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BACKGROUND OF THE INVENTION
This invention relates to guidance systems for laser surgery. More
specifically, the present invention relates to a control system which is
able to precisely maintain the proper position of a laser beam during
surgery. This invention is particularly, but not exclusively useful for
controlling a laser surgical device during ocular surgery without
immobilizing the eye.
DISCUSSION OF THE PRIOR ART
Medical surgical procedures using laser beams to alter tissue in the target
area have been practiced for many years with great success. Fortunately,
as our knowledge of lasers is increased, there is a concomitant
recognition of new ways in which lasers can be effectively employed.
Ophthalmic surgery is one field of use where new uses for laser surgery
have been efficaciously applied.
A real problem with ophthalmic or ocular surgery, however, is the fact that
the eye is so easily moved. Additionally, of course, the sensitivity of
this very delicate organ creates a whole set of unique concerns. Thus,
there is a widely recognized need to be extremely precise with any
procedure which alters tissue of the eye. It happens that the more precise
the surgical instrument can be in its operation, the more there is a
desire to benefit from that precision by having accurate control over the
instrument in its relationship with the eye.
In the past, various attempts have been made to fixedly hold the eye during
ocular surgery. An example of a device intended for this purpose is found
in U.S. Pat. No. 4,665,913 to L'Esperance Jr. wherein an eye retaining
fixture is disclosed. Specifically, a device of this type is suctionfitted
onto the eye to establish external means by which the eye can be held. As
might be expected, the use of such a device can be very painful for the
patient and the efficacy of such a device can be questioned.
Unlike the devices of the prior art which require immoblization of the eye
in order to achieve their precision, the present invention achieves the
required operative precision while allowing the eye some degree of
movement. Specifically, the present invention recognizes that precise
ophthalmic surgical procedures can be accomplished without immobilizing
the eye. This is possible because, unlike the prior art devices, the
present invention further recognizes that an ophthalmic surgery device can
be programmed to track the eye by monitoring references which are marked
on the eye in a known relationship with the visual axis of the eye.
In light of the above, it is an object of the present invention to provide
an eyetracker with will keep an ophthalmic surgery device in a
preprogrammed relationship with the visual axis of the eye during surgery.
Another object of the present invention is to provide an eyetracker which
allows the accomplishment of ophthalmic surgical procedures without
immobilizing the eye. Still another object of the present invention is to
provide a device which is useful for identifying either the visual axis or
the symmetrical axis of the eye. Yet another object of the present
invention is to provide a reference grid on the cornea of the eye which
has a known relationship with the visual axis of the eye and which
precisely identifies the areas of the cornea on which surgical procedures
are to be performed. A further object of the present invention is to
provide an eyetracker for use with ophthalmic laser surgical instruments
which is easy to operate, relatively easy to manufacture and which is cost
effective.
SUMMARY OF THE INVENTION
A preferred embodiment of the novel eyetracker comprises a visual light
source on which a patient can fixate to coaxially align the visual axis of
the patient's eye with a segment of the axis of the visual light beam. A
laser source also coaxially aims its beam along this axis segment of the
visual light beam. Thus, when the patient's eye is fixated, a reference
alignment is established between the eye's visual axis and the laser beam
since the eye's visual axis and the laser beam are both coaxially aligned
on the same axis segment. While the patient's eye is initially in
reference alignment, the laser source is activated to mark a grid on the
cornea of the patient's eye which fixes a known relationship between the
grid and the eye's visual axis.
A source of diffused infrared light is provided to illuminate the grid
marked cornea. Reflections therefrom are optically directed to a sensor
where movements of the grid out of its reference alignment are detected.
At the sensor, variations in the intensity of the reflected infrared light
are used to generate signals which are representative of any grid
movement. These signals are then transmitted by electronic means from the
sensor to a comparator. In the comparator, each grid movement signal is
compared with a reference signal which is representative of the grid
position when in reference alignment. An error signal, proportional to the
difference between the "grid in reference alignment" signal and the "grid
movement" signal, is generated and transmitted to a guidance system which
steers the laser beam in a manner that reduces the error signal to a null.
While maintaining the error signal at a null, the laser beam is steered to
make controlled external or internal ablations of the cornea in accordance
with a predetermined computerized program.
In addition to the use of diffused infrared light for monitoring grid
movement to establish a closed loop feedback control for the laser
guidance system, the source of diffused infrared light may also be used to
help identify the eye's axis of symmetry. For this latter purpose, the
sensor is focused on the plane of the iris rather than on the cornea. When
focused on the plane of the iris, the sensor is focused along a plurality
of lines to detect the intensity of the light that is reflected from the
sclera, the iris and the pupil. The sensor then transmits this information
to a computer which uses it to precisely determine the exact relationship
between sclera, iris and pupil. Also, a beam of collimated infrared light
from a laser diode is focused onto the cornea. In alternation with its
focus on the iris plane, the sensor is focused on the cornea to scan the
cornea and detect specular reflections from the beam of collimated
infrared light. Due to the shape of the cornea, the specular reflection of
greatest intensity will come from the apex of the cornea. Since the sensor
is preferably a line diode, the apex of the cornea is identified when the
most intense specular reflection falls on the center of the line diode.
This information is also sent to the comparator where information from the
iris plane concerning the relationship of the sclera, iris and pupil, and
information from the corneal plane concerning the apex of the cornea are
used together to determine the eye's axis of symmetry.
With the information obtained as discussed above, surgical laser operations
on the cornea can be accomplished using the reference alignment without
immobilizing the eye. This is accomplished whether the reference alignment
is made directly on the visual axis of the eye or on the visual axis as
empirically determined by its relationship with the eye's axis of
symmetry.
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 side elevation view of surgical equipment comprising the
present invention operatively positioned for surgery on a patient's eye;
FIG. 2 is a schematic view of the eyetracker and its associated optical
elements shown in relationship with a cross-sectional view of a portion of
an eye;
FIG. 3 is a cross-sectional view of a portion of an eye;
FIG. 4 is a front plan view of an eye marked with a grid;
FIG. 5 is a front plan view of an eye shown with superposed representative
scan lines;
FIG. 6 is a graph of the intensity of light reflected from the iris plane
of an eye along a preselected scan line;
FIG. 7 is a graph of the intensity of light reflected from the cornea of an
eye under prescribed conditions; and
FIG. 8 is a functional block diagram of a closed-loop feedback system as
incorporated into the electronic circuitry of the eyetracker.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring initially to FIG. 1, it is to be appreciated that the eyetracker
of the present invention is a component of a surgical laser device
generally designated 10. As shown in FIG. 1, surgical laser device 10 with
its component eyetracker is operatively positioned for surgical procedure
on patient 12. Although patient 12 shown sitting in a chair 14 for
purposes of ophthalmic or ocular surgery, it is to be understood that
patient 12 may be placed in a reclining position on an operating table
(not shown) without compromising the effectiveness of the present
invention.
The patient 12, while sitting in chair 14, is placed with his/her head 16
positioned within restraint 18 for purposes of restricting movement of
his/her head during surgical operations. In accordance with the present
invention, the eye 20 of patient 12 need not be immobilized during
surgery. Instead, the present invention is intended to compensate for
modest movements of eye 20.
It is to be understood that the present invention is preferably intended
for use with a laser guidance system of the type disclosed in our
co-pending application for an invention entitled "3 Dimensional Laser Beam
Guidance System" which was filed on Jan. 27, 1988 and which is
incorporated herein by reference. The major components of this guidance
system are shown in FIG. 2 in cooperation with elements of the present
invention and are shown to generally comprise a fine tuner 24, a focusing
element 26 and a laser source 22 of a type disclosed in our co-pending
application entitled "Multiwavelength Laser Source" which was filed on
Feb. 2, 1988 and which is incorporated herein by reference. A comparator
(computer) 28 is shown electrically connected to provide programmable
input for fine tuner 24 and focusing element 26 of the guidance system.
Referring for the moment to FIG. 3, a brief description of eye 20 and
certain of its geometric properties will be helpful. Specifically, in FIG.
3 it will be seen that a cross-section of eye 20 reveals a cornea 30 which
is set apart from the iris 32 of eye 20. It will also be appreciated that
iris 32 establishes pupil 34 of eye 20 and that a lens 36 is functionally
positioned relative to the iris and behind the pupil, substantially as
shown. A greater detailed description of the human eye 20 need not be
provided for an adequate description of the functioning of the present
invention. However, some very important concepts with regard to the
alignment of eye 20 are important insofar as the present invention is
concerned.
It is well known that the human eye can be discussed with regard to two
separate axes. First, there is the axis of symmetry 38. Essentially, as
the name implies, the axis of symmetry 38 is that axis about which a
rotation of a section of the eye will generate a three dimensional model
of the complete eye. It happens, however, that the visual functioning of
the eye does not occur along the axis of symmetry 38. Instead, visual
acuity occurs along a visual axis 40. As indicated in FIG. 3, there is a
slight off-set 42 between the axis of symmetry 38 and the visual axis 40.
Although the magnitude of this off-set will vary slightly from individual
to individual, it can be empirically determined and is generally around
five degrees. It is also helpful for an understanding of the present
invention to define the corneal plane 44 as that plane which is tangential
to the cornea 30 at the point where the axis of symmetry 38 is normal to
cornea 30. Also, it is helpful to define the iris plane 46 as that plane
which generally passes across the pupil 34 between the diametrical
extensions of iris 32.
Referring back to FIG. 2, it will be seen that the eyetracker of the
present invention includes a source of visible light 48. Preferably,
source 48 is a quadratic array of light emitting diodes which radiate
blue-green light. Light beam 50, which radiates from visible light source
48, passes through a negative lens 52 and is reflected by turning mirror
54 toward a selectively reflective mirror 56. After passing through mirror
56, beam 50 of visible light passes through objective lens 58 and is
incident upon cornea 30 of eye 20. Preferably, and for reasons to be more
clearly apparant after subsequent discussion, selectively reflective
mirror 56 is transparent for visible light and for all wavelengths used
for the cutting laser beam for laser source 22. On the other hand,
selectively reflective mirror 56 should be reflective for infrared
components of light and, more specifically, mirror 56 should be reflective
of infrared light having wavelengths which are radiated by diode array 66.
It will be recalled that visual axis 40 is the most important reference
insofar as vision is concerned. Therefore, in ocular surgery, some
operative reference to visual axis 40 needs to be established. This is
done, in accordance with the present invention, by directing visible light
from source 48 toward eye 20 to establish a spot of light upon which the
patient 12 can fixate. Accordingly, if patient 12 fixates on the beam 50
of light coming from source 48, the visual axis 40 of eye 20 will be
coaxially aligned with beam 50. This coaxial alignment is extremely
important insofar as the present invention is concerned since, as
inferenced above, in ophthalmic or ocular surgery it is desirable that
corneal surgical procedures be accomplished with reference to the visual
axis 40.
With visual axis 40 of eye 20 coaxially aligned along a portion of beam 50
of visible light, a reference grid 60 is marked on the cornea 30 of eye
20. This marking of grid 60 is accomplished by laser beam 64 which
originates at source 22. Specifically, laser beam 64 is coaxially aligned
within device 10 along that portion of beam 50 which is directed by
turning mirror 54 toward eye 20. Thus, when patient 12 fixates on beam 50,
laser beam 64 is coaxially aligned with the visual axis 40 of eye 12. This
caged coaxial alignment between visual axis 40 and laser beam 64 is
referred to herein as the reference alignment and is used by the
eyetracker of the present invention as a base from which subsequent
movement of laser beam 64 is monitored.
Optically, device 10 uses movement of grid 60 to control movement of laser
beam 64. Therefore, grid 60 must be marked when visual axis 40 and laser
beam 64 are in reference alignment. Specifically, as patient 12 fixates on
beam 50 from light source 48, laser source 22 is activated and steered by
proper programmed input to fine tuner 24 in a manner which will establish
a grid or template of small incisions on cornea 30 of eye 20. Momentarily
referring to FIG. 4, a grid generally designated 60, as envisioned by the
present invention, is shown marked on cornea 30 of eye 20. From FIG. 4, it
will be appreciated that a series of spots 62 are cut into cornea 30 in
any manner desired by the operator for subsequent procedures. As shown in
FIG. 4, groups of spots 62 may be arranged in a coded fashion. Such coding
can be subsequently used at comparator 28 to further identify the
orientation of eye 20. Preferably, spots 62 are small incisions created in
cornea 30 by the cutting laser 64 from laser source 22. Further, it is
preferable that spots 62 be approximately ten microns in depth and
approximately ten microns in diameter. It has been found, in accordance
with the present invention, that these small spots when cut into cornea
30, provide a sufficient optical reference for further operation of the
eyetracker.
Referring back to FIG. 2, it will be seen that a monitoring system is
provided to effectively track movement of eye 20. For this purpose, an
annular array of diodes 66 is positioned in device 10 between eye 20 and
objective 58 to radiate diffused infrared light onto cornea 30 of eye 20.
This light will be reflected from cornea 30 with information concerning
the spots 62 which have been cut into cornea 30 to establish grid 60. This
reflected light passes through objective 58 and is incident on selectively
reflective mirror 56 where it is further reflected along a path 68 toward
galvanometric mirror 70. Although selectively reflective mirror 56 can be
established to reflect various wavelengths of light, this reflectivity
must be compatible with the light emitted from diode array 66. Therefore,
preferably, mirror 56 is reflective of light having wavelengths
approximately 940 nanometers. Such wavelength would be typical for an
infrared source such as intended for diode array 66. On the other hand, it
will be understood that selectively reflective mirror 56 must be
transparent for light coming from source 48 and source 22.
The infrared light reflected from cornea 30 with information regarding grid
60 is reflected by galvanometric mirror 70 where it is directed to pass
through a pair of convex lenses 72a and 72b. After passing through convex
lenses 72 a and 72b, this infrared light is incident on 50% mirror 74
where half the light is passed toward linear diode 76. Linear diode 76 is
preferably a line of individual light sensitive sensors or pixels. Thus,
galvanometric mirror 70 can be continuously rotated in a well known manner
to provide linear diode 76 with a scanning coverage of eye 20. Signals
generated by such a device can then be used in a manner well known in the
art.
FIG. 2 also shows that linear diode 76 is operatively connected with
comparator 28. Through this connection, information concerning the
movement of grid 60 can be transmitted to comparator 28 for comparison
with a signal representative of the reference alignment. Comparator 28
then generates an error signal proportional to the difference between the
actual position of grid 60 as detected by the sensor linear diode 76, and
the desired position of grid 60 in its reference alignment. This error
signal is used by fine tuner 24 to guide laser beam 64 in a manner which
reduces the error signal to a null.
Under certain conditions, i.e. the patient is uncooperative or unable to
cooperate, it may be difficult or impossible to identify visual axis 40.
Therefore, it may be necessary to just identify the axis of symmetry 38
and then empirically determine visual axis 40. For this purpose, FIG. 2
also shows that an infrared light emitting diode 78 can be provided as a
source of infrared light. Infrared light from diode 78 passes through a
collimating lens 80 before being incident on 50% mirror 74. It will be
understood that upon reflection from mirror 74 collimated infrared light
from source 78 is directed by mirror 70 and mirror 56 and through lens 58
onto cornea 30 of eye 20. As previously indicated, galvanometric mirror 70
is moveable to scan the beam of collimated light from source 78 along scan
lines on cornea 30. Reflections of light from these scan lines are
detectable by linear diode 76 and information contained in the reflections
can be transmitted to comparator 28 for use by device 10. Specifically,
this use relates to identification of the symmetrical axis of eye 20 in a
manner to be disclosed below.
It will be understood that with the movement of galvanometric mirror 70,
device 10 is able to reflect collimated light from source 78 and diffused
light from diode array 66 to linear diode 76 where the reflections will be
sensed as line scans. Referring for the moment to FIG. 5, examples of such
scan lines in iris plane 46 are shown as lines 82 and 84. Still referring
to FIG. 5, it can be appreciated that a scan along line 84 will reflect
light from the sclera 86, the iris 32 and the pupil 34 of eye 20.
Importantly, the intensity of reflected light from these parts of eye 20
will vary according to a graph generally shown in FIG. 6.
Cross-referencing FIG. 5 with FIG. 6 indicates that the intensity of
reflections at point 88 on sclera 86 corresponds to comparable point 88 in
FIG. 6 relative to the intensity of the reflected light. Likewise, points
90, 92, 94 and 96 respectively are shown in FIG. 6 as intensity variations
along line 84.
Providing information for comparator 28 in the form of signals which are
proportional to the intensities of reflected light, as generally shown in
FIG. 6, allows for a profile mapping of eye 20. The precision of the
profile is enhanced by taking a plurality of such measurements along a
series of scan lines. Scan lines 82 and 84 are only exemplary.
Recall that information in the collimated infrared light from light
emitting diode 78 which is reflected from cornea 30 is also available for
use by comparator 28. It happens that this light is specularly reflected
from the cornea and that its intensity, sensed by linear diode 76, is
dependent on whether the reflected light is focused. Therefore, since the
cornea is not perfectly spherical, apex 98 of cornea 30 will reflect more
intense light than other spots on cornea 30 when objective 58 is focused
on corneal plane 44. The result, when mirror 70 moves to scan cornea 30,
is an intensity variation of light which is reflected to linear diode 76
that generally follows the graph 100 depicted in FIG. 7. Making linear
diode 76 responsive to the intensity whenever it passes some threshold
value 102 allows a rather precise identification of apex 98. It follows
that by alternating the focus between iris plane 46 and the corneal plane
44 in which apex 98 lies, information concerning the location of apex 98
relative to iris 32 and pupil 34 can be obtained. From this, the axis of
symmetry 38 of eye 20 is determined.
Based upon an empirical determination of the off-set 42 between the
symmetrical axis 38 of eye 20 and the visual axis 40 of eye 20, it will be
possible to control laser beam 64 with reference to either axis. This is
so since the reference alignment can be related through the visual axis 40
to symmetrical axis 38. Thus, when referencing symmetrical axis 38,
movement of eye 20 can be detected by linear diode 76 by monitoring the
specular reflection from the apex 98 of cornea 30.
OPERATION
The actual operation of the eyetracker for surgical laser device 10 will be
best appreciated by reference to the functional block diagram of a closed
loop control system shown in FIG. 8. Specifically, when conforming the
elements of device 10 with the block diagram of FIG. 8, it will be
appreciated that a reference input 106 is provided to comparator 28 which
represents the reference alignment. In accordance with previous
disclosure, the reference alignment contains information pertaining to the
actual location of beam 50 in an established relationship with laser beam
64.
Briefly, the reference alignment is established by first having patient 12
fixate on beam 50. This fixation coaxially aligns visual axis 40 of eye 20
with the axis of beam 50. Since device 10 is built with beam 50 and laser
beam 64 in coaxial alignment, this fixation also coaxially aligns visual
axis 40 with laser beam 64. This relationship is what has been referred to
herein as the reference alignment. Then, with visual axis 40 in reference
alignment, laser beam 64 is used t | | |
