an autocollimating video camera whose sensitivity is so adjusted as to register only those portions of the target image which are sharply in focus.
As the image plane of the inventive device is moved toward the cornea, a dot image will appear in the center of the screen when the ~mage plane first contacts the epithelium of the cornea. As the image plane is moved further toward the eye, a gradually outwardly mov-ing ring of dot images will appear on the screen. Even-tually, as the image plane reaches the endothelium of the cornea, a new dot image will appear on the screen, and a second set of outwardly moving dot images will appear on further movement of the image plane.
By correlating the position of the image plane with the distance of the dot images from the cent~r of the screen on a continuing basis, a mathematical ex-pression can be generated which exactly defines the ~ 3 -contours of both the epithelium and endothelium of the cornea, as well as the thickness of the cornea itself.
It should be noted that in this procedure, a movement of the cornea will produce a simultaneous displacement of all the dot images in the direction of the movement.
Thîs simultaneous displacement can readily be compensated for by well-known electronic software/hardware techniques.
In addition to measuring the contour and thick-ness of the cornea, the inventive method can be used to calculate a three-dimensional representation of the anterior chamber of the eye by continuing to move the focal plane of the target image until it reaches the iris.
The invention there~ore provides an optical method of continuously measuring the topography of a cornea simultaneously throughout its surface. Furthermore, the invention accomplishes the foregoing result by tracking the reflections of a target image as the image plane of the target image is moved through the cornea.
The invention will now be described further by way of example only and with reference to the accompanying drawings.
Brief Description of the Drawings Fig. 1 is a schematic representation of the device of this invention;
Fig. 2 illustrates one possible type of target which may be used in the preferred embodiment of the invention;
Fig. 3a is a schematic representation of the apparatus of this invention in a position where the focal plane first contacts the epithelium of the cornea;
Fig. 3b illustrates the dot image seen on the television screen when the focal pIane is in the position of Fig. 3a;
~ Fig. 4a illustrates the shift of the image as the focal plane moves toward the interior of the eye;

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- 3a -Fig. 4b illustrates the shi.ft in the circular dot imàges as the image plane is moved between the positions shown in Fig. 4a;
Fig. 5a illustrates the reflection from the endo-thelium when the center of the focal plane has reached the interior surface of the cornea;

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Fig~ 5b illustrates the dot image seen on the ; television screen when the focal plane is in the posi-tion of Fig. 5a; and Fig. 6 represents the calculated representation of the cornea and anterior chamber when a full set of measurements have been taken.
Description oE the Preferred Embodiment Fig. 1 shows an optical system lO including a focusing objective 12, a beam splitter 13 and a video camera 14 provided with an appropriate objective 15.
An image of a target 16 illuminated by an illuminator 17 is projected toward the eye l9 through collimating optics 21, beam splitter 13, and focusing objective 12.
The focusing objective 12 produces a sharply focused im-age of the target 16 in the image plane 18. The elements of the optical system lO are so chosen, in accordance with known optical principles, to give the image plane 18 a very shallow field depth. As a result, the image of target 16 is sharp in the image plane 18, but becomes substantlally blurred at even a very small distance in front of and behind the image plane 18.
Reflections of the target image from the eye 19 are conveyed through focusing objective 12 and beam splitter 13 to the objective 15 of the video camera 14.
Thus, the camera 14 sees the reflection of the target 16 in the eye 19.
In the preferred embodiment of the invention, the focusing objective 12 is movable in a horizontal direction in Fig. l, toward and away from eye 19, by a stepping motor 25. The position of motor 25 is digitally encoded by conventional means into a position signal 47, which ,~ ~ is transmitted to the computer 45 for a purpose described below. Alternatively, the focusing objective 12 may be ~! fixed, and the target 16 may be movable.
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In either event, in the position of image plane 18 shown in Fig. 1, the reflection of the target 16 by the eye 19 will be blurred. In accordance with the invention, the video output of camera 14 is electron-ically clipped by conventional clipping circuitry 23 sothat only those portions of the reflected target image which are sharply in focus (and which therefore have the highest intensity) are transmitted by the clipper 23.
Consequently, in the position of image plane 18 shown in Fig. 1, there is no video output from clipper 23.
When the actuation of the motor 25 moves the image plane 18 toward the eye 1~ in Fig. 1, the image plane 18 will eventually contact the eye 19, and at least a por-tion of the target 16 will be re~lected in sharp focus.
That reflection is seen by the camera 14, passed by the clipper 23, and applied as the image through a conven-tional image digitizer 27 as the image input to computer ~5 for contour mapping purposes.
As shown in Fig. 2, the target 16 may preferably consist of lines 2~ radiating in all directions from a central point 26; however, other target con~igurations may be used to accommodate various algorithms which may be used in interpreting the moving dot images hereinafter described.
Turning now to Figs. 3a and 3b, it will be seen that when the apparatus of this invention is placed in front of the curved surface of a cornea and is moved toward the interior of the eye, the image plane 18 will first contact the epithelium 26 of the cornea 28 at a central point 30. Due to the curvature of the epithel-` ium 26, all portions of the target image except theportion reflected from point 30 will be out of focus, and will therefore be clipped out by the clipper 23.
As a result the image seen by computer 45 and by the monitor 22 will look as depicted in Fig. 3b when the ~L2~6~

apparatus is in the position of Fig. 3a. The dot 32 seen on the monitor 22 in this position is a representa-tion of the center 26 of the target 16.
; As the image plane 18 is moved toward the interior of the eye, point 30 will go out of focus, and a ring of locations including points 34 and 36 will come into focus. At the output of clipper 23, this translates into a ring of dots representing portions of the lines 24 of the target. The position of the dots in the ring including points 34 and 36 is an indication of the posi-tion of the epithelium 26 when the image plane is in the rightmost position of Fig. 4a. If the cornea is not spherical (as in the case of astigmatIsm), the locus of the dots will be oval rather than circular.
As the image plane 18 moves to the leftmost posi-tion in Fi~. 4a, points 34 and 36 go out of focus and points 38, 40 come into focus. The dot image 43 now applied to computer 45 and produced on the monitor 22 is again a circle of dots, but now farther outward from the center of the screen than was previously the case, As the lmage plane 18 moves farther to the left, it will eventually hit the endothelium as shown in Fig.
5a. This second reflection will appear, as shown in Fig. 5b, as a second central dot 44 in the center of the still existing dot ring from points 46, 48. Continued movement of the image plane 18 to the left will then produce another ring of dots radiating out from the central dot 44 as the central dot 44 disappears.
The position of the dots in the digitized dot image scan by computer 45 can be analyzed within computer 45 by conventional techniques and can be corre-lated by the position input 47 with the position of the image plane 18 that produced them to provide the input to a conventional graphics generator 49 ~Fig. 1). ~now-lng the coordinates of the dots in the dot image 43 ~ . .. .

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and the corresponding position of the image plane 18, the computer 45 and graphics generator 49 can trans-late the image data at successive image plane posi-tions into a three-dimensional or sectional representa-~ 5 tion of the cornea ~8 in a format suitable for use by ; the surgeon~ The output of the computer 45 may, for example, be used to produce a representation such as that of Fig. 6 on a printer 51 for any given sectional plane corresponding to one of the lines 24 of the target 16. In such a representation, the solid lines 50, 52 representing the cornea and 54 representing the iris may be actual measurements, whereas the dotted lines 56, 58 and 60 may be extrapolated from the measured lines 50, 52 and 54.
Eye movement during the mapping of the cornea does not interfere with the accuracy of the measurements in the apparatus o this invention~ Whereas the progress-ive displacement o the image plane 18 causes the coord-inates of the image dots to increase radially outwardly from a central point, eve movement causes all the dot coordinates to move in the same direction. This dis-tinction is readily recognized b~ the computer 45, and allows it to disregard any dot shift caused by eye movement.

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Claims

1. An ocular topology device, comprising:
(a) means for projecting toward the eye a target image sharply focused in an image surface substantially perpendicular to the direction of projection;
(b) means for moving said image surface through at least part of said eye;
(c)means for detecting the in-focus portions of the reflection of said target image by surfaces of said eye; and (d) means for producing a representation of the location of said in-focus portions of said reflected target image with respect to a reference point for a plurality of positions of said image surface.

2. The device of claim 1 further comprising:
(e) means for monitoring the directions of movement of said in-focus target image portions as said image surface is moved; and (f) means for disregarding, for the purposes of producing said location representation, any movement of said image portions in which all of said image portions move in the same direction.

3. An ocular topology device, comprising:
(a) optic means for projecting toward an eye a target image focused in an image surface of shallow field depth substantially perpendicular to the direction of projection, said image surface being movable toward and away from said eye;
(b) position sensing means for producing a signal representative of the position of said image surface;
(c) camera means for detecting the reflections of said target image by reflecting surfaces of said eye as said image surface is moved through said eye;

(d) clipping means for deleting from said target image reflections those image portions which are not sharply in focus;
and (e) computer means connected to said position sensing means and said clipping means for storing the coordinates of the undeleted image portions with respect to a reference point in relation to successive positions of said image surface, and for producing therefrom information representative of the topology of said eye.

4. The device of claim 3, further comprising:
(f) movement selection means associated with said computer means for shifting said reference point to compensate for eye movement whenever all of said undeleted image portions move in the same direction.

5. The device of claim 3, in which said target image consists of lines radiating outwardly from a central point.

6. The device of claim 5 in which said central point is said reference point.

7. The device of claim 3, further comprising:
(g) display means associated with said computer means for displaying a representation of the topology of said eye computed by said computer means from said coordinates and image surface positions.

8. The device of claim 7, in which said representation is a tomographic representation.

9. The device of claim 3 in which said target image is two-dimensional in said image surface.

10. An ocular topology device, comprising:
(a) optic means for projecting toward an eye a target image focused in a flat plane substantially perpendicular to the direction of projection, said plane being movable toward and away from said eye;
(b) position sensing means fox producing a signal representative of the position of said plane;
(c) camera means for detecting the reflections of said target image by reflecting surfaces of said eye as said plane is moved through said eye;
(d) clipping means for modifying those image portions which are not sharply in focus; and (e) display means for displaying said target image reflections.

11. The device of claim 10 in which said modification is a deletion, and said display means display only the undeleted portions of said target image reflections.

12. The device of claim 10 in which said target image is two-dimensional in said plane.

13. A method for optically determining the topology of an eye, comprising the steps of:
(a) projecting toward the eye a target image sharply focused in an image surface substantially perpendicular to the direction of projection;
(b) moving said image surface through at least part of said eye;
(c) detecting the in-focus portions of the reflection of said target image by surfaces of said eye; and (d) producing a representation of the location of said in-focus portions of said reflected target image with respect to a reference point for a plurality of positions of said image surface.

14. The method of claim 13, further comprising the step of:
(e) using said representation to produce topological data for said eye.

15. The method of claim 14, further comprising the steps of:
(f) monitoring the directions of movement of said in-focus target image portions as said image surface is moved; and (g) disregarding, for the purposes of producing said location representation, any movement of said image portions in which all of said image portions move in the same direction.

16. The method of claim 14, in which said topological data is a tomographic representation of said eye.

17. The method of claim 13, in which said target image is two-dimensional in said image surface.