U.S. patent application number 09/949254 was filed with the patent office on 2002-01-17 for device for ophthalmologically treating the eye using a fixation light beam.
This patent application is currently assigned to Wavelight Laser Technologie AG. Invention is credited to Donitzky, Christof.
Application Number | 20020007178 09/949254 |
Document ID | / |
Family ID | 7627383 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020007178 |
Kind Code |
A1 |
Donitzky, Christof |
January 17, 2002 |
Device for ophthalmologically treating the eye using a fixation
light beam
Abstract
A device for ophthalmologically treating the eye has a treatment
laser beam (UV) for ablating parts of the cornea (12) and a
fixation light beam (24). A fixation light spot in the vicinity of
the fovea (30) and the fovea are imaged by means of a camera (40).
This makes it possible to check whether the patient has reliably
fixated the fixation light source (22). In addition, the pupil can
be recorded and both recordings can be superimposed.
Inventors: |
Donitzky, Christof;
(Eckental, DE) |
Correspondence
Address: |
Andrew V. Smith
Sierra Patent Group
P.O. Box 6149
Stateline
NV
89449
US
|
Assignee: |
Wavelight Laser Technologie
AG
|
Family ID: |
7627383 |
Appl. No.: |
09/949254 |
Filed: |
September 7, 2001 |
Current U.S.
Class: |
606/5 ;
606/10 |
Current CPC
Class: |
A61B 2018/2025 20130101;
A61F 2009/00872 20130101; A61F 9/008 20130101; A61F 9/00804
20130101; A61B 3/113 20130101; A61F 2009/00846 20130101 |
Class at
Publication: |
606/5 ;
606/10 |
International
Class: |
A61B 018/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2000 |
DE |
100 01 131.4 |
Jan 15, 2001 |
EP |
PCT/EP01/00393 |
Claims
1. Device for ophthalmologically treating the eye, comprising a
treatment laser beam (UV) for ablating parts of the cornea (12) and
a fixation light beam (24) that is provided to be fixated by the
patient, characterized by a camera (40) for imaging a light spot
(Sp) generated by the fixation light beam (24) on the retina, in
particular the fovea, and the fovea (F).
2. Device according to claim 1, characterized by a device (36) for
imaging the pupil (18, P) in such a way that the image of the light
spot (Sp) of the fixation light beam (24) and the image (P) of the
pupil are superimposed.
3. Device according to claim 2, characterized in that the device
for imaging the pupil is the said camera.
4. Device according to claim 2, characterized in that the device
for imaging the pupil is a second camera (40).
5. Device according to claim 4, characterized in that the second
camera (40) is also assigned to an eye tracker.
Description
[0001] The invention relates to a device for ophthalmologically
treating the eye using at least one treatment laser beam to ablate
parts of the cornea and a fixation light beam that is provided to
be fixated by the patient.
[0002] In photorefractive keratectomy (PRK), an ametropia of the
human eye is corrected by partly reshaping the cornea. A special
PRK method that is appreciably gaining importance at the present
time is LASIK. In the LASIK method, a lid ("flap") is cut in the
cornea and folded back. Then a UV laser beam (normally an excimer
laser beam having a wavelength of 193 nm) is directed at the
exposed parts (laid bare by the lid) of the cornea in order to
remove (to ablate) material at that point. After the desired
ablation, the lid is shut again and consolidates with the
cornea.
[0003] The present invention relates generally to PRK and, in
particular, the LASIK method.
[0004] In the photorefractive ophthalmological method, it is
important to position the eye precisely with respect to the laser
radiation, in particular the ablation beam, used, i.e. in the case
of every laser pulse that impinges on the eye in an ablating
manner, the system must "know" precisely the point at which the
laser beam impinges on the eye. For this purpose, so-called "eye
trackers" are used in the prior art. These are devices with which
the respective instantaneous position of the eye can be determined
in order to control the laser beam in accordance with said
determined position. In this connection, the laser beam is guided,
for example by means of a scanner, temporally and spatially over
the eye surface to be treated (in the case of LASIK, for example,
in the stroma), the temporal and spatial control of the laser spot
(focus spot) being such that a desired ablation profile is removed
(ablated). In this connection, a so-called fixation light beam has
to be used in the prior art. A fixation light source is positioned
in such a way that the patient can fixate it visually. The patient
is asked to do this. This has the object of arresting the eye as
constantly as possible in that the patient fixates uninterruptedly
the fixation light source. Since the patient has to recognize the
fixation light source in this process, it goes without saying that
the fixation light source emits a fixation light beam having
wavelengths in the visible range, for example, in the green
range.
[0005] However, the patient does not generally succeed in fixating
the fixation light beam in a completely uninterrupted manner. In
the prior art, therefore, the said "eye trackers" are known, i.e.
optoelectronic systems, with which movements of the eye can be
detected. Such movements occur if the patient (involuntarily)
"loses sight", literally, of the fixation light source for a more
or less short period of time. If the patient fixates the fixation
light source in the ideal way, the fixation light is imaged
precisely on the fovea. If, on the other hand, the patient loses
sight of the fixation light source, the fixation light is no longer
imaged on the fovea, but on another point of the retina, that is to
say next to the fovea, to a greater or lesser extent remote from
it. The said eye tracker of the prior art, which is assumed herein
to be known, detects, for example, movements of the eye by
recording the pupil by means of a camera and an image evaluation in
which movements of the pupil are detected. The treatment laser beam
is then controlled in such a way that such detected eye movements
are taken into account and the ablation takes place precisely in
accordance with the desired ablation profile despite the eye
movements.
[0006] The present invention is based on the insight that, in the
event of relative positioning of eye and ablation laser beam,
special problems may arise if fixation light is used because the
patient does not correctly fixate the fixation light source at all
during the stress of the operation or for other reasons. The object
of the invention is to provide a remedy in such a situation.
[0007] According to the invention, said object is achieved by a
camera that records the image of the fixation light on the retina,
in particular in the region of the fovea, and the fovea. In the
ideal case, the spot of the fixation light is situated precisely on
the fovea if the patient fixates the fixation light source
correctly. If, on the other hand, the patient does not fixate the
fixation light source correctly, the fixation light is not imaged
precisely on the fovea but at another point in the retina. This can
then be detected with the camera provided according to the
invention. The optical imaging elements of said camera are so
designed that images are imaged sharply in the retina plane in the
region of the fovea with the camera. Consequently, the
ophthalmologist can establish before and during the operation
whether the patient has "his eye" on the fixation light source as
prescribed. This check can also be automated by image processing.
In this connection, the camera system is at least approximately
sharply focused on the retina surface at the level of the fovea,
and an image of the fovea and of the adjacent regions of the retina
is recorded. Image processing can then establish whether the spot
of the fixation light is situated sufficiently precisely on the
fovea or not. If it is established that the patient does not fixate
the fixation light source sufficiently precisely, i.e. with
sufficiently small deviations (viewed temporally and according to
distance), the physician can then draw conclusions from this for
the treatment and, if necessary, take measures. The checking of the
relative position of the fixation light spot on the retina with
respect to the fovea can take place before and/or during the
operation.
[0008] In accordance with a preferred refinement of the invention,
not only is the position of the fixation light spot with respect to
the fovea measured and evaluated, but the pupil is also
additionally recorded by a camera. Both recordings, that is to say
the recording of the fixation light spot and fovea, on the one
hand, and the recording of the pupil, on the other hand, are
performed with respect to a common, constant axis, for example the
fixation light axis. The two images differ therefore in the
focusing plane: the imaging of the pupil is offset with respect to
the imaging of the fixation light spot and fovea by about the
diameter of the eye, that is to say 2 to 3 cm.
[0009] The recording of pupil and fovea by the fixation light spot
can preferably be superimposed on one another, i.e. a picture is
produced in which, on the one hand, the pupil is imaged and, on the
other hand, the fovea and the fixation light spot. This makes it
possible not only to check whether the patient has reliably fixated
the fixation light source, but also to determine a beneficial
central axis for the ablation. If the patient fixates the fixation
light source then there appear on the superimposed image explained
above the pupil, the fovea (more precisely: the macula lutea) and
the fixation light spot, the latter precisely centrally in the
fovea. Under ideal conditions, the fixation light spot (and the
fovea) is situated centrally in the pupil. Under real conditions,
however, the fixation light spot together with fovea is frequently
not central with with respect to the pupil, i.e. on the
superimposed image, the fixation light spot with the fovea is
offset with respect to the centre of the pupil. With this finding,
the subsequent ablation takes place in a centred manner in
accordance with a preferred refinement of the invention with
respect to a point that is eccentric with respect to the pupil and
that is defined by the centre point of the fixation light spot and
fovea (with precise fixation, this is one and the same point).
[0010] The two recordings mentioned above, that is to say pupil, on
the one hand, and fixation light spot with fovea, on the other
hand, can be generated by a single camera if the imaging plane
(focus) is varied for the two recordings in the manner of an
autofocus effect. This can be done with suitable zoom means
periodically in a short time sequence so that the said
superimposition image can be produced.
[0011] On the other hand, however, it is also possible, and
preferred at the present time,.to work with two cameras, i.e. a
first camera records the pupil and another camera simultaneously
records the fovea with the fixation light spot. Both images can be
combined and superimposed in a computer, and can be displayed on a
display in the superimposed state or fed to an automated image
processor in the manner described above.
[0012] The cameras used are preferably video cameras, particularly
preferably solid-state video cameras, for example CCDs or the
like.
[0013] In this connection, a camera can preferably be used that is
available in any case in the ophthalmological treatment system for
the purposes of "eye tracking".
[0014] An exemplary embodiment of the invention is explained in
greater detail below by reference to the drawing. In the
drawing:
[0015] FIG. 1 shows diagrammatically a device for the
photorefractive keratectomy of the eye, in particular in accordance
with the LASIK method; and
[0016] FIGS. 2, 3 and 4 show diagrammatically superimposition
images of pupil, fovea and fixation light spot in various
situations.
[0017] The eye 10 shown diagrammatically in FIG. 1 has a cornea 12,
an iris 14, a lens 16 and a pupil 18.
[0018] A fixation light source 22 known per se emits a fixation
light beam 24 that penetrates the front surface of the cornea 12 at
the point 20. The wavelength of the fixation light beam 24 is such
that it is visible to the patient, that is to say, for example, in
the green region of the spectrum. A diode is normally used as
fixation light source 22. The fixation light beam 24 is stationary
and the patient is urged to fixate the fixation light source, which
appears to him to be punctiform.
[0019] An excimer laser Ex emits the actual ablation beam, that is
to say the beam with which the cornea 12 is reshaped. Said ablation
beam UV (for example, 193 nm) is deflected via a mirror UV-S and
guided over the cornea 12 in accordance with an ablation algorithm
so that the desired ablation profile is removed. The ablation beam
is therefore not stationary. The means for moving ("scanning") the
ablation beam are known per se and not shown in greater detail in
the figure.
[0020] The fixation light beam 24 passes through the cornea and the
pupil 18 and is imaged on the fovea 30. It is therefore also
described as the "line of sight". Said line of sight therefore
joins on the object side the fixation point (that is to say the
point of the fixation light source 22) to the centre of the entry
pupil. The "entry pupil" is the virtual image of the real pupil
that an observer sees on viewing the eye.
[0021] The position 20 at which the fixation light beam 24 passes
through the front surface of the cornea 12 may be chosen as the
centre for the ablation, i.e. the ablation profile in accordance
with which the ablation beam UV is guided ("scanned") over the
cornea 12 is centred on the point 20 at which the fixation light
beam 24 passes through the exposed front surface of the cornea 12.
In the LASIK method, the front surface of the cornea is in this
context the exposed surface after folding back the so-called lid
(flap). In order to determine the penetration point 20 on the
cornea 12, a centring light source 32 is used that, in the
exemplary embodiment shown, emits a laser beam in the infrared
range. Said centring light beam 34 is directed via a partly
transparent mirror 26 coaxially with the fixation light beam 24
onto the cornea 12. In the figure, the fixation light beam 24 and
the centring light beam 34 are shown in parallel next to one
another, but they actually extend coaxially, i.e. on a common
central axis. This means that the centring light beam 34, which is
stationary during the operation, also passes through the front
surface of the cornea 12 at the penetration point 20. In the
exemplary embodiment, the centring light beam 34 has a wavelength
in the infrared range, for example, in the range from 800 to 1100
nm. It is important that the centring light beam 34 has a
wavelength that is different from the wavelength of the fixation
light beam 24 so that reflections and images that are generated by
both beams can be discriminated from one another, i.e. because of
the different wavelengths, it is possible to measure a reflection
of the centring light beam 34 at the front surface of the cornea 12
without interference by the fixation light beam. Accordingly, the
penetration point 20 is measured by measuring the
scattered-light/Fresnel reflection of the centring light beam at
the front surface of the cornea. For this purpose, a partly
transparent mirror 28 is used that directs the
scattered-light/Fresnel reflection 34' of the centring light beam
onto a camera 36. The camera 36 is, in the exemplary embodiment
shown, also for other reasons part of the device, namely as a
so-called "eye-tracking camera" (cf. DE 197 02 335 and the prior
art mentioned therein).
[0022] The use of a special centring light beam 34 to determine the
penetration point 20 of the fixed radiation at the front surface of
the cornea has, compared with the use of the fixation light beam 24
for this purpose, the advantage that a relatively high-power
reflection not swamped by other images can be evaluated by means of
the camera 36 and a downstream evaluation computer 38. The
scattered-light/Fresnel reflection of the fixation light is also
itself swamped by the Purkinje-Sanson image, with the result that
this reflection is difficult to evaluate.
[0023] The camera 36 and the computer 38 into which the camera
measurements are inputted, form a so-called eye-tracking system
(cf. the abovementioned prior art). For this purpose, the eye is
illuminated with independent radiation, for example IR radiation
46, generated by a light source 44 and the pupil 18 is, for
example, measured by means of its rim in order to determine, in
particular, the geometrical centre of the pupil (the so-called
"centre of gravity of the pupil"). In addition, the system
comprising camera 36 and computer 38 now also measures the position
of the scattered-light/Fresnel reflection of the centring light
beam 34 at the front surface of the cornea 12, i.e. at the position
of the penetration point 20. The camera 36 is consequently IR
sensitive in the exemplary embodiment shown. Preferably, the system
comprising camera 36 and computer 38 determine the relative
position between penetration point 20 and geometrical centre
("centre of gravity") of the pupil 18.
[0024] In order to check whether the patient fixates the fixation
light source 22 (i.e. the fixation light beam 24) sufficiently
precisely, a further camera 40 is provided in the exemplary
embodiment in accordance with FIG. 1. The camera 40 is, for
example, a video camera (CCD) and the image signals are likewise
electrically inputted into the computer 38. The optical means (not
shown) of the camera 40 are designed in such a way that they record
an image in the plane of the fovea 30 of the eye 10. The imaged
spot of the fixation light beam 24 is, in the ideal case, i.e. if
the patient fixates the fixation light source 22 precisely,
situated precisely on the fovea 30. If the fixation on the part of
the patient is inaccurate, the fixation light spot is situated
alongside the fovea 30. The camera 40 therefore receives via a
partly transparent mirror 41 radiation with which the image plane
of the fovea and its surroundings is recorded in the camera 40. The
image thus produced with fovea and fixation light spot can be
displayed to the physician on a viewing screen so that he can check
the fixation on the part of the patient. The evaluation of the
images can also be automated in the computer 38 using the
technology of image processing.
[0025] In FIG. 1, the individual laser radiation sources and the
deflection mirrors are shown only diagrammatically for the purpose
of facility of inspection. In practice, the excimer laser beam will
be coupled in a different way to that shown, in particular as near
as possible to the eye since special requirements are imposed on
partly transparent mirrors for UV radiation. Consequently, the
arrangement of the partly transparent mirrors in practice will be
such that the mirror UV-S is still underneath the mirror 41.
[0026] In accordance with a preferred refinement of the invention,
the cameras 36 and 40 shown in FIG. 1 are used (the camera 36
additionally to the function described above) in such a way that an
image of the pupil 18 is recorded by the camera 36 and inputted
into the computer 38, while the abovedescribed image in the plane
of the fovea 30 with the fixation light spot on the retina is
recorded by the camera 40 and likewise inputted into the computer
38. Both images are recorded with respect to a fixed common axis so
that both images can be superimposed on one another in the computer
38, with the result that a conclusion is possible relating to the
relative positioning of the fovea, fixation light spot and
pupil.
[0027] This is shown diagrammatically in FIGS. 2, 3 and 4. These
figures each show the superimposed images mentioned.
[0028] FIG. 2 shows the rim P of the pupil, the fovea F and the
fixation light spot Sp, such as are obtained with the aid of an
abovedescribed superimposed image of the two cameras 36 and 40 in
the computer 38 and can, optionally, be displayed on a suitable
viewing screen. In the situation in accordance with FIG. 2, the
patient does not fixate the fixation light source correctly. The
physician detects this from the fact that the fixation light spot
Sp is not precisely in the fovea F but is offset with respect to
it. This has the consequence that the physician has to take
appropriate measures to induce the patient to make a precise
fixation.
[0029] FIG. 3 shows the ideal case, in which the patient fixates
precisely and consequently the fixation light spot Sp is situated
precisely concentrically with the fovea F. With the finding in
accordance with FIG. 3, the fovea F is also central in the pupil P.
This does not always have to be the case. FIG. 4 shows an example
in which, although the patient correctly fixates the fixation light
source 22 so that the fixation light spot Sp is situated precisely
in the fovea F, the fovea F is not central in the pupil P. Still
more complicated findings are possible in which the pupil rim shown
does not at all have the ideal circular shape shown in the figures,
but deviates from it. This applies, in particular, after
ophthalmological operations already performed earlier.
[0030] In the case of a finding in accordance with FIG. 4, the
position of fixation light spot Sp and fovea F found to be
eccentric with respect to the pupil P can be chosen as ablation
centre, i.e. the ablation takes place in a centred manner with
respect to the central point of Sp and F. This option has also
given good results.
* * * * *