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Description  |
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FIELD OF THE INVENTION
This invention relates to the control of Curie Point material for medical
and other purposes by magnetic and high frequency electrical fields.
BACKGROUND OF THE INVENTION
It has previously been proposed to localize the application of chemotherapy
agents for the treatment of cancer tumors by the physical association of
magnetic materials and chemotherapy agents in protein microspheres, such
as human serum albumin, or other microscopic carriers. In this regard,
reference is made to Medical World News, Aug. 21, 1978, p. 57; "Magnetic
Targeting of Microspheres in Blood Flow", by C. F. Driscoll et al.,
Microvascular Research 27, 353-369 (1984), and "Experimental Methods in
Cancer Therapeutics", by Kenneth J. Widder et al, Jl. of Pharm. Sciences,
Vol. 71, No. 4, Apr. 1982 pp. 379-386. Related patents include K. J.
Widder U.S. Pat. No. 4,247,406; R. T. Gordon U.S. Pat. No. 4,106,488; G.
H. Czerlinski U.S. Pat. No. 4,454,234. The Widder patent relates to the
process mentioned above. The Gordon patent relates to the heating of
particles which may be magnetic, by high frequency magnetic fields, as a
cancer treatment method. The Czerlinski patent involves aggregation and
re-suspension of fine particles in solution where the particles include
magnetic material having a predetermined Curie point, and the aggregation
and re-suspension is accomplished by varying the temperature above and
below the Curie point.
With regard to the use of magnetic particles and chemotherapy agents in
microspheres for the treatment of cancer, this technique has been
successful with tumors which are localized near the surface of the human
body. However, for deep seated tumors, it is not practical to provide a
magnetic field which is localized deep within the body. More specifically,
the magnetic field is provided by permanent magnets or electromagnets
which are mounted outside of the body, and therefore provide a magnetic
field which extends from the skin to the deep-seated tumor. Accordingly,
if the procedure is employed in such cases, many of the carrier
microspheres will be held outside of the desired cancerous zone. Further,
because the magnetic field strength drops off with distance from the
magnet, higher magnetic field strengths, and correspondingly higher
concentrations of the chemotherapy agent will be located near the skin,
outside of the deep-seated cancerous zone.
A principal object of the present invention is to overcome the limitations
as outlined above on the treatment of deep-seated cancerous tumors or the
like.
SUMMARY OF THE INVENTION
In accordance with the present invention, the magnetic material employed in
the microsphere technique as described above, is formed of a Curie point
material wherein the magnetic material loses its magnetic properties at a
temperature slightly above the normal temperature of the human body; and a
high frequency electric field is applied to heat up the magnetic material
within the magnetic field zone but outside of the area where the tumor to
be treated is located. With the magnetic particles only being magnetic in
the vicinity of the tumor, the microspheres and the chemotherapy agent
will be concentrated at the desired localized zone. The chemotherapy or
other therapeutic agent to be associated with the microspheres may be any
of the known types of chemotherapy or therapeutic substances, or may
involve radioactive materials, as noted below.
In order to verify the location of the active microspheres, they may also
include Technetium 99 m, or other suitable gamma ray emitting radioactive
material. As a further alternative, when radiation treatment is also
desired, I-131 or a comparable radioactive isotope may be used, as this
iodine isotope emits both gamma rays for imaging location and beta
particles for radiation treatment. Known types of gamma ray imaging
equipment may be employed to determine the location and concentration of
the particles, and suitable adjustments in strength and/or location of the
magnetic field and high frequency electromagnetic heating arrangements,
may be made to shift the concentration as needed or desired.
For the steady magnetic field, an electromagnet or several electromagnets
are normally preferred for ease in adjustment, but permanent magnets may
also be employed. To provide the electromagnetic radiation, waveguides
with output horns may be used; high frequency coils could be employed; or
dipole type radiating antenna type elements may be used, to directionally
apply the radiation to the desired location. Constructive and destructive
interference from more than one radiating source may be employed with
cyclic phase shifting when appropriate, to heat the magnetic particles
where they are not wanted, and to avoid heating the magnetic particles in
the desired zone within the magnetic field where the particles are to be
concentrated.
From a broad standpoint, the present invention involves the use of Curie
point magnetic material, at least one magnet for providing a steady
magnetic field for exerting a restraining force on the magnetic material,
and the application of a high frequency electric field to selected areas
within the steady magnetic field, such as close to the magnet, so that the
Curie Point magnetic material may be restrained only at the desired
location or locations within the magnetic field.
Other objects, features and advantages of the invention will become
apparent from a consideration of the following detailed description, and
from the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an electromagnetic therapy system illustrating the principles
of the invention;
FIG. 2 is a pair of plots of magnetic field strength versus depth for two
different sets of electrical currents in the electromagnets of FIG. 1;
FIG. 3 shows corresponding plots of magnetic gradient versus depth for the
same magnets;
FIG. 4A is a comprehensive showing of an alternative embodiment of the
invention;
FIG. 4B shows the fields of the embodiment of FIG. 4A;
FIG. 5 is a series of plots of the interference of electromagnetic waves
from two different sources such as those of FIG. 4;
FIG. 6 is a diagrammatic showing of electromagnetic wave interaction;
FIG. 7 is another diagrammatic showing of electromagnetic wave combination
from four dipoles; and
FIG. 8 is a plot of magnetization versus temperature, for Curie point
materials.
DETAILED DESCRIPTION
General Considerations
As mentioned above, the present invention involves the selective restraint
of magnetic material having an accessible Curie point temperature, and the
use of (1) a magnetic field to hold the magnetic material and (2) the use
of a high frequency electromagnetic field to selectively heat the magnetic
particles to a temperature above the Curie point.
In order to effect restraint of particles within a selected field zone, two
conditions must be simultaneously met therein--(1) the particles must be
magnetically responsive i.e., at a temperature sufficiently below the
Curie point to exhibit substantial ferromagnetic exchange coupling, and
(2) the static magnetic field gradient must be of adequate strength to
restrain magnetically responsive particles within capillary vessels in the
selected field zone.
It is necessary and sufficient that either one of these conditions be
absent at sites external to the selected field zone (where it is desired
to concentrate the microspheres) in order to effect free unrestrained flow
of the particles.
The appropriate presence and absence of these conditions is regulated by
the geometrical intersection of an oscillatory electromagnetic field and
the static magnetic field, as set forth below. The effect of the
oscillatory electromagnetic field is to heat up the magnetic particles and
render them substantially nonmagnetic.
It is a general feature of this invention that the oscillatory
electromagnetic wave intensity be absent or of negligible value in the
selected target zone. Oscillatory electromagnetic waves may be locally
diminished (1) by natural exponential attenuation upon passage through
lossy material, and (2) cancellation of waves oppositely phased emanating
from two or more sources.
SPECIFIC EXAMPLES
One example illustrating the principles of the invention utilizing the
former, diminution principle noted at (1) in the preceeding paragraph, is
shown in FIG. 1 in cross-section.
A static magnetic field may be generated unilaterally with respect to a
patient, 12, by means of a concentric coil array, including an inner coil
14 and an outer coil 16, in FIG. 1. By adjustment of the polarity and
current in the individual coils 14 and 16, a region 18 of static magnetic
field gradient equalling or exceeding the requisite strength is
controllably projected symmetrically on the axis of the coil array 14, 16
to a desired depth. The central region 19 has a relatively low magnetic
field gradient.
The magnetic field strength would appear as shown in FIG. 2 along the axis
of the coil array with selected energization of coils 14 and 16. The
curves depict the general form of the magnetic field strength B with
respect to depth in two separate examples with plot 14-1 corresponding to
currents flowing principally in the inner, smaller coil 14, and plot 16-1
corresponding to substantial currents in the outer, larger coil 16. The
curve characteristic of particular interest is the region of steepest
slope which correlates to the maximum magnetic gradient critical to
restraint. In FIG. 2, this value occurs at depth zones centered at points
a and b on curves 14-1 and 16-1 respectively. The coil array is placed
such that its central axis intersects the targeted zone 20 in FIG. 1, and
the coil currents are adjusted until the depth of the maximum gradient
coincides with zone 20.
The significance of the curves in FIG. 2 may be better understood when
graphed as the derivative (slope) of B with respect to depth as shown in
FIG. 3, with plot 14-2 relating to plot 14-1, and gradient plot 16-2
corresponding to plot 16-1. Thereby, it may be recognized that regions
centered about some depth, e.g. points a and b in FIG. 3, exhibit a
maximal, moderately constant, gradient.
Without an oscillatory electromagnetic field, particles would be restrained
and accumulate in the entirety of region 18, FIG. 1. It is necessary
however, to restrict such accumulation to a sub-region of volume 18,
namely region 20 which is the targeted zone.
The confinement of particles to region 20 is accomplished in the example
shown in FIG. 1 by means of an antenna array 22 directing an oscillatory
electromagnetic field toward the general region of the static magnetic
gradient region 18. Elements of said array are waveguide horns 22.
For example, at a frequency of 100 megahertz (MHz), human tissue attenuates
the intensity by a factor of 1/e squared, or reduces the intensity to
about 0.137 of its original value for every 7 cm traversed. If the closest
border of the target zone were 14 cm deep, then natural attenuation would
reduce the incident oscillatory wave intensity by a factor of 0.137 of
0.137 or approximately 0.02, or one-fiftieth of its original value. In
order to confine the magnetic particles to the targeted zone 20, it is
necessary to deliver a threshold oscillatory wave intensity I at the
closet target zone border point 24 FIG. 1, 14 cm deep in this example.
This value I is selected to increase the particle temperature to a
marginal limit of magnetic responsiveness, or non-responsiveness.
It may be noted in this example, that the incident intensity substains an
attenuation to 0.02 of its original value to ultimately deliver a
threshold intensity of I at point 24. Conversely, the incident intensity
must be 1/0.02 or 50 times as large as I. At depths less than that of
point 24, FIG. 1, the oscillatory wave intensity, sustaining less
attenuation, exceeds I, thereby heating the particles and rendering the
particles magnetically non-responsive. With appropriate spatial separation
and oscillatory frequency, antenna elements 22 are driven with sufficient
amplitude to cause the marginal limit zone 26 to exhibit a spatial
curvature, within the patient, which extends substantially beyond the
limits of the restraining static magnetic field gradient, 18. Thereby, the
oscillating field within the bounds 26 nullifies the magnetic
responsiveness of particles throughout the high gradient region 18 except
in zone 20. Because of the relatively inverted curvatures of the fields 18
and 26, zone 20 is roughly spherical in shape. At depths greater than that
of point 24 FIG. 1 within zone 20, the natural attenuation reduces the
oscillatory field intensity below the intensity level I, whereby the
magnetically responsive particles are restrained within the targeted zone.
At depths exceeding that of zone 20, the magnetic gradient falls off
sufficiently so that restraint does not occur.
Thereby, in this example shown in FIG. 1, the requisite conditions are met
whereby particle restraint occurs exclusively in the targeted zone 20.
Incidentally, the flexible container or bag 28 contains a dielectric fluid
providing a transition for the electromagnetic waves permitting a
controlled radiation pattern into the body 12 without undue reflection.
The fluid within the dielectric or plastic bag 28 may be deionized water.
This bag 28 may, for example, be of dielectric or plastic material, to
avoid reflections.
Within the scope of this invention, oscillatory wave interference or
cancellation may also be utilized in effecting intensity diminution at the
selected target zone. One such example is depicted in cross-section in
FIG. 4.
The patient 12-1 is positioned next to a static magnetic field which may be
generated by a coil array including coils 14-1 and 16-1 similar to that
depicted in FIG. 1. A magnetic gradient region 32 in FIG. 4 is of
sufficient strength to restrain magnetically responsive particles.
Restraint, however, is to be restricted to a targeted zone 34 within
region 32. It is then necessary to cause the particles to be rendered
non-magnetic in all regions of zone 32 exclusive of the targeted zone 34.
Opposing planar antenna arrays 36 and 38 shown in cross-section in FIG. 4
are utilized to direct oscillatory electromagnetic radiation toward region
32. The amplitude and phase of each element in each array are controlled.
If all elements within each array have the same phase, planar waves
essentially parallel to the array plane are generated from each respective
array. The instantaneous amplitude of such waves from arrays 36 and 38 may
be described respectively as Asin(2.pi. ft) and Bsin(2.pi. ft+k), upon
arrival at the target zone plane enclosed by the dashed lines 40-a. The
amplitude coefficients A and B depend upon two variables each, the
attenuation sustained by the respective waves in traversing the respective
distances to the target zone plane and, of course, the initial amplitude
of emission from arrays 36 and 38. Accordingly, the latter variables may
be adjusted to compensate for the former so that the amplitude
coefficients A and B each equal a common value which we will designate as
A at the target zone plane. The attenuation is dependent upon the
frequency f of the waves. The relative phase of the waves in the target
zone plane can be arbitrarily selected simply by retarding or advancing
the phase of one of the arrays as indicated by the constant k in the
following equation (1). In particular, for the amplitudes from arrays 36
and 38 to add to zero implies that:
Asin(2.pi. ft)+Asin(2.pi. ft+k)=0 (1)
for which k equal to .pi. (or 180 degrees) is a solution. With this
relative phase condition, the waves from arrays 36 and 38 of FIG. 4,
moving right and left respectively as shown in the plots of FIG. 5,
totally cancel at all times at the target zone plane 40; but the
intensities average to a non-zero value in the regions on either side of
said plane 40 as shown in FIG. 5 for three successive instants in time,
t.sub.1, t.sub.2 and t.sub.3, in the three showings of FIG. 5.
Effectively, a planar slab delineated in cross-section by lines 40-a in
FIG. 4 defines a region of zero or negligible oscillatory wave intensity
wherein particles remain magnetically responsive, and outside of which
particles are heated by the field and rendered magnetically
non-responsive.
By simultaneously retarding and advancing the individual antenna elements
across the arrays 36 and 38 of FIG. 4, the aforementioned planar slab can
be tilted at any desired angle. Instantaneously, the slab might be defined
by plane 40-b. The rotation of this slab about an axis 42 extending
centrally through arrays 36 and 38 from 40-b to 40-c and thence back to
40-b is effected by a periodic retardation and advancement of the
individual antenna elements. Averaged out, only the targeted zone 34 would
constantly remain in a region of sufficiently low oscillatory wave
intensity for particles therein to remain unheated and magnetically
responsive. All other regions of zone 32 are subjected to sufficiently
substantial oscillatory wave intensity to render the particles therein
magnetically non-responsive.
The foregoing examples demonstrate the principles of this invention with a
circular coil array constituting a static magnetic field generator. It may
be appreciated that other magnetic field generators can also be used such
as a "C" shaped iron yoke magnet where the patient is placed between the
magnetic poles. The magnet can be either an electromagnet or a permanent
magnet.
Likewise a variety of oscillatory electromagnetic field antenna arrays and
operating modes are applicable within the scope of this invention.
ENERGY ABSORPTION IN PARTICLES
A central feature of this invention is the spatially controlled disposition
of oscillatory electromagnetic energy in said particles. In an idealized
circumstance, such energy disposition would be zero at the targeted field
zone and abruptly very high elsewhere. Specific physical interactions
mediate to diminish the abruptness of the absorption transition in and out
of the target field zone. However, using the techniques as described
herein, together with materials having appropriate absorption
characteristics and moderately abrupt Curie temperature, effective
restraint in the target zone is achieved.
The absorption of oscillatory electromagnetic radiations in magnetic and in
conductive matter will now be considered. For example, from the American
Institute of Physics Handbook (McGraw-Hill, New York, 1957), Sec. 5 p. 90,
tin and magnetic iron have very similar conductivities, being in a ratio
of 1:1.2. Nevertheless, the absorption of energy flux is in a ratio of
1:16 based upon the relative penetration depths at which the flux has
diminished to 1/e squared for radiation in the range of 1 to 3000 MHz.
This rather marked absorption difference is attributed to the relative
magnetic permeabilities which are in a ratio of 1:200. Electromagnetic
radiation, which consists of oscillatory electric E and magnetic B vector
components, is absorbed in relation to electric conductivity and magnetic
permeability, respectively. Accordingly, it may be understood that tin and
magnetic iron both absorb a certain similar proportion of the electric
component but the magnetic iron additionally absorbs a very large
proportion of the magnetic component. If both components are radiated at
equal amplitudes, it may be expected that magnetically responsive
substances will absorb energy predominantly from the magnetic component.
The relevance of this interaction to the present invention may now be
understood. The particles of this invention have a magnetic permeability
which is very sensitively temperature dependent. In the targeted field
zone, the particles are to be maximally magnetically responsive in order
to effect restraint with respect to the static magnetic field. In regions
immediately exterior to this zone, the particles are to be minimally
magnetically responsive in order to allow unrestrained flow into the zone.
If, for example, the electromagnetic radiation immediately exterior to the
zone were ten times as high as in the zone, then the particles would be
expected to sustain a ten-fold higher energy absorption and a concurrent
temperature rise outside the zone. However, since the particles are
deliberately designed to exhibit a substantial reduction in magnetic
permeability in response to a substantial temperature rise, the absorption
of the magnetic component of oscillatory electromagnetic energy is
severely diminished. If the magnetic component is the predominant source
of energy, then the desired effect partially cancels the means to achieve
that effect. That is, an initially high temperature rise brought about by
a strong absorption of the magnetic component is quickly followed in
equilibrium by a partial loss in temperature as the magnetic component is
less strongly absorbed. Since the final equilibrium temperature is not as
high as the brief initial temperature, the particles immediately exterior
to the zone sustain only a partially reduced magnetic responsiveness and
may exhibit a degree of undesired restraint in response to the static
magnetic field. Effectively, the minimum size of the targeted field zone
is increased somewhat and the concentration of restrained particles is not
as abruptly delineated by the zone.
As developed below, however, the multiplicity of antenna elements may be so
configured and phased so as to substantially cancel the oscillatory
magnetic components and augment the oscillatory electric components in the
aforementioned regions exterior to the targeted field zone. Since the
interaction of the particles with regard to the oscillatory electric
component is effectively independent of temperature, the energy absorption
of the electric-enhanced oscillatory field is essentially proportional to
the intensity of the field.
This type of arrangement increases the sharp delineation of the particle
restraint zone.
Specifically, consider FIG. 6 where the instantaneous oscillatory field
components are generated from a pair of equally driven antenna dipole
elements 52(a) and 54(b). The respective resultant magnetic components
B.sub.a and B.sub.b at the point 56 are oppositely oriented, perpendicular
to the plane of the page, thereby cancelling. The electric components add
vectorially giving a value E.sub.tot significantly larger than the
components themselves. Extending this configuration to a second pair of
antenna elements 58 and 60, where all four elements are on the vertical
edges of a box-like geometrical shape of square cross section, as shown in
FIG. 7, allows the generation of a strong electric oscillatory field
located centrally above as indicated at reference numeral 62. The
corresponding net magnetic component remains at a constant zero magnitude.
It will be appreciated from the mutually perpendicular orientations of the
electric and magnetic components for any oscillatory field source, that a
variety of antenna element types may be operated conjunctively so as to
nullify the net magnetic components within a region. Accordingly, this
invention is not restricted to the illustrated configurations which are
presented as examples.
PARTICLE PROPERTIES
A number of substances called ferromagnetics, such as iron, may be very
strongly magnetized while in the presence of a magnetic field. Most of
these substances exhibit magnetization versus temperature curves similar
in shape to FIG. 8 but differing in scale. For example, the magnitude of
the maximum magnetization M.sub.m and the temperature T.sub.c on the
absolute scale varies considerably among the known ferromagnetics. The
value T.sub.c is the temperature at which the extrapolated curve
intersects the axis, and is known as the Curie point. A substance
responding as in FIG. 8 is said to be ferromagnetic when below the Curie
point, T.sub.c. At temperatures above the Curie point T.sub.c, the curve
descent levels off somewhat wherein a substance is said to be
paramagnetic.
The very large magnetization exhibited by ferromagnetic substances is a
collective quantum mechanical phenomenon known as exchange coupling. When
aggregates of certain atomic species are formed, a very large percentage
of the individual atomic magnetic moments align together. The broad
gradually sloping region of FIG. 8 below T.sub.c shown in FIG. 8,
indicates nearly 100% alignment. As temperature increases up to T.sub.c,
this exchange coupling is disrupted by thermal agitation with a concurrent
decrease in magnetization. The paramagnetic state, above T.sub.c, is said
to exist when sufficient disruption occurs such that the coupling is
totally broken and the atoms act independently in their alignment
response. The maximum magnetization M.sub.m for the purposes of this
invention, should be substantial, ideally comparable to iron and other
strong ferromagnetics. The particles of this invention should also exhibit
response wherein human body temperature, which is 310 degrees K., or 98.6
degrees Fahrenheit, should fall at a point T.sub.O on the shoulder of the
curve at the onset of rapid descent as in FIG. 8. For a value of T.sub.O
so situated, T.sub.c is typically a modest increment higher on the order
of magnitude of 10 degrees Kelvin. While it is not necessary that the
induced temperature increase actually reach or exceed T.sub.c, it is
essential that a very large relative decrease in magnetization be
effected. Nevertheless, substances having Curie points slightly above 310
degrees K. are indicative of good candidates for the particles.
Pure iron for example is inappropriate, having a Curie temperature of 1040
degrees K. Several possible choices and their Curie temperature in degrees
Kelvin include, CrTe, 320; Cr.sub.3 Te.sub.4, 325; Nd.sub.2 Fe.sub.7, 327;
Ni-Cr (5.6% atomic % Cr), 324; and Fe-Ni (about 30% Ni) 340 as well as
many other combinations. Furthermore, it is known in the art that small
percentage variations in composition can increase or decrease the Curie
temperature by several degrees. For instance, the Fe-Ni alloy can be
altered to provide a lower Curie temperature of perhaps 320.
The Fe-Ni alloy is also desirable since it is a moderately good conductor,
essential to absorption of the oscillatory electric component. Fe-Ni also
exhibits magnetization comparable to that of pure iron, Fe. Biologically,
the elements Fe and Ni do not exhibit the undesirable toxicity common to
an element such as chromium, Cr, included in some of the afore-mentioned
combinations, and the material is therefore substantially medically inert.
ENERGY ABSORPTION AND TEMPERATURE RISE
The purpose of the oscillatory wave generator is to significantly raise the
particle temperature in regions exterior to the targeted zone. The
temperature rise is caused by the preferential conversion of
electromagnetic energy to thermal energy by the particles. Conversely, the
temperature of surrounding tissue is not significantly raised when
subjected to the same oscillatory waves.
The underlying physical principles are readily understood in conjunction
with the relative absorptivity of good conductors and patient tissue. For
example, at 100 MHz, the intensity decreases by a factor 1/e squared in
0.0007 cm of copper and in 7 cm of tissue, indicating that a good
conductor such as copper is 10,000 times as absorptive as tissue.
The thermal energy of the particles is subsequently dissipated to
surrounding tissue. However, the total mass of injected particles is many
orders of magnitude less than that of the patient. Consequently, the
patient is effectively an infinite heat sink negligibly increased in
temperature by the relatively small total heat content transferred from
the particles.
Thereby, the particles are readily increased in temperature whereas direct
and indirect energy transfer to tissue is negligible resulting in an
insignificant rise in overall patient temperature.
IMPLEMENTATION OF APPARATUS
The oscillatory electromagnetic field may be provided by devices such as a
MA-150 waveguide antenna horn coupled to a BSD-1000 RF power generator,
both manufactured by BSD Medical Corporation, Salt Lake City, Ut.
These devices are conventionally used to achieve regional hyperthermia by
selectively directing radio frequency (RF) electromagnetic waves of high
intensity at a tumor site within a patient. Certain tumor types are
temperature sensitive compared to normal tissue. In this regard, a
temperature increase of about 5 degrees K. sustained for approximately 20
minutes is often effective in killing tumor cells, while normal cells are
left undamaged.
A coaxial conductor cable interconnects the BSD-1000 to a termination
within the MA-150 waveguide antenna horn consisting of plate electrodes
across a dielectric layer.
The antenna horn facilitiates directivity of the projected electromagnet
waves. A flexible water bag affixed to the mouth of the antenna horn is
pressed against the patient over the site targeted for the application of
electromagnetic energy. The water efficiently couples the RF waves into
tissue and minimizes reflections. Thermal energy generated in the water is
continuously removed by pumping through an ice-filled heat exchanger. By
this means, the surface of the patient is cooled through a thermal
conductive process which allows for additional control of temperature
within the patient.
The BSD-1000 RF power generator provides fully adjustable power from 5
watts to 250 watts over the frequency range of 95 MHz to 1000 MHz.
Although heating may be obtained over a wider range, for the purposes of
the present invention, a frequency range of about 50 megahertz or
50,000,000 cycles per second, up to about 200 megahertz is preferred. The
reason that this range is preferred is that above 50 megahertz, there is
more absorption by the particles and less by the human body; and above 200
megahertz, hot spots may develop near the horns. However, effective
heating may be accomplished over a much broader range of frequencies.
More than one MA-150 antenna horn may be driven by the BSD-1000 using power
splitters. The MA-150 units may be arranged in an array such that each
unit represents an antenna element of this invention. The power output
from the BSD-1000 to each MA-150 unit may be phase shifted and attenuated
to control of the oscillatory wave intensity as described with respect to
this invention.
E-field sensors available from BSD are placed in skin contact on the
patient to monitor the incident electric field and estimate the resultant
internal temperature distribution.
The MA-150 horns project electromagnetic waves with the electric and
magnetic vectors mutually perpendicular to each other and also to the
direction of the wavefront propagation as is common to all such
electromagnetic propagation. Thereby, as described hereinabove, two
adjacent MA-150 horn units may be placed to produce total cancellation of
the magnetic vector and augment the electric vector in the neighborhood of
a mid-plane between the units. Correspondingly, opposing MA-150 units
produce an intermediate null plane by destructive interference, as
described herein, using opposite relative phase.
The component devices used in hyperthermia are necessarily operated at high
power levels to produce gross regional temperature increases of about 5
degrees K. in and around targeted tissue.
For the purposes of this invention, sub-therapeutic power levels with
respect to hyperthermia, are used such that actual regional tissue
temperature at all sites is never increased by more than 2 degrees K., and
generally by less than 1 degree K. Nevertheless, when such tissue contains
particles as described herein, then said particles locally sustain a
substantially higher temperature increase of approximately 10 degrees K.
as demonstrated by loss of magnetic responsiveness.
Furthermore, the objective of hyperthermia is, ideally, a focal heating of
targeted tissue e.g., a tumor. This focal heating may be augmented by
constructive interference of horn antennae at the depth of the tumor
whereas in the context of the present invention, a significantly reduced
RF intensity exists at the targeted tissue. It may be appreciated that
attenuation by tissue absorption, and by phase inversion of the electric
vectors from opposing horn antennae and destructive interference, or
cancellation, may be used to produce this reduced RF intensity.
The static magnetic field may be produced by Model HS-1785-4A DC power
supplies combined with circular coil elements such as those in the Model
M-4074 assembly, both av | | |
