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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. 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 constant
laser power 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 destruction of the lesion is uncontrolled and is
accompanied by thermal injury to the vessel walls immediately adjacent the
obstruction. In an effort to avoid such thermal injury and to provide
better control of the tissue removal, 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. See, for example, International patent
application PCT/US84/02000, published June 20, 1985. One example of a
laser for producing this higher level energy is known as the Excimer
laser, which employs a laser medium such as argon-chloride having a
wavelength of 193 nanometers, krypton-chloride (222 nm), krypton-fluoride
(248 nm), xenon-chloride (308 nm) or xenon-fluorine (351 nm). The light
produced by this type of laser appears in short bursts or pulses that
typically last in the range of ten to hundreds of nanoseconds and have a
high peak energy level, 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 each single 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 Excimer and other pulsed lasers 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. Typically, to couple a large-diameter laser
beam into a smaller diameter fiber, the fiber input end is ground and
polished to an optical grade flat surface. Residual impurities from the
polishing compound and small scratches on the surface absorb the laser
energy. These small imperfections result in localized expansion at the
surface of the fiber when the laser energy is absorbed. The high-energy
Excimer laser pulses contribute to high shear stresses which destroy the
integrity of the fiber surface. Continued application of the laser energy
causes a deep crater to be formed inside the fiber. Thus, it is not
possible to deliver a laser pulse having sufficient energy to ablate
tissue in vivo using a conventional system designed for continuous wave
laser energy.
This problem associated with the delivery of high energy laser pulses is
particularly exacerbated in the field of coronary angioplasty because of
the small diameter optical fibers that must be used. For example, 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. 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 efficiently ablate the obstructing
tissue or plaque without thermal damage.
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 pulsed laser light 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 ultraviolet laser
energy in vivo for the ablation of atherosclerotic plaque. In this regard,
it is a particular object of the present invention to provide a highly
efficient waveguide for use in such a delivery system.
Briefly, in accordance with one aspect of a delivery system embodying the
present invention, 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 a preferred form, this energy coupler comprises a
fused or ground lens at the input end of a synthetic silica fiber. This
lens could be part of the fiber itself or a separate element that is
attached to the fiber. A second lens, preferably a plano-convex lens,
focuses a large collimated laser beam to a point source. The lens at the
end of the fiber is located at a distance greater than the focal length of
the second lens, so as to receive light from the point source and couple
it into the fiber. Preferably, the two lenses are housed within a chamber
filled with a liquid which matches the index of refraction of the fiber.
Another step that is utilized in the implementation 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.
Another significant feature of the present invention is the structural
makeup of the fiber optic waveguide. The fiber is preferably constructed
of a synthetic silica having a lower metallic impurity content than that
which can be found in naturally occurring silicas, e.g., quartz. This
synthetic silica is doped with a material which repairs the structural
defects inherent in any silica. Preferred doping materials include an OH-
radical, fluorine, or a combination of the two. The doping of the silica
enhances the transmittance of high-energy pulsed laser light in the
ultraviolet range.
When a delivery system embodying the present invention is to be used in the
field of angioplasty, the laser waveguide is preferably comprised of a
closed-packed bundle of small diameter fibers. The smaller diameter fibers
provide greater flexibility than a single fiber having the same overall
diameter as the bundle, thus accommodating the high bends that are
encountered when the delivery system is maneuvered through body cavities,
particularly the vascular system. Further in this regard, to avoid damage
that might be occasioned by the light losses which occur at such high
bends, each fiber is provided with a cladding made from an organic
material that is not degraded by high-energy ultraviolet light.
The efficiency of an angioplasty system incorporating the novel delivery
system can be increased by maintaining a continuous layer of fluid between
the distal end of the fiber and the lesion to be ablated. The fluid
functions to keep ablate tissue from the fiber as well as to cool the end
of the fiber. In addition, it provides an index of refraction that is
higher than that of air, so that smaller shock waves occur within the
fiber as a result of the laser pulses.
Further features of the present invention and preferred modes for
implementing them 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 energy
Excimer laser light utilizing a funnel-shaped energy coupler;
FIG. 3 is a cross-sectional side view of a second embodiment of an energy
coupler;
FIG. 4A is a side view, partly in section, of a third embodiment of an
energy coupler;
FIG. 4B is an enlarged view of a portion of FIG. 4A, illustrating the
principle of operation of this embodiment;
FIGS. 5A and 5B are illustrations of the light pattern which emerges from
the distal end of the lensed fiber-optic waveguide;
FIG. 6 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. 7 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;
FIG. 8 is an end view of the system of FIG. 7 as incorporated in an
angioplasty system; and
FIG. 9 is a perspective view of an alternate embodiment for gauging
distance and/or size within a blood vessel.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
In the following specification, a laser delivery system is described with
particular reference to the use of Excimer laser energy in an angioplasty
system, in order to facilitate an understanding of the invention and its
uses. Referring to Figure 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
proximal end of an optical fiber 12. The 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 proximal 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 pulsed ultraviolet
laser, such as an Excimer laser, is activated to ablate the obstruction.
In a preferred implementation of the invention, the laser light is
conducted along the same optical fiber 12 as the visible light. To
accomplish such a result, the output beam 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 pulsed 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 is particularly suitable for use in the
delivery of high energy pulsed ultraviolet 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 substantially pure synthetic fused silica, i.e. amorphous silicon
dioxide. This material preferably has a metallic impurity content of no
more than 30 parts per million, to provide better conduction of the
transmitted laser energy than that which is obtainable with natural fused
quartz. The term "metallic impurity" includes both metals per se and
oxides thereof.
Even with such a low level of metallic impurity, defects in the silica
fiber can serve as linear and non-linear absorption sites for the photons.
These defects can vary from oxygen vacancy to unbonded silicon atoms found
in any silica glass. They can result in lowered transmittance of
ultraviolet radiation. Increasing the intensity (or energy) of the laser
light that is introduced into one end of a fiber exhibiting such defects
will not necessarily result in proportionally increased output at the
other end. Rather, the increased intensity level can reduce the threshold
level at which bulk damage occurs to the silica glass, and thereby destroy
the delivery system.
In accordance with one aspect of the present invention, the transmittance
of high energy UV laser light in a fiber made of synthetic silica is
enhanced by lightly doping the silica with a material which functions to
repair some of the inherent structural defects of the silica. The silica
is preferably doped with an OH.sup.- radical, to thereby form so-called
"wet" silica. It is believed that defects in silica that affect UV light
transmission comprise oxygen hole centers and unbonded silica atoms. It is
theorized that the doping of the silica with the OH.sup.- radical
functions to repair these defects by eliminating the oxygen holes or
vacancies in one case and by bonding to the silicon to form the SiO.sub.2
double bond. It has been reported that pure silica having only about 5
parts per million (ppM) of an OH radical has an absorption coefficient
which is 2-3 times greater than silica having about 1200 ppM of the
radical. See J. H. Stathes et al, Physical Review B. ; Vol. 29, 12, 1984,
pp. 70-79. Other investigations have reported that an optical absorption
band appears in silica fibers having a low OH.sup.- content as a result of
the fiber drawing process. See Kaiser et al, J. Opt. Soc. Am. 63, 1973, p.
1141 and J. Opt. Soc. Am. 63, 1974, p. 1765. Apparently, an increase in
the OH.sup.- content of silica reduces both types of absorption sites
described above, and in accordance with the present invention this concept
is applied to a system for delivering high peak energy ultraviolet laser
pulses to thereby enhance the efficiency of the energy transmittance.
Preferably, the silica that makes up the fibers contains about 200 to 2000
ppM of the OH.sup.- radical, most preferably 1200 ppM.
In another embodiment of the invention, the silica that is used to produce
the fibers of the delivery system is doped with fluorine. Fluorine doped
silica exhibits even lower attenuation than high OH silica. It appears
that the fluorine functions to shift the absorption band gap in the
SiO.sub.2 structure, to facilitate the transmittance of a large number of
photons at low wavelengths. For multimode fibers having diameters in the
range of 100 micrometers to 1500 micrometers, the silica preferably should
contain between 0.25 and 2.0 wt % fluorine, most preferably 1.0 wt %.
As a further feature of the invention, the silica can be doped with both
the OH.sup.- radical and fluorine. When both of these materials are used
in combination, the OH radical content should range between 200 and 2000
ppM, and the fluorine should comprise between 0.5 and 3 wt % of the
silica.
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. A bundle of close-packed small-diameter fibers is preferred
because they provide greater overall flexibility and thereby more easily
accommodate the twists and tight turns that are required to feed the
delivery system through body cavities. This is particularly desirable
where a larger diameter waveguide is required to deliver a relatively
large diameter beam, such as in vascular angioplasty. This entire
structure can be surrounded by a protective flexible jacket 28 made of a
material which is not damaged by ultraviolet light. More particularly,
when the fiber undergoes sharp bends, for example at the juncture of two
arteries, light losses occur. These losses may be enough to melt some
types of jacket materials such as Silicone and nylon. However, UV light
resistant materials, for example UV cured acrylate compounds or
Teflon.RTM., can sustain high bending losses without degradation and are
therefore more desirable for the jacket.
In a preferred form of the invention, the protective jacket is incorporated
as part of the fiber itself, rather than being a separate piece of
structure which surrounds all of the fibers. As noted previously, every
fiber comprises a core and a cladding which surrounds the core to maintain
the transmitted light energy within the core. The cross-sectional area of
the fiber might normally have a core/cladding ratio of 80/20 to provide
suitable flexibility. Typically, both the core and the cladding are made
of glass, with the cladding being appropriately modified (e.g., doped) to
provide it with a lower index of refraction. In this conventional
structure, the protective jacket comprises a third layer which surrounds
the core and cladding.
In accordance with one aspect of the invention, the conventional glass
cladding is eliminated and the core of the fiber is directly surrounded by
a coating of organic material. One specific preferred material is UV-cured
acrylate. It has a lower index of refraction than silica, and thereby
functions to maintain the laser energy within the core. It also serves to
protect the silica glass, and hence eliminates the need for a third layer.
This reduces the overall size of the fiber and hence enables the net
cross-sectional area of the core to be increased for a delivery system
having a given outer diameter.
Further details regarding the composition of preferred coatings can be
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