Ophthalmologic instrument for determining the topography of the cornea

BACKGROUND OF THE INVENTION

1.

Field of the Invention

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This invention relates in general to certain new and useful improvements in ophthalmic pachymeters for aiding in the determination of the thickness and relative optical density of the cornea of the eye on a real-time basis.

2.

Brief Description of the Prior Art

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Planning for anterior segment surgery is a topic which has received increasing attention in recent years, as, for example, in a paper by Lehrman, et al., "Measurement of Anterior Chamber Diameter" and "Biometry of Anterior Segment by Scheimpflug Slit Lamp Photography", reported in Investigative Ophthalmology and Visual Science, Volume 32, No. 3, March 1991, pages 529-532.

The slit lamp is an instrument employed by many optometrists and ophthalmologists for examination of the anterior portion of the eye. Many different versions of the instrument have been produced over the last century, but all of the slit lamps have three major elements in common and which include a projector for providing a collimated image of an optical slit focused on the eye, a bio-microscope or camera for viewing the image and a mechanical support system. In this slit lamp system, the bio-microscope or camera is designed to view the image formed by the projector and is confocal with the projector. The mechanical support system must be elaborate to at least support the subject and the projector and viewing system. Furthermore, the elements must be positioned relative to one another for appropriate examination of the eye.

Pachymetry attachments are available for the slit lamp to be used in clinical environments. These attachments operate so as to displace half the image by a plane parallel glass block interposed in the viewing path. In this way, corneal thickness is measured at a single point. The reading of the drum forming part of this attachment is then recorded by hand as the local corneal thickness. While these modified forms of slit lamps, which operate as pachymeters, can accurately define corneal thickness at an unknown location, they are slow, expensive and fragile. Moreover, they are quite difficult to operate, and require substantial training on the part of the operator.

In the prior art, there is no effective means to actually measure distances between portions of the eye on an accurate and easy basis. In fact, distance measurements are made usually by approximations and which frequently are inaccurate at best. The present invention overcomes these problems in that it enables one to use a relatively simple pachymetry process to determine distances between eye tissue segments based on the use of a pair of illuminated slits which have different angles with respect to one another during scanning movement of the eye.

OVERVIEW OF THE INVENTION

Densitometry is a term applied to measurement of the optical density of areas of photographs. Densitometers commonly measure the log of the reciprocal of the percentage of light transmission for a defined area at a stated wavelength or wave band. Measurement of the relative reflection of scattered light is commonly employed to define turbidity in water samples. The amount of light scattered and thus retro-reflected is thereupon compared to a known reference value in this process, in order to determine scattered light reflection.

The densitometer of the present invention measures the relative amount of reflected light from scattering within ocular tissue as an indication of the optical density. In the strictest sense, this is not a true density measurement but is a measure of the relative transparency of the corneal tissues. The corneal interface with the air is not affected by stromal opacification and serves as a relative density reference for the measurement. The minimum reflectance value for calibration is derived from the signal representing the anterior chamber over the pupil where the average reflectance is the smallest. Optical density of small and large areas of the cornea often provides diagnostic data for deciding the need for surgical intervention. In addition, surface contour and the thickness of the cornea are quantified for producing a complete, three dimensional thickness map of the cornea including local thickness of the membrane.

The present invention utilizes light from an incandescent lamp for analysis of the thickness and optical density of the cornea at one or more wavelengths. By making successive exposures with a small linear movement of a slit image between each exposure, a series of slice density images can be generated. These images are then stored in a digital format.

The present invention is also effective for mapping localized opacities and used in planning corneal replacement surgery. Corneal wounds and ulcers can also be mapped to provide accurate diagnostic information to the physician by use of the pachymeter of the invention. Changes in index of refraction associated with scaring or ulceration create foci of light scattering and loss of transparency. The index of refraction in the bulk of the cornea is less in the fluid than in the fiber cytoplasm and scattering results from the optical discontinuities. The degree of opacification can be monitored by loss of visual acuity but the loss of night vision due to loss of image contrast may be debilitating occasionally where the Snellen acuity is only slightly affected.

Slit lamp examination will reveal the presence of these abnormalities but direct visual examination does not provide accurate and repeatable assessment of the lesion location, density, area and any changes in size or opacity with time. The present invention provides a tool for repeatable assessment of potential vision loss under adverse lighting conditions. Focal illumination is employed in the present invention by a modified conventional Köhler projector. The object at the focal plane is one or more optical slits that may be moved either manually or by an associated computer controlled mechanism.

The "Tyndall phenomenon" is the term employed to describe this method for generation of an optical section of the eye that is well known in the ophthalmic art. The diffuse reflection from the first of the layers of the cornea that provides the optical section image is used in the present invention as a reference against which the diffuse reflection from the other portions of the cornea are compared for determining the relative transparency.

The present invention does not use the conventional bio-microscope of the common slit lamp, but rather employs a television camera system for producing digital images for analysis of the anterior portion of the eye. The correlation of a sequence of fixed image individual photographs is difficult due to the change of fixation of gaze relative to the optical axis of the camera. The present invention solves this problem by employing a rectilinear scan system to provide rapid density and thickness measurement of the entire cornea in an easily comprehended format.

The ophthalmic pachymeter of the present invention measures the elemental brightness of a selected portion of a Tyndall image of the eye. Selection of a portion of the field of view of a slit lamp system is accomplished with computer control through the use of a table of valued fiducial marks delineating selected areas in the video display.

In the system disclosed in the present invention, a "clock" signal is derived from a highly accurate crystal controlled oscillator to provide subdivision of the raster lines into well-defined time/size elements. The preferred embodiment uses a medium resolution solid state television camera.

The present invention provides a system for further reducing the number of loci used in the calculations involved in the mapping by circumscribing portions of the visible frame to encompass the area of interest. In this way, the resultant information is stored in more compact form without loss of resolution or accuracy. The gamma curve for film images shows the relationship between log exposure and log density of the image.

The shape of both anterior and posterior surfaces and the thickness of the cornea of the human eye can be mapped by means of the slit projection system of this invention. The line of gaze fixation is made coincident with the optical axis of the camera by a target viewed by the subject via a beam splitter. The beam splitter and fixation target are so positioned as to cause the desired alignment of the eye and camera, and thus, the slit beam.

After generating the anterior surface of the cornea the posterior surface can be defined using the refractive index of the cornea and the angle of incidence for each ray derived from the anterior surface data by application of Snell's law.

Calculation of the surface shape in the present invention is performed by means of analysis of similar triangles, as illustrated in Figures 13 and 15. The anterior surface contour is defined first. The method involves simple geometric analysis of the Tyndall image. The angle between the slit beam and the optical axis is fixed at 45 degrees at center. The distance between the mirror and the eye is also fixed at a known distance. The image point for each pixel is displaced from the optical axis as a direct function of point height above the reference base plane. In the preferred embodiment of the present invention the image to be analyzed is produced by projection of narrow bands of light into the eye by optical projectors of conventional "Köhler" design.

In one of the preferred embodiments a pair of projectors are employed and each move an individual slit assembly. Further, each slit assembly is moved under computer control perpendicular to the longitudinal axis of the slit. Each slit is also moved from an opposite side of the eye.

Computer generated alignment points, or so-called "fiducial points", are displayed with the picture of the eye. These fiducial points are placed over the corneal image by movement of the instrument by the user. The operator moves the instrument until the corneal image lies within the defined area, focuses the camera by axial motion until the desired focus figure is centered on the limbus within the frame, and then operates a switch to accept the data sample and to initiate the data collection sequence.

BRIEF SUMMARY OF THE DISCLOSURE

Generally speaking, the present invention generally provides both a new system for measuring corneal thickness and optical density, as well as a process for measuring the corneal thickness and optical density. The system and the method both provide relative transparency mapping of may types of lesions of the cornea. The invention further provides pachymetry with an almost instant display of the data to permit assessment of need for surgery. Thus, one of the important contributions of the present invention is that it essentially operates on a real-time basis, rendering almost immediate results.

The pachymeter of the present invention, as indicated previously, constitutes three major subsystems which are a modified slit lamp for projecting beams of light onto the eye to be examined, a television camera and lens system for obtaining the image of the eye and electronic circuity for defining and quantifying a portion of the television image. Drive mechanisms are also provided for moving the optical slit to produce successive images for analysis. Associated computer software performs the requisite drive mechanism control, image selection, digital conversion of the analog television signals for computer processing and numerical analysis. This converts the information into a viewable measurement of surface shape, thickness, optical density area and even a display of derived information for clinical use. Obviously, a video output could be connected for display teaching and/or record keeping.

The clinical pachymeter of the invention also includes amplitude detection circuitry to derive the locus of image brightness discontinuities which may be associated with a lesion. The invention includes memory storage for these image points which are digital representations of a magnitude of the image brightness discontinuities. These image brightness discontinuities exist in pixel terms. A conventional electronic computer is used to derive the relative optical density and thickness profile of the cornea. The pachymeter of the present invention also generates a display of the derived density and thickness information for immediate use.

In broad terms the invention can be described as an ophthalmic pachymeter for aiding in the determination of the thickness, surface contour and transparency of the anterior segment of an eye. The ophthalmic pachymeter comprises, in broad terms, a light projecting means, such as a projector, for illuminating a defined area of the cornea. An imaging means, such as a television camera, provides for a television image of selected portions of the illuminated area of the eye. A video means, such as a video amplifier, receives the image of the eye and generates and transmits a video signal representing the image of the eye.

The video signals, which are derived by the video means, operates in conjunction with a converter means for converting these video signals into a digital format. The converter means may adopt the form of an analog-to-digital converter which operates in conjunction with a computer having a data memory. A fiducial means delineates portions of the video image for focusing and alignment. A suitable computer program delineates these portions of the video signal to be converted into this digital format. Finally, an analysis means is provided for detecting and storing the relative brightness levels within the delineated areas in the aforesaid data memory. An address counter is also employed to enable subsequent addressing of the stored information.

The invention may further be described as a system for producing surface contour maps of the cornea of the eye and comprises projection illumination means for producing a definable spacial delineation of corneal contour. A television camera means renders the illuminated areas into electrical analog form signals. The analog signals are converted in a digitizing means for computer-legible operation. A computer means calculates the corneal surface shape from the digital signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will now be made to the accompanying drawings (seven sheets) in which: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a schematic view showing some of the major components of the system of the present invention;</li><li>Figure 2 is a top plan view, partially in horizontal section, of the optical pachymeter constructed in accordance with and embodying the present invention;</li><li>Figure 3 is a perspective view of an alternate construction of a focus aid mechanism forming part of the system of the present invention;</li><li>Figure 4 is a front elevational view of the ophthalmic pachymeter of the present invention;</li><li>Figure 5 is a side elevational view of the ophthalmic pachymeter of the present invention, partially in section, and illustrating the major components in the interior thereof;</li><li>Figure 6 is a schematic view showing the optics and optical paths involved in producing a slit image at the eye of a subject;</li><li>Figure 7 is a plan view of a television screen and showing, in enlarged detail, a portion of the television raster therefor;</li><li>Figure 8 is a schematic diagram of a portion of the electric circuitry employed in the system of the present invention;</li><li>Figure 9 is a graphical illustration showing a television wave form which may be produced in the system of the present invention;</li><li>Figure 10A illustrates a front elevational view of the eye with half-Tyndall images for focus and alignment superimposed;</li><li>Figure 10B illustrates a front elevational view of the eye, similar to Figure 10A, with the half Tyndall images aligned and in the position where they would be centered in a fiducial mark;</li><li>Figure 11 is a schematic view showing the fiducial figure employed for alignment in the present invention;</li><li>Figure 12 is an illustration of a horizontal cross section of the eye for reference purposes;</li><li>Figure 13 is a graphical illustration showing the geometry of the image analysis employed in the Placido method and in the present invention;</li><li>Figure 14 is a schematic illustration of an optical ray trace for deriving thickness of a transparent member, and</li><li>Figure 15 is a schematic illustration showing the geometric relationship of the image obtained in accordance with the present invention with respect to a projector lens and camera lens.</li></ul>

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now in more detail and by reference characters to the drawings, and particularly to Figure 1, it can be seen that the ophthalmic pachymeter of the present invention comprises a television camera 20 having a conventional lens and which is aligned with and receives an image of the eye 22 of a subject through a beam splitter 24 for later quantification and for providing a television image of the eye for analysis. Referring to Figure 3, it can be seen that the apparatus comprises a conventional incremental motor 26 for positioning an elongate aperture, e.g. a slit 28, of a slit form 30 in the focal plane of light projector represented in Figure 1 by a lamp 32. The slit form 30 may be operatively connected to a suitable slide assembly 31, as also best illustrated in Figure 3 of the drawings.

The slit form 30, and particularly the slit 28 thereof, in conjunction with the lamp 32, will produce an image at the eye 22 through the action of a projection lens 34 as shown in Figure 1 for selecting sequential images for analysis. Referring now to Figure 6, there is illustrated a Köhler arrangement 36 which comprises the lamp 32 having a dense filament 38. In this case, by further reference to Figure 6, it can be seen that the image of the filament 38 is formed at the entrance pupil of the projection lens 34 by means of a condenser lens 40.

Adjacent to the condenser lens 40 and in optical alignment with the condenser lens 40 is the slit form 30 carrying the optical slit 28. This slit form 30 is preferably mounted on a carrier (not shown in detail). The carrier is moveable in a direction perpendicular to the slit 28 by the aforesaid incremental motor 26. The images of the slits are brought into focus in the same plane as the television camera 20 by the projection lens 34 and a system of mirrors and prisms, designated schematically by reference numeral 42 in Figures 1, 2, 4 and 5. A fixation lamp 44, sometimes referred to as a "target lamp" or "fixation target lamp", as shown in Figure 1, is also provided for operation with the beam splitter 24, as shown. This combination of the lamp 32, lenses 34 and 40 and mirrors and prisms 42, as well as the slit form 30, function as a slit lamp projector.

Referring again to Figure 1, it can be seen that the television camera 20 generates a signal representative of the image of the eye which is transmitted to a video amplifier 46 for amplification and mixing video signals for analysis. A flash analog-to-digital converter 48 receives the output of the video amplifier 46 for processing and digitizing analog signals received from the television camera 20. A data buffer 50 receives an output from the analog-to-digital converter 48 for directing the digital data to and from a storage in the form of a digital data memory 52. For example, the data buffer 50 and the data memory 52 may form part of a conventional computer which is not illustrated in detail herein. In this respect, it can be observed that many of the components are shod in schematic form (rectangular boxes) in Figure 1.

The digital data which is directed to the digital data memory 52 constitutes a storage of the numerical brightness of each element within the fiducial boundary. An address counter 54 is provided for determining the location and storage of the pixel brightness data for each pixel in the image. A mode controller 56 is connected to the address counter 54 and is provided for determining the sequence of operations of the system. The mode controller 56 receives an input from a computer interface 58 which, in turn, is connected to the data buffer 50 and is also connected to the analog-to-digital converter 48, as illustrated in Figure 1.

The computer interface 58, operating in conjunction with a computer, controls the system elements through the associated computer. In this case, a display driver 60 is provided for controlling the aforesaid fixation target lamp 44 visible by reflection by the beam splitter 24. This serves to render the apparent location of the fixation target lamp 44, namely from the beam splitter 24 coincident with the optical center of the television camera 20 and its associated lens system.

Turning now to Figure 10, it can be observed that there is a representative image of the eye 22. The slit beam 28 illuminated anatomical features are visible as a Tyndall image 62 representing those portions of the eye, such as the corneal epithelium, the stroma and the endothelial layer which scatter the light. An iris 64 in the eye is not the area to be measured and therefore, the illumination of this area is an artefact of Tyndall illumination. The iris image 64 may further be diminished by limiting the spectral distribution of the slit beam 28 through the use of a color filter (not shown). In addition, a slit projector, or slit projectors if more than one is used, produce specular reflections 66, as shown in Figure 10, and which are located in "X, Y" coordinate space, depending upon the surface curvature of the cornea of the eye 22.

The operator of the ophthalmic pachymeter is provided with a computer-generated figure which is used as a fiducial mark system illustrated by reference number 68 in Figure 11. The fiducial marks in this fiducial mark system 68 are located around the center of the display monitor.

Referring now to Figures 4 and 5, it can be seen that the slit lamp illuminator components, e.g. the lamp 32 and slit form 30, as well as the television camera system 20 are mounted on a moveable base 70 which comprises a frame casting. A vertical positioning element, in the nature of a vertically arranged support shaft 72 is operative mounted on the base castings 70, as illustrated. Generally axles 74 which support toothed wheels (not shown) are located in the base casting 70, as best shown in Figure 5, for motion toward and away from the subject. The device also comprises dust covers 78 which cover the toothed wheels. The toothed wheels 76, located under the dust covers 78, serve to constrain the motion relative to a table 80 and hence, the patient so that movement occurs only in a specified area. The base casting 70 is provided with an upstanding handle 82 for manual manipulation by an operator of the apparatus to enable positioning of the instrument with respect to a subject and which is also hereinafter described in more detail.

The beam splitter 24 may be mounted on the base plate 84 of a housing 86 which houses many of the components of the ophthalmic pachymeter, such as, for example, the television camera 20 the condenser lenses 40, the slit form 30, the lamp 32 and the mirrors and/or prisms 42. Located beneath the beam splitter 24 is a printed circuit assembly (not shown). This printed circuit assembly may contain the fixation lamp 44. Otherwise, the fixation lamp 44 may be mounted above the beam splitter 24 in the manner as best illustrated in Figures 4 and 5 of the drawings.

The motors 26 which move the slit form 30 will slew the full length slits 28 of Figure 3 across the eye from each side sequentially to provide the data sequence which will ultimately be stored for analysis. The data is masked by software in order to eliminate extraneous material. The arc of the Tyndall images lies on only one side of the iris section illuminated by the light which is passed through the cornea and has a definable maximum number of pixel loci at the apex from the iris line.

The television display is in the form of a raster as shown in Figure 7. The television has a monitor 100 which displays the visual information in time sequence. The beam current is low for black areas 102 and high for white areas 104 and scaled in magnitude to recreate the brightness range of the original scene. The television camera 20 generates the voltage analog of scene illumination that is provided with synchronizing signals to assure that the time sequence as reproduced is a faithful recreation of the scene being photographed. <u>IMAGE PROCESSING AND OPERATION</u>

The following section more specifically describes the process employed in determining thickness and topography of the cornea. However, and while the circuitry as shown in Figure 8, literally constitutes a part of the apparatus, it is nevertheless described in connection with this image processing and operation, since it is integrally related to the image processing and operation.

In Figure 13, the relationship between the Tyndall image 62 and the topography of the cornea is shown. Along each raster line in the television display, there is a detectable edge of the Tyndall image which has a virtual image location displaced by delta d (Δd). This displacement distance is from the point at which the beam would have intersected the optical axis, if undeflected, as best shown in Figure 15. From this image pixel locus the height of the datum above the reference plane, delta h (Δh), can be calculated. The calculations are performed for all intercepts in all data frames to provide a matrix of X coordinate loci from which the topography can be plotted.

Referring now to Figure 9, the voltage waveform produced by the television camera of the pachymeter, is illustrated. As indicated previously, the beam is low for black areas 102 and high for brighter areas 104. The brightness amplitude ratio of the anterior edge of the corneal section to the dark pupillary area representing the anterior chamber is used as a reference value for lens reflection assessment. The pixel amplitudes for all elements of the reference areas are averaged to provide the baseline reflectance value.

The television signal voltage wave form, as shown in Figure 9 is a single raster line of video information in which there are bright areas 108 from the image of the cornea and a brighter image of the iris 64 (represented by the bright areas 104) illuminated by the slit beam after the latter passes through the cornea. A sync pulse signal 112 precedes each line of pictorial information carrying voltage levels. After the sync pulse 112 a short period of a low level blanking pulses 114 follows. The blanking pulse 114 insures that the display is off while the beam is retraced to the start of a new line. The black level, represented by reference numeral 116, is the most negative of the pictorial data voltages in the video composite signal. This level is determined by a keyed clamp circuit of conventional design where a selected spot in the image representing the anterior chamber signal is sampled and used as a minimum brightness determinant. As the voltage increases, the brightness also increases in the displayed image from black to peak white 118 representing saturation of the signal. The voltage level produced at saturation by a "white" image 118 is shown by the dotted line at the top of the illustration. The brightness profile of the corneal image will vary as the local optical density and index of refraction varies.

At the leading edge in time of the corneal reflex signal, the signal rises to a peak 119 which represents the cornea to air interface. The amplitude of this signal is quite constant from subject to subject and from time to time. This constant interface signal is used for signal reference against which reflex measurements are made to quantify corneal transparency. Each succeeding raster line will then provide a density profile for a different portion of the cornea.

Each exposure containing the Tyndall image or images 62, is converted to digital form by the analog to digital converter 48. Through the action of the data buffer 50, the mode controller 56 the address counter 54 and the digital data memory 52 these sequential amplitude values are stored for use. The data in storage represents the pixel brightness versus locus for each slice of the cornea to be analyzed. Each successive pixel of each successive frame is then multiplied by a constant derived from the cornea to air interface signal average and the optical constant that corrects for the lower normal brightness. As each point is calculated, it is returned to storage in the same sequence for later computation and display. Tyndall illumination provides three-dimensional data sequences of data that are transferred to the computer by the action of the computer interface 58.

Figure 8 represents a schematic diagram of part of the electronic circuitry employed in the preferred embodiment of the present invention. The composite video signal from the television camera 20 is applied to the input 120 of a signal conditioning amplifier. The terminated signal is buffered by an emitter follower 122 which drives DC restoration and sync stripper networks 124 and 126. The DC restored and limited video is buffered by a second emitter follower and serves to drive clamping and mixing amplifiers 128 and 130.

A computer derived black reference timing signal 132 is generated in temporal synchronism with the area of the picture from the television camera 20 which defines the pupillary area near the center of the picture. This pulse is conditioned by mono-stable circuits 134 to provide a constant amplitude and constant width sampling pulse. This sampling pulse, via a capacitor 136 allows the capacitor to store a voltage sample of the raw video that represents the "black" level. The black reference level thus generated biases the amplifier 130 for use in the analog to digital converter 48.

Signals from the computer are used for regeneration of the television timing in a conventional integrated circuit device 140 which makes use of a composite sync signal 142 and dot clock signal 144 from the computer display driver. The computer generated fiducial signal 146 and regenerated composite sync are mixed via resistors 148 with the video signal from an emitter follower 150 for providing the monitor signal. The monitor signal is used to drive a conventional CRT display for use as a viewfinder by the user of the pachymeter of the present invention.