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BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates in general to microwave balloon angioplasty,
and pertains more particularly to a microwave or radiofrequency catheter
system for the heating of plaque in arteries or blood vessels. Also
described herein are improvements pertaining to features of the microwave
catheter system, including improved antenna constructions and associated
fiberoptics.
II. Background Discussion
Balloon angioplasty is now a relatively well-accepted alternative to bypass
surgery for high-grade obstructive atherosclerotic lesions of the
peripheral, renal and coronary vessels. In this regard, U.S. Pat. No.
4,643,186, entitled "Percutaneous Transluminal Microwave Catheter
Angioplasty," by Rosen et al., describes a coaxial cable and antenna for
microwave heating of artery plaque. This system suffers from several
shortcomings which make it difficult, if not impossible, to develop a well
controlled volume of heat within the plaque material. Also, for small
arteries where catheter diameter and flexibility are critical, the system
described by Rosen et al. does not allow for sufficient transmission of
microwave power to the plaque for welding purposes. Recent work with laser
balloon angioplasty demonstrates the need to heat the plaque to nominally
100.degree. C. for 30 seconds. For a 1.34 m. length of commercial
microwave coax, the insertion loss at 10GHz is approximately 10 dB. This
frequency corresponds to a depth of penetration in plaque of 3 mm.
Therefore, a 35 watt load requirement for heating plaque to nominally
100.degree. C. requires a 350 watt power supply (RF). This situation is
not practical. The transmission line itself would heat up, because 315
watts is dissipated by it during power transmission to the plaque (load).
The monopole antenna described in Rosen et al. does not provide radiation
confined solely to the distal end inside the balloon. A very nonuniform
radiation pattern is developed with antenna current leaking back up the
outside surface of the outer conductor, which forms the coax. The
resulting heating pattern is sharply peaked at the point along the coax
where the inner conductor protrudes outside of the outer conductor and a
secondary heating pattern develops along the length of the coax back to
the generator. Leakage currents produce the secondary heating pattern.
This may result in melting of the catheter plastic material.
Accordingly, it is an object of the present invention to provide an
improved technique for the heating of plaque in arteries, veins or blood
vessels, such as in association with microwave balloon angioplasty.
Summary of the Invention
To accomplish the foregoing and other objects, features and advantages of
the invention, there is provided a microwave or RF catheter system for
heating arterial plaque. In accordance with one embodiment of the present
invention, there is provided a flexible catheter member adapted for
positioning in the artery and adapted to support at the distal end thereof
an inflatable balloon. A microwave signal generator is disposed at the
proximal end of the catheter member. A transmission line means couples
from the signal generator through the catheter member and includes at its
distal end an antenna means. Optic fiber means extends through the
catheter member between proximal and distal ends thereof, and has one end
thereof disposed in the balloon in juxtaposition with the antenna means.
Channel means extend through the catheter member between proximal and
distal ends thereof for coupling a fluid to the balloon for inflation
thereof.
In accordance with further features of the present invention, the channel
means has an entrance port to the balloon and further includes a
pressurized fluid source for introducing fluid to the balloon under
pressure. The signal generator is excited for a predetermined period of
time upon injection of the inflating fluid. The optic fiber means has a
sensor at the distal end within the balloon to measure temperature in or
at the surface of the balloon, and in one embodiment, a pair of sensors
are employed for measuring temperature at separate locations within the
balloon. The antenna means may comprise a collinear array antenna. This
antenna may be disposed inside of the balloon, or may be disposed within
the skin forming the balloon. The collinear array antenna may be formed in
a spiral to provide full balloon circumferential coverage, or may,
alternatively, be formed in a helix. In still a further embodiment of the
invention, the collinear array antenna may include separate antenna
sections in combination with a power divider for intercoupling the
transmission line to the separate antenna section. The separate antenna
sections may be disposed in opposite locations in the balloon. In still
another embodiment of the invention, the antenna means may comprise a
plurality of separate collinear array antennae. There may also be provided
separate transmission lines in the catheter member for each of the
collinear and array antennae. In a further embodiment of the invention,
the antenna means may comprise a microstrip radiator. The microstrip
radiator may be comprised of a conductive strip and a ground plane,
separated by a dielectric substrate. The radiator may be of annular
configuration, having an outer radiating strip. In one embodiment, the
radiator includes a hollow member coated with a conductive film to form a
ground plane, a thin dielectric film over the ground plane and a
conductive antenna pattern printed over the dielectric film surface. The
antenna pattern may be in a spiral or helix configuration.
In accordance with still further embodiments of the present invention, the
transmission line may be in the form of a stiff guide member that retains
sufficient stiffness and yet is flexible. In this embodiment, there is
provided a guide wire forming a center conductor. Impedance matching means
are provided along the center conductor at locations where the center
conductor enters and leaves the balloon. The transmission line has an
outer conductor except at positions within the balloon and the tip of the
center conductor extends beyond the balloon in this embodiment. In still
another embodiment of the present invention, there may be provided a
plurality of metallic filaments, each having a length of one-half
wavelength or less at the microwave frequency of operation. An active
antenna within the balloon is used for exciting these filaments. The
filaments may be disposed either inside the balloon or within the skin of
the balloon. In another embodiment of the invention, the antenna means is
comprised of a plurality of spacedly disposed antenna wires arranged about
the balloon near the inside surface thereof, and commonly coupled to the
transmission line.
In accordance with still a further feature of the present invention, there
is provided a triaxial fiberoptic/RF cable that is in the form of a fiber
core having multiply deposited layers on the core, including a conductive
layer defining a conductor, a dielectric coating defining an insulating
layer and an outer conductive layer defining an outer conductor.
In accordance with still a further embodiment of the present invention, the
balloon itself is constructed of a compliant material that is either
loaded with a lossy material or coated with a flexible material
sufficiently loaded with lossy particles to allow for absorption of
microwave energy in the balloon directly. Also, the fluid within the
balloon may be of a type having lossy particles in suspension. The lossy
material used may include ferrite or graphite materials.
BRIEF DESCRIPTION OF THE DRAWINGS
Numerous other objects and advantages of the invention should now become
apparent upon a reading of the following detailed description, taken in
conjuction with the following drawings, in which:
FIG. 1 is a diagram of one embodiment of the present invention, employing a
microwave balloon catheter with a fiberoptic temperature sensor;
FIG. 2 is a cross-sectional view at the balloon end of the apparatus;
FIG. 3 illustrates a spiral configuration of the collinear array antenna;
FIG. 4 illustrates a spiral configuration of the collinear array antenna
embedded inside the skin of the balloon;
FIG. 5 illustrates two collinear array antennae in the balloon skin and fed
by a power tee or power splitter;
FIG. 6 illustrates separate collinear array antennae, each fed from a
separate transmission line;
FIG. 7 illustrates a microstrip geometry in accordance with the present
invention;
FIG. 8 is a perspective view illustrating an alternate embodiment of a
microstrip antenna;
FIG. 9 illustrates a microstrip spiral radiator adapted to be placed in a
balloon;
FIG. 10 illustrates a microstrip wrap-around radiator in accordance with
the present invention;
FIG. 11 illustrates a guidewire antenna system in accordance with the
present invention;
FIG. 12 illustrates a further antenna arrangement employing an active
antenna and associated parasitic array elements;
FIG. 13 illustrates a further embodiment of the invention employing plural
separate antenna wires fed from a common coax;
FIG. 14 schematically illustrates the use of a pair of ferrite sleeves
disposed a distance apart along an antenna axis but outside of the
balloon;
FIG. 15 is a fragmentary view illustrating a balloon skin with lossy
loading;
FIG. 16 is a fragmentary view of a balloon skin with an external lossy
coating;
FIG. 17 is a fragmentary view of a balloon skin with an internal lossy
coating;
FIG. 18 illustrates the triaxial fiberoptic/RF cable as in accordance with
the present invention adapted to transmit RF energy to a ferrite sleeve;
FIG. 19 is a further view of the embodiment of FIG. 18 showing further
details;
FIG. 20 is a cross-sectional view of the antenna of the antenna
construction of the present invention showing, in solid lines, a
cross-section of one-half of the far field antenna array pattern, each of
the three antenna elements, and in dotted lines the near field heating
pattern resulting from the superposition of the electromagnetic energy
pattern generated by the three antenna elements;
FIG. 21 is a cross-sectional view of the antenna of FIG. 1 along the lines
21--21;
FIG. 22 is a cross-sectional view of the antenna of FIG 20 along the lines
22--22;
FIG. 23 is a cross-sectional view of the antenna of FIG. 20 along the lines
23--23;
FIG. 24 is a cross-sectional view of the antenna of FIG. 20 along the lines
24--24;
FIG. 25 is an illustration of an insulated dipole in an ambient medium used
to depict the algebraic parameters needed for calculating the optimum
transformation of parameters;
FIG. 26 is a plot of frequency versus power ratio in decibels for the
antenna of the invention;
FIG. 27 is a side view of an optional embodiment of the invention employing
a lossy sleeve;
FIG. 28 is a cross-sectional view of an alternate embodiment of the
outermost end of the antenna construction;
FIG. 29 is an enlarged fragmentary view of FIG. 28; and
FIG. 30 is a cross-sectional exploded view of a flexible coaxial connector
adaptor system for use with the antenna of FIG. 20 as shown in the process
of being assembled;
FIG. 31 schematically illustrates a four wire transmission line antenna
system;
FIG. 32 illustrates a five wire transmission line antenna system;
FIG. 33 illustrates helix radiation patterns;
FIG. 34 illustrates various tapered axial mode helical antennae;
FIG. 35 illustrates axial mode helices; and
FIG. 36 illustrates a further embodiment of antenna construction.
DETAILED DESCRIPTION
Reference is now made to the drawings herein that illustrate a number of
different embodiments of the present invention. The concepts of the
present invention are explained herein in association with a microwave
balloon angioplasty technique. However, the concepts of the present
invention may also be used with energy in frequencies of the
electromagnetic spectrum outside of the microwave range. Also, the
concepts of the present invention may be employed in higher temperature
ranges, such as for ablation purposes.
It is desired to supply microwave heat to the plaque material only in
microwave balloon angioplasty. In this connection, experimental work with
laser balloon angioplasty has demonstrated that welding of the plaque from
heat and pressure results in reduced restenosis. Microwave energy, when
delivered to the plaque in a sufficient amount, likewise is helpful in
preventing restenosis by application of heat and pressure. Laser energy
absorption in plaque for melting may be the result of interaction with the
water molecules' vibration energy levels, whereas microwave energy
absorption in plaque may be the result of interaction with the water
molecules' dipole moment or rotation energy levels.
In accordance with the present invention for successful delivery of
microwave energy to the plaque, a highly flexible miniature transmission
line is used, that can transmit sufficient radiofrequency or microwave
power to the load (plaque). This transmission line is to be kink-free,
because of the requirement of relatively small turning radii.
In accordance with the present invention, the antenna system is to be
designed to deliver microwave energy to a specific layer of plaque without
heating wall tissue during pressure application by the balloon. The liquid
that inflates the balloon preferably does not absorb any substantial
microwave energy. It is instead preferred that the energy be concentrated
at the plaque rather than in the liquid itself that causes the balloon's
expansion.
In connection with certain fabrication techniques for the highly flexible
miniature transmission line, reference is made to description set forth
hereinafter relating to FIGS. 20-30.
In accordance with the present invention, there are now described a number
of techniques for providing control of the quantity of microwave energy
that is coupled to coronary vessel plaque without heating vessel tissue. A
collinear antenna array is provided inside the balloon or between two
balloon surfaces (balloon inside a balloon). In accordance with one
embodiment of the invention, a printed microstrip circuit radiator or
antenna pattern is configured in one of several ways, such as inside the
balloon, between balloon surfaces or outside the balloon.
In accordance with another embodiment of the invention, the antenna may be
formed from a guide wire. In another embodiment of the invention, a
collinear array antenna configuration may be provided inside the balloon
and the balloon may be fabricated with either a magnetic or dielectric
lossy coating on its surface or the balloon itself may be loaded with a
similar lossy material so as to provide direct balloon heating.
In accordance with another embodiment of the invention, to be described in
further detail hereinafter, there is provided an array of resonant thin
wire dipoles over the entire balloon surface or embedded within the
balloon material. These dipoles may be parasitic elements driven by an
active antenna.
In still a further embodiment of the present invention described
hereinafter, a wire balloon monopole is provided. A group of thin parallel
wires is connected at one end to the feed coax and forms an expanded
center conductor. Each wire lies on the surface or inside the balloon
material.
All of the above mentioned embodiments will be described hereinafter in
further detail. These various embodiments may be employed to precisely
deliver microwave or radiofrequency energy to plaque during a pressure
treatment. An alternate heating approach involves much higher temperatures
than 100.degree. C. (for example, 400.degree. C.-500.degree. C.), and
involves an embodiment in which the microwave antenna axis of the
collinear array is extended beyond the end of the balloon. In this regard,
refer to FIG. 14 herein. Also refer to FIG. 27 that illustrates the
employment of a ferrite sleeve 80 associated with the antenna.
FIG. 14 shows the antenna A extending through the balloon B and having at
its tip T a concentric layer of ferrite material that may have a Curie
temperature in the 400.degree. C.-500.degree. C. range. Microwave energy
is rapidly absorbed in the ferrite when this material is at a current
maximum of the antenna. The primary function of this hot tip (when the
ferrite is at the far end of the antenna) is to melt plaque (ablation).
This is used for those cases where the artery is fully blocked by plaque,
and it would therefore be necessary to remove some plaque in order to
insert the balloon. In FIG. 14, note the plaque volume at V. Once some
plaque has been removed, the balloon may be inflated and the microwave
angioplasty carried out.
As indicated previously, FIG. 27 herein teaches the use of a lossy sleeve
80 for focused heating. An alternate embodiment is to employ two ferrite
sleeves F1 and F2, as illustrated in FIG. 14, some distance apart along
the antenna axis but outside of and essentially in front of the balloon.
In this regard, the arrow A1 in FIG. 14 illustrates the direction of
insertion of the antenna structure.
As indicated previously, FIG. 14 shows a two-ferrite geometry. The ferrites
F1 and F2 heat through the plaque (occluded artery) using microwave
frequency F1. To withdraw the antenna back through the plaque and avoid
sticking, the ferrite F2 is tuned to a frequency F2. It remains hot to
allow the antenna to be withdrawn prior to inserting the balloon and using
the antenna in its normal temperature plaque welding mode. Also, this
ferrite, hot tip antenna may be completely removed from the catheter in a
different antenna design employed for low temperature operation.
Reference is now made to one embodiment of the present invention
illustrated in FIGS. 1 and 2 herein. This employs a collinear antenna
array that may be of the type to be described in further detail herein in
FIGS. 20-24. This arrangement provides localized microwave heating energy
to provide a circumferential heat treatment during balloon angioplasty for
the purpose of sealing, preventing abrupt reclosure, and preventing
restenosis in patients with vascular disease.
As indicated previously, one embodiment of a collinear array antenna is
described in further detail hereinafter in FIGS. 20-24. The collinear
array antenna is positioned inside the balloon and preferably as near to
the balloon's surface as is practical. The balloon is inflated with a low
dielectric loss fluid that may be a liquid or gas. A low loss dielectric
material is preferred so as to minimize microwave energy coupling to the
fluid. Several different embodiments will be described hereinafter in, for
example, FIGS. 1-13.
In accordance with the present invention, it has been found that the use of
microwave energy is considerably less expensive than a laser probe with
its associated driver. In accordance with the invention, heat is also
controlled by a fiberoptic sensor in close proximity to the antenna for
providing accurate temperature readings during microwave power
application. This ensures precise temperature control to avoid excessive
heating. The outer diameter of the coaxial cable is preferably 0.02". This
allows easy placement of the cable in a standard balloon catheter.
Alternatively, in accordance with the invention, the antenna may be used
in combination with a ferromagnetic sleeve in the ablative mode to
eliminate plaque buildup. The ferrite sleeve permits high localized
temperatures to be generated by microwave energy absorption within the
sleeve volume. Further embodiments of the invention cover this feature,
such as will be illustrated and described in further detail herein in
FIGS. 18 and 19.
Reference is now made to one embodiment of the present invention
illustrated in FIGS. 1 and 2. This embodiment illustrates the microwave
balloon catheter 1 with a fiberoptic temperature sensor. A cross-section
schematic of the balloon end is illustrated in FIG. 2. The balloon 12, it
is noted, is secured to the distal end of the catheter member 1. The
catheter member 1 has three lumens for carrying, respectively, the
microwave coaxial transmission line 2, the fiberoptic cable 3, and the
channel 4, which is for the coupling of the electrically low loss fluid 13
to the balloon 12 for inflation purposes.
FIG. 1 also illustrates, in the system, a microwave signal generator 7 that
includes fiberoptic temperature processing circuitry. The generator 7, it
is noted, couples with the cable 6 and also the fiberoptic cable 5. These
cables are continuations of the aforementioned cables 2 and 3.
FIG. 1 also illustrates the fluid source 8, which may comprise a pump for
pressurizing the fluid, connected to the channel 4. As indicated
previously, the fluid 13 pumped from the source 8 is preferably a low loss
tangent liquid or gas that is adapted to minimize microwave or
radiofrequency energy absorption.
The balloon 12 is inflated by means of the liquid 13 injected into it under
pressure at the entrance port 10. FIGS 1 and 2 also illustrate a microwave
antenna 11 that supplies electromagnetic energy to the liquid 13 for, say,
a period of 30 seconds. The liquid 13 is low loss so that the energy from
the antenna is concentrated outside the balloon skin rather than in the
liquid itself.
The antenna 11 is adapted to provide an axially uniform power pattern along
its active length, which is contained within the balloon. The antenna
construction is such that no microwave power leaks back along the feed
cable 2. It is preferred to employ a collinear array antenna as described
in further detail hereinafter in FIGS. 20-24.
FIGS. 1 and 2 also show the temperature sensor 9 within the balloon 12.
Instantaneous temperature rise is measured by the sensor 9, preferably at
two points within the balloon. The fiberoptic sensor is coupled to a
fiberoptic transmission line that may have a diameter of 0.01" for a
single temperature measurement point or may be 0.02" for simultaneous
two-point temperature measurement. A typical balloon length and diameter
are 2 cm and 3 mm, respectively, for coronary artery angioplasty. The
antenna may be coated with a lossy magnetic material to provide
temperatures in the range of 450.degree. C.-500.degree. C. for ablation
purposes. In such an application, the antenna is employed before balloon
angioplasty.
Further embodiments of the collinear array antenna are now described in
FIGS. 2-6. In these embodiments it is desired to provide an
omnidirectional and uniform heating pattern and, thus, the antenna may be
wound in helical or spiral fashion, so that its radiation pattern provides
a full 360.degree. of balloon circumference.
The frequency of operation and, therefore, the antenna-design parameters,
are controlled by the desired depth of penetration of microwave energy
into the plaque. At a 3 mm depth of penetration, the frequency is
nominally 10 GHz, with a 1.5 cm antenna length. The balloon length and
length of the plaque deposit should coincide with the antenna length, as
measured along the balloon axis. Alternatively, the collinear array may be
positioned along the axis of the balloon and inside the balloon, as
depicted previously in FIG. 1.
A reference is now made to FIGS. 3-6 for further embodiments of the antenna
construction. FIG. 3 illustrates the collinear array antenna 65 coupled
from the coaxial transmission line 2. The antenna 65 is provided in a
spiral or helical configuration. In this embodiment of the invention, it
is noted that the antenna is disposed substantially exclusively inside the
balloon. However, in the alternate embodiment of FIG. 4, it is noted that
the spiral or helical configuration of the collinear array antenna is
embedded in the balloon skin.
Reference is now made to FIG. 5 for still a further embodiment of the
present invention. This embodiment employs two separate collinear array
antennae 68A and 68B embedded in opposite sections of the balloon skin.
These antennae are fed from the coaxial line 2 by means of a power tee or
power splitter, illustrated at 69 in FIG. 5.
Reference is now made to FIG. 6 for a further embodiment of the present
invention employing three separate collinear array antennae 70A, 70B and
70C. In this embodiment, each of these antennae is provided inside the
balloon skin, as illustrated. Each of these antennae may couple to its own
separate coaxial microwave transmission line. For this purpose, the
catheter member, such as member 1, illustrated in FIG. 1, may be provided
with means for accepting each of these separate transmission lines. In all
embodiments of FIGS. 2-6, a fluid 13 is contained in the balloon and is
used for the purpose of inflating the balloon. This fluid is preferably a
low loss fluid as indicated previously.
Reference is now made to FIGS. 7-10 for various microstrip printed antenna
constructions. Microstrip is a type of open wave guiding structure that is
a simple construction and can be fabricated readily in miniature size. The
microstrip antenna, such as that illustrated in FIGS. 7 and 8 herein, is
manufactured using printed circuit board techniques. In this connection, a
relatively simple patch radiator is shown in FIG. 7. A somewhat different
configuration is illustrated in FIG. 8.
In FIG. 7, the microstrip geometry includes a dielectric substrate 73 that
has supported on its upper surface a printed conductive strip 72 of metal
which is suitably contoured. The lower surface of the dielectric substrate
73 is backed by a conducting metal forming a ground plane 71. The
microstrip patch radiator illustrated in FIG. 7 can be used in various
applications where a flat radiator is appropriate. Such a conformal design
is suitable for microwave balloon angioplasty.
FIG. 8 shows a slightly different version of the microstrip antenna,
employing an antenna 77 supported on a dielectric substrate 75 and also
illustrated in a ground plane 76. It is noted that the antenna 77 is fed
from the center conductor of coax 74. In this connection, the dielectric
substrate is preferably a low loss substrate such as titanium dioxide.
Reference is now made to FIG. 9 for a practical application of a microstrip
geometry to microwave balloon angioplasty. The microstrip spiral radiator
of FIG. 9 is adapted to be placed in a balloon with the long axis of the
balloon parallel to the long axis of the spiral radiator. The overall
antenna structure of FIG. 9 may be comprised of a cylindrical core 82.
This core 82 may be of a rubberlike material, and may be hollow, so as to
accept an optical fiber. The surface of the core 82 may be coated with a
thin film of highly conductive metal to provide a ground plane, as
indicated at 83 in FIG. 9. Next, a thin dielectric coating or film is
provided over the entire ground plane surface. This is illustrated at 84
in FIG. 9. The dielectric coating may be, for example, titanium dioxide. A
conductive antenna pattern is printed, as illustrated at 85 in FIG. 9,
over the dielectric film surface. The pattern 85 may be provided in a
continuous spiral patch, as illustrated in FIG. 9, or, alternatively, a
wraparound radiator may be provided as illustrated at 87 in FIG. 10.
In the embodiment of FIG. 9, the coaxial transmission line that feeds
microwave or radiofrequency energy to the printed radiator pattern may be
connected at one end of the radiator. The cylindrical geometry is well
suited for the application of balloon angioplasty. However, a simple
version for heating plaque may employ a flat antenna geometry as
illustrated in FIG. 8. In FIG. 10 the ground plane may be curved to match
the balloon curvature lengthwise or remain straight and parallel to the
balloon axis.
Reference is now made to FIG. 11 for still another embodiment of the
present invention in the form of a guide wire antenna system. The guide
wire essentially provides some stiffness to the catheter and makes it
easier to guide the catheter along the artery channel. The guide wire
itself may be employed to form the center conductor of a feed coaxial
transmission line. In this regard, in FIG. 11, note the coaxial line 90
and the guide wire 91, which furthermore extends through the balloon to
the tip 92.
As indicated previously, the guide wire is used as the center conductor of
the flexible coax 90. It is coated with a dielectric and then a metallic
film outer conductor, as illustrated at 93, except for the region within
the balloon 94, and, furthermore, except for the area at the tip 92.
In the embodiment of FIG. 11 chokes A and B are formed. These chokes may
also be referred to as impedance matching transformers, and may be of the
type to be described hereinafter in association with FIG. 20. The chokes A
and B are formed where the guide wire 91 enters and leaves the balloon 94
to prevent antenna currents from forming between the choke B and the tip
92 and the choke A and the transmitting end (c | | |
