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Description  |
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BACKGROUND OF THE INVENTION
The present invention is directed to a system for delivering high energy
laser by means by an optical waveguide, and in one particular application
is concerned with laser angioplasty.
The use of laser energy to ablate atherosclerotic plaque that forms an
obstruction in a blood vessel is presently being investigated as a viable
alternative to coronary bypass surgery. This procedure, known as
angioplasty, essentially involves insertion of a fiberoptic waveguide into
the vessel, and conduction of laser energy through the waveguide to direct
it at the plaque once the distal end of the waveguide is positioned
adjacent the obstruction. In order to enable the physician to ascertain
the location of the waveguide as it is being moved through the vessel,
additional waveguides for providing a source of illuminating light and for
conducting the image from inside the vessel back to the physician are fed
together with the laser waveguide. Typically, the three waveguides are
encapsulated within a catheter.
Most of the experimentation and testing that has been done in this area has
utilized continuous wave laser energy, such as that produced by Argon Ion,
Nd:YAG or Carbon Dioxide lasers. The light produced by this type of laser
is at a relatively low energy level. Ablation of the obstruction is
achieved with these types of lasers by heating the plaque with the laser
energy over a period of time until the temperature is great enough to
destroy it.
While the use of continuous wave laser energy has been found to be
sufficient to ablate an obstruction, it is not without its drawbacks. Most
significantly, the removal of the obstruction is accompanied by thermal
injury to the vessel walls immediately adjacent the obstruction. In an
effort to avoid such thermal injury, the use of a different, higher level
form of laser energy having a wavelength in the ultra-violet range (40-400
nanometers) has been suggested. This energy, known as Excimer laser
energy, can be provided, for example, by a laser medium such as
argon-chloride having a wavelength of 193 nanometers, krypton-chloride
(222 nm), krypton-fluoride (240 nm) or xenon-chloride (308 nm). The output
energy from this type of laser appears in short bursts or pulses that can
last for 10-85 nanoseconds and have a high peak energy, for example as
much as 200 mJ. Although the destruction mechanism involving this form of
energy is not completely understood, it has been observed that one pulse
of the Excimer laser produces an incision which destroys the target tissue
without accompanying thermal injury to the surrounding area. This result
has been theorized to be due to either or both of two phenomena. The
delivery of the short duration, high energy pulses may vaporize the
material so rapidly that heat transfer to the non-irradiated adjacent
tissue is minimal. Alternatively, or in addition, ultraviolet photons
absorbed in the organic material might disrupt molecular bonds to remove
tissue by photochemical rather than thermal mechanisms.
While the high peak energy provided by the Excimer laser has been shown to
provide improved results with regard to the ablation of atherosclerotic
plaque, this characteristic of the energy also presents a serious
practical problem. Specifically, a laser pulse having sufficient energy
density to destroy an obstructing tissue also tends to destroy an optical
fiber. The high energy density pulses break the fiber tip at the input
end, first at the glass/air interface. Continued application of the laser
energy causes a deep crater to be formed inside the fiber. Thus, it is not
possible to deliver high-power ultraviolet laser energy in vivo using a
conventional system designed for continuous wave laser energy.
This problem associated with the delivery of high energy Excimer laser
pulses is particularly exacerbated in the field of angioplasty because of
the small optical fibers that must be used. A coronary artery typically
has an internal diameter of two millimeters or less. Accordingly, the
total external diameter of the angioplasty system must be below two
millimeters. If this system is composed of three separate optical fibers
arranged adjacent one another, it will be appreciated that each individual
fiber must be quite small in cross-sectional area.
A critical parameter with regard to the destruction of an optical fiber is
the density of the energy that is presented to the end of the fiber. In
order to successfully deliver the laser energy, the energy density must be
maintained below the destruction threshold of the fiber, which might be
around 25-30 mJ/mm.sup.2. Thus, it will be appreciated that fibers having
a small cross-sectional area, such as those used in angioplasty, can
conduct only a limited amount of energy if the density level is maintained
below the threshold value. This limited amount of energy may not be
sufficient to ablate the obstructing tissue or plaque.
OBJECTS AND BRIEF STATEMENT OF THE INVENTION
Accordingly, it is a general object of the invention to provide a novel
system for delivering high energy Excimer laser energy using an optical
waveguide.
It is a more specific object of the invention to provide such a delivery
system that is particularly well suited to deliver Excimer laser energy in
vivo for the ablation of atherosclerotic plaque.
It is a further object of the invention to provide a novel angioplasty
system.
It is another object of the invention to provide a novel image scope that
can be used in an angioplasty system.
Briefly, in accordance with one aspect of the present invention, the
delivery of high energy Excimer laser light by means of an optical
waveguide is carried out by using a three-step approach. The first step is
the selection of a proper material for the waveguide itself. It has been
found that an optical fiber having a core of pure silica works best for
the transmission of high energy ultraviolet laser light. Preferably, the
fiber is a multimode fiber having a high core to cladding area ratio.
As a second step, the density of the energy that is conducted within the
waveguide is increased by means of an energy coupler at the input end of
the optical fiber. In one embodiment of the invention, this energy coupler
comprises a container of liquid that essentially functions as a buffer to
protect the input end of the fiber and enable energy of a higher density
to be presented thereto. In a second embodiment of the invention, the
energy coupler is provided by designing the input end of the fiber in a
funnel shape so that the end of the fiber has a relatively large
cross-sectional area which tapers down to the small diameter fiber needed
for the desired application.
The third step that is utilized in the context of the present invention is
to increase the density of the energy once it exits the fiber at the
distal end. This increase is provided by means of a lens on the fiber
itself. This lens can be produced by melting the flat polished tip of the
fiber to a predetermined spherical curvature. The lens causes the laser
beam to converge on a focal spot that is smaller than the fiber itself,
thus reducing the beam area and increasing its energy density.
In accordance with another aspect of the present invention, the
characteristic pattern of the light which exits from the lensed distal end
of the optical fiber is utilized to provide a novel angioplasty system.
Basically, the system utilizes only two optical fibers to perform the
three functions of illumination, lasing and imaging. One optical fiber is
dedicated to the imaging function. The other optical fiber conducts both
visible light energy and the laser energy to accommodate the two functions
of illumination and lasing. The use of only two fibers enables a fiber of
larger diameter to be utilized than those which can be accommodated in a
three-fiber angioplasty system.
In a third aspect of the invention, the angle of the illuminating light is
adjusted relative to the field of view of the image waveguide to provide a
reference viewing plane which enables a physician to determine the
location of the distal end of the waveguide relative to an object being
viewed, and hence the size of the object. Preferably, the beam of
illuminating light intersects the field of view at a reference plane to
allow the physician to determine when the viewed object is located at a
preset distance from the end of the image guide.
Further features and advantages of the present invention will become
apparent from the following detailed description of preferred embodiments
of the invention illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a laser and image delivery system that can
be used for angioplasty;
FIG. 2 is a cross-sectional side view of a delivery system for high power
Excimer laser energy utilizing a funnel-shaped energy coupler;
FIG. 3 is a cross-sectional side view of an alternate embodiment of an
energy coupler;
FIGS. 4A and 4B are illustrations of the light pattern which emerges from
the distal end of the lensed fiber-optic waveguide;
FIG. 5 is a cross-sectional end view of the two fibers that are employed in
the laser and image delivery system of the present invention;
FIG. 6 is a side view of an alternate embodiment of a laser and image
delivery system that provides a reference viewing plane within a narrow
conduit; and
FIG. 7 is an end view of the system of FIG. 6 as incorporated in an
angioplasty system.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
In the following description of preferred embodiments of the invention, an
Excimer laser delivery system is described with particular reference to
its use in an angioplasty system, in order to facilitate an understanding
of the invention and its uses. However, it will be appreciated that the
practical applications of the delivery system are not limited to this
single environment. Rather, the invention, in its broader aspects, can
find utility in any application in which it is desirable to deliver high
peak energy ultraviolet laser light by means of a fiber-optic waveguide,
such as in a cutting tool or in arthroscopy for example.
Referring to FIG. 1, an angioplasty arrangement that can employ the
delivery system of the present invention is shown in schematic form. The
angioplasty system must be capable of performing three functions within
the blood vessel. The first two of these relate to the illumination and
imaging of the interior of the vessel to enable a physician to
successfully propagate the distal end of the system through the vessel to
the location of the obstruction. Accordingly, the output from a source of
visible light, such as a Halogen or Xenon lamp 10, is directed to the
input end of an optical fiber 12. The other (distal) end of this fiber is
housed within a catheter (not shown) to enable it to be fed through a
blood vessel. A second optical fiber 14 located adjacent the fiber 12
within the catheter receives the image from the illuminated interior of
the blood vessel and transmits it to a video camera 16 by means of a video
coupler 18 connected between the output end of the fiber 14 and the
camera. The image presented to the camera 16 by the fiber 14 is converted
into a video signal and fed to a suitable monitor 20 for viewing by the
physician as the catheter is being positioned inside the blood vessel.
Alternatively, the video coupler, camera and monitor can be replaced by an
eyepiece that is attached to the output end of the fiber 14.
Once the distal ends of the fibers 12 and 14 have been appropriately
positioned adjacent the obstruction, a high energy laser, such as an
Excimer or Argon laser, is activated to ablate the obstruction. In a
preferred form of the invention, the laser energy is conducted along the
same optical fiber 12 as the visible light. To accomplish such a result,
the output of the laser is directed at a beam splitter 24 which also
transmits the visible light from the source 10. These two forms of light
energy are propagated along the same path and presented to the input end
of the optical fiber 12 by means of an energy coupler 26.
Referring now to FIG. 2, one embodiment of the delivery system for high
energy Excimer laser light is illustrated in greater detail. The delivery
system essentially comprises three basic elements. The first of these is
the optical fiber 12. A fiber that has been found to be particularly
suitable for use in the delivery of high energy Excimer laser light is a
multi-mode fiber which has a relatively large core, or active area,
relative to the area of its cladding, i.e., the outer skin of the fiber.
The core is made of pure silica, e.g. quartz, which has been fused, and
the cladding which surrounds this core comprises silica that has been
doped with fluorine. In the context of the present invention, the fiber
can be a single fiber or a bundle of fibers having a total diameter in the
range of 100-2,000 microns. This entire structure can be surrounded by a
protective flexible jacket 28 made, for example, of steel, nylon or a
suitable polyurethane material.
A fiber of this construction can typically accommodate input energy up to a
level around 30 mJ/mm.sup.2. If the density of the energy is increased
above this level, the input end of the fiber will be damaged or destroyed.
Unfortunately, this density level is about the minimum that is required to
produce ablation of calcified plaque, thus providing no margin of safety
if the intended use of the delivery system is for angioplasty.
Accordingly, in order to enable a higher level of energy to be conducted
in the fiber, an energy coupler 30 can be provided at the input end of the
fiber. In the embodiment illustrated in FIG. 2, this energy coupler
comprises a section of fiber that has a larger cross-sectional area than
the main portion of the fiber. This larger cross-sectional area gradually
tapers to the nominal diameter of the fiber, so as to provide a
funnel-shaped input section.
Production of such a shape on the end of the fiber can be accomplished
through appropriate design of the die through which the silica is drawn to
produce the fiber. By interrupting the drawing of the fiber, a bulbous
mass remains at one end of the fiber. This mass can be cut and polished to
produce the funnel-shaped input section.
In operation, the increased area of the funnel-shaped coupler decreases the
input energy density for a given level of energy within the fiber.
Accordingly, the area of the input end can be appropriately dimensioned to
enable a sufficient amount of energy for ablation of tissue to be coupled
into the fiber without damaging the input end. Once it has been coupled
in, the density of the energy is increased by decreasing the
cross-sectional area of the fiber within the tapered section, so that a
greater amount of energy can be conducted within the fiber than would be
possible without such a device.
An alternate embodiment of an energy coupler is illustrated in FIG. 3. In
this embodiment, the optical fiber has a uniform diameter along its length
and terminates at a flat polished end. The end section of the fiber is
encased within a ferrule 32 made of a suitable material such as brass, for
example. An aluminum casing 34 having an annular ring 36 projecting from
the inner wall thereof is threaded onto the ferrule. A teflon O-ring 38
disposed between the end of the annular ring 36 and the ferrule provides a
watertight seal between the casing and the ferrule. A second O-ring 40 is
disposed on top of the annular ring and supports a glass plate 42 made of
z-cut quartz, for example. This arrangement forms a fluid-tight cavity 44
between the ferrule 32, the casing 34 and the glass plate 42. The glass
plate can be held in place by means of a third O-ring 46 and a clamping
ring 48 disposed on the top of the casing. The fluid tight cavity is
filled with liquid which acts as a buffer to the input end of the fiber,
enabling laser energy having a relatively high density to be coupled into
the fiber without damage thereto. The liquid within the cavity can be
distilled and de-ionized water or it can be a transparent oil having an
index of refraction that is matched to that of the fiber 12, for example.
The third feature of the delivery system is a lens 50 that can be provided
at the distal end of the fiber. This lens operates to further increase the
density of the energy once it emerges from the distal end of the fiber by
reducing its cross-section to an area smaller than the fiber itself.
Referring to FIGS. 4A and 4B, two examples of the focused light pattern
which emerges from the lens at the end of the optical fiber are shown. As
can be seen, a majority of the light emerging from the optical fiber
converges upon a focal point or plane 52. Basically, the distance of this
focal point from the end of the fiber is determined by the radius of
curvature of the lens. In addition, a small amount of the light diverges
upon exiting from the optical fiber, so that the resulting light pattern
at the focal point consists essentially of an extremely bright spot at the
center of the focal point surrounded by a concentric area of lower
illumination. As described in greater detail hereinafter, this outer area
of lower illumination that is produced by the divergent light rays can be
used with advantageous results in an angioplasty system.
Preferably, the lens 50 on the end of the optical fiber is integral with
the fiber, i.e. formed from the material of the fiber itself. For example,
a micro-torch can be used to melt the flat polished distal end of the tip
to a predetermined spherical curvature under a microscope. Alternatively,
the lens can be a discrete element separate from the fiber itself and
adhered thereto by suitable means which minimizes light reflection at the
fiber/lens interface.
Thus, with the combination of the pure silica fiber, the energy coupler 30
that enables a greater level of energy to be conducted through the fiber
and the lens 50 at the distal end which converges the output energy onto a
smaller area to thereby increase its density, an amount of high power
laser energy that is sufficient to produce an incision can be safely
transmitted through an optical fiber waveguide without the risk of damage
to the fiber.
As noted above, one particular application for which the laser delivery
system is particularly well suited is the field of angioplasty. In such an
application, the optical fiber for the delivery of the laser energy can
also be used to deliver the visible light that illuminates the interior of
the vessel. While it is desirable to focus the laser energy so as to
increase the density level, the opposite effect is normally preferred for
the visible light. In other words, it is preferable to illuminate as wide
an area as possible in order to give the physician a full view of the
blood vessel in the vicinity of the end of the fiber. As shown in FIGS. 4A
and 4B, although most of the energy is concentrated at the focal point 52,
some of the light rays diverge upon emergence upon the fiber. It is
possible to make use of this divergent light to perform the illumination
function. It has been found that the amount of light which diverges away
from the focal point is generally sufficient to provide enough
illumination in the blood vessel to enable the physician to adequately
observe the ambient area.
Thus, the angioplasty system need only utilize two optical fibers 12 and
14, one to deliver the laser and visible light energy and the other to
return the image to a video camera for monitoring. These two fibers can be
placed in a side-by-side arrangement, as illustrated in FIG. 5.
Preferably, each fiber has a diameter of about 0.5 mm. They can be encased
in a catheter 54 which has an outer diameter that is only slightly greater
than 1 mm. The extra space present between the inner wall of the catheter
and the optical fibers can be used as a flushing channel, thus making
possible the use of the system within most coronary arteries.
Alternatively, separate light waveguides can be used for the laser energy
and the visible light to provide a depth of field reference for the
physician. More particularly, one of the more difficult tasks in the
viewing of a body cavity or blood vessel through an endoscope or an
angioscope is the determination of the size and location of a given object
in the field of view. An inherent characteristic of the wide angled lens
found in these devices is the fact that it distorts the scene and has no
specific focal point. This problem is particularly noticeable when the
viewing takes place through a single image guide that is located within a
tunnel-like environment, such as a blood vessel that is obstructed by a
non-uniform three-dimensional lesion.
In accordance with another feature of the present invention, however, this
drawback can be eliminated by utilizing an illumination beam which has a
smaller divergence angle than the field of view provided by the objective
lens on the imaging waveguide. Referring to FIG. 6, the image waveguide
56, which can consist of a single optical fiber or a bundle of fibers,
terminates in an objective lens 58 having a field of view which subtends
the angle .alpha.. This image waveguide is surrounded by illuminating
light waveguides 60 which project light that diverges over an angle
.beta.. In a conventional endoscope or angioscope, .theta. is greater than
.alpha. so that the entire field of view is illuminated. However, in
accordance with one aspect of the present invention, .theta. is less than
.alpha.. These angles are determined by the numerical aperture (N.A.) of
each of the waveguides 56 and 60, which is in turn related to the index of
refraction of the material from which the core and cladding of the
waveguide are made. Through appropriate adjustment of these two angles by
proper selection of the materials for the waveguides, the illuminating
field can be made to intersect the field of view of the lens 58 at a
reference plane 62. For example, the N.A. of the image waveguide 56 could
be 0.35 and that for the light wave-guides 60 can be 0.20 to provide a
reference plane that is about 2 mm from the end of the lens 58.
In practice, when an object is viewed through the image waveguide, the
light reflected from that object will completely fill the field of view
only when it is positioned at the reference plane 62. If the object is
farther away from the lens than the reference plane, the illuminated
portion of the object will be less than the total field of view, i.e., a
dark circle will appear around the object. Alternatively, if the object is
closer to the lens than the reference plane, it will appear blurred. Thus,
the physician can determine the exact location of the distal end of the
fiber relative to the viewed object, and hence the size of the object, by
adjusting the position of the fiber until the illuminated image completely
fills the field of view.
The incorporation of this principle into an angioplasty system is
illustrated in FIG. 7, which comprises the distal end view of the optical
fibers. The image waveguide 56 and a lensed laser waveguide 64 are located
in a side-by-side arrangement. A plurality of smaller light waveguides 60
are provided around most or all of the remaining circumference of the
image waveguide, so that the beam of illuminating light will be concentric
with the field of view of the image waveguide. By way of example, the
image waveguide can have a diameter of about 1 mm., the laser waveguide
can be about 0.5 mm. in diameter and the light waveguides can have a
diameter between 0.10 and 0.15 mm.
It will be appreciated by those of ordinary skill in the art that the
present invention can be embodied in other specific forms without
departing from the spirit or essential characteristics thereof. The
presently disclosed embodiments are therefore considered in all respects
to be illustrative and not restrictive. The scope of the invention is
indicated by the appended claims rather than the foregoing description,
and all changes that come within the meaning and range of equivalents
thereof are intended to be embraced therein.
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