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CROSS REFERENCE TO RELATED APPLICATIONS
This application contains subject matter set forth in the co-pending U.S.A.
application by the inventor named herein, Dr. Robert Rand et al., Ser. No.
223,727 filed Jan. 9, 1981 entitled "Induction Heating Method For Use In
Causing Necrosis of Neoplasm" now U.S. Pat. No. 4392040. The entire
disclosure of this co-pending application is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
The invention set forth in this specification pertains to a new and
improved method and apparatus for causing necrosis of neoplasm by
hyperthermia.
Aberrant cells within the body, such as are frequently referred to as
neoplasms, are the subject of a great deal of concern to society and
civilization because of the very undesirable consequences of various
growths such as are commonly referred to as tumors, cancers and the like.
Tremendous sums of money have been and will continue to be spent in
connection with the problem of causing necrosis of neoplasm with a minimal
detrimental effect to a person at a minimal cost. It is considered that no
one approach to this problem of causing necrosis of neoplasms will ever be
completely satisfactory for use with all different types of neoplasms.
It has been recognized that certain types of neoplasms can be effectively
necrosed as the result of the use of hyperthermia. In the past, the
heating used in this type of treatment has been achieved either through
the use of the electromagnetic radiation associated with a radio frequency
or microwave field or through the use of an alternating magnetic field
such as is associated with induction heating. It is considered that an
understanding of the present invention requires an understanding as to
essential differences between these two distinctly different types of
heating.
A radio frequency or microwave field can only be utilized in causing
heating at or adjacent to the surface of a body because of the tendency of
the body to absorb radio frequency electromagnetic radiation. The depth to
which such radiation will penetrate the body will vary depending upon the
frequency of the radiation. Such body tissue heating as is caused by radio
frequency radiation has two principal components: eddy current heating and
dielectric heating. Both result from the electromagnetic field employed
passing through the tissue. Effective heating of this type can be achieved
in the absence of any foreign material in tissue being heated.
As opposed to this, induction heating of body tissue as a result of the
presence of a material having hysteresis characteristics may be referred
to or at least considered as hysteresis heating. It is based upon the use
of a magnetic field which is normally presumed to be unattenuated by
tissue. Hence, induction heating can reach any desired depth or level
beneath the surface of tissue. In order to achieve this manner of heating
in tissue, it is necessary to locate a material exhibiting magnetic
hysteresis at or about the location where heat is desired. Any such
induction-hysteresis heating will be accompanied by some eddy current and
dielectric heating.
The amount of eddy current and dielectric heating can be minimized by
utilizing a comparatively low frequency alternating magnetic field. The
amount of such heating varies directly with the square of the frequency of
the field. As opposed to this, the amount of hysteresis heating forming
the operative "part" of induction heating for use with the present
invention varies directly with the frequency. Thus, in general, the lower
the frequency of the field the less the amount of undesired eddy current
heating achieved relative to the amount of the desired hysteresis heating
achieved. The frequency used, however, should not be so low as to preclude
the achievement of a desired degree of hysteresis heating.
The limited amount that a radio frequency field can normally penetrate body
tissue has been and remains an important factor limiting the use of this
type of heating in the treatment of various different neoplasms.
Obviously, this radio frequency heating can not be utilized effectively in
causing hyperthermia in aberrant growths which are sufficiently beneath
the surface of tissue so as not to be reached by a radio frequency field.
The use of such a field to cause hyperthermia at or adjacent to the
surface of the skin or at or adjacent to the interior of an incision
within the body is limited by another important factor--the problem of
selectively concentrating the heat developed so that the neoplasm is
heated to a point sufficient to cause necrosis without there being a
related or corresponding heating of adjacent normal tissue.
This can be easily illustrated by referring to specific temperatures such
as have previously been recognized to be important in causing necrosis as
a result of hyperthermia. It is usually considered that necrosis will not
be caused by the exposure of most body tissue to a temperature of about
40.degree. C. for a reasonably prolonged period. It is also generally
considered that if commonly found body tissue--including neoplasms--are
held at a temperature of at least 42.degree. C. for a reasonably prolonged
period that such cells or tissue will be destroyed if the period is
adequately long for the purpose.
It is also generally conceded that commonly encountered cells and
tissue--including neoplasms--will be necrosed by exposure to a temperature
of from about 60 degrees centigrade or, of course, a higher temperature
for a very minimal or limited, practically instantaneous time period. It
should be noted that body tissue should not normally be subjected to a
temperature reasonably approaching 100.degree. C. for even a very short,
instantaneous time period because of the possibility of the vaporization
of water in the body and the attendant possibility of damage being caused
by the sudden expansion of vaporization of water. As a practical matter,
it is considered that internally the body should never be heated past 90
degrees centigrade to avoid any possibility of such vaporization.
Apparently, the relationship between the time that neoplasm or other body
growth need be exposed to a specific temperature to cause necrosis and the
time necessary for such exposure to cause necrosis are not related in
exactly usual time-temperature reactions. Further, certain parts of the
body or neoplasms are more susceptible to necrosis at an elevated
temperature than other parts of the body and/or other certain neoplasms.
Because of these factors, it is impossible to give exact relationships
between the time and the temperature necessary to accomplish necrosis and,
in particular, necrosis of neoplasms. Another factor which will be
important in considering the amount of hyperthermia to cause necrosis in
neoplasm will involve the tendency of the body and/or a particular portion
of the body to serve to at least a degree as a heat insulator so as to
tend to concentrate heat developed, as for example, at a specific source.
With radio frequency or microwave hyperthermia the difficulty in confining
the field necessary to cause necrosis of a specific area or region will
normally cause necrosis of healthy tissue in an adjacent area or region.
While to a degree this can be combatted through the use of specialized
electrodes, it is not considered that it is possible to adequately control
radio frequency or microwave heating in many applications so a to avoid
detrimental damage to normal cells or tissue. While it may be possible to
improve the concentration of the heating effects achieved with radio
frequency or microwave heating through the implantation of a metallic
conductor or another similar material in a specific area where
concentrated heating is desired, it is considered that such a use of a
conductor is not always advantageous because such a conductor will not
improve the depth of pentration of tissue by a radio frequency field.
The use of induction heating in causing necrosis of neoplasm is considered
desirable in overcoming several problems involved in utilizing a radio
frequency field to cause hyperthermia leading to necrosis of neoplasm as
noted in the preceding. When induction heating is used for this purpose,
it is necessary to utilize an invasive technique to locate a material
having a magnetic permeability greater than unity and capable of exibiting
hysteresis losses--for example, ferromagnetic particles such as a
conventional ferrite or conventional iron or steel powders--in or
immediately adjacent to the neoplastic growth. The use of such a magnetic
material in an alternating magnetic field results in hysteresis heating
within the magnetic material itself. This localized or focused hysteresis
heating overcomes the problem of containing or limiting the heated area
obtained with a radio frequency field. Because of the penetration of a low
frequency magnetic field with respect to body tissue, it is possible to
use a magnetic material well beneath the surface of the body.
From this it is believed that it will be apparent that the utilization of
induction hysteresis heating in connection with the hyperthermia of
neoplasms has a great deal of advantageous potential. Unfortunately, it is
not considered that the prior efforts for accomplishing necrosis of
neoplastic growth by induction heating have been sufficiently effective
and desirable for this purpose. It is considered that several factors may
be important in connection with this.
One of these concerns the lack of availability of induction heating
equipment which is specifically adopted for use in treating a body such as
a human or animal body in which magnetic material has been implanted so as
to produce localized heating. In connection with this, it is noted that
prior induction heating coils have not normally been designed with the
intent that they be utilized to provide a very intense AC magnetic field
over a comparatively large volume so as to cause heating in a specific,
very limited area or region coming within the "scope" of the complete
magnetic field produced by the coil in which material capable of
hysteresis heating is located. It is considered that this factor has led
to a degree of inefficiency of prior induction heating coils when such
coils have been utilized in conjunction with body tissue so as to cause
inductuon heating of materials or particles embedded within such tissue.
It is considered however, that the prior procedures for accomplishing
necrosis of neoplastic growths by hysteresis heating have also been
comparatively unsatisfactory for other reasons relating to the technique
or procedure followed in implanting the magnetic material or particles
used. In general, such materials have not been utilized in such a manner
as to be effectively immobilized either within or immediately adjacent to
a neoplasm so as to be capable of providing a localized heating without
danger of the magnetic material used moving from its initial location
within the body. Any such movement would be highly disadvantageous because
of the possibility of undesired interference with the normal operation of
the body.
BRIEF SUMMARY OF THE INVENTION
Because of these various related considerations, it is considered that
there is a need for a new and improved process of causing necrosis of
neoplasm as a result of hyperthermia of the neoplasm which may be easily
and conveniently carried out at a comparatively nominal cost and which is
effective for its intended purpose. The invention is intended to provide
such a process. A further "process" objective of this invention is to
provide a procedure as indicated which can be utilized in connection with
a wide variety of different neoplasms located in various different regions
of the body without significantly endangering the life or health of the
body in which such neoplasms are located.
In accordance with this invention, those aspects of the invention which are
directed towards the process as noted are achieved by providing a process
of causing necrosis of neoplasm as a result of hyperthermia of the
neoplasm in which magnetic particles are injected into tissue in proximity
with the neoplasm and are subjected to an alternating magnetic field so as
to cause hysteresis heating of said particles in which the improvement
comprises: said particles are mixed with a carrier fluid as they are
injected into said tissue, said carrier fluid being biologically inert and
being capable of becoming non-fluid within said tissue, and causing said
carrier fluid to become non-fluid within said tissue. Said carrier fluid
may include radiopaque material which renders said fluid more visible
under radiographic examination.
BRIEF DESCRIPTION OF THE DRAWINGS
Because of the nature of this invention, it is considered that it is best
more fully explained with reference to the accompanying drawing in which:
FIG. 1 is a diagrammatic view illustrating the essential features or
concepts of a presently preferred embodiment or form of an apparatus in
accordance with this invention; and
FIG. 2 is an isometric view showing the configuration and utilization of
the coils explained.
It is to be understood that the illustration of a particular apparatus
shown is not to be taken as limiting this invention in any respect. This
invention involves the concepts or principals set forth and defined in the
appended claims. Those skilled in the field of utilizing induction heating
in treatment of neoplasm will realize that these concepts or principals
may be utilized in various different ways without departing from the scope
of the subject matter set forth in the claims.
DETAILED DESCRIPTION
As indicated in the preceding summary, an important aspect of the invention
involves the formation of a mixture of magnetic particles and a carrier
fluid. This mixture should have fluid or fluid like properties enabling
the complete mixture to be easily and conveniently injected into a body
through the use of a cconventional hypodermic needle or any reasonably
related procedure such as, transarterial catheterizaton. This intended
manner of location of a mixture within the body places several important
restrictions upon the ingredients of the mixture.
Within the broad purview of the present invention, it is possible to
utilize any magnetic material having a magnetic permeability greater than
unity, which exhibits hysteresis heating. As a practcal matter, it is
desired to utilize only magnetic materials which present a minimum of a
toxicity problem when used within the human body and in addition which
possess or exhibit as large a hysteresis loop when subjected to an
alternating magnetic field as reasonably obtainable. It is the area within
such a loop that is of prime importance in the present invention since
such area corresponds with the efficiency of hysteresis caused heating.
Because of this consideration, the exact shape of the hysteresis loop of
the material used is not important in the sense that it is relatively
unimportant as to whether or not a square wave type of hysteresis type
loop is achieved.
It is considered preferable to utilize with the present invention only
ferromagnetic particles such as various conventional ferrites and various
commercially obtainable iron and steel particles known to have or exhibit
comparatively large magnetic losses. It is considered quite fortunate that
such materials are relatively inert as far as most metabolic processes are
concerned and can be utilized within most parts of the body without
significant concern. Other biologically inert particles capable of
exhibiting hysteresis heating can, of course, be employed.
An interesting aspect of the invention lies in the fact that magnetic
particles may be chosen having a Curie temperature or Curie point such as
to control the maximum temperature which can be achieved during induction
heating. Thus, for example, it is possible to choose a particular
paramagnetic or ferromagnetic composition having a Curie point anywhere
within the range of from about 42.degree. C. to about 90.degree. C.
specified to the preceding so that hyperthermia of a neoplasm may be
accomplished without fear of heating tissue beyond a desired temperature.
In connection with this matter, it is noted that normally the magnetic
characteristics of a material will change over a comparatively small range
of temperatures and that therefore the Curie point in many respects
corresponds to a limited range of temperature.
The particles utilized should be sufficiently small so as to be capable of
either going into colloidal suspension within a carrier fluid as
subsequently indicated or so as to be capable of forming a slurry with the
carrier fluid. Any such slurry should have a sufficiently low viscosity so
as to be capable of being compared with a suspension of aluminum hydroxide
antacid in water capable of being easly handled and being poured from a
bottle to a spoon and the like. As a practical matter, it is considered
that this method of utilization indicates that the particles used should
have an average diameter of no greater than about 1 millimeter. It will be
realized, however, that on occasion particles which are of slightly larger
dimension can be effectively poured and handled.
It is considered that the particles used should not be sufficiently fine so
that their fineness will tend to detract from the ability of a magnetic
field to cause these particles to heat. In general, if the particles are
undesirably small this will tend to make the hysteresis loop exhibited by
the material smaller than the loop which would be obtained with somewhat
larger sized particles. Because of this, the particles should be of such
dimension that there is no diminution of the hysteresis loop exhibited as
a result of the size of the particles used. With at least some materials,
comparatively long, thin particles appear to have better hysteresis
heating characteristics than particles of a reasonably spherical shape.
As a practical matter, it is considered that the lower level of desirable
size of the particles used and that the shape of the particles used should
be determined on a empirical basis as a result of testing particles of
various sizes. Also as a practical matter, it is considered that in
general, the particles used should be of a dimension no less than the
dimension of the magnetic domains of the material within these particles.
In general, it is considered undesirable to utilize particles any finer or
smaller than those which are conventionally utilized within ferro-fluids
in which ferromagnetic particles are held in colloidal suspension.
Within the broad aspects of the present invention, virtually any type of a
biologically inert liquid capable of being easily tolerated by the body
can be utilized as a carrier fluid so as to facilitate the insertion of
magnetic particles as indicated in the preceding. Such carrier fluid may
be of either two broad generalized types. It may be a type which will
remain fluid after being located in place within the body and/or tissue.
Such a fluid may be of a type which will form a solid or at least a
semi-solid or non-newtonian solid within such tissue.
A carrier fluid of the first type is preferably utilized when particles are
to be injected in a location within a neoplasm or even within normal
tissue adjacent to neoplasm where there is no reasonable possibility of
the particles entering the bondstream, since there is always a possibility
that any such articles in the bloodstream would cause damage. As opposed
to this, carriers of the second type which become a solid or semi-solid
may be utilized in any location relative to neoplasia--including in blood
vessels since the second type of carrier fluid will preclude the movement
of particles within the bloodstream, by becoming solidified.
Within the broad concepts of the present invention, it is even possible to
utilize water as a carrier vehicle in those cases where the particles used
will in effect, be isolated from the remainder of the body by normal
tissue surrounding the specific location where such particles are used.
Other permanently liquid or at least semi-liquid fluids capable of being
utilized are such liquids as Ringer's solution, albumen, essentially
fluid-like starch gels and the like. It is not considered necessary to
enumerate in this specification many other different carrier fluids which
are capable of being utilized within the broad concepts of the present
invention inasmuch as various fluids which are compatible with body
functioning are well known. When water or any other liquid or semi-liquid
carrier is used it is considered that it will be obvious that minor
amounts of biologically inert surfactants and the like may be employed so
as to facilitate the suspension of the particles used and so as to aid in
controlling viscosity of the mixture.
Whenver a carrier is used which will not become solid or semi-solid in time
in connection with magnetic particles as noted it is considered preferable
to completely surround such particles with a biologically inert, adherent,
conventional protective coating. While in a sense the use of such coated
particles is optional, it is nevertheless considered highly preferable to
minimize any chance of any sort of unintended interference with normal
metabolic processes. Many different types of such coatings are, of course,
known. It is considered that coatings such as various polyolefin coatings
are particularly suitable. The coatings used on the particles should, of
course be as thin as reasonably possible. A nominal amount of
agglomeration of the particles caused by the inherent character of the
coating material employed will not normally prove detrimental.
With the present invention it is preferred to utilize a carrier fluid which
is of such a character that it will change from a liquid or fluid to
essentially a solid or semi-solid after it has been injected into a
neoplasm or into a body adjacent to a neoplasm either as a result of a
polymerization or similar action initiated as a result of one or more
components present in the carrier fluid or as a result of the application
of external "energy." Such "energy" may be derived from normal body heat.
The energy used may be derived directly from the magnetic field used in
causing hysteresis heating or may be the heat resulting from such heating
of the particles within the carrier fluid.
The primary reason why the use of a carrier fluid which changes so as to
become non-fluid or substantially non-fluid in character is preferred with
the present invention relates to the desirability of creating a physical
structure within the body which will physically hold the particles used so
that such particles cannot move throughout the body in the blood and so
that such particles cannot become lodged in various passages or cells
where they might cause at least a degree of damage. The precise method or
mode of forming a solid or a non-newtonian semi-solid from the mixture of
the particles and carrier fluid employed is in many respects immaterial to
the present invention.
Further, the solidity of the non-fluid body or composition created is also
a matter of choice. From the point of view of patient comfort it is
considered that the ultimate non-fluid body created from the carrier fluid
should either be of a somewhat flexible, somewhat elastomeric character
corresponding to a reasonable degree to the character of human tissue or
should be of a non-newtonian character capable of being deformed while
still remaining as a coherent body or unit.
As a result of these considerations, it is considered preferable to utilize
as the carrier fluid a elastomer which will develop elastomeric, non-fluid
properties within the body after being mixed with an amount of particles
which is effective in the production of heat by hysteresis heating. Any
such fluid composition may contain a wide variety of different
biologically inert secondary ingredients such as fumed silica, various
surfactants, and the like serving various secondary functions such as, for
example, a degree of control of the ultimate elasticity of the polymer
body produced. Such a fluid composition may also include radiopaque
material, such as barium sulfate to enhance its visualization under
radiographic examination.
The liquid or fluid composition employed may be a pre-polymer or partially
polymerized solution containing one or more catalysts. Such a liquid
carrier composition may include inert liquid such as water which will not
enter into an ultimate or final solid or non-newtonian semi-solid
composition but which will serve as a dilutent viscosity control agent
until such time that the composition becomes non-fluid in character.
It is presently preferred with the present invention to utilize as the
carrier fluid a silicone elastomer composition which is capable of being
injected into the body as a liquid and which will set up when in place
within the body as a polymer serving to physically hold the various
magnetic particles used in virtually any intended location within the
body. The use of this material is considered to be especially desirable
because of the fact that mixtures of this type of composition and of
particles as noted are normally adequately coherent so that they will not
separate or break up into smaller "units." Further, silicone elastomers as
noted herein are normally acceptable for use within the human body.
It is considered quite important that a carrier which is intended to become
solid or semi-solid within the body is reasonably able to "wet" various
magnetic particles as noted so that such particles are completely coated
by the carrier fluid in such a manner that they are effectively isolated
from normal body fluids in the final non-fluid body created. This
minimizes any chance of the material within the particles used interfering
with normal metabolic processes. It also makes it possible to utilize with
the invention particles of magnetic material which are somewhat
biologically active. The use of any such biologically active material is
not, however, preferred because of the possibility of some sort of
unintended interference with normal metabolic processes.
The relative proportions of particles and the carrier fluid which should be
utilized together in a mixture as described will depend upon the physical
properties of the ingredients used. In general, the mixture should contain
the minimum amount of the fluid carrier which is effective to suspend and
hold the particles so that they will not separate out of the mixture
during the time period when the mixture is being injected into a body as
indicated in the preceding discussion. Further, the proportions should be
such that adequate carrier fluid is present so that the inherent physical
cohesive "character" of the fluid will tend to hold the mixture together
as a unit after the mixture is injected into the body.
This is considered to be particularly important as, for example, when a
mixture as used is inserted into a blood vessel. The mixture should not
fragmentize within such a blood vessel but should be of such a character
as to hold together as a result of cohesion until such time as it is
rendered non-fluid or solidified as indicated in the preceding discussion.
In certain applications of the present invention it is highly desirable
for the mixture of particles and the fluid carrier to be sufficiently
coherent so that such a mixture may to a degree be guided to a specific
location or held in such a specific location within or adjacent to
neoplasm through the use of an external magnetic field until such time as
the mixture is rendered sufficiently non-fluid that it is held in place as
a result of physical engagement with adjacent walls or tissue.
In some cases, to increase the amount of hysteresis heating attainable at a
given magnetic field intensity, it may be desirable to apply a DC magnetic
field to the magnetic mixture while it is solidifying. This will align all
of the magnetic particles in one direction. Subsequent applications of the
AC magnetic field along the same axis will cause more hysteresis heating
than if the particles were randomly oriented.
Radiopaque materials such as barium sulfate may be added to the mixture to
increase its visibility under radiographic examination. The radiopacity of
the mixture facilitates the location of, and monitoring of, the mixture by
radiographic examination.
A mixture having these properties can be compounded by using a
dimethylpolysiloxane liquid silicone preparation available under the trade
name Dow Corning Silastic 382 which is diluted 50% with a silicone fluid
available under the trade name Medical Grade Dow Corning Silicone fluid
360. These materials are available from the Dow Corning Company of
Midland, Mich. A 1 gram portion of finely ground ferrite powder and a 1
gram portion of finely powdered barium sulfate are homogenously dispersed
in 15 ml. of this solution by thorough mixing to form a colloid-like
suspension.
In addition, the mixture may contain temperature-indicating ingredients
that can be monitored by X-ray, ultrasonic, or other means while the
induction heating is in progress. The amount of these materials should be
small enough so as not to appreciably alter the properties of the mixture.
After a mixture as indicated in the preceding has been located with respect
to a neoplasm and, except in those cases where the carrier fluid is "set
up" as a result of hysteresis heating as subsequently discussed, the
tissue containing the neoplastic growth is subjected to an alternating
magnetic field as discussed in the preceding, which will cause adequate
hyperthermia to cause necrosis of the neoplasm adjacent to the magnetic
particles present.
Although such heating may be caused with any sort of a coil capable of
being utilized for induction heating purposes, it is considered preferable
within the concepts of the present invention to utilize a specialized
induction heating apparatus 10 as illustrated in the accompanying drawing.
For convenience this apparatus 10 is referred to as including circuit
components as hereinafter described.
This particular apparatus 10 includes a series of coaxial, identically
constructed and dimensioned flat, "pancake" coils 12. Each of these coils
12 consists of a series of turns (not separately numbered) of an
electrically non-conductive tube 14 such as an extruded polyvinyl polymer
tube located in two flat parallel planes, and a conductor 16 going through
the interior of each tube 14. These conductors 16 are sufficiently small
so as to permit a cooling liquid such as, for example, a dielectric oil or
distilled water to be circulated through the tubes 14 around the
conductors 16.
Appropriate conventional fittings 18 are provided at the ends (not
separately numbered) of the tubes 14 for the purpose of joining the ends
(not separately numbered) of the tubes 14 to manifolds or distributing
pipes 20 used in connection with a storage reservoir 22 and a pump 24.
These fittings 18 are formed in a conventional or known manner so that the
conductors 16 extend through them so that these conductors 16 may be
connected as hereinafter indicated.
With the present invention each conductor 16 constitutes the electrically
"active" part of a coil 12 and is connected into a tank circuit 26 across
two separate capacitors 28 of equal capacitance value located in series
with one another. All of the coils 12 and the associated capacitors 28
together constitute a tank circuit having a specific resonant frequency.
A known adjustable power supply 30 capable of being adjusted so as to
operate at various frequencies is used as a means for supplying a current
at the noted resonant frequency to a transformer 32 which in turn is
connected to only one of the tank circuits 26. In the particular
embodiment illustrated, the transformer 32 is connected across only one of
the two capacitors 28 in such a circuit 26. However, if desired, this
transformer 32 can be connected across both the capacitors 28 in this
particular circuit 26.
All of the circuits 26 are grounded by conventional grounds 34. In order to
promote safety it is preferred to ground each circuit 26 between the
capacitors 28 in such circuit as shown. With the present invention the
particular coil 12 which is driven by the power supply 30 and the
transformer 32 may be any one of the individual coils 12 used in the
complete apparatus 10.
The coils 12 are assembled closely together as shown in a "stack" of
aligned, identical pancake-type coils 12. They preferably are secured
together as a unit (not numbered) by the use of a conventional,
appropriate adhesive (not shown). Because they are used in this manner, in
effect, the coils 12 constitute a series of closely coupled transformer
coils. As a consequence of this, the use of the power supply 30 with a
single coil 12 has the effect of "driving" the remainder of the coils 12
and, of course, the associated tank circuits 26 including these coils 12.
In order to achieve the degree of hysteresis heating desired with the
present invention in connection with a comparatively small area or region
of a mixture of a carrier and particles as indicated in the preceding
discussion it is necessary to drive the particular circuit 26 connected to
the power supply 30 utilizing a comparatively significant amount of power.
The exact amount of such power will, of course vary in accordance with
many factors. At this time, it is believed that the exact amount of power
used in various different reasonably related treatments of neoplastic
tissue should be judged on an empirical basis as an outgrowth of
experience, taking into consideration, factors as are subsequently
indicated.
The frequency of the AC current supplied to a coil 12 and its associated
circuit 26 is normally determined on the basis of the factors indicated in
the preceding discussion relative to the background of the invention so as
to minimize eddy current type heating losses while concurrently maximizing
the efficiency of the hysteresis heating obtained. At this time, it is
believed that a frequency of from about 1000 to about 5000 Hz--and
preferably of about 2000 Hz--initially represents the most desirable
"balance" between competitive factors which are involved in achieving the
desired hyperthermia in accordance with the invention and that the
individual tank circuits 26 should be constructed to be tuned to a
specific frequency as noted and that the power supply 30 should be
operated at such frequency.
As power is supplied to a circuit 26 and its associated coil 12 a cooling
liquid will, of course, be supplied to the individual coils 12 in an
effort to prevent any damaging temperature rise in these coils or their
manner of operation. However, in spite of this some heating will normally
occur in the individual conductors 16 in the various coils 12 and in
connection with the various associated capacitors 28. Because of such
heating, the individual circuits 26 will change slightly as they are used
and this in turn will affect the resonant frequency of the complete
apparatus.
Because of this, as the complete apparatus 10 is operated the frequency of
the power supply 30 used to drive directly one circuit 26 and its
associated coil 12 and the remainer of the circuits 26 and coils 12
indirectly should be adjusted so as to always match the new resonant
frequency of the complete collection of circuits 26. In order to further
minimize power losses it is considered highly preterable to utilize as the
conductors 16 either wires composed of a twisted multitude of strands of
individual wires or conventional Litz wires.
The utilization of such conductors may also be advantageous for another
reason. During the use of the coil 12 at high power levels, as indicated,
a cooling fluid must be circulated through the tube 16, as previously
indicated, so as to prevent damage to either the tubes 14 or melting of
the conductor 16. It is considered that the surface configuration of
either a twisted or a Litz wire conductor, as noted, is effective in
promoting efficient heat transfer from the conductor to the cooling liquid
employed.
During the utilization of the apparatus 10 power will be supplied from the
power supply 30 to a connected tank circuit 26 at a frequency matching the
resonant frequency of all of the circuits 26 as a "unit." As this specific
circuit 26 is driven or powered in this manner it will set up a flux field
which will in turn couple to the adjacent coils 12 so as to power these
adjacent coils in a transformer-like manner by mutual induction. This will
have the effect of utilizing one specific coil 12 to drive all of the
various coils 12 solely as the result of the mutual induction between the
coils 12. This has several significant advantages. First, only
comparatively low voltages are required at the terminals of the individual
coils since they are electrically in parallel; a series-wound coil of the
same total number of turns would require many kilovolts of excitation
voltage, with accompanying high-voltage insulation requirements, corona
discharge problems, and safety hazards. Second, the circuits 26 are
electrically isolated so that they need not all have exactly the same
voltage, a requirement that would exist if all of the circuits 26 were
connected to a common voltage source.
It is not to be assumed from the above that the apparatus 10 requires the
utilization of coils 12 which are symmetrical relative to an axis. In
increasing the efficiency of the apparatus 10, it is considered desirable
to construct all the coils 12 used so that they are of an identical oval
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