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Claims  |
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What is claimed is:
1. In an interventional therapeutic apparatus for remote delivery of heat
to body tissue, the combination which comprises:
(a) a oscillator for providing an r.f. output signal within the range of 10
MHz to 2 GHz;
(b) a transmission line having a proximal end connected to the oscillator
for receiving the output signal and a distal end, the transmission line
being constructed and arranged to pass through the interior of a body
cavity; and
(c) an inductive load dispose at the distal end of the transmission line,
the inductive load comprising a magnetic material and operating to convert
the r.f. signal transmitted through the transmission line into heat, the
conversion being optimized at a predetermined frequency, the frequency of
the output signal from the oscillator being set at substantially said
predetermined frequency, whereby the inductive load is heated to a
temperature sufficient for therapeutic effects.
2. The apparatus of claim 1 adapted for thermal angioplasty wherein the
transmission line is constructed and arranged to pass through a blood
vessel so that the occlusive effect of plaque residing within the vessel
may be reduced.
3. The invention of claim 1 further including means for measuring the power
reflected from the distal end of transmission line back to the proximal
end of the transmission line.
4. The invention of claim 3 further including means for measuring the
incident power applied to the proximal end of the transmission line by the
oscillator, and means for adjusting the magnitude of the incident power.
5. The invention of claim 4 wherein the inductive load is characterized by
a temperature rise up to a maximum value which is proportional to the
magnitude of the difference between the incident and reflected power,
whereby the temperature of the load and distal end of the transmission
line can be controlled by adjusting the magnitude of the incident power
while maintaining the reflected power at a predetermined level.
6. The invention of claim 5 wherein the inductive load is further
characterized by a loss of ability to convert r.f. energy into heat at a
preset maximum temperature.
7. The invention of claim 3 wherein the inductive load is ferrite.
8. The invention of claim 7 wherein the reflected power measuring means
includes reflected power signal generating means for producing a reflected
power signal representative of the magnitude of the power reflected from
the distal end to the proximal end of the transmission line.
9. The invention of claim 8 wherein the oscillator further includes means
for adjusting the frequency of said output signal and further including
means responsive to the reflected power signal for controlling the
frequency adjusting means to minimize the reflected power.
10. The invention of claim 9 further including incident power adjustment
means for controlling the magnitude of the power delivered to the
transmission line.
11. The invention of claim 10 further including means for providing an
incident power signal representative of the magnitude of the power
delivered to the transmission line by the oscillator and wherein the power
adjustment means is responsive to the incident power signal for
maintaining the delivered power at a present level.
12. The apparatus of claim 1 wherein the oscillator further includes means
for adjusting the frequency of said output signal.
13. The invention of claim 12 further including means for measuring the
power reflected from the distal end of the transmission line back to the
proximal end of the transmission line, the reflected power measuring means
being arranged to produce a reflected power signal and wherein the
frequency adjusting means is responsive to the reflected power signal and
arranged to minimize the magnitude of the reflected power.
14. A delivery system for thermal angioplasty comprising:
(a) a coaxial transmission line having distal and proximal ends with a
center conductor and an outer conducting shield and being arranged to pass
through the interior of a blood vessel, the proximal end of the
transmission line being adapted to be coupled to a source of r.f. energy;
(b) a core of magnetic material disposed between the center conductor and
outer shield at the distal end of the transmission line, the magnetic
material being arranged to convert r.f. energy into heat;
(c) a heat conductive cap closing the distal end of the transmission line
and in heat conducting relationship to said core, whereby the application
of r.f. energy to the proximal end of the transmission line will cause the
core and cap to heat sufficiently to allow the distal end of the
transmission line to be forced through plaque restricting deposits in the
vessels and reduce the occlusive effect thereof.
15. The invention of claim 13 wherein the magnetic core is characterized by
a temperature rise up to a predetermined maximum level which is
proportional to the magnitude of the r.f. power applied to the proximal
end of the transmission line with substantially minimum power being
reflected back to the proximal end from the distal end of the line.
16. The delivery system of claim 15 wherein said magnetic material is
ferrite.
17. The delivery system of claim 16 wherein the diameter of the distal end
of the transmission line is approximately 2 mm.
18. A device for reducing the occlusive effect of plaque in blood vessels
in living tissue which comprises:
(a) a coaxial transmission line having proximal and distal ends and a
center conductor and outer conducting shield, the line being adapted to
pass through the interior of a blood vessel;
(b) a ferrite core disposed between the center conductor and outer shield
at the distal end of the transmission line;
(c) an electrically and heat conducting cap closing the distal end of the
transmission line in electrical contact with the center conductor and
outer shield and in heat conducting relationship with the ferrite core for
providing a closed electrical circuit at the distal end of the
transmission line and for receiving and dissipating heat generated in the
ferrite core; and
(d) power supply means coupled to the proximal end of the transmission line
for applying r.f. energy thereto, whereby the r.f. energy is converted
into heat by the ferrite core which heat is transmitted to the cap thereby
enabling the distal end of the transmission line to ablate plaque deposits
in contact with the cap and reduce the occlusive effect of such plaque
within the vessel in which the transmission line is inserted.
19. The device as defined in claim 18 including means to adjust the
frequency of the r.f. energy output from the power supply means to
minimize the magnitude of energy reflected back to the proximal end from
the distal end of the line.
20. The device as defined in claim 19 further including reflected power
measuring means for measuring the magnitude of the reflected power.
21. The device as defined in claim 20 wherein the reflected power measuring
means is further arranged to produce a reflected power signal
representative of the magnitude of the reflected power and wherein the
frequency adjusting means is responsive to the reflected power signal.
22. The device as defined in claim 21 further including means to adjust the
magnitude of incident r.f. power applied to the proximal end of the
transmission line and means to measure the magnitude of the incident r.f.
power.
23. The device as defined in claim 22 wherein the incident power measuring
means is further arranged to produce an incident power signal
representative of the magnitude of the incident power and wherein the
power adjustment means is responsive to the incident power signal and
arranged to maintain the magnitude of the incident r.f. power at a preset
level.
24. The device as defined in claim 18 wherein the power supply means is
further arranged to apply r.f. energy to the transmission line within the
frequency range of 1 MHz to 3 GHz.
25. The device as defined in claim 24 wherein the frequency of r.f. energy
applied to the transmission line is within the range of 500 MHz to 1.2
GHz. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates to an apparatus and method for applying heat
to tissue within a body cavity for therapeutic purposes.
2. Description of the Prior Art
As a natural consequence of aging and other biochemical factors,
atherosclerotic obstructions consisting of fatty deposits, fibrous tissue
and eventually calcium, tend to form on the vessel walls of the human
coronary, peripheral and cerebral vasculature. As this accumulation
progresses, the lumen of the artery is narrowed (or, sometimes completely
blocked), restricting or preventing adequate supply of oxygenated blood to
supply the muscles of the heart or legs.
The state of inadequate oxygenation, known as "ischemia," when it occurs in
the coronary arteries, results in abnormalities of the biochemical,
electrical and mechanical functions of the heart. The clinical
manifestation of this condition may include angina pectoris, acute
myocardial infarction or ventricular arrhythmia that can lead to sudden
death.
In the peripheral arteries, the ischemia condition commonly occurs with
exercise and is termed "intermittent claudication," i.e., the pain that
occurs in a muscle with inadequate blood supply that is stressed by
exercise. Pain may also occur at rest and may be continuous in the toes
and foot. Occasionally numbness or paresthesias may be present. Ulceration
and gangrene of the toes and distal foot are common when the disease
reaches advanced stages. Amputation is sometimes required.
Many therapeutic interventions have been available to treat coronary,
peripheral and cerebral vascular disease. Drugs that dilate the vessel and
lower blood viscosity are prescribed to prevent or lessen an ischemic
attack.
The introduction of polyvinyl balloon tipped catheters marked the beginning
of a new era in revascularization techniques. These catheters are threaded
through the vasculature, via a percutaneous incision, to the site of
occlusion. The balloon is then inflated using an accessory device located
outside the body. As the balloon inflates, the lumen of the artery is
increased to allow for greater blood flow resulting in the alleviation of
symptoms.
Bypass surgery is frequently prescribed. Grafting a healthy vessel around
the diseased one provides a new connection for blood flow to the portion
of the artery that is distal to the diseased section.
More recent treatment techniques have focused on the use of high energy
laser light pulses conducted through a fiber optic bundle to remove
atherosclerotic deposits.
The use of laser energy has also been used to heat a metal tipped catheter
which then melts or dissolves the plaque. Lasers, however, are complex,
relatively large, expensive and very inefficient. The use of lasers for
removing plaque deposits is still in the formative stages.
In addition to the use of expandable balloons and lasers, it has been
suggested in U.S. Pat. No. 4,643,186 that the plaque deposits be radiated
with microwave energy via a transmission line inserted through the
affected artery. Such a technique has several inherent problems. For
example, the plaque will provide a variable load to the microwave source
depending upon the moisture content of the plaque. Such a variable load
makes it difficult to control the amount of power applied to the plaque to
vaporize or otherwise remove the same without creating a risk of damaging
the vessel wall.
There is a need for a simple and effective apparatus for reducing the
occlusive effect of plaque deposits within blood vessels.
SUMMARY OF THE INVENTION
In accordance with the present invention, an interventional therapeutic
apparatus is provided for the remote delivery of heat to body tissue. The
apparatus may be employed, for example, to reduce the occlusive effect of
plaque deposits in blood vessels. The apparatus includes a probe in the
form of a transmission line such as a coaxial cable having proximal and
distal ends. The transmission line probe is adapted to pass through the
interior of a body cavity such as a blood vessel. An inductive load such
as a ferrite bead or core is disposed at the distal end of the
transmission line, the load being arranged to convert radio frequency
(r.f.) signals transmitted through the line into heat, with the conversion
being optimal at a predetermined frequency. A variable frequency
oscillator is connected to the proximal end of the transmission line for
applying r.f. energy at a suitable frequency, e.g., 10 MHz to 3 GHz, to
frequency of the r.f. energy is adjusted to substantially the
predetermined frequency so that the inductive load is heated to a
temperature sufficient to deliver the desired heat to the body tissue
undergoing treatment. For example, the distal end of the line may be
heated sufficiently to melt or otherwise remove plaque with which the
distal end comes into contact. This enables the transmission line to be
pushed through restrictive plaque deposits and reduce the occlusive effect
thereof.
The magnitude of the power applied to the proximal end of the line (i.e.,
incident power) and the magnitude of the power reflected back from the
distal end of the line (i.e., reflected power) may be measured to enable
an operator to adjust the level of power delivered to the load and thereby
control the temperature at the distal end of the transmission line or
probe. If desired signals representative of the incident and applied power
may be utilized in a closed loop system to maintain the incident power at
a preset level (and thus the distal end of the line at a given
temperature) and the reflected power at a minimum.
The features of the present inventions can best be understood by reference
to the following descriptions taken in conjunction with the accompanying
drawings, wherein like numerals indicate like components.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective view of an apparatus of the present invention
showing the probe thereof inserted into an artery in the leg of a patient.
FIG. 2 is a block diagram of an electronic circuit and a diagrammatic view
of a section of a probe in accordance with one embodiment of the
invention.
FIG. 3 is an enlarged cross-sectional view of the distal end of the probe
of FIG. 2.
FIG. 4 illustrates the distal end of the probe of FIG. 2 inserted into an
artery and abutting a plaque formation which totally occludes the artery.
FIG. 5 illustrates how the insertion of the distal end of the probe through
the plaque deposit opens a passage and reduces the occlusive effect of the
plaque.
FIG. 6 is a block diagram of an alternative electronic circuit for use in
the invention.
FIG. 7 is a schematic circuit diagram of an attenuator which may be used in
the circuit of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and more particularly to FIG. 1, there is
illustrated a housing 10 containing an oscillator or r.f. power supply,
and meters 12 and 14 for providing visual indications of certain
parameters, i.e., incident and reflected power as will be explained. Knob
16 allows manual control of the magnitude of the r.f. power applied to the
distal end 20 of a coaxial transmission line or probe 22, and knob 18
allows manual control of the frequency of the r.f. signal or energy
applied to the line. The distal end 24 of the probe, or line 22, is
illustrated as being inserted into an artery in a patient's leg 26.
Referring now to FIG. 2, an r.f. power supply, or oscillator unit 30,
provides an r.f. signal at its output 32 which is connected to the
proximal end 20 of the coaxial line 22. An r.f. power supply marketed
under the Model No. M445 by Eaton Corp. with M187 Plug-In Unit may be
employed as unit 30. The output power and r.f. frequency of the oscillator
may be controlled by manually operated potentiometers or the like (via
knobs 16 and 18), as is well known in the art, and such controls are
incorporated in the unit identified above. The frequency of the r.f.
energy is greater than 1 megahertz (MHz). The frequency of the r.f. energy
is preferably within the range of 10 MHz to 3 gigahertz (GHz) and a most
preferred range is from 500 MHz to 1.2 GHz.
A directional coupler 34 is inserted between the oscillator 30 and the
proximal end 20 of the transmission line 20. The coupler 34 includes a
power input 36 connected to the output 32 of the oscillator and power
output 38 connected to the proximal end of the transmission line. The
coupler 34 includes two signal outputs 40 & 42 which are connected to the
reflected power meter 14 and the incident power meter 12, respectively.
The signal on output 42 is an analog signal representing the magnitude of
the power applied to the proximal end of the transmission line by the
oscillator (i.e., incident power) and the signal on output 40 is an analog
signal representing the magnitude of the power reflected back to the
coupler 34 from the distal end 24 of the line 22 (i.e., reflected power).
Meters 12 and 14, which may be of the D'Arsonval type, display the level
of incident and reflected power. The coupler 34 may be of the type
marketed by the Bird Electronics Corp. under the name Power Sensor and
Model No. 4162W.
Referring now to FIG. 3, there is illustrated a cross-sectional view of the
distal end of the transmission line 22. The line is in the form of a
flexible coaxial cable having a center conductor 46, an annular insulating
layer 48, an outer conducting shield 50, which may be in the form of a
wire mesh, and an insulating covering material 52. An inductive load such
as a ferrite bead or core 54 is disposed at the distal end of the line
between the central conductor 46 and the outer conducting shield 50, as
shown. A cap 56 made of a suitable metal such as stainless steel, a
platinum, silver or gold alloy closes the distal end 24 of the line or
probe 22. The cap 56 closes the transmission line circuit at the distal
end 24 by electrically connecting the center conductor and the metal
sheath 50. The cap 56 is also placed in good heat-conducting relationship
with the ferrite bead 54 to transfer heat therefrom to the plaque to be
melted.
The inductive load or ferrite bead 54 acts as a lossy inductor load to r.f.
energy transmitted down the line 22 by converting the r.f. energy to heat.
The diameter of the coaxial line or probe 22 must be small enough to be
inserted into the blood vessels of interest. Preferably the diameter of
the probe is within the range of 1 to 2 millimeters.
In operation, the distal end of the probe or line 22 is inserted into an
artery (in which stenotic plaque is to be removed or reduced) and guided
through the artery by conventional fluoroscopy techniques until the distal
end of the probe abuts the plaque deposit. The frequency of the oscillator
30 is then manually tuned, e.g., by knob 18, until the reflected power is
approximately at a minimum. At this frequency (and there may be more than
one such frequency) the load is matched to the oscillator and line and
essentially all of the power applied to the line is converted into heat
within the ferrite bead or inductive load. The temperature of the probe
tip (distal end 24) may then be controlled by adjusting the magnitude of
the incident power (e.g., via knob 16).
The reflected power measurement at meter 14 provides an indication of the
temperature of the probe tip since ferrite changes its magnetic properties
as a function of temperature. When the Curie point is reached, the ferrite
loses its magnetic properties and ceases to act as an inductive load,
thereby limiting the maximum temperatures achievable at the probe tip.
The Curie temperature of the ferrite depends upon the alloy used. For
example, a nickel-zinc ferrite alloy marketed as ferrite No. 61 by
Fair-Rite Corp. of New York reaches its Curie point at about 350.degree.
C.
Referring now to FIGS. 4 and 5, the manner in which the probe 22 may be
used to reduce the occlusive effect of atherosclerotic plaque is
illustrated. The distal end 24 of the probe 22 (closed by the cap 56) when
heated to a sufficient temperature melts the plaque 57 which it comes in
contact with and causes a reshaping of the plaque so that a relatively
smooth and open passage through the artery wall 58 is provided as
illustrated in FIG. 5.
Another embodiment of the invention is illustrated in FIG. 6 in which the
incident and reflected power signals are used in a closed loop to maintain
the magnitude of the reflected power at a minimum and the magnitude of the
incident power at a preset level. The system of FIG. 6 includes a voltage
controlled oscillator 60 which has a voltage control input 62 and an r.f.
output 64. The r.f. output 64 is connected to an input 66 of an attenuator
or modulator 68. The attenuator 68 has a voltage control input 70 and an
output 72 connected to the r.f. input 36 of the directional coupler 34 via
a power amplifier 74. The incident and reflected power signals are applied
via analog to digital converters 76 and 79 to input ports 78 and 80 of a
microprocessor 82. The microprocessor 82 processes the incident and
reflected power signals and applies output signals to the control inputs
62 and 70 of the VCO 60 and the attenuator 68 via digital to analog
converters 84 and 86, as shown. The microprocessor is arranged (by
appropriate programing) to change the amplitude or voltage level of signal
applied to the input 62 of the VCO and thereby changing the frequency of
the r.f. output at 64 as needed to minimize the level or magnitude of the
reflected power signal at input port 80. The microprocessor is also
arranged (by appropriate programing) to adjust the level of the signal
applied to control input 72 of the attenuator 68 so that the incident
power will be maintained at a level preset into the microprocessor in a
well-known manner. The attenuator/modulator may be arranged to either
attenuate or modulate the applied r.f. signal (from the oscillator) to
thereby change the magnitude of the incident power as is well known in the
art.
FIG. 7 illustrates one type of attenuator that may be employed as unit 68
in FIG. 6. The attenuator of FIG. 7 varies the amount of the attenuation
of the input r.f. signal and thus the magnitude of the output signal (with
a constant r.f. input at 66) by changing the amount of bias voltage on a
P/N diode 90. A pair of bypass capacitors 92 and 94 are connected between
the cathode of the diode and the r.f. input and output, as shown. An
inductor 96 is also connected between the control input 70 and the cathode
of the diode. The level of diode conduction (resulting from the level of
positive bias voltage supplied by the microprocessor 82) determines how
much the diode shunts the load (connected to the output 72). When the
diode is reverse biased, the attenuation is effectively zero and the load
is matched allowing all of the r.f. power to be transmitted to the load.
As the diode turns on due to a positive bias voltage, it shunts the load
and part or all of the signal is reflected back to (and absorbed by) the
source or VCO 62.
In the operation of the circuit of FIG. 6, the operator merely presets the
desired incident power into the microprocessor 82 and follows the
procedure outlined with respect to the circuitry of FIG. 4, except that
the oscillator (and attenuator) is automatically controlled.
There has thus been described a simple, efficient and reliable apparatus
for the delivery of therapeutic heat to body tissue, for example to melt
stenotic plaque and reduce the occlusive effect thereof in blood vessels.
The above description presents the best mode contemplated in carrying out
our invention. Our invention is, however, susceptible to modifications and
alternate constructions from the embodiments shown in the drawings and
described above. Consequently, it is not the intention to limit the
invention to the particular embodiments disclosed. On the contrary, the
invention is intended and shall cover all modifications, sizes and
alternate constructions falling within the spirit and scope of the
invention, as expressed in the appended claims when read in light of the
description and drawings.
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