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BACKGROUND OF THE INVENTION
This invention relates generally to optical imaging systems. More
particularly, this invention relates to an imaging system which detects
specular reflections from the interface between media having different
indices of refraction. The present invention is particularly, but not
exclusively, useful in ocular or ophthalmic surgery for identifying tissue
boundaries or cavities within tissues.
DISCUSSION OF THE PRIOIR ART
Advances in the various medical arts have now made surgical procedures
possible which just a few years ago were only speculative. One important
tool in this progress is the surgical laser. As is well known in the
medical field, surgical lasers can be very efficaciously employed for
certain purposes. This efficacy, however, is extremely dependent on the
controllability of the laser and an exact identification of the tissue
being operated on.
Not surprisingly, whenever a laser is used to perform very intricate and
precise surgical procedures, there is a need for monitoring the subject
tissue. This is particularly so in ophthalmic surgery where sensitive and
delicate tissue is involved. Accordingly, several devices have been
proposed which are capable of imaging portions of the tye for the purpose
of observing the surgical procedure. Exemplary of such devices is U.S.
Pat. No. 4,598,311 to Bille which discloses optical means for viewing an
operative field. This device does not, however, precisely identify the
location of a tissue boundary. Instead, its information relates to surface
topography rather than the establishment of a reference datum. Imaging
systems have also been proposed which are useful for diagnostic purposes.
Exemplary of such a system is the disclosure In re application of Bille,
application Ser. No. 706,619 filed on Oct. 11, 1987 for an invention
entitled "Apparatus for, and Method of, Examining Eyes." Unlike the
present invention where the desired information is primarily the geometric
relationship between tissue masses, a diagnostic image is more concerned
with detecting aberrations or abnormalities of the tissue. While these
examples evidence means by which information concerning certain physical
characteristics of the tissue within the field of view can be obtained,
additional information concerning the size, shape and geometrical
relationships of a tissue mass may be of equal or even more value in
certain procedures.
The present invention recognizes there is a need in certain surgical
procedures to precisely determine the boundary of the tissue being
operated on. Specifically, corneal surgery represents such a requirement.
Because of the layered structure of the cornea, the present invention
recognizes that the ability to accurately identify the boundary between
the cornea and the aqueous humor can be extremely helpful. With such
information, operations at the interface between the cornea and the
aqueous humor can be performed. Equally important, by using this interface
as a reference datum, operations within the cornea can be performed
without compromising the interface. Additionally, the present invention
recognizes that the ability to monitor affected tissue within the cornea
during the actual cutting procedure can be very helpful when creating
internal ablations in the stroma. Further, the present invention
recognizes that such identification and monitoring can be accomplished by
detecting intensity variations in the light that is specularly reflected
from the interface between media having different indices of refraction.
In light of the above, it is an object of the present invention to provide
an imaging system for use in conjunction with a surgical laser that will
accurately identify the interface between materials or media having
different indices of refraction. Another object of the present invention
is to provide an imaging system which will give indications of the
location of internal ablations in the cornea either during or after the
time the internal ablation is being created. Still another object of the
present invention is to provide a system which will identify a reference
datum from which an ophthalmic surgical laser operation can be performed.
Yet another object of this invention is to provide an imaging system which
is cost effective, relatively inexpensive to manufacture and easy to use.
SUMMARY OF THE INVENTION
A preferred embodiment of the novel imaging system of the present invention
includes a light source which directs a beam of light toward the eye. An
aperture is positioned along the path of the beam and an optical steering
means is provided which can move the beam across the surface of the
cornea. A first focusing means focuses the beam at the aperture, a second
focusing means focuses the beam at desired points on the path of the beam,
and a third focusing means confocally focuses specularly reflected light
from the eye at the aperture.
Identification of a tissue boundary or interface is made by varying the
focus of the system on the eye until a specular reflection is obtained
from the interface of media or materials having different indices of
refraction. Using signals generated by such reflections, the imaging
system of the present invention identifies the boundary of tissue masses
and uses this information to establish a datum from which subsequent
ophthalmic operations with surgical lasers can be performed.
As implied above, the imaging system of the present invention may be
operatively employed in conjunction with a surgical laser beam. With this
combination, the light beam of the imaging system and the surgical laser
beam are coaxially aligned and coincidentally focused by the focusing
means. Thus, the light beam can be used both to identify a tissue
interface and to visually monitor movement of the cutting laser.
The novel features of this invention, as well as the invention itself, both
as to its structure and its operation, will be best understood from the
accompanying drawings, taken in conjunction with the accompanying
description, in which similar reference characters refer to similar parts,
and in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the optical elements of the present
invention shown in operative relationship with a cross-sectional view of
an eye;
FIG. 2 is a detailed cross-section of the cornea of the eye as seen in FIG.
1; and
FIG. 3 is a detailed cross-section of the cornea as seen in FIG. 2 with an
internal ablation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, an imaging system, generally designated 10, is shown in its
operational relationship with an eye 12. As shown, imaging system 10
includes a light source 14 which radiates a beam 16 along an optical axis
18. For illustrative purposes, beam 16 is contained within an envelope
defined by boundaries 20 and 22. Although several active media can be used
within the contemplation and spirit of the present invention to produce
beam 16, light source 14 is preferably a HeNe laser which emits light
having a wavelength of approximately 632 nanometers.
After leaving light source 14, beam 16 is incident on turning mirror 24 and
is reflected therefrom toward fifty percent (50%) mirror 26. At this
point, beam 16 passes through 50% mirror 26 and continues along optical
axis 18 to pass through filter 28 before being reflected by selectively
reflective mirror 30 toward a spatial filter (unnumbered). For purposes of
the present invention, the filter 28 is transparent to light from light
source 14 and selectively reflective mirror 30 is completely reflective of
light from light source 14.
From mirror 30, beam 16 is directed through the spatial filter which
comprises, in order along optical axis 18, a convex lens 32, an aperture
34 and a convex lens 36. As is well known to the skilled artisan, such a
combination focuses beam 16 at the pin hole 38 of aperture 34 for the
purpose of limiting the quantity of light which is to be passed along
optical axis 18. More specifically, convex lens 32 focuses beam 16 onto
pin hole 38 of aperture 34. Further, as a confocal arrangement, convex
lens 36 will focus light reflected along optical axis 18 onto pin hole 38
of aperture 34. Preferably, for the purposes of imaging system 10, pin
hole 38 is approximately ten microns in diameter so that the focal depth
of imaging system 10 will be approximately 5-10 microns.
The steerability of beam 16 within the imaging system 10 is provided by the
concerted operation of galvanometric mirrors 40 and 50. This operation is
disclosed in detail in our copending application Ser. No. 151,569 for an
invention entitled "3-Dimensional Laser Beam Guidance System" which was
filed on Feb. 2, 1988 and which is incorporated herein by reference.
Further to this, however, it will be seen in FIG. 1 that a galvanometer 42
is operatively connected with galvanometric mirror 40 for rotating mirror
40 around the axis 44 in the directions of arrow 46. As will be
appreciated by the skilled artisan, this movement of mirror 40 directs
optical axis 18 of beam 16 from mirror 40 on paths which lie in a plane
that is perpendicular to the plane of FIG. 1. From mirror 40, beam 16
passes through convex lenses 48 and 52 before being incident on
galvanometric mirror 50. Clearly, the operation of mirror 50 is similar to
that for mirror 40. Specifically, however, galvanometric mirror 50 is
rotated by galvanometer 54 about the axis 56 in the directions of arrow 58
to steer beam 16 onto paths which radiate from mirror 50 in the plane of
FIG. 1. From the above, and the referenced co-pending application, it will
be understood that the beam 16 can be effectively steered in an x-y plane
by either preprogrammed or manual movement of mirrors 40 and 50.
A focusing lens 60 is provided on optical axis 18 for the purpose of
focusing beam 16 at a desired point on optical axis 18 that is coincident
with a selectable point in the target area. Specifically, lens 60 is
operatively connected with a focusing element 62 which will move lens 60
in the directions indicated by arrow 64 to focus beam 16. This arrangement
is well known in the art and any focusing element 62 can be effectively
used for the intended purposes. It is to be noted that, as mentioned
above, imaging system 10 provides a confocal arrangement with both convex
lens 32 and convex lens 36 focusing light onto pin hole 38 of aperture 34.
It will be understood that this confocal arrangement is compatible with
the operation of focusing lens 60. Importantly, beam 16 must be focusable
at both pin hole 38 in aperture 34 and at a selectable point in the target
area.
Various optical elements 66 can be incorporated into imaging system 10. For
example, element 66 will preferably include components disclosed in our
co-pending applications for an invention entitled "3-Dimensional Laser
Beam Guidance System," which is cited above, and another application Ser.
No. 154,859 for an invention entitled "Eyetracker and Method of Use" which
was filed on the same date as this application and which is incorporated
herein by reference. It is to be understood, however, that the imaging
system 10 of the present invention is useable with any surgical laser
system in which enhanced controllability is a prerequisite.
After being incident on eye 12, portions of beam 16 will be reflected back
through system 10 along axis 18. Part of this reflected light will pass
through pin hole 38 of aperture 34 and then be reflected by selectively
reflective mirror 30 through filter 28. 50% mirror 26 will direct part of
the reflected light along path 72 toward photomultiplier tube 74 where a
signal proportional to the intensity of the light reflected from eye 12 is
generated. This signal is then transmitted by electrical connection 76 to
microprocessor 78 where the signal is used in accordance with a
predetermined program for the operation of the imaging system 10.
It is to be appreciated that imaging system 10 is intended to be used with
a surgical laser system. Thus, a cutting laser source 80 may be used and
incorporated generally as shown in FIG. 1. Preferably, a laser source such
as the one disclosed in our co-pending application entitled
"Multiwavlength Laser Source" is incorporated. Regardless what source 80
may actually be, the generated cuttng laser beam 82 is to be coaxially
aligned with beam 16 in imaging system 10 and confocally focused
therewith. More precisely, cutting laser beam 82 and beam 16 can be placed
in coaxial alignment and both steered and confocally focused while in such
alignment by imaging system 10. It will be appreciated, however, that
filter 28 blocks light from cutting laser source 80 to protect light
source 14 and photomultiplier 74 from adverse effects which may be caused
by cutting laser beam 82.
OPERATION
Although the actual layout of components for the imaging system 10 is best
seen in FIG. 1, its operation is best understood by reference to FIGS. 2
and 3. More specifically, the operation of imaging system 10 is understood
by recognizing the way in which it causes a beam 16 to interact with eye
12.
In FIG. 2, a cross-section of cornea 70 is shown with its various layers
identified. Specifically, cornea 70 comprises epithelium 84, Bowman's
membrane 86, stroma 88, Descemet's membrane 90 and endothelium 92.
Together, these layers form the cornea 70 which separates the aqueous
humor 94 inside eye 12 behind cornea 70 from the air 96 which is outside
eye 12 and in front of corea 70. For purposes of the present invention,
the refractive index of air will be taken as 1.00. For all practical
purposes, the refractive index of each layer in cornea 70 is the same as
the refractive index of every other layer. The value of this refractive
index is approximately 1.376. Importantly, the refractive index of aqueous
humor 94 is approximately 1.336. Thus, there are discernable differences
between the refractive index of air 96 and that of cornea 70, and between
the refractive index of cornea 70 and that of aqueous humor 94.
Still referring to FIG. 2, it can be appreciated that as beam 16 passes
from air 96 with refractive index 1 into cornea 70 with a refractive index
of 1.376, part of beam 16 will be reflected at the interface 98
therebetween. The actual amount of light which is reflected will depend,
of course, on the brilliance (i.e. concentration of photons) of beam 16 as
it is incident on interface 98. Thus, if beam 16 is focused on interface
98, its brilliance is increased and the amount of reflected light is also
increased. The same is true for beam 16 as it is incident on interface 100
between cornea 70, with refractive index 1.376, and aqueous humor 94, with
refractive index 1.336. It happens, however, due to the magnitude of the
differences between the respective refractive indices, focused light will
be reflected form interface 98 with approximately one hundred (100) times
the brilliance of that for focused light reflected from interface 100.
Nevertheless, when beam 16 is focused at point 102 on interface 100, its
brilliance in specular reflection significantly surpasses the brilliance
of unfocused light reflected from interface 98. Further, as reflected
light from interface 100 is radiated back through imaging system 10 along
axis 18, the effect is enhanced by aperture 34 which effectively
suppresses the unfocused light reflected from interface 98. On the other
hand, because imaging system 10 is a confocal system, beam 16 is focused
at both point 102 on interface 100 and at pinhole 38 of aperture 34. The
result is that specularly reflected light from point 102 is passed through
aperture 34, without loss of intensity, and is transmitted to
photomultiplier 74 and microprocessor 78 for further processing in
accordance with selected procedures.
This same explanation applies whenever beam 16 is focused at any other
interface between media having different indices of refraction. Thus,
interface 98 could likewise be imaged if beam 16 were focused thereon. On
the other hand, whenever beam 16 is focused at a point, such as point 104
where there is no change in refractive index, there will be no specular
reflection. In such cases, imaging system 10 generates no operational
signal at photomultiplier 74. Interfaces 98 and 100, however, will
generate operational signals and can thereby be indentified. In accordance
with the operation of system 10, this is done merely by steering beam 16
with the concerted operation of mirrors 40 and 50 while simultaneously
varying the focal point of beam 16 along optical axis 18 by operation of
focusing element 62.
FIG. 3 presents a special situation of interest for the imaging system 10
of the present invention. Specifically, FIG. 3 shows an internal ablation
106 formed into stroma 88 of cornea 70. As will be appreciated by the
skilled artisan, creation of internal ablation 106 creates interface 108
which will be detectable by imaging system 10 in the manner previously
described. Thus, in accordance with the imaging system 10 of the present
invention, beam 16 can be used to identify interfaces 98, 100 or 108
according to the desires and needs of the operator. Thereafter, cutting
laser beam 82 can be selectively activated to cut at desired focal points
relative to the established interface. Further, beam 16 and cutting laser
beam 82 can be operated concurrently.
While the particular imaging system as herein shown and disclosed in detail
is fully capable of obtaining the objects and providing the advantages
herein before stated, it is to be understood that it is merely
illustrative of the presently preferred embodiments of the invention and
that no limitations are intended to the details of construction or design
herein shown other than as defined in the appended claims.
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