U.S. patent application number 09/828148 was filed with the patent office on 2001-12-27 for method of corneal anlysis using a checkered placido apparatus.
Invention is credited to D'Souza, Hery M., Sarver, Edwin J., Wakil, Youssef S..
Application Number | 20010055095 09/828148 |
Document ID | / |
Family ID | 26799796 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010055095 |
Kind Code |
A1 |
D'Souza, Hery M. ; et
al. |
December 27, 2001 |
Method of corneal anlysis using a checkered placido apparatus
Abstract
A method for analysis of the curvature of the surface of a
cornea using a checkered placido comprises, projecting the image of
the checkered placido onto a patient's cornea, detecting the image
of the checkered placido reflected off of the cornea, detecting a
plurality of nodal points from the reflected image, determining the
mean curvature at a plurality of nodal points and analyzing the
mean curvature at a plurality of nodal points in order to produce a
graphic display of the estimated actual curvature of the
cornea.
Inventors: |
D'Souza, Hery M.; (Cypress,
TX) ; Sarver, Edwin J.; (Merritt Island, FL) ;
Wakil, Youssef S.; (Houston, TX) |
Correspondence
Address: |
MANELLI DENISON & SELTER PLLC
7th Floor
2000 M Street, N.W.
Washington
DC
20036-3307
US
|
Family ID: |
26799796 |
Appl. No.: |
09/828148 |
Filed: |
April 9, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09828148 |
Apr 9, 2001 |
|
|
|
09102839 |
Jun 23, 1998 |
|
|
|
6213605 |
|
|
|
|
09102839 |
Jun 23, 1998 |
|
|
|
07891961 |
Jun 2, 1992 |
|
|
|
Current U.S.
Class: |
351/212 |
Current CPC
Class: |
A61B 3/107 20130101;
G01B 11/255 20130101 |
Class at
Publication: |
351/212 |
International
Class: |
A61B 003/10 |
Claims
What is claimed is:
1. A method of corneal analysis using a checkered placido
comprising the following steps: a) projecting the image of said
checkered placido onto a patient's cornea; b) detecting an image of
said checkered placido reflected off of said cornea; c) detecting a
plurality of nodal points from said reflected image of said
checkered placido; d) determining mean curvature of said cornea at
a plurality of said nodal points; and e) analyzing said mean
curvature at said plurality of said nodal points to produce a
graphic display of the estimated actual curvature of said cornea.
Description
[0001] This is a continuation in part of application Ser. No.
07/891,961, filed Jun. 2, 1992.
CROSS REFERENCE TO RELATED PATENT APPLICATIONS
[0002] This patent application is related to the patent application
for a "VIDEO TO PRINTER INTERFACE METHOD AND APPARATUS" by Edwin J.
Sarver, Henry M. D'Souza, Steven Woo filed concurrently on Apr. 10,
1992. This patent application is related to the design patent
application for "DESIGN FOR AN ABSOLUTE DIOPTRIC SCALE
REPRESENTATION" filed concurrently with this application on Apr.
10, 1992 by Edwin Jay Sarver, Ph.D. and assigned to EyeSys
Laboratories, Inc. This patent application is also related to U.S.
Ser. No. 07/817,868, "A" Contact Lens Sculpturing Device" by Wakil,
D'Souza, Baumgartner, and Carbonari and assigned to EyeSys
Laboratories, Inc. (divisional application of 607,640 filed Jan. 7,
1992); U.S. Ser. No. 07/818,659, "A Method of Using A Placido" by
Wakil, D'Souza, Baumgartner, and Carbonari and assigned to EyeSys
Laboratories, Inc. (divisional application of 607,640 filed Jan. 7,
1992); and U.S. Ser. No. 07/819,364, and "A" Placido Apparatus" by
Wakil, D'Souza, Baumgartner, and Carbonari and assigned to EyeSys
Laboratories, Inc. (divisional application of 607,640 filed Jan. 7,
1992).
MICROFICHE APPENDICES FOR COMPUTER PROGRAM SOURCE CODE LISTINGS
[0003] This patent specification includes a microfiche appendix.
The appendix is a source code listing for the CORNEAL ANALYSIS
SYSTEM software, which consists of 11 microfiche, for a total of
997 frames.
BACKGROUND OF THE INVENTION
[0004] A number of forms of eye surgery including lamellar corneal
surgery, keratomileusis, epikeratophakia, cataract surgery,
penetrating keratoplasty, corneal transplantation radial keratotomy
as well as laser refractive keratectomy involve a consideration of
corneal surface topography. In radial keratotomy, for example, a
number of cuts are made into the cornea in order to change its
curvature and correct refractive power so that images focus closer
to the retina, if not upon it for best visual acuity. It has been
reported that after radial keratotomy "about 55 percent of the
patients function without glasses and the remaining 45 percent have
some degree of improvement." Origination of the technique of radial
keratotomy and other techniques in refractive surgery are generally
credited to Dr. Svyatasklav Fyodorov of the Soviet Union who is
reputed to have performed thousands of such operations.
[0005] While ophthalmic surgery is often successfully performed,
the results obtained have been subject to variation occasioned by
the particular operating "style" of the individual surgeon which
dictates the number, location and depth of incision. Elements of
subjective judgment are paramount. It would be useful to provide a
device that could assist the surgeon in more quantitatively
assessing pre-operative and post-operative corneal contours.
[0006] The present system relates to improvements in the art of
photokeratometry and more particularly to the use of digital image
processing techniques to ascertain the radius of curvature,
refractive power and contour of the cornea. A keratometer is an
instrument for determining the curvature shape of the corneal
surface which generally uses a Placido or other illuminated target
that is centered around the patient's line of sight. The reflection
of a Placido or other illuminated target by the patient's cornea or
by the tear film on the anterior surface of the cornea is
subsequently analyzed to determine the surface contour of the
eye.
[0007] The technique in modern form dates from the early thirties
when the Zeiss optical company of Germany introduced a "Photo
Keratoscope". In general, the art has required the image reflected
by the eye to be photographed and the image on the film measured in
a second step to derive the quantitative data from which the
contour map is generated.
[0008] Recent improvements have been in the area of automating this
photogrammetric analysis by re-imaging the photograph with a
television apparatus and digital signal conversion. After
digitization, computer analysis of the resultant information is
performed with conventional image analysis algorithms. This type of
data analysis is computer intensive and the image formed by the
television system contains a large amount of redundant and
extraneous information. For adequate resolution the sampling rate
must exceed the data frequency by at least three to one, thus
generating a huge number of data points for mathematical analysis.
Consequently the systems are costly, complex, slow and often lack
real resolution in the image analysis. Other means have been used
for clinical measurements such as direct casting of the eye surface
in plastic or wax and coating the cornea with talcum powder and
projecting a grid on this surface for photogrammetric analysis.
[0009] The initial development in keratometry came from Gullstrand
in 1896. Gullstrand disclosed the foundation for the current
technology but his apparatus had no provision to compensate for
aberrations in the optical system other than limiting the
photographic coverage of the cornea to a 4 mm area. As a result,
multiple exposures and calculations were necessary to map the
corneal surface. Much of the modern technique was developed by
Amsler in 1930 and embodied in his "Photo-Keratoscope" which also
required measurement and calculation as a separate step to derive
the corneal shape data.
[0010] At present, the clinical standard is the Bausch and Lomb
Keratometer, which is sold commercially. The Bausch and Lomb
Keratometer only measures the average of the corneal radius in two
meridians of the central 3 mm "cap" of the cornea. The standard
technology does not provide total surface topography of the cornea
and thus is inadequate for many diagnostically significant
abnormalities, contact lens fitting, or the needs of ophthalmic
surgical procedures. In addition, the prior art technique is
cumbersome and involves great potential for error.
[0011] The standard instrument which is in most common use for
central optical zone shape measurement is the Bausch and Lomb
Keratometer. Several companies offer similar devices with similar
principles of operation. In these devices a single Mire image is
projected on a small central portion of the anterior surface of the
cornea usually 3 mm in diameter. The user is required to operate
several controls to bring the optically split Mire images reflected
from the cornea simultaneously into focus and alignment. In
addition, the operator manually records the data obtained at two
perpendicular axes. Other instruments are also available, such as
the Haag-Streit Javal Schiotz device which measures only one axis
at a time, but is slightly easier to use and tends to be more
accurate in practice than the Bausch and Lomb system. In addition
there exists a photographic system made by International Diagnostic
Instrument Limited under the trademark "CORNEASCOPE" (and a similar
system made by Nidek in Japan), as well as autokeratometers by
several manufacturers. The CORNEASCOPE produces instant photographs
of the reflection of a Placido disc and requires a second
instrument separate from the camera assembly to analyze the data.
This system is fairly accurate, but expensive and tedious to use.
The autokeratometers all are limited to a single zone of
approximately 3 mm diameter and, in cases where the magnitude of
the astigmatism is low, are inaccurate in their assessment of axes
of astigmatism. Also available are three computer-direct systems
which use conventional image analysis algorithms in conjunction
with a mini-computer. These are the Corneal Modeling System (CMS)
introduced in 1987 by Computed Anatomy, Inc. of New York, N.Y. and
the ECT-100, introduced into the market by Visioptic of Houston,
Tex. and a system using light emitting diodes disposed in
concentric rings built by Zeiss of Germany. The Placido disc-photo
technique is superior to the Bausch and Lomb Keratometer because of
the much greater amount of corneal surface analyzed from the
Placido reflection as opposed to the mires of the Keratometer.
[0012] A number of patents have been issued that relate to
keratometers. U.S. Pat. No. 3,797,921 proposes the use of a camera
to record the Placido reflection from a patients eye. From this
photograph, the radius of surface curvature of the cornea is
determined at several points and calculated using a complex
computer system. The use of a ground glass focusing screen with the
small aperture of the optical system and large linear magnification
makes use difficult and requires a darkened room for operation.
[0013] U.S. Pat. No. 4,440,477 proposes a method and device for
measuring the corneal surface, comprising a slit lamp for
illuminating the corneal surface, a camera for recording the
reflection from the corneal surface, and a processor to calculate
the image distance and the radius of curvature of the eye. The
operation of the processor is not detailed in U.S. Pat. No.
4,440,477.
[0014] A more recent entry into the market is the "Corneal Modeling
System" manufactured by Computed Anatomy Incorporated of New York
which uses a light cone Placido target in conjunction with a "frame
grabber" to digitize and store for conventional image analysis the
pictorial data. The Placido is in cylindrical form and illuminated
from one end. This cylindrical Placido maintains a small aperture
optical system creating a large depth of field of focus for the
imaging system and, consequently, requires a sophisticated focus
determining apparatus to assure accurate and reproducible image
evaluation. This system is said to produce corneal thickness data
using a scanning laser, as well as the surface contour but is very
expensive and does not lend itself to clinical applications which
are increasingly cost driven.
[0015] The prior art systems discussed above tend to be both
expensive and difficult to use. Many of the prior art devices have
a significant potential for error, due to complexity of the
calculation, the imaging of the corneal surface and the difficulty
in operating these systems.
[0016] Since even a normal human cornea will not be perfectly
spherical, the illuminated rings will generally be reflected from
the corneal surface as a pattern of shapes variously distorted from
the circular. The data pertaining to the coordinates of points in
the two-dimensional video image is processed to define a
three-dimensional corneal surface yielding the equivalent spherical
radius of curvature (or dioptric power) for each of the acquired
points.
SUMMARY OF THE INVENTION
[0017] Accordingly, there is provided herein a new technique for
image analysis that provides full topographical mapping of the
cornea, with almost instant display of the corneal radius of
curvature at enough points to permit accurate assessment of the
surface shape. The improved photo keratometer includes a
transilluminate target or "Placido", which is reflected by the
surface of the eye to be examined. A CCD camera and lens system is
mounted behind the Placido so that the optical axis is coincident
with the visual axis of the eye being examined and is generally
centered in the target member to provide an image of the reflection
of the target by the eye. The image information of multiple "rings"
on the cornea from the CCD camera is then captured by a frame
grabber board and processed by an edge detection algorithm to
derive the locus of image brightness discontinuities which are
associated with the target reflection from the eye. These image
points are, in turn, transferred to storage in the internal memory
as digital representations of the x, y locus of the image
bright/dark transitions representing the Placido ring edges.
[0018] The stored data associated with the CCD image of the target
reflection are then treated by an image processing algorithm in a
conventional electronic computer to derive the surface contour of
the eye and to generate the display of the derived shape
information for use by the operator. The Multi-Functional Corneal
Analysis System described herein can serve as a sensitive method to
determine proper contact lens fit by measuring the shape of both
front and back surfaces of the contact lenses and comparing these
shape measurements with the shape of the eye to which the said lens
is to be applied.
[0019] An illustrative system in accordance with the invention the
Eyesys Multi-functional Corneal Analysis System which combines the
features of an automatic keratometer, photokeratoscope and corneal
topography device into a single instrument. Comprehensive
keratometric results and quantitative corneal surface measurements
provide multi-functional corneal evaluation capabilities. Multiple
analysis routines offer information from basic keratometric
readings to intermediate zone values and graphics to full surface
topography color mapping. An easy to use joy-stick and positioning
aid provides precise patient alignment and image focus.
User-friendly menus guide users to quick and reproducible exams. An
on-line operators manual provides rapid assistance. For most exams,
processing time is under 10 seconds for 360 meridians. Corneal
information is reported as numerical values with graphic
presentations for the 3 mm, 5 mm and 7 mm zones, corneal contour
profile graphics of any two meridians and topographical color
surface maps according to either dioptric power or millimeter
radius of curvature. Up to four surface maps can be displayed for
comparative analysis. Patient exams can be archived to hard disc or
floppy disc and recalled at any time. Permanent records may be
produced via optional Polaroid camera or color graphics
printer.
[0020] An illustrative system in accordance with the invention
utilizes a unique data-acquisition design to perform rapid,
cost-effective quantitative photokeratoscopy. The system obtains a
complete 360 degree measurement (approximate corneal zone diameter
0.9-9.5 mm) with only a single data-acquisition "shot," eliminating
the need for camera rotation. The system has a more precise and
user recognizable focusing target and improved optics over other
systems known in the art, which further enhances the accuracy and
reproducibility of corneal topographic profiles. The interval
between electronic data capture and complete display for all
meridians is less than 10 seconds. The system is packaged either as
a single tabletop unit with a base dimension of roughly
18".times.23" which includes an integrated IBM-compatible computer
and a photographic port so that standard photokeratoscopic
photographs on Polaroid or 35-mm film can easily be obtained or as
a modular unit on a mobile pedestal with dimensions of
32".times.24" with computer housing separate from photokeratoscope.
Video output is available for video image storage if desired. In
addition to standard numerical displays, new color graphics for
corneal profiles and for isodioptric color-coded contour maps can
be selected. The system in accordance with the invention is easy to
use and therefore suitable for use in a standard clinical setting.
Its data-acquisition design provides rapid data capture and display
and offers distinct advantages for clinical and research
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is an overview of the system.
[0022] FIG. 2 is a cross section of the system.
[0023] FIG. 3 is a detail of the system menus and graphic
presentations.
[0024] FIG. 4 is a diagram of optical principles.
[0025] FIG. 4A and 4B are a graphic presentation of the operation
of the focusing aid.
[0026] FIG. 5 is a detail of the construction of the focusing
aid.
[0027] FIG. 6 is an operational sketch of the focusing aid.
[0028] FIG. 7 is operational depiction of the optical assembly and
patient positioning assembly.
[0029] FIG. 8 is a mechanical drawing of the optical assembly
housing.
[0030] FIG. 9 is a schematic of the power supply.
[0031] FIG. 10 is a block diagram and schematic and PAL equations
for the frame grabber board.
[0032] FIG. 11 is the optical path layout and design
methodology.
[0033] FIG. 12 is the system menus and high level description of
the software.
[0034] FIG. 13 is a cross section of the eye.
[0035] FIG. 14 is a front view of the eye.
[0036] FIG. 15 is a graphical representation of the checkered
Placido apparatus in operation.
[0037] FIG. 16 is a front view of a checkered Placido
apparatus.
[0038] FIG. 17 is a rearview of a checkered Placido apparatus.
[0039] FIG. 18 is a diagonal view of a checkered Placido
apparatus.
[0040] FIG. 19 is a front view of a checkered Placido
apparatus.
[0041] FIG. 20 is a front view of a Placido.
[0042] FIG. 21 is a side view of a checkered Placido image
projected onto a cornea and an expanded view of a selected nodal
point.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0043] I. System Overview
[0044] FIGS. 13 and 14 illustrate the more important features of
the eye as they relate to keratometry. The pupil of the eye is
defined by the central area surrounded by the iris. The iris
opening size is controlled by the autonomic nerve system in
relation to the brightness of illumination as well as other factors
and may be as small as one millimeter in diameter in bright light
to five millimeters in diameter in dim light. The constriction of
the iris in bright light also provides an increase in depth of
focus such as is observed in conventional photography. The
reflection of the one or more concentric rings of the Placido off
of the anterior corneal surface will appear as more or less
circular bright rings superimposed on the pupil and iris when
viewed by the television camera. The interior of the eye is shown
as a horizontal cross-section to show the more important
structures. The globe is enclosed in a semi-rigid white membrane
called the sclera. The transparent membrane at the front is called
the cornea. The cornea is a thin membrane which is supported in
shape by the pressure of the fluid behind the membrane and in front
of the crystalline lens. The lens is supported by a system of
filaments and muscle tissue which cooperate to change its thickness
and, in consequence, the focal length of the lens.
[0045] The primary focusing power of the optical system of the eye
is provided by the light refraction curvature of the cornea and the
fluid filling the anterior chamber, while the lens serves to permit
the change of plane of focus from near objects to distant scenes.
The light entering the eye through the iris opening is brought to
focus on the surface of the retina which lines a large portion of
the inner globe and contains the photo-receptor cells. These cells
are of two general types, rods and cones. The rods predominate in
the areas peripheral to central image and are highly sensitive to
light but devoid of color sensitivity. The rods provide "scotopic"
or night vision. The cones predominate in the central retina and in
the "fovea", where critical central vision takes place. The center
of vision is located in the fovea which is displaced from the
optical axis of the eye by some five to seven degrees. Because the
surface of the cornea is not a perfect spherical section the
curvature of the surface is asymmetrical around the center of
vision or visual axis and must be taken into consideration in
keratometry.
[0046] As noted in U.S. Pat. Nos. 3,542,458 and 4,440,477 the
reflection of an object in a convex mirror will produce an image
which is "virtual" (cannot be formed on a screen, but can be viewed
directly) erect, and reduced in size by an amount which is a
function of the radius of curvature of the mirror. In this system,
the tear film and/or the surface of the cornea acts as such a
mirror. The formula often used to define the light reflected from a
transparent surface is dependent upon the index of refraction of
the optical medias-involved.
[0047] The commonly used values of the indices of refraction, n,
for the three media of transmission in this case, air to tear film
and cornea are 1.000 for air, 1.333 for the tear film and 1.3375
for the cornea. There exists approximately a two percent reflection
at both of these optical interfaces, i.e. the air/tear film and
tear film/anterior corneal surface. The small thickness of the tear
film places both reflections in close proximity so that they are
indistinguishable from each other for instrumental purposes. As a
result these reflections are lumped together for clinical
applications. However, the small amount of light in the reflected
pattern influences the system design, as discussed below.
[0048] The anterior surface of the normal cornea is not quite
spherical, as it appears to have been assumed in the construction
of many of the prior art devices such as the Bausch and Lomb
Keratometer, but is more nearly an ellipsoid. The central two or
three millimeters of the normal cornea does conform reasonably to
the spherical form so the simplistic model will serve to illustrate
the optics of the system for rays at or near the common optical
axis.
[0049] The user is most often interested in data presentation in
terms of diopters of focusing power of the cornea. The radius
information can then be converted to this form in the commonly use
formula as follows:
d=(n-1)/r
[0050] where the index of refraction of the cornea n is assumed to
be 1.3375 and the radius of curvature of the corneal surface r is
expressed in meters. It should be noted that there is not an
agreement on the actual value of the effective index of refraction
of the cornea to be employed in keratometry and that the
calculation of corneal curvature in dioptric form also involves
optical correction factors to compensate for the effectively
negative "lens" formed by the rear surface of the cornea. In
practice the value of index of refraction used by several systems
for this conversion range from 1.332 (Zeiss) 1.336 (American
Optical) to 1.3375 (Haag-Streit and Bausch & Lomb). The
"normal" range of curvature in the central zone ranges from 7.2 to
8.3 mm with a mean value of 7.8 mm. Some representative values for
the Bausch & Lomb instruments converting the readings into
diopters are shown in the following table:
1 Dioptral curvature Surface radius in mm 61.0 5.53 60.0 5.63 47.0
7.18 45.0 7.50 44.0 7.67 42.0 8.04 41.0 8.23
[0051] From the foregoing it follows that the conversion of the
data into dioptric form is not difficult and involves the use of a
selected constant but that the data so expressed is subject to
variable error inherent in the technique. The common keratometer
has been used for many years with data in dioptric form, even
though the magnitudes are not precisely accurate. The choice of
display form either in diopters or millimeter radius of curvature
is selectable in this system to permit the user to choose between
the more accurate and the more common form. The display of the
derived data may be in graph form for ease of assimilation and
application by the user.
[0052] The data of interest to the user is generated from the pixel
radii of each chord of Placido ring reflection in the acquired
image in any of the possible directions from center. The millimeter
radius of curvature and dioptric curvature of the surface at each
of these points is then provided to the user for his evaluation.
The keratometer known in the art and in common use measures two
perpendicular meridians at each selected angle and produces data in
the form of "K1, K2", cylinder and axis. These terms refer to the
average dioptric curvature from both sides of visual axis in each
of the two meridians which have the greatest and least curvature,
assumed to be 90 degrees apart in "regular" astigmatism the
magnitude of the difference between the two, and the angle relative
to the horizontal of the larger of the two. The terms are commonly
used and are recognized by the user as definitive of these
descriptive elements as derived by conventional keratometry. The
axis can either be measured or assumed to be regular (90 degrees
apart), however, in today's applications more comprehensive data is
necessary. K values are obtained for a full 360 degrees by a
process of repeated measurement and recordation.
[0053] To reduce the amount of data required to define the ring
image size in radial terms, only those pixel loci which define a
change of brightness greater than a threshold value are stored.
Each ring reflection produces one data point at each reflection
edge. These points can be used to determine the actual locus of the
center of the ring reflections. The optical system is preferably
provided with an optical fiber which defines the optical center of
the system and provides a bright point of light for the patient to
fixate upon. The reflection of this small point from the cornea
provides a true center from which all measurements are made.
Furthermore the numerical scatter of the data points is a function
of the focus and overall image quality which permits the evaluation
of each measurement for minimum acceptable quality. The decision to
reject any measurement which does not fulfill the quality standard
is set into the software. This is due to the requirement that the
object distance be known and fixed for accurate data analysis.
Small errors in focus can degrade the measurement and so an optical
system with a small depth of field of focus and a software scatter
determination are used to insure accuracy. The central fixation
target reflection from the optical fiber is also examined for
relationship to the true center of the picture and if the image is
decentered in either axis by a predetermined amount the measurement
is invalidated. The shadow cast by the nose, brow, lashes, etc. as
well as the lid margin which may lie within the camera field will
cause some data points to be missing from the theoretical maximum
number. The lash shadows will not completely obscure the area to be
measured and so some minimum number of valid points may be selected
which will permit the areas thus partially masked to be defined
with a large degree of confidence. The entire picture is examined
for brightness transitions in this manner and the axis determined
by mathematical algorithms in the computer. Given that, for
example, the image resolution of the system provides a pixel size,
Placido image referred, of 0.014 mm (750 pixels=10 mm so one
pixel=1/75 mm or 0.0133 mm) then an estimate of the minimum
curvature difference and radial interval detectable by the system
can be derived.
[0054] For best accuracy, each instrument should be calibrated
periodically to compensate for minor differences in system
magnification and linearity to obtain maximum accuracy of the
derived data. For this reason calibration means preferably are
provided as a part of the computer software and the user may check
the calibration and reset the table values at any time.
[0055] Referring now to FIG. 4, the optical theory diagram shows
some of the relationships which are of interest in the present
system.
[0056] An object (the cornea of the eye to be measured) with a size
h is imaged by reflection at plane d with an image size h'. The
magnification is derived by the usual formula:
m=hl'h
[0057] The focal length of a convex mirror is negative and equal to
one-half of the radius of curvature. The sum of the reciprocals of
the object and image distances is equal to the reciprocal of the
focal length. These two can then be combined to the form:
1/o+1/i=1/-f=-2/r
[0058] or
i=or/[2(or)-r]
[0059] It follows that the remainder of the image is formed in a
similar fashion and that the same figure applies in any meridian.
(These formulae are only true for rays which are very close to the
optical axis). From the size of the object, size of the image, the
distance and the optical magnification, the radius of curvature can
be calculated as follows:
R=M(2U/0)I
[0060] Where:
[0061] M is the magnification constant of camera and optics;
[0062] U is the distance from object to cornea;
[0063] I is the observed size of image; and
[0064] 0 is the actual size of object.
[0065] The objects imaged are the several rings of the target which
yield the curvature of the eye at several distances from the center
of the cornea. For the i.sup.th ring, all the constants are lumped
into one, K.sub.i, thus:
R.sub.i=K.sub.iI.sub.i
[0066] R.sub.i is the radius of curvature of cornea of the i.sup.th
ring;
[0067] I.sub.i is the observed size of i.sup.th ring; and
[0068] K.sub.i is the i.sup.th ring conversion constants.
[0069] Thus, all that is needed for computation of curvatures are
the K.sub.i constants. The K.sub.i's can be calculated but it is
much easier, and more accurate, to measure them by calibrating the
instrument with balls of known, precise diameter R.sub.o and
setting all K.sub.i=1. The values of V.sub.i are measured which
provides a measurement of I.sub.i since V.sub.i=1.times.I.sub.i.
Thus, the constants are determined by:
K.sub.i=R.sub.o/V.sub.i
[0070] Where:
[0071] R.sub.o is the known radius of calibration ball; and
[0072] V.sub.i is the measured radius of calibration ball with
K.sub.i set to 1.
[0073] According to conventional techniques a table is constructed
to provide a look up system for conversion of measured reflex
diameters, representing a range of known surface curvature values.
In this manner the necessary degree of precision may be achieved to
assure accurate output data accuracy for the intended application.
Interpolation between table entries is quite practical and reduces
the number of table entries needed to assure accurate
measurements.
[0074] A more exact surface shape characterization could, in
theory, be obtained by the method iterated by Wittenberg and Ludlam
in a paper published in the Journal of the Optical Society of
America Vol. 56 No. 11, November 1966 but the simpler form provides
adequate accuracy for clinical use. The magnification factor and
the effective numerical aperture are chosen as a compromise between
the most desirable small relative aperture and acceptably small
depth of field to facilitate the setup and focusing step. This
provides an acceptable error from subject positioning resulting
from inability to judge small differences in subject distance due
to the depth of focus of the optical system as well as adequate
image brightness for noise reduction. In most, if not all cases,
the exact surface contour is of less interest to the clinician than
the relative contour. For example, in a surgical application, the
object is to arrive at a smooth, regular corneal surface, which has
a similar shape in two perpendicular axes. That is to say that the
corneal astigmatism is minimal. The errors of measurement are least
at, or near, the center of the cornea which is the main image
forming surface of the eye. Therefore small error accumulation in
the periphery of the cornea are tolerable. In surgical procedures
where the cornea is cut, suture tension and location can alter the
surface shape. The peripheral curvature must be maintained as
closely as possible to the same value in all axes if there is to be
no induced post-operative astigmatism. The keratometer can provide
information for post-operative adjustment of sutures to better
achieve this result. The shape derivation for contact lens fitting
is also a comparative process in that the lenses may also be
measured by the instrument and so small errors from true surface
derivation cancel and the resulting data are usable in a clinical
context.
[0075] Because the eye is centered in the picture by adjustment of
the instrument and headrest at the time of setup and because the
subject is fixating on a target which is coaxial with the system's
optical axis, the center of the reflected image and thus, the
cornea can be located exactly by a rather simple software
technique. The largest difference between the two central image
points from the fixation lamp reflection constitutes a measurement
that is equivalent to a diameter of the inner Placido ring
reflection (in pixel terms). One-half of that measured value is the
center of the figure. The remainder of the analysis is based upon
similar technique and is much less software intensive than the
classical image analysis algorithms which make more complex
decisions about a much larger number of pictorial elements each of
which may have one of many numerical values which may represent
intensity, saturation and hue. Thus it can be seen that this system
substitutes novel means and method for the conventional image
analysis technique to permit the construction of a very inexpensive
system which can be used to produce clinically useful data when
operated by unsophisticated users within the economic constraints
imposed by current clinical fee structures.
[0076] The computer program controls measurements, data analysis
and display format. Each single measurement consists of measuring
the edges of the Placido reflection in view. Subsequent to the data
gathering step, the curvatures are computed from the available
edges. Any values falling outside of a window of selectable size
are considered "bad". Then the half chord measurements for each
ring from the selected data points are derived. The values of
curvature are similarly computed for each ring image on each side
of center at enough angles to permit accurate assessment of major
and minor axis angles.
[0077] The formula used for computing the curvatures is:
R.sub.i=K.sub.iR.sub.i
[0078] Where:
[0079] R.sub.i is the radius of corneal surface curvature of
i.sup.th ring;
[0080] K.sub.i is the lumped constant of i.sup.th ring; and
[0081] R.sub.i is the measured radius of i.sup.th ring.
[0082] (The lumped constant depends on magnification, ring size,
local rate of curvature etc.) The constants K.sub.i are determined
by calibrating the instrument by measuring objects of known radius.
These data are stored on a disk, in an EPROM (Erasable Programmable
Read Only Memory). For some similar means for use by the main
program. The provision of a variable focal length camera lens would
permit adjustment to compensate the magnification errors which will
result from the tolerance of focal length of commercial lenses if
desired but the calibration table method is the preferred
embodiment.
[0083] II. Processing Circuitry And Operation
[0084] Referring now to FIG. 2, the keratometer of the preferred
embodiment comprises a Placido or similar target, a lens system, a
CCD (Charge Coupled Device) camera 50 for receiving the reflection
of the Placido 2 from the eye and an image processing sub-system
48.
[0085] The eye to be examined is positioned according to
conventional techniques preferably at a distance of 3 inches from
the Placido and centered on the optical system. Referring to FIG.
15 for more detail the Placido is in the form of a
trans-illuminated surface of translucent material 220 with the CCD
camera lens centered in the Placido, and with the lens, in turn,
surrounded by concentric circles of opaque material 221. The
Placido is illuminated by one or more lamps 222 placed behind the
disc surface so the translucent areas are bright circles as viewed
by the subject. By this technique an image is provided in a plane
223 posterior to the normal corneal surface of the eye 224.
Referring back to FIG. 2, the reflection of this image is received
by a CCD camera after passing through the lens. The lens preferably
includes an objective lens 53 located at or near its focal length
from the eye. A beam splitter or mirror 15 may be included along
with a second lens 52 whereby a portion of the image formed by the
objective lens may be diverted to a camera port for photographic
recordation of the eye and the Placido reflection. Otherwise the
remaining image portion is brought into focus at the
photo-sensitive surface of the CCD camera 51.
[0086] The subject is placed in front of the instrument with the
chin supported in a rest (24) which may be adjusted for subject
size in terms of chin to eye dimension. This adjustment (25) is
typically a screw operated device. The optical assembly (58) is
mounted in suitable "slides" or rollers (45, 46) which permit
motion in two perpendicular planes without rotation so that either
eye may be aligned on the optical axis and the image brought into
critical focus by the motions. The Placido (2) is illuminated from
behind by a lamp (27) which may be a circular fluorescent tube or
other type as desired. The assembly is also moveable in the
vertical axis which could be by a means of a slide (32, 33) under
control of a screw (31). The rotation of the screw may be by a knob
or a motor drive comprising a motor (40), pulleys (37, 39) and a
cooperating belt (38) or other suitable means to permit the
elevation of the optical axis to be under operator control for
alignment of the optical axis of the instrument with the eye to be
measured. The action of a "joy-stick" (42) mounted in a ball and
socket system (43) under operator control via a cam or friction
member (44) preferably propels the instrument on the slides,
rollers or wheels (45, 46) to facilitate the positioning and focus
steps. The present system in accordance with the invention utilizes
a positioner assembly from SCO, Scandicci, Florence, Italy. A brow
rest (26) may be mounted on the head support system (49) to insure
the fixed position of the eye to the instrument while the
adjustment and measurement are made. The patient is requested to
focus his eye on the fixation target (79) to assure the coincidence
of the optical axes of the instrument with the eye (1). After the
positioning and focus step, the operator presses the switch (41) or
a foot operated switch, at which time the portions of the image
relating to the measurement to be made are captured by the
electronic assembly (48) and suitable power supply (47) operatively
associated therewith.
[0087] An object (the Placido) is reflected from the surface of the
cornea and the size of the reflection is measured. The focal length
of a convex mirror is one-half of the radius of curvature and the
image and object sizes can be related to the focal length. The
object in this case is preferably a Placido or Placido's disc.
[0088] The data points which are recorded in memory comprise pixel
numbers which denote the locus in X, Y terms of each brightness
transition in the picture which are over threshold magnitude. These
points are contaminated to some extent by random noise and so must
be treated to remove the noise, establish centering and focus
accuracy and general quality prior to being converted into final
form for use in standard display algorithms. As the data points are
stored in memory at the time of recordation, the points which
define line numbers can be identified by addition of a flag bit in
the position commonly occupied by the sign bit. This is possible
because the data points all bear a common positive sign and makes
sorting simpler by making use of a sign compare instruction
available in most computers. The line numbers are stored as a
paired table with the data points provided by the pixel numbers in
the measurement and the process continues until all data points are
so sorted. The end of data in storage is indicated by either a line
number or a pixel number being equal to zero which is caused by
clearing the entire data memory to zeros prior to each measurement.
This technique reduces the number of data points to be treated in
the ensuing calculations. A numerical mask is set into the software
to define a small area at the center of the picture which defined
the location in which the fixation target reflection will be found
if the instrument is properly aligned with the eye (1). The
reflection of the fixation target should be inside this mask for
best accuracy. The data points within a slightly larger area are
averaged to define the optical center of the data to be
treated.
[0089] If the average data point is within the mask area, it is
stored as the center point for polar data form conversion; if
outside the mask the measurement is aborted. The operator may be
notified of the error or an automatic repeat measurement for some
given number of tries, commonly (3) can be done prior to the
notification as desired.
[0090] After the data format conversion from Cartesian to polar
form the angle count is set to zero and the points in radial
sequence are stored in a table. This is repeated for as many angles
as are desired. The increase in the number of angles enhances later
display use but increases the calculation time so the number of
angles is user selected.
[0091] After all desired angles have been converted, the data
points are examined by distance from center as groups. It should be
noted that this is in sequence terms as opposed to discrete
distance terms in that the reflection will be closed, nested
curves, but not circles or other regular figures in most cases.
[0092] The radially selected groups are subjected to a smoothing
process such as least squares or moving average window to define
the shape of the reflection of the Placido. To provide the common
form of central K1, K2, Cylinder and Axis, the innermost smooth
curve may be presumed to be an ellipse and the calculations produce
the "best fit" ellipse from the smoothed data. From this the K1 and
K2 are determined by look up and interpolation from the calibration
data table and the numerical difference becomes "Cylinder" or
astigmatism. The Axis is, of course, the major axis angle of the
determined ellipse in anti-clockwise form from zero degrees in the
horizontal plane extending to the right of the origin.
[0093] The remaining steps take each set of points for successive
concentric reflections and smooth them in like fashion. Any data
point which fails to fit the smooth curve by more than two standard
deviations or other like threshold parameter is then deleted and
the data resmoothed. The smoothed data are then converted to X, Y
and millimeter radius of surface curvature form by table look up
for use in any desired display format.
[0094] The area of corneal coverage is 0.9 mm-9.0 mm. (@42.5D). The
axis range is 0-360 degrees (1 degree increments). The diopter
range is 9D-99D. The resolution is +/-0.25 diopters. The dimensions
of an integrated system embodiment are 23"D.times.18"W.times.24"H,
80 pounds, otherwise, the system components can be modularized and
provided on a compact mobile pedestal table.
System Components
[0095] The system in accordance with the invention is comprised of
a photokeratoscope, a Placido 2, a patient focusing assembly 202, a
computer 203, a high resolution CCD video camera, a 14" VGA color
monitor 200 and an image processing subsystem. The system is
mounted on a table top which is attached to a moveable pedestal
205.
[0096] The illustrative system in accordance with the invention
includes the following components:
[0097] Photokeratoscope, case, CCD camera, Placido, light chamber,
optics assembly, patient focusing assembly, positioning
base/chinrest, IBM AT compatible computer or 80386 based computer,
101 Key Enhanced Keyboard, 40 Megabyte Hard Disc Drive, 1.44
Megabyte Floppy disc Drive, High Resolution CCD Video Camera, 14"
VGA Color Monitor, Image Processing Sub-System, Image processing
algorithms, frame grabber board, power supply board, pedestal,
tabletop.
Image Processing
[0098] The image processing software and all other software used in
the system in accordance with the invention is set out in the
appendix. The software is adapted for speed and performance in
numerous ways. The software uses integer mathematics in lieu of
floating point mathematics to obtain a substantial increase in
speed on the family of processors used by the system, the Intel X86
family of processors, available from Intel Corporation, Santa Clara
Calif. These techniques improve performance on any processor,
however. Integer math is even faster than using a co-processor for
floating point operations. The math uses a fixed point operator.
For instance, to use the number 3.279 the proxy number 3,279 is
manipulated instead using integer math; the decimal point is later
placed in the result as necessary. This is much faster than
floating point math. The system in accordance with the invention
also uses integer math for sines and cosines, simply scaled by 1000
to give a significant increase in performance. Because only three
significant digits are necessary, this scale by 1000 operation
works adequately to give three significant digits.
[0099] Numerous performance enhancements are detailed in the source
listing. An important factor in performance enhancement is the
architectural design of the software as well as the selection of
steps and sequence used to perform the image processing and other
function. Additionally the technologies of image processing,
parallel processing and expert systems are incorporated into the
software design.
[0100] The software design is parallel. It can be executed on a
parallel processor such as a super computer and would not be forced
to be sequential. Therefore the architecture has been designed so
that it can be executed in a parallel implementation.
Edge Detection
[0101] The system in accordance with the invention uses an edge
detection algorithm implemented in software. Each Placido ring that
is reflected in the cornea is seen as two edges by the edge
detector. Other known systems use peak amplitude to detect Placido
ring locations which is less accurate and generates fewer data
points for post image capture analysis. The system in accordance
with the invention uses the edge detection software to sense the
interior and exterior edge of each ring (see FIG. 12K). The image
processing software then counts the number of pixels to each edge
of a Placido ring, and then rotates 1 degree and repeats the
process of counting pixels.
[0102] For example, if nine Placido rings were used and reflected
off of the cornea they would generate eighteen ring edges; this
implies 360 degrees.times.18 edges=5760 points of corneal
topographic information. Older style keratometers only utilized
four data points, the radius to a single mire or ring measured 90
degrees apart and then photographed the mire reflected in the
cornea to quantify a spherical characteristics of the cornea. The
total analysis alone took over 20 minutes to complete. The system
in accordance with the invention performs a full 360 degree
analysis in under 15 seconds.
[0103] The number of pixels counted to the edge of a Placido ring
corresponds to a particular radius of curvature when compared to
the calibration curve for the system. The edge detector which
resides as software in the computer looks at the pattern of rings
reflected from the cornea on the CCD camera and captured by the
frame grabber board and counts the number of pixels to the edge of
each concentric ring. The number of pixels counted to each ring
edge is proportional to the radius of curvature of the corneal
surface of the eye at that point.
[0104] The image processing software resides in the integrated
computer performing edge detection to find the edge of each Placido
ring reflected on the corneal surface, then image processing
algorithm software creates a table of pixel distances to each
Placido ring edge thus generating a pixel count or distance to a
ring edge that is proportional to the corneal radius of curvature.
The system in accordance with the invention uses sub-pixels
{fraction (1/10)} resolution to determine position of edges and
diopter measurements. These calculated pixel distances are compared
to the calibration curve to generate the topographic curvature for
the object cornea.
[0105] Software also includes function for patient history, data
base management, displays, driving the video board, writing pixels
to the display board buffer, site specific profiles for
communications parameters, doctor preferences for number of colors
on the screen, file manipulation code, menus and numerous other
functions evident upon examination of the software source listings
in the appendix.
Calibration Curve
[0106] The illustrative system in accordance with the invention
generates a calibration curve by imaging objects with a known
radius of curvature. The calibration routine calculates and stores
a look up table (essentially a calibration curve) for each of four
calibration spheres in the current design, each table corresponding
to the number of pixels counted for this known radius of curvature.
The number of calibration spheres can easily be increased or
decreased. Presently these four tables are used to generate an
interpolated calibration curve (pixel versus diopter or radius of
curvature). This best-fit curve, presently calibrated to four known
radius-of-curvature calibration objects, gives the radius of
curvature for a given pixel count when imaging an object with an
unknown radius of curvature. The software source code is listed
fully in the appendix.
Data Presentation and Display
[0107] Corneal information can be reported as a set of numerical
values or may be displayed in a color-graphic presentation. The
system in accordance with the invention is capable of graphical
presentation of the 3 mm, 5 mm and 7 mm zones, or as a corneal
contour profile graphic of any meridian. The system in accordance
with the invention can also generate topographic color coded
surface maps in either diopter or millimeter radius of curvature
scales. Up to four surface maps can be displayed together for
comparative analysis. Patient exams can be archived to floppy disc
and recalled at any time. Permanent records can be produced via
optional Polaroid camera or color graphics printer.
[0108] The colored graphics presentation can be utilized to show
where to make correcting incisions into the cornea during a radial
keratotomy or laser sculpturing procedure of the cornea. A video
display monitor is utilized to view the graphic presentation. Video
graphics can be saved on the system printer or on disc for archival
purposes.
[0109] Graphical displays can also be useful to record the
topographic history of the cornea during the healing process. The
cornea can take months and sometimes even years to heal: the cornea
has no blood in it, so it repairs similar to a missing finger nail;
it does not scab over and heal within a week or so. Such historical
topographic data enables a doctor to make necessary adjustments as
the cornea heals. The doctor can tighten or loosen sutures or make
other correcting adjustments to optimize the corrective effects of
surgery on the shape of the cornea.
[0110] Graphical presentations can also be used to compare the
topographic corneal characteristics before and after surgery. The
difference between the two can also be shown so that a physician
can observe how the surgery has affected the topography of the
cornea. Examples of the graphic presentations are presented in
FIGS. 3A-3L.
[0111] The graphic display can be chosen from menus as shown in
FIG. 3A using the "Select Display Format" menu. FIG. 3B is an
example display of keratometric data (orthogonal). FIG. 3C is an
example display of keratometric data showing astigmatism in a
"torque display" in which the 3 mm, 5 mm and 7 mm topographies are
overlaid in one display. FIG. 3D is a profile graph which can be
generated for any 2 meridians. FIG. 3E is a tabular display of
keratometric data which can be generated for any 2 meridians. FIG.
3F is an example display of a contact lens fitting map showing the
dioptric correction for different points on the cornea. FIG. 3G is
an example of a comparative isodioptric mapping that can be
generated to compare 1, 2, 3 or 4 eyes. FIG. 3H is a color map with
nonnalized dioptric scale. FIG. 31 is an example of a data overview
display. FIG. 3J is an example of display eye image which is a
display of the eye and the Placido ring image upon it. FIG. 3K is
an example of the contact lens fitting display. FIG. 3L is an
example of comparative iso-dioptric color mapping. FIG. 3M is an
example of a tabular display of curvature data in any two selected
meridians.
[0112] The dataorganization and presentation software source code
is listed in appendix 1.
Precise Placido Positioning and Focusing Aid
[0113] The size of the reflection of the Placido in the cornea, and
therefore the calculated perceived radius of curvature for a
particular zone of the cornea, is a function of the distance from
the eye to the Placido. Therefore it is desirable that this
distance be the same each time the doctor analyzes the topography
of the cornea. A change in the distance would render an erroneous
calculation as to the topography of the cornea. For instance,
should the Placido be positioned slightly closer to the cornea on a
second "snapshot," the rings would appear farther apart and more
pixels would be counted between the rings even though the shape of
the cornea has not changed, and the results would erroneously
indicate an increase in the millimeter radius of curvature.
[0114] In the present embodiment the actual Placido is
approximately 3" from the eye; however, a focus aid is employed to
exactly position the Placido to the same position for each
diagnostic session. The focusing aid is much closer to the eye than
the actual Placido and it projects focusing cross-hairs on to the
eye. These focusing cross hairs constitute a "synthetic Placido"
which represents a Placido which is much closer to the eye than the
actual Placido. Therefore when the focusing aid or "synthetic
Placido" is positioned precisely the positional errors in the
actual Placido are negligible. Therefore the deviations in the
position of the actual Placido become insignificant and aid
accurate and reproducible positioning of the actual Placido, thus
reducing inaccuracies in corneal topographic calculations due to
positioning errors.
[0115] The focusing aid projects focusing cross hairs onto the eye.
The focusing aid acts as an optical range finding system to
determine when the cornea is in focus. The focusing aid promotes
accurate and reproducible eye placement to get exact comparative
readings between pre-operative and post-operative corneal
topography. Comparative readings are also useful in determining how
the corneal shape may change over time during the healing process.
The position of the Placido with respect to the eye is important in
determining the corneal topography, in both an absolute or a
comparative sense. The corneal topographic characteristics are also
useful in determining and predicting the after effects of surgery
and detecting possible errors that may have occurred in surgery
using other diagnostic techniques than the system in accordance
with the invention.
[0116] The space between the reflected Placido rings is a function
of the distance from the eye to the Placido 2. Because the Placido
may appear in focus during travel through the depth of field for a
particular lens, there can be significant variance in the distance
from the Placido to the eye for two different points within the
depth of field. A difference in this distance from the Placido to
the eye causes a difference in the distance between the Placido
concentric circles, inducing an error in measurements of the
distance between the Placido lines. The Placido should be
positioned at the same distance from the eye each time a
measurement is taken so that variations in the distance between
reflected Placido lines are caused by variations in corneal
topography and not by variations in the distance s from the Placido
to the cornea.
[0117] Now referring to FIGS. 4A and 4B, prior art systems use a
triangulation method (100) as shown in FIG. 4A. The prior art
method employs converging laser beams 100 from lasers 102, to
position the apex of the cornea, 101 relative to the optical
assembly 58. This method introduces an error in post-surgery
keratometric readings, as the tip or apex of the cornea 101 may be
depressed substantially from its pre-operation position. This
depression causes a flattening of the corneal apex as shown in FIG.
4B. This flattening causes the cornea to be positioned closer to
the optical system under the prior art and thus exaggerating the
corneal flattening resulting from the surgery. The prior art
triangulation focusing method induces an error in the distance to
the reference point, the apex of the cornea. The prior art system
therefore is less likely to give consistent and repeatable results
or measurements because the distance from the optical assembly to
the entire cornea changes after surgery inducing an error in
postoperative measurement. Moreover, the use of lasers projected
onto the cornea is also dangerous as they can damage the
tissue.
[0118] The illustrative system in accordance with the invention
offers an improvement in that these induced errors are reduced to
enhance repeatable and accurate results. The present system in
accordance with the invention uses two light emitting diodes (LEDs)
104, however another illuminating source or means of projecting an
image could be used. These LED projected images do not converge but
are pointed at the limbus area at the periphery of the eye. These
LEDs project a focus aid image consisting of an "x" or cross hairs
103 onto the outer portion or limbus area of the eye. This outer
portion is less susceptible to change through either flattening or
steepening than the apex area of the cornea. The change in this
limbral area of the cornea after surgery is negligible compared to
the change in the apex. Therefore the measurements are more
accurate and comparable using the system in accordance with the
invention of the present system for pre and post operative corneal
curvature changes.
[0119] FIGS. 5A, 5B and 5C show the focusing aid in detail. An LED
121 is held in place by an LED holder 122. The tube 126 encases the
focusing aid. A spacer 123 fits inside the tube 126 along with lens
one (f=84 mm) 125 and lens two (f=48 mm) 124. The focusing aid is
secured to the optical assembly 58 by the focusing aid mounting
collar 127. The LED holder 122 has 38 gauge wire cross hairs 129
attached with epoxy. Focusing aid 130 projects these cross hairs
129 onto the limbus area of the eye. The cross hairs 129 are
reflected by the eye back into the optics system and displayed on a
video monitor for the operator to observe. In FIG. 9, the operator
actuates joy stick 42 bringing the Placido 2 into focus. The
operator via the joystick moves the optical assembly 58 along the
optical axis 151 as shown in FIG. 6A. This motion moves the optical
assembly 58, focusing aid 130 and Placido 2 along the optical axis.
The operator observes the focus aid image 103 in FIG. 4A on the
video monitor 200 in FIG. 1. The focus aid image is reflected off
of the limbus region of the cornea into the optics assembly into
the camera where it is displayed on the video monitor 200. The
angle of incidence 170 in FIG. 6A of the focus aid image upon the
eye changes as the focusing aid travels along the axis 151. When
the angle of incidence is proper, the cross hairs split the circle
projected from the focusing aid into equal quadrants and the
focusing aid is properly focused at the correct distance to take a
shot of the Placido properly focused on the cornea That is when the
focus aid is at the proper distance and in focus as indicated by
the cross hairs positioned shown as in FIG. 6C, the Placido is also
at the correct distance and properly in focus. This technique gives
repeatable and consistent results.
[0120] When the cross hairs 129 in FIG. 5B are seen as shown in
FIG. 6C the Placido is focused at a repeatable distance from the
eye each time before and after corneal surgery because of this
precise focusing technique upon the limbral region of the cornea.
Changes in the central shape of the cornea have a negligible effect
upon the focus distance and therefore introduce little error into
the analysis of the cornea. Hence the measurements are repeatable
and negligible error is induced by a change in the reference point.
The operator adjusts the focusing aid and placid reference distance
using a calibrated sphere with a known radius of curvature.
Frame Grabber Board
[0121] The present system in accordance with the invention takes a
digital picture of the Placido as it is reflected by the cornea
using the CCD camera. The frame grabber board stores this image by
grabbing two (even and odd) consecutive NTSC video fields {fraction
(1/60)} of a second apart, storing them in memory to form a NTSC
video frame giving a composite image for viewing by the operator.
The board also enables the operator to actuate the frame grabber
via a foot switch. The board is designed to work at high speed with
the computer and software. The design details and schematic, as
well as the programmable logic array equations, are presented in
FIGS. 10A through 10K.
Edge Detection and Analysis
[0122] The illustrative system in accordance with the invention
looks at the Placido reflection from the cornea and determines the
position of the edges of the light and dark pattern generated by
the Placido. The edge detection and analysis software is set out in
the appendix.
Placido Selection and Design
[0123] Different types and shapes for the Placido may be used. In a
cylindrical Placido the rings are marked around the inside of the
tubular surface to generate a pattern of rings when projected onto
the cornea. However, in this arrangement the distance from the
Placido to the eye is very short, usually less than 1" and most
likely right on top of the eye. A planar Placido increases the
distance from the Placido to the eye and in the present embodiment
is at approximately 3". Increasing this working distance from the
Placido decreases the effect of positioning errors. What is an
error of {fraction (1/10)}" is a much smaller percentage of 3" than
it is of 1" so that as an error of {fraction (1/10)}" has much less
effect on the measurements using a planar Placido with a working
distance of approximately 3", than a cylindrical Placido with a
working distance of 1".
[0124] The generic design of the Placido is set out in the
mathematical model below. In a planar Placido the inner bands of
the Placido are thinner than the outer bands to generate a 50% duty
cycle between light and dark edges in the reflected Placido image
off of a normal cornea. The Placido can be designed to generate a
50% duty cycle of light and dark edges in the reflected Placido
image or any other duty cycle or variable duty cycle desired. The
Placido can be designed for any shape also using the mathematical
model set forth below. The design of the Placido is a generalized
design and is set out below as a mathematical model. This model
works for any shape Placido. The design model tells the operator
where to put the Placido edges on any shape Placido. For any shape
Placido the operator must mark edges on the shape and the
mathematical model tells the operator where to mark the edges on
the shape.
[0125] The Placido generates a virtual image on the convex cornea.
The image actually exists behind the surface of the cornea, so when
the operator focuses, the focus is on a point internal to the eye
where the virtual image exists. This virtual image of the Placido
is object of the camera. The virtual image is a series of rings.
Using this design method and a planar Placido increases the working
distance, which is more comfortable for the patient and less
difficult to position.
Mathematical Model for Placido
[0126] Each larger successive concentric Placido ring is wider to
reflect a nominally uniform width set of rings in the cornea. The
normal angle with respect to optical axis at point yi (2.y1 dia
zone) is given by a =sin-1 [y1/7.937], where 7.937 is the radius of
curvature at a 42.5 diopter surface. For a reflected ray from point
y1 to be parallel to the optical axis, it follows that the angle of
incidence of that ray (on point y1) be
<ia=<r.
[0127] This ray emanates from a ring edge on the Placido making the
enter of curvature of a 42.5 diopter surface our origin in a
Cartesian frame of reference, we have the locus for the Placido
point yielding a reflection at the 2Y diameter zone as
[0128] (y-y 1)=M (x-x1) (equation of a line)
[0129] M=Tan 2a (angle of incident ray with respect to x axis)
[0130] Since (y1)2+(x1)2=(7.937)2 (equation of 42.5 Diopter
surface)
[0131] Therefore, Y=X tan 2a+[Y1-(tan 2a) (7.9372-Y12)1/2] 1 3) ie:
Y = X [ tan 2 ( sin - 1 Y1 7.937 ) ] + [ Y1 - tan 2 ( sin - 1 Y1
7.937 ) ] ( 7.95 - Y1 2 ) ] 1 / 2
[0132] is the focus of Placido points yielding a 2 Y1 diameter
reflect on a 42.5 D surface.
[0133] The tip of the 42.5 D surface is at 7.937 mm=0.3125".
[0134] Choosing an x (eye clearance is x-0.3125 inches) yields an
ordered pair (X,Y) for the Placido profile. Note that Y Max will be
the overall diameter of the Placido (X Max, Y Max) if Y1=Max
desired zone covered. Choose Y2=Min desired zone covered Y=7/8"/2
yields (X Min, Y Max) for Placido inside circle for a conical
Placido, the focus of Placido points is 2 X - X Max Y - Max + X - X
Min Y - X Min 4) ie: Y = X ( Y Max - Y Min ) ( X Max - X Min ) + (
X Max Y Min - X Min Y Max ) ( Y Max - X Min )
[0135] and the solution of 4)+3) yield edge radii.
[0136] NOTE: Conical Placido profile has been used here but the
theory obviously extends to any desired profile, from cylindrical
to planar
Optical Assembly
[0137] The optical assembly houses a power supply and electronics,
illuminating lamp, a camera, the optics, a Placido and the focusing
aid. The camera sits inside the optical assembly and behind a
plate. The camera has an optical tube that contains a lens which
passes through the plate and projects all the way forward to the
Placido. The tube surrounds the optical path of the Placido image.
The fluorescent lamp that illuminates the Placido sits in front of
the plate. The shape of the housing eliminates the need for a
reflector pan as the housing serves as a reflector behind the lamp
that illuminates the Placido. The purpose is to have a homogeneous
light source to illuminate the Placido.
[0138] The optical assembly is detailed in FIGS. 8A-8I. The layout
and design of the optical path is set out in FIGS. 11A and 11B. The
optical path in the current embodiment is a single lens system
taking a magnification of 0.58 for 12 mm coverage so the doctor can
see slightly more area than the eye itself. The design can
accommodate various lens sizes. For example for a 75 mm lens, the
total path length of the tube is approximately 8 inches. The
outside diameter of the tube is 1 1/4" and has 3 baffles with 3/4"
apertures.
[0139] Magnification is important to the resolution of the system.
The pixel resolution of the system is proportional to the
magnification of the lens. More magnification means more pixels per
millimeter and less resolution. The more pixels per millimeter that
are present the better one can analyze small changes across that
distance. For example, if you have 5 pixels per millimeter you can
resolve a pixel 1/5 of a millimeter in size. If you have 10 pixels
per millimeter you can resolve a pixel {fraction (1/10)} of a
millimeter in size.
Power Supply Board
[0140] The power supply board is specifically designed to work with
the present system in accordance with the invention and is
presented in detail in FIG. 9.
Contact Lens Fitting System
[0141] The system in accordance with the invention includes a
contact lens fitting system including software in which the corneal
analysis generated inputs into a transformation function operating
in software. The transformation function converts the corneal
topographic profile parameters into contact lens design parameters.
These contact lens design parameters are sent to a contact lens
lathe, well known in the art, to sculpt a custom contact lens to
fit the eye that has been analyzed. The contact lens design
parameters may be checked for quality control before sending the
parameters to the lathe or the parameters can be sent to the lathe
without such a quality control checking function and simply let the
patient and physician determine if the lens is satisfactory. A
software function (or equivalent) to transfer the contact lens
parameters from the corneal analysis computer to a lathe for
sculpting a lens is necessary to implement the system. One such
quality control function has been developed by Polytech, Division
of EMI-MEC, Limited, a Sunleaigh Company, School Lane, Chandler
Ford, East Leigh, Hempshire, England, S05 3ZE. The present system
in accordance with the invention does not claim the Polytech
version of the quality control function. The software for the
transformation from corneal parameters to contact lens design
parameters and the link software from the design parameters are
available in source listing form in the appendix to the
specification.
[0142] The communications software that sends a file from the
corneal topography analyzer computer to any other computer utilizes
an off the shelf package available from Blaise Computing, Inc.,
2560 Ninth St., Suite 316, Berkley, Calif., (415) 540-5441.
Checkered Placido
[0143] A ray passing through a point on a checkered Placido,
reflected off a cornea, and detected by a CCD camera is graphically
depicted in FIG. 15. The focal plane 40 of the CCD camera, the
plane of the lens 42, the plane of the Placido 44 and the plane 46
tangential to the apex of the cornea are depicted in FIG. 15. Each
plane contains a local XY coordinate system. The origin of the XY
coordinate system existing in each plane is intersected by a line
48 representing the optical axis of the eye. Each plane is parallel
to the other planes. The optical axis is coincident with the origin
of the coordinate system in each plane. Point "A" 50 lies on the
Placido 44. A line intersecting point "A" 50 and the origin of the
XY coordinate system lying in the Placido plane 44 forms an angle
"a" 52 with the horizontal axis or X axis of the XY Placido
coordinate system.
[0144] As shown in FIG. 18 the Placido in an illustrative
embodiment is shaped like a cone. In an alternative embodiment, the
Placido could be a paraboloid. In yet another alternative
embodiment, the Placido could be yet another shaped surface. The
patient looks into the concave surface of the conical Placido in a
preferred embodiment. The exterior or convex surface of the conical
Placido in a preferred embodiment is backlit by a light source.
Point A 50 represents a point on the Placido. A ray of light from
the light source will pass through point "A" on the Placido and
strike a reflection point 58. This ray is called the incident ray
56. The incident ray 56 passes through Placido point "" 50 and is
reflected at the reflection point 58. The reflected ray 60 is
detected point "A" 62 on the CCD focal plane 40. A line 64 passing
through detected at point 62 and through the origin of the
coordinate system existing in CCD plane 40 forms an angle "a.sup.I"
66 with the horizontal axis or X axis of the CCD plane coordinate
system.
[0145] Referring now to FIG. 19 a front view of the checkered
Placido, the Placido is laid out in a checkered pattern. In a
preferred embodiment, the checkered Placido is made up of black and
white sections. In an alternative embodiment, the checkered Placido
could be made up of another set of contrasting colors. The
checkered pattern is designed so that black and white transitions
are encountered when traveling along a radius drawn from the origin
74 to the outer edge 76 of the Placido as concentric rings of
contrasting color are encountered. The design also provides for
black and white transitions when traveling along an arc 78. The arc
78 is generated by angularly rotating a point drawn a distance R
from the origin 74 where R is less than the radius of the Placido
perimeter 76. Thus, edge transitions are encountered when traveling
along a concentric circle drawn inside the perimeter of the Placido
circumference as adjacent sections of contrasting color are
encountered. These sections are formed by drawing a plurality of
radii to form the triangular shaped sections shown in FIG. 19. Upon
drawing a plurality of radii there will be edge transitions
encountered both radially and concentrically in the pattern. There
will exist points on this pattern which will have edge transitions
both radially and concentrically. These points will be defined as
nodal points where it is possible for direct measure of orthogonal
radius of curvatures, radially and concentrically.
[0146] A benefit of the checkered Placido is that, as shown in FIG.
15 with the checkered Placido, the meridian of the incident ray can
be determined by construction. Therefore when the meridian of the
reflected ray is measured it is possible to determine the precise
orientation of the surface normal at the reflecting point.
Referring again to FIG. 15, in the past it was assumed that the
incident ray, the surface normal and the reflected ray were
contained in a single plane. This plane was assumed to contain the
principle or optical axis. However, this is not necessarily true.
It depends on the precise orientation of the surface normal at the
reflecting point or the shape of the surface at the reflection
point.
[0147] When the reflection point is located on a perfectly
spherical surface the principle axis or optical axis is located in
the plane containing the incident ray, the surface normal and the
reflected ray. However, when the reflection point is located on a
non spherical surface, such as a cornea having non spherical
characteristics, then the optical axis is not located in the plane
containing the incident ray the surface normal and the reflected
ray.
[0148] In the checkered Placido the angle or meridian of the
incident ray can be determined because the checkered Placido has a
marking or identifying line at the X axis in the coordinate system
for the Placido plane. Thus, the deflection angle 79 of a line
drawn through a point "B" 50 on the Placido plane can be
determined. Thus, the XY coordinates of the point "A" 50 on the
Placido plane are known. At nodal points there will be a unique
surface normal in three dimensions readily defined since the edge
transitions occur in orthogonal curvatures.
[0149] Referring now to FIG. 15, in a system with a Placido
consisting only of concentric rings, not having the checkered
pattern of the present embodiment, it was assumed that the angle
"a" 52 measuring the angular deflection from the horizontal of the
point "A" on the Placido plane, and the angle "" 66 measuring the
angular deflection from horizontal of the point "" 62 the detected
point on the CCD image plane, were the same. That is, it was
assumed that the angular deflection of the point on the Placido
plane and the angular deflection of the point on the detected CCD
image were the same. However, this assumption is not necessarily
true when the reflection point is located on a surface that is not
perfectly spherical.
[0150] The point "A" 50 is reflected at the reflection point 58 and
passes through the lens center represented by the origin of the
lens plane coordinate system 42 and forms an image on the CCD focal
plane. There is also a parallel ray from a virtual image, which is
behind the eye. The parallel and the principal ray or chief ray
converge at point 62 to determine where the image is formed. The
system graphically depicted in FIG. 15 has been designed to form
the image at the CCD focal plane so that only the principal ray is
a concern in the calculation.
[0151] Previously it was assumed that the angle 52 was equal to the
angle 66 because there was no way easy to determine where the point
50 was located in the Placido plane 44. However, with the checkered
Placido the location of the point 50 on the Placido plane can be
determined because the meridian or the angle "a" 52 is known by
construction. The deflection angle or meridian 66 of the detected
point dan be measured on the detected GCD image. The angle "a" 52
is known by construction because it lies on or near the
intersection of a black to white or color transition edge on the
checkered Placido. The angle measured from horizontal to each edge
of the black to white or color transition on the Placido is known.
Thus, points on or near these transition "edges" can be
determined.
[0152] The deflection angle "" 66 of a detected point can be
measured on the CCD. The angle 66 of the detected point will not
equal the angle "a" 52 of the point on the Placido when the
reflection point is located on a nonspherical surface. Also, when
the reflection point is located on an nonspherical surface, the
surface normal will be twisted such that the surface normal at the
reflection point 58 will not be contained in that plane containing
the optical axis. It will be contained instead in a skewed plane.
When the angle 52 is not equal to the angle 66, the surface normal
is not contained in the plane of the optical axis and the surface
containing the reflection point is not spherical.
[0153] Therefore the location of the point on the Placido helps
determine more precisely the shape of the eye. The checkered
Placido helps to determine the location of a point on the Placido
by construction. The most obvious points are nodal points where
orthogonal edge transitions occur by construction. The
determination of the unique surface normal in three dimensions is a
direct consequence of knowing the tangent plane at each nodal point
common to each of orthogonal local radius of curvature measurements
calculated from the respective edge transition locations. The
location of any point on the Placido can be determined because the
Placido is constructed of black and white or contrasting sections
whose edge transitions can be detected and mapped or located so
that a map is formed of these known locations of points on the
Placido. The angular deflection from horizontal for each black to
white or color transition edge is known because the checkered
Placido is manufactured with a known deflection angle for each of
these transition edges. Knowing the measured angle 66 of the
detected point, one can use solid geometry to go from point 62 on
the CCD image through the lens center back to the reflection point
on the cornea of the eye and back to a known point "A" on the
checkered Placido. The angle 52 determines the surface normal of
the reflection point in three-dimensional coordinates. These
three-dimensional coordinates define precisely the orientation of
the surface normal at the reflection point (N.sub.X, N.sub.Y, and
N.sub.Z). This triplet identifies the surface normal at the
reflection point. The surface normal triplet can be determined for
every point on the cornea where a measurement is taken.
[0154] Any line originating from the center of a sphere and
intersecting the surface of the sphere is a normal. In nonspherical
volume there may be a significant perturbation away from a triplet
for a normal on a spherical surface. The checkered Placido helps to
determine this perturbation or delta. Determining this perturbation
or delta enables the corneal analysis system to determine more
precisely the topographical or non-spherical characteristics of the
surface of the cornea at the reflection point.
[0155] Points on the Placido are mapped based on the angular
deflection of the point from that zeroth meridian or horizontal
axis. Each edge at a color transitions is a known number of degrees
from the horizontal. The radial Placido sections are constructed by
drawing radii so that each section is a known number of degrees
from horizontal. For example, if each radial section is 10 degrees
wide the angular deflection between the horizontal or zeroth
meridian and the edge of the first section would be 10 degrees, 20
degrees to the second section edge, 30 degrees to the third section
edge, and so on. The black to white edges or color transition edges
formed by the adjoining radial sections encountered in the angular
direction are detected by the same edge detection and location
method as used for the edges of the concentric circles encountered
in the radial direction. In a preferred embodiment, adjacent
sections on the Placido are alternately black and white, however,
the adjacent sections can be another set of contrasting colors, as
long as a detectable edge is formed between adjacent sections, to
facilitate locating points on the Placido.
[0156] The edges or color transitions are determined by the
mathematical process of differentiation. The derivative function
highlights these edges by generating an impulsive change. The
impulsive change is used to determine the precise position of the
edge of the black to white or color transition between two pixels.
The position is determined to a sub-pixel position by a process of
weighting and using the surrounding pixel information to determine
where between the two pixels the edge is precisely located.
[0157] The surface normal is calculated by drawing a line from the
detected point 62 to the reflection point and drawing a line from
the reflection point to point 50 on the Placido. The angle
bisecting the angle between these two lines is the surface
normal.
[0158] A successive approximation process is used to determine the
reflection point at which the surface normal intersects the
eye.
[0159] Knowing the location of the detected point 62, the lens
center point and the location of the Placido point 50, and
reflection point one can calculate the surface normal. A line is
drawn from point 62 through a point at the origin of the lens
coordinate system 42 or the center of the lens. This line 60 is
extended to intersect the plane of the reflection point. A plane
which contains the line extended through point 62 and the center of
the lens is rotated until it touches the point 50 on the Placido.
The X and the Y coordinate of the surface normal are determined.
The Z coordinate of the surface normal is then determined. Once the
three coordinates of the surface normal are known-for a number of
points, a plane can be drawn orthogonal to each surface normal. The
planes can be joined together to form a multi-faceted surface area.
This area of facets or joined planes can be smoothed to represent
the surface contour of the cornea. That is, the faceted surface is
integrated to smooth the faceted surface to represent the actual
contour of the cornea. Points in between the surface normal are
calculated by the process of interpolation. The actual contour of
the cornea can be further described; for example, by calculating,
among possible curvatures, the mean curvature, either arithmetic or
geometric (Gaussian), at an infinitely small area when two
orthonogal or principal radius of curvatures are known. The local
area surrounding an nodal point can be directly analyzed to provide
these mean curvature measurements either arithmetic or geometric
(Gaussian) to describe more precisely the local radius of curvature
of the cornea's contour. FIG. ______ demonstrates this calculation
where a nodal point analysis containes orthogonal edge transition
for determining the local radius of curvature in orthonogal
direction which can be mathematically manipulated to provide the
arithmetic or geometric (Gaussian) mean of the lcoal radius of
curvature to provide more precise analysis of the cornea's contour.
This process provides novel capability to determine the contour of
the cornea and therefore its refractive power as an optical element
necessary for vision by the eye. Historically, curvature
measurements had only been made in a radial fashion from the
optical axis of the cornea outward, peripherally. Mean curvature
calculation from measurement of orthonogal local radius of
curvatures will further enhance our understanding of the cornea
contour and its shape in addition to its optical performance. This
process more precisely defines the surface of the eye. 3 Arithmetic
R mean = R 1 + R 2 2
[0160] or
Geometric R mean={square root}{square root over
(R.sub.1.circle-solid.R.su- b.2)}
[0161] At times it may be difficult to know exactly what ring on
the Placido corresponds to the ring detected in the CCD digital
image. This may be due to corneal distortion, surgical scarring, or
some other optical aberation that obliterates data in the digital
image so that an edge becomes undetectable. As explained above, it
is advantageous to know which ring or section on the Placido
corresponds to the ring or section detected in the CCD digial
image. Ring seven may be mistaken for ring six, for example, if the
edge between ring five and six is not detected and not counted,
thus skipped. In an illustrative embodiment, as shown in FIG. 19, a
reference mark 76 or numeral is placed on the Placido to a ring or
section on the Placido.
[0162] As shown in FIG. 20, reference marks or numerals enhance
correlation of points on the CCD digital image and corresponding
points on the Placido. In an illustrative embodiment, reference
marks or numerals are placed in each white ring. Four lines of
reference numerals are placed respectively at zero degrees, ninety
degrees, one hundred eighty degrees, and two hundred seventy
degrees to facilitate checking of each of the quadrants.
[0163] Even though the reference marks may undergo distortions,
including scale, perspective, and rotation, when reflected of a
non-spherical surface, the system can find the section containing
the reference mark. The system will do a mapping back into a
normalized space to obtain a normalized detected reference mark. In
normalized space there is a library in which templates are stored
for the reference marks or numerals as detected when reflected from
calibrated spheres. The system correlates the stored template mark
or numeral from the library with the normalized detected reference
mark or numeral to confirm which ring or section on the Placido
corresponds to the detected ring or section.
[0164] The normalization of reference marks allows the system to
recognize numerals or marks that have undergone translation,
rotation, and rubber-sheeting type of perspective distortion.
[0165] While an embodiment of the present system in accordance with
the invention has been described herein, it will be understood that
a person skilled in the art may make minor alternations or
substitute circuitry and apparatus other than that described
without departing from the spirit of the invention. For example,
those of ordinary skill having the benefit of this disclosure will
of course recognized that the "hard-wired" discrete logic function
described herein may alternatively and equivalently be implemented
in software, i.e., through suitable programming of a processor
system equipped with a suitable processor and a memory or other
storage device. Such a software implementation would be a matter of
routine for those of ordinary skill having the benefit of this
disclosure and knowledge of the processor system in question.
Software functions disclosed in this application could likewise be
implemented in hardware by a person having ordinary skill and the
benefit of this disclosure.
* * * * *