Electromagnetic Scalpel for the Heating of Biological Tissue
Inventor: Kent Davey New Smryna Beach, Florida
Related Applications
The present application claims the priority of U.S. Provisional Application Serial
No. 60/145,754, filed July 27, 1999, and U.S. Provisional Application Serial No.
60/147,868, filed August 9, 1999, the contents of which are hereby fully incorporated
herein by reference.
Field of the Invention
The present invention relates to devices and methods for heating and excising
tissues of the human body.
Background of the Invention
Collagen connective tissue is the major protein found in the human body. It
provides the cohesiveness of the musculoskeletal system, the structural integrity of the
viscera, and the elasticity of integument. Intermolecular cross links provide collagen
connective tissue with unique physical properties of high tensile strength and substantial
elasticity.
A previously recognized property of collagen is hydrothermal shrinkage of collagen
fibers when elevated in temperature. This unique molecular response to temperature
elevation is the result of rupture of the collagen stabilizing cross-links. When heated, the
protein fibers adjust their linkage causing the fiber length to shorten, with an immediate
contraction of the collagen fibers to about one-third of their original dimension. In the
process the cells within the matrix are usually destroyed. The new cell growth takes place
within either of the ends.
Two prominent patents that use radio-frequency injected current to accomplish the
dimensional contraction are U.S. Patents 5,569,242 to Lax, et. al, Oct. 29, 1996 and United
States Patent 5,458,596 to Lax, et. al., Oct. 17, 1995. There, a probe is employed with a
lead injection electrode and a trailing band electrode as part of a 2.3 mm laproscopic probe.
There are multiple patents that utilize RF electrical energy with electrodes in a
catheter or endoscope to accomplish tissue ablation. Among those are: United States
Patent No. 5,843,075, issued to Taylor Dec. 1, 1998, and entitled "Probe for Thermal
Ablation"; United States Patent No. 5,906,613 issued to Mulier, et. al. May 25, 1999, and
entitled, "Method for R-F ablation"; United States Patent No. 5,893,885 issued to Webster,
Jr. on Apr. 13, 1999 and entitled "Multi-electrode Ablation Catheter"; United States Patent
No. 5,837,001 issued to Mackey on Nov. 17, 1998 and entitled "Radio Frequency Energy
Delivery System for Multipolar Electrode Catheters"; United States Patent No. 5,807,395
issued to Mulier, et. al. on Sept. 15, 1998, and entitled "Method and Apparatus for RF
Ablation and Hyperthermia"; United States Patent No. 5,540,681 issued to Strul, et. al. on
Jul. 30, 1996, and entitled "Method and System for Radiofrequency Ablation of Tissue";
United States Patent No. 5,545,161 issued to Imran on Aug. 13, 1996, and entitled
"Catheter for RF Ablation Having Cooled Electrode with Electrically Insulated Sleeve";
United States Patent No. 5,472,441 issued to Edwards, et. al. on Dec. 5, 1995, and entitled
"Device for Treating Cancer and Non-malignant Tumors and Methods"; United States
Patent No. 5,348,554 issued to Imran, et. al. Sept. 20, 1994, and entitled "Catheter for RF
Ablation with Cooled Electrode"; United States Patent No. 5,125,928 issued to Parins, et.
al. Jun. 30, 1992, and entitled "Ablation Catheter with Selectively Deployable Electrodes";
and United States Patent No. 4,945,912 issued to Langberg on Aug. 7, 1990, and entitled
"Catheter with Radiofrequency Heating Applicator". The disclosures of those patents are
fully incorporated herein by reference.
Discussed in those patents are various shapes and configurations of electrodes,
methods of cooling the electrode tips, and control of their excitation. The devices are used
both in excising tissue and in hyperthermia treatment for cancer.
Other applications of such methods include subluxations and dislocations, both of
which are a common occurrence and cause for a large number of orthopedic procedures
each year. Symptoms include pain, instability, weakness, and limitation of function. If the
instability is severe and recurrent, functional incapacity and arthritis may result. Surgical
attempts are directed toward tightening the soft tissue restraints that have become
pathologically loose. These procedures are typically performed through open surgical
approaches that often require hospitalization and prolonged rehabilitation programs.
More recently, endoscopic (arthroscopic) techniques for achieving these same goals
have been explored with variable success. Endoscopic techniques have the advantage of
being performed through smaller incisions and therefore are usually less painful. They are
performed on an outpatient basis and often under local anesthesia, and are associated with
less blood loss, lower infection risk, and have a more cosmetically acceptable scar.
Postoperative recovery is often faster than using open techniques.
It has been shown that cartilage and even the disks in the spinal cord exhibit the
same property of contraction upon heating as discussed above. When cartilage becomes
torn, and the ends remain attached to their respective joints, the usual procedure is to
surgically reattach the lengthened cartilage. The healing time for this prior art procedure
is typically approximately 6-8 weeks. Localized heating of the same lengthened cartilage
tightens the joint and greatly reduces the healing time to approximately 2-4 weeks.
Heating a herniated disk in the back has the effect of shrinking the disk, allowing it to pull
away from the irritated nerves. If the energy source is localized enough, it can be used to
cauterize blood vessels that might have grown into the enlarged disk. The ideal
temperature for heating tissue seems to be about 149 ° F.
There has been discussion in the existing literature regarding alteration of collagen
connective tissue in different parts of the body. One known technique for effective use of
this knowledge of the properties of collagen is through the use of infrared laser energy to
effect tissue heating. The use of infrared laser energy as a corneal collagen shrinking tool
of the eye has been described and relates to laser keratoplasty, as set forth in U.S. Patent
No. 4,976,709 (which is fully incorporated herein by reference). Controlling the
localization, timing and intensity of laser energy delivery is paramount in importance for
providing the desired soft tissue shrinkage effect without creating excessive damage to the
surrounding non-target tissues.
U.S. Patent No. 5,458,596, which is incorporated herein by reference, discloses an
apparatus and method for controlled contraction of soft tissue utilizing RF energy. The
shift from laser light to radio waves reduces the frequency of the radiation and,
consequently, the treatment temperature. The technology of the '596 patent utilizes an
electrode which can be inserted into the connective tissues of the human body (as a probe)
for the delivery of energy to the tissue to achieve a desired contraction of the collagen
fibers.
These newer probes can also be applied for treating damaged spinal disks. A spinal
disk consists of rings of ligaments around a gel-filled center. When the ligaments are torn,
blood vessels grow into the tear, bringing nerve endings with them. Bending of the spine
squeezes the nerves and causes pain. Traditionally there have been two approaches to this
problem: remove the damaged disk or fuse together the vertebrae on either side of it using
screws with plates or rods. Another version of that surgery involves going through the
patient's abdomen from the front, pushing past the stomach and other organs to the spine
and inserting a titanium to immobilize adjacent vertebrae. These cage insertions seem to
require a long time for healing.
The spinal probe is less invasive. Under local anesthetic, using an X-ray machine,
the damaged disc is pinpointed and the probe is inched through the ligaments into the gel-
filled center of the disk. Once the flexible, wirelike probe is coiled around the inside of
the disk, the power is turned "on" at the probe's control console. The heat of the activated
probe shrinks the stretched-out rings of collagen and cauterizes the blood vessels and
nerves that have crept into the ligament layers.
To minimize damage from the intrusion, the diameter of the probe has to be very
small. However, it is difficult to achieve the required frequency of radiation using a small
probe. Accordingly, there is a substantial interest in the field for new devices and methods
to accomplish tissue heating in connection with the above described problems and
procedures.
Summary of the Invention
It is an object of the present invention to accomplish the various tasks of heating,
ablation, and even cauterization without electrodes.
It is an object of the present invention to efficiently direct adequate heating energy
to biological tissue using a small probe for precise delivery to biological tissue.
It is another object of the present invention to non-invasively induce heating
through the skin to heat tissue located beneath the skin.
In accordance with the invention, various electromagnetic scalpels are provided
herein. The scalpels allow action to be taken on biological tissue using electromagnetic
energy.
In accordance with one embodiment of the present invention, the various tasks of
heating, ablation, and even cauterization without electrodes, by using frequencies outside
of the RF band is accomplished. Eliminating electrodes allows the procedures to be
performed less invasively, since the device need not be in direct contact with the tissue to
be treated. This is especially advantageous when the objective is only heating. Eliminating
electrodes reduces the risk of electrode contamination since half cell reactions are
eliminated. Operating outside the RF band increases the efficiency of tissue energy transfer
since the water molecules absorb more energy to reorient in their polarization direction;
the permittivity has a much stronger imaginary component from 900 MHz to 2 Ghz. The
process does not depend on thermal heat transfer since the energy is induced preferably at
the site of interest. Both external and miniaturized internal embodiments are presented
towards this end.
In a further embodiment of the present invention, a novel construction of the
probe is provided employing a ferrite core and a steel wire. The combination of these two
elements permits reduction of the size of the probe while offering a low reluctance
magnetic path. The path would normally contribute to fringe field production, but in this
case those fringe fields can be effectively used to induce heating in the surrounding tissue.
In a further embodiment, the heating is realized non-invasively by utilizing a
concentric cylinder arrangement. The resulting cup structure can be placed against the
skin. When the windings within the cup are excited, heating is induced through the skin.
If the surface of the cup is cooled, the surface of the skin can be cooled while heat is
injected underneath the skin. The depth of penetration is controlled by altering the inner
and outer diameter of the cylinder.
Further embodiments, objects, advantages and features of the inventions will
become apparent in conjunction with the disclosure provided herein.
Brief Description of the Drawings
A full understanding of the invention can be gained from the following description
of the preferred embodiment when read in conjunction with the accompanying drawings
in which:
Figure 1 is a cross-sectional view of one embodiment of the present invention, a
probe used for inducing eddy currents into surrounding connective tissue. Only one half
of the probe, the other half is formed by rotation about the vertical central axis.
Figure 2 is a partial cross-sectional view of another embodiment of the probe,
showing the right half only (for simplicity of illustration) of a disk-shaped probe. The
segment in the figure is the right half of the probe shown extending off of the central axis
of the disk, the central axis being shown in dotted outline. (A rotation of the section of
probe 18 around the central axis results in the disk shape of the entire probe.)
Figure 3 is a partial cross-sectional view of the embodiment shown in Figure 2,
where the winding has a greater radial spread.
Figure 4 is a front perspective view of the stimulation unit comprised of four
separate "U"-shaped cores, which is utilized in accordance with the present invention to
accomplish heating of the tissue non-invasively.
Figure 5 is a back perspective view of the stimulation unit shown in Figure 4, also
comprised of four separate "U"-shaped cores.
Figure 6 is a cross-sectional view of the arrangement shown in Figs. 4 and 5;
Figures 7(a) and (b) are perspective views of one of the "U"-shaped cores of Figures
4, 5 and 6, showing its preferred dimensions, in inches;
Figure 8 shows a nanocrystalline slab upon which a 1 turn coil has been placed,
with Figure 8(a) being a top view and Figure 8(b) being a perspective view of the slab;
Figure 9 is a schematic diagram illustrating an excitation circuit to cancel the
inductive reactance and yield a 50 ohm load for the transmission line coupling.
Figure 10 shows a magnetic thermal heater and ablation tool comprised of a steel
winding and ferrite core, with Figure 10(a) being a cross-sectional view of the embodiment,
and Figure 10(b) illustrating typical magnetic field lines for the reduced in size device
shown in Figure 10(a).
Figure 11 is a perspective view of a toroidal core with a winding placed at the tip of
a laparoscope.
Figure 12 is a perspective view showing the use of a leaky resonant cavity to
provide the high frequency field induction to the collagen tissue.
Table I show the range of materials which can be used for the devices shown in
Figure 10 and the resulting induced power.
Detailed Description of the Figures and the Preferred Embodiments
In accordance with the present invention, electromagnetic (EM) scalpels are
provided for use on biological tissue within the anatomy. The EM scalpel directs
electromagnetic energy into the desired area of the anatomy resulting in heating of the
tissue or even excising it. The scalpels can either be totally non-invasive (e.g. by directing
the radiation through the skin) or can operate by insertion into the appropriate body area.
In accordance with one embodiment of the present invention, as shown in Figure 1,
a small electromagnetic scalpel or probe 10 is provided by insertion into the ligaments,
joint capsules and other connecting tissues of the human body. The probe is inserted for
the purpose of delivering RF energy to these tissues to induce their contraction.
As shown in Fig. 1, the probe 10 is comprised of a small diameter ferromagnetic
core 12 and a winding 14. The diameter of the probe is preferably about 2 mm, 1 mm for
the core and Vi mm on each side of the coil winding. In one embodiments, an air core and
a copper or steel winding can be used. In the preferred embodiment, however, the
inventor has found that a 42% improvement in heating power is achieved through the use
of steel wire in place of a copper wire.
Insertion of a ferrite core instead of the air core further increases the heat intensity,
compared with an air core probe, allowing for increase in power without increasing the
size of the probe. The combination of the ferrite core and steel winding offers a low
inductance magnetic path. The path would normally contribute to fringe field production,
but in this case, the fringe fields are used to induce heating in the surrounding tissue.
Alternatively, the small diameter of the probe can also be achieved by using an iron
wire and a solenoidal winding. Such combination allows the injection of heat into the
ligaments or other connective tissue while minimizing the damage to the surrounding
tissue.
Another possible design of the probe, in accordance with the present invention, is
shown in Fig. 2. In this embodiment, the probe 16 is in the form of a disc preferably 2.1"
in diameter and 0.25" thick (with only the right half of the disks being shown in the
illustrations of Figures 2 and 3). Although 0.25" in thickness is preferred, the thickness of
the device can be made smaller as far as the fields are concerned. The thickness has been
added in this embodiment to give the device some structural integrity. The ferrite core 18
is notched to house the steel winding 20. The annular winding extends around the disk
near the disk's outer circumference. The width of the notch 22 is preferably 0.22" and its
height is 0.0625". The inner diameter of the coil notch is preferably 1.5". A wider
winding 20, shown in Figure 3, will reduce the injected power due to eddy currents in
winding. Not properly winding the core also reduces the power.
This embodiment is more suitable for delivering higher EM frequency. At the
frequency of 915 MHz, and an excitation of 1.78 A, this device is capable of injecting 50
watts of power into the connective tissue. If, alternatively, the winding 20 has a greater
spread, as shown in Fig. 3, the field also spreads out and delivers 50 watts of power with
the same frequency when 2.8 A are driven through the winding.
Another embodiment of the present invention is shown in Figures 4, 5 and 6. In
this embodiment, the heating is realized non-invasively by utilizing a concentric cylinder
arrangement 24. This consists of four (4) U-shaped cored with a winding encircling one leg
of each of the cores (the center common legs). The resulting cup structure can be placed
against the skin. When a winding 28 within the cup is excited, heating is induced through
the skin. If the surface of the cup is cooled, the surface of the skin can be cooled while heat
is injected underneath the skin. The depth of penetration is controlled by altering the
inner and outer diameter of the cylinder.
As shown in Figs. 4 and 5, one stimulation unit is comprised of four separate "U" -
shaped ferrite cores 30, 32, 34, 36. The white "U"s 30 and 32 and the black "U"s 34 and 36
are constructed of a suitable magnetic material. Preferably, grain oriented steel is used (4%
grain oriented steel in particular), wound on a mandrel using 2 mil stock. The smallest 2
mil laminations from National Magnetics are suitable for this application. In the preferred
embodiments of all of the inventions of the present application, the cores are
ferromagnetic. Although 4% grain oriented steel is one preferred material for the cores,
alternatively, the cores can be constructed of nanocrystalline materials, vanadium
permendur, orthinol, metallic glasses (metglass), permalloy, supermalloy, powdered iron,
ferrite, or a different silicon iron or silicon steel (e.g. 3% grain oriented steel, also known as
magnesil).
Two "U"-shaped cores are placed back to back, the white pair 30 and 32 in Figs. 4,
5 and 6, while the other two, the black pair 34 and 36, are placed against the first two (the
second pair preferably being at a 90 degree angle to the first pair). Preferred dimensions of
the "U"s are shown in Fig. 7. In that embodiment, all four of the ferrite cores have the
same dimensions. The single winding 28 is wound around the interior legs of the cores,
i.e. located within the cut out portions 38, the space between the legs of the "U"s. When
all cores are arranged as shown in Fig. 4, the primary field is driven up the common center
40.
According to empirical analysis, when the cylindrical arrangement 24 is run with
linear ferrite, the power dissipation is 25 W. When, however, it is run with nonlinear
saturable ferrite, the power dissipation drops to 5 W. In this case, ferrite is heavily
saturable. If 100 MHz frequency is applied to the device 24, shown in Figs. 4 and 5,
utilizing steel cores, the injected heat has a skin penetration depth of 1.25 mm. To achieve
this result, it is preferable to use a 100 MHz sine wave function generator and an amplifier
with a bandwidth from 1 MHz to 100 MHz. These results assume a copper winding; the
power coupling is increased if the winding is altered to steel. It is also preferable to
associate a temperature probe transducer, along with the device, to monitor function and
prevent overheating of the skin.
Alternatively, it is possible to use the above described stimulation unit with
relatively high frequencies, preferably 915 MHz. At this frequency, water has both a real
and an imaginary permittivity, when the fluid begins to absorb energy in conjunction with
the electric dipole reversals.
In a further embodiment of the invention, the electromagnetic scalpel, as shown in
Figure 8, consists of a ferromagnetic wafer or laminated slab 110 upon which is placed a
one turn winding 120. Typical slab dimensions are 1.25" square on the face, by 0.5" of
depth of thickness. The slab is laminated with ferromagnetic material having a tape
thickness of one mil or less. It is important that the material be low loss, having both a
high resistivity and low hysteresis losses. In one embodiment, the typical excitation
frequency is 915 MHz. Since the device will radiate electromagnetic energy, it is desirable
that it radiate away from the slab; hence the slab material is chosen to be ferromagnetic. In
the preferred embodiment, a laminated nanocrystalline slab is used, which meets the
requirements of keeping the energy radiating away from the slab, while the 1 mil
laminations help curtail losses. Nanocryatalline is also considerably lower in hysteresis
and magnetostrictive losses than silicon steel. The nanocrystalline material also has a high
permeability, although that property is not necessary for this embodiment.
Operation of this embodiment at high frequencies helps for two reasons. First, the
power induced in the body is proportional to frequency squared. Second, water becomes
more absorptive at higher frequency; the permittivity has an imaginary component
indicating that power is being consumed just to reorient the polar molecules comprising
the fluid. The higher frequencies lead to problems with the inductive reactance. This
reactance is extremely high (> 400 Ω). It is desirable to both cancel out that inductive
reactance and to couple power into the device using a 50 Ω transmission line. One circuit
for accomplishing this end is shown in Figure 9. The ferromagnetic wafer 110 appears as a
large inductive load. A capacitor 130 (C,) is chosen to resonate with the inductive load 110.
The coil of the wafer 120 may be shimmed off the wafer to aid in getting the system into
resonance. Approximately 1/8 of a wavelength back from the load, a capacitor 140 (Cj) is
placed across the line to cancel the imaginary component of the load. The exact distance is
found in the conventional manner from a Smith chart when the real component of the line
impedance becomes 50 Ω. The capacitor is chosen to cancel the reactive component at that
point.
The first two patents cited in the Background of the Invention section above discuss
an RF electrode which is configured to be placed in a small laparoscope or endoscope
about 2.3 mm in diameter. Both ablation and heating are accomplished by the direct
injection of RF current through electrodes at the tip of the probe. In contrast, the present
inventions accomplish the same results but by using magnetic fields, i.e. without electrodes
and without the commensurate fear of electrode - tissue contamination.
Shown in Figure 10 is another embodiment by which this is accomplished. As
shown in Figure 10(a), this embodiment consists of a solenoidal winding 160 with an
internal core or ferrite 170. (This figure is similar to Figure 1). This apparatus constitutes
the tip of a laparoscopic instrument. The outer medium 150 is the polyelectrolytic solution
to be heated. Each of the regions 160 and 170 are 0.5 mm radial thickness by 10 mm long
in an axisymmetric geometry. The outer test region 150 has a typical radius of 5 mm by 20
mm long. Typical field lines 180 for this axisymmetric geometry are shown in Figure
10(b).
There are 4 options for the composition of regions 160 and 170, listed in Table I.
The winding 160 can be wound with copper or steel wire. The inner sleeve of the solenoid
170 can be filled with the ambient polyelectrolytic (saline, for example) by leaving it open,
or it can be filled with ferrite. Each case was worked out with a boundary element code
and the loss in the surrounding polyelectrolytic fluid 150 computed with a current density
of 3.0 x 106 A/m2 and 108 Hz. The results, shown in Table I, clearly indicate the advantage
of using steel wire with a ferrite core.
Another way of injecting heat magnetically with a miniaturized scope probe is
shown in Figure 11. A toroidal core 190 is preferably wound with copper winding 211.
The core is wound with a very fine tape of ferromagnetic material on a mandrel. Typical
dimensions of the tape is 1 mil thick by 8 mm width, and it extends past the tip of the
nonconducting laparoscope or endoscope 210 by about 2 -5 mm. Preferably, the copper
winding or core coil is wrapped from the outer cylindrical surface of the toroid and into
the inner cylindrical surface of said toroid. When excited with current at high frequency,
interstitial current is induced to flow down or through the center of the toroid, in the
same loop that the exciting current follows.
An additional embodiment is shown in Figure 12. A closed conducting waveguide
220 such as aluminum, with a cavity therein, is designed to resonate at a frequency near
915 MHz. The waveguide may be dielectrically loaded so that smaller dimensions can be
employed to yield this resonance frequency. A coaxial loop antenna 222 or dipole antenna
extending into the cavity of the waveguide, excites the waveguide, injecting energy from an
RF oscillator. A hole 224 is cut in the upper sidewall of the waveguide to allow leakage of
the field into the surrounding space. The advantage of the resonant cavity approach is that
the field pattern witnessed by the patient is less dependent on the shape of the exciting
antenna. The present embodiment avoids the problem of concentrating the field in the
location of the antenna, and allows a broader field to emanate from the hole in the cavity.
Having described this invention with regard to specific embodiments, it is to be
understood that the description is not meant as a limitation since further embodiments,
modifications and variations may be apparent or may suggest themselves to those skilled in
the art. It is intended that the present application cover all such embodiments,
modifications and variations.