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BACKGROUND OF THE INVENTION
The subject invention relates to microwave probes useful in hyperthermia
therapy, and more particularly, to probes utilizing a coaxial antenna
construction and useful primarily for the interstitial or invasive
hyperthermia treatment of tumors.
It has long been known that certain cancer cells can be destroyed at
elevated temperatures which are slightly lower than temperatures normally
injurious to healthy cells. In recent years, hyperthermia therapy
utilizing electromagnetic radiation has been found to be particularly
effective and many processes and types of apparatus utilizing microwave
hyperthermia are known in the art. These include non-invasive types in
which external applicators are utilized and the microwave energy is
allowed to penetrate the skin and underlying tissue, including a tumor to
be treated. Obviously, this results in the heating of healthy tissue as
well and this lack of control is one reason why more direct and exact
means of applying microwave hyperthermia have been sought.
Thus, invasive methods and related apparatus have been developed in which a
hyperthermia probe or probes may be inserted to the point of treatment via
a normal body opening or may be inserted interstitially through the skin
directly to the site of the tumor to be treated. Such invasive methods and
apparatus provide the advantage of potentially better control of
temperature of the mass or volume of tissue to be treated.
Since many types of malignancies cannot be reached and effectively treated
with non-invasive techniques or with invasive probes designed to be
inserted into a normal body opening, much attention has been recently
given to long, narrow needle-like probes which can be inserted directly
into the body tissue and to the site of the tumor or malignancy to be
treated. Such probes must of necessity be of a very small diameter, both
to aid the ease with which they may be inserted and used and to reduce the
trauma associated therewith to the patient.
Interstitial microwave hyperthermia probes may be of a rigid or semi-rigid
type with a needle-like point for direct insertion into the body tissue.
Alternately, the probe may be more flexible and adapted to be used inside
a catheter first inserted into the body tissue by ordinary, well-known
methods. The advantages of using a probe inserted into a catheter include
avoiding the need to separately sterilize the probe and to take advantage
of catheters which may already have been inserted for other types of
concurrent treatment, such as radiation therapy. Nevertheless, the
convenience of more rigid probes adapted for direct interstitial insertion
allow their alternative use in certain situations.
Regardless of the type of microwave probe, a problem common to all of them
has been to provide a uniform pattern of radiated energy and heating
axially along and radially around the effective length of the probe or to
otherwise control and direct the heating pattern. A known and predictable
heating pattern is, of course, important so that the heating may be
confined to the greatest extent possible to the tissue to be treated and
excessive heating of healthy tissue avoided.
Microwave probes of two kinds have been used, one comprising a monopole
microwave antenna and the other a dipole coaxial antenna. In a monopole
antenna probe, a single elongated microwave conductor is exposed at the
end of the probe (sometimes surrounded by a dielectric sleeve) and the
microwave energy radiates generally perpendicularly from the axis of the
conductor. However, so-called monopole probes have been found to produce
non-uniform and often unpredictable heating patterns and the heating
pattern does not extend beyond the probe tip. As a result, more recent
attention has been directed to so-called "dipole" antennas of a coaxial
construction. These include constructions having an external reentrant
coaxial "skirt" around the distal end of the outer conductor.
The typical coaxial probe includes a long, thin inner conductor extending
along the axis of the probe, surrounded by a dielectric material, and an
outer conductor surrounding the dielectric. To provide the effective
outward radiation of energy or heating, a portion or portions of the outer
conductor can be selectively removed. This type of construction is
sometimes referred to as a "leaky waveguide" or "leaky coaxial" antenna.
Obviously, variations in the location, size and area of the outer
conductive material removed along the effective length of the probe can
significantly affect the heating pattern provided. One of the primary
goals in such construction has, thus, been to provide a uniform heating
pattern generally or a more narrowly controlled and directed pattern in a
selected region of the probe tip. U.S. Pat. No. 4,204,549 discloses the
removal of short semi-cylindrical sections of the outer conductor in a
coaxial probe to provide directional control of the heating pattern, but
uniformity in the control of the heating pattern is not discussed. U.S.
Pat. No. 4,669,475 discloses a coaxial antenna probe in which a full
circumferential cylindrical portion of the outer conductor is removed over
a selected intermediate axial length of the probe. However, the heating
pattern per se is not disclosed or described, and the heating pattern of
individual or multiply oriented probes is controlled by varying the
microwave energy supplied to the probes. U.S. Pat. No. 4,658,836 discloses
the removal of long axial segments of the outer conductor to provide full
length heating. Control of the heating pattern is attained by varying the
thickness of supplemental outer dielectric covering or by a unidirectional
variation in the axial outer conductor segments. In an alternate
embodiment, the outer conductor is attached in a uniform double spiral
pattern to provide the necessary open space for radiation leakage.
However, the utility of the spiral pattern is disclosed as providing the
probe with flexibility and to provide relative rotation of the heating
pattern along the probe length. Finally, the probe of this patent is
intended particularly for insertion into a body passage or cavity and not
direct interstitial insertion into body tissue.
Control of the heating pattern in coaxial probes for interstitial
hyperthermia treatment continues to be a problem. It is important to be
able to control and predict the heating pattern axially along the
effective length of the probe. Because of the need to maintain the very
small diameter of these probes, it is impractical to utilize heating
pattern control means which increase the effective diameter, such as an
outer dielectric layer. Thus, any improved means of heating pattern
control should be compatible with the typical constructions of
interstitial probes, whether they be of the flexible or rigid type.
SUMMARY OF THE INVENTION
In accordance with the present invention, heating pattern uniformity is
provided in a coaxial microwave probe, both radially and axially along its
effective length, by varying the open area in the outer conductor axially
along the effective length of the probe such that there is a maximum open
area in the axial center portion and smaller open areas in both opposite
directions.
In a preferred embodiment, the typical continuous outer conductor is
removed or eliminated along substantially the full effective heating
length of the probe and replaced with a spiral winding of a conductive
wire in which the winding has a varied spacing to provide the varying open
area in the outer conductor. An extremely broad range of winding patterns
may be used depending on specific probe construction characteristics.
Typically, the effective length of the probe is divided into sections of
different winding pitch. The sections may provide an overall symmetrical
pattern or be asymmetric, the pitch within a section may be uniform or
varying, transitioned pitch sections may be provided between the major
sections, and each section itself may comprise subsections with variations
in the windings.
In an alternate embodiment particularly applicable to rigid or semi-rigid
probes in which the outer conductor is typically a thin solid metal
covering, the axially varying open area in the outer conductor is attained
by providing a series of axially spaced slots of varying length and depth.
The slots are preferably transversely disposed in axially spaced planes
perpendicular to the axis of the probe. As with the spirally wound
embodiment, the slots are sized or spaced to provide a maximum open area
in the center portion of the effective length of the probe and relatively
smaller open areas axially in both directions therefrom. The slot pattern
may be symmetrical or asymmetric in an axial direction, and may be
otherwise varied in manner similar to the spirally wound embodiment.
The general arrangement of both embodiments has been found to provide the
desired uniformity in the heating pattern, both radially and axially along
the probe. In both embodiments, healing extends beyond the tip.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a longitudinal view, partly in section, of the embodiment of the
invention particularly adaptable for use with flexible coaxial probes.
FIG. 2 is a view similar to FIG. 1 showing a pattern in the wound outer
conductor particularly suitable for longer probes.
FIG. 3 is an enlarged detail of a portion of the probe shown in FIG. 1.
FIG. 4 is a side view of a probe of a more rigid construction showing an
alternate embodiment of the invention.
FIG. 5 is a top view of the probe shown in FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a microwave probe 10 is constructed from a section of
conventional small diameter flexible coaxial cable. The cable includes a
wire-like center conductor which runs axially along the length of the
cable and is surrounded by a cylindrical layer of suitable dielectric
material 12. Many types of dielectrics are usable and a plastic material,
such as PTFE, is often used. The dielectric material is, in turn,
surrounded by an outer conductor 13 comprising a braided metal
construction. Both the unitary center conductor 11 and the braided wire
outer conductor 13 may be made of a copper base metal which is preferably
silver-plated to enhance conductivity. Other suitable metals may, of
course, also be used. The proximal end of the probe 10 includes a standard
coaxial connector 14 for connection to a conventional source of microwave
energy (not shown).
The conventional coaxial cable thus far described is modified to provide
the present invention by removing a cylindrical section of the braided
outer conductor 13 along the distal end of the probe. The axial length of
the section of the outer conductor removed determines the effective length
of the probe from the standpoint of the length along which an effective
heating pattern may be produced. A short section of braided outer
conductor 16 may be left at the probe tip to facilitate the conductive
connections to be hereinafter described, but that short section may be
eliminated as well and the conductive connections made in another manner.
In place of the section of the braided outer conductor 13 removed from the
end of the cable, a thin wire 17 is wound in a helical pattern over the
dielectric 12 and between the outer conductor 13 at the proximal end of
the probe to the free end thereof. Referring also to FIG. 3, appropriate
conductive connections 18 and 19 are made between the wire 17 and the ends
of the outer conductor 13 between which the spiral winding is made.
Alternately, if the short section of outer conductor 16 at the end of the
probe is eliminated, the spiral winding is simply run to the probe tip.
In a specific embodiment of the probe shown in FIG. 1, which may have an
effective length (or axial heating pattern length) of about 4 cm
(approximately 1.5 inches), the center conductor 11 has a diameter of
0.007 inch. The cylindrical dielectric covering 12 has an outer diameter
of 0.033 inch and the braided outer conductor is about 0.010 inch in
thickness. The conductive wire winding 17 is 0.007 inches in diameter.
Thus, the nominal OD of the probe along its effective length is 0.047
inch.
The spiral winding pattern shown includes a center portion 20 and end
portions 21 adjacent thereto. The wire 17 is preferably wound with a
spaced pitch over the full length of the winding and with the maximum
spacing in the center portion 20 to provide the maximum open area in the
outer conductor for microwave energy leakage or radiation. In the probe
shown, the center portion 20 has a length of about 0.250 inch and the end
portions 21 lengths of 0.295 inch each. The spaced pitch of the winding of
wire 17 in the center portion 20 is 0.0281 inch and the spaced pitch of
the end portions 21 is 0.0095 inch. Suitable pitch transition zones may be
provided between the portions 20 and 21.
The actual heating pattern provided by the probe extends axially in both
directions beyond the conductive connection 18 and 19 between the wire 17
and the outer conductor 13. Typically, the heating pattern will extend
about 0.2 inch (0.5 cm) beyond the tip of the probe. Thus, the effective
heating pattern provided by the probe is approximately 4 cm.
FIG. 2 shows an alternate embodiment of the spirally wound probe shown in
FIG. 1, adapted particularly for longer probe constructions. Overall, the
components of the probe 23 in FIG. 2 are the same as those in the
embodiment of FIG. 1 and are, therefore, identically numbered. Thus, a
conventional coaxial cable has a conductive inner member 11, surrounded by
a dielectric 12, and around which is disposed an outer conductor 13. The
outer conductor, which may be of the braided construction previously
described, is cut away to expose the dielectric material 12 at the end of
the probe 23.
Beginning with a conductive connection 18 to the end of the outer conductor
13, a wire 12 is wound in a helical pattern which, in this embodiment,
runs to the end of the probe. The wire is wound in a spaced arrangement
and the pitch of the winding varies over the length of the probe with a
maximum spaced pitch in the center portion. However, in this embodiment,
the center portion 24 of the winding itself comprises a winding having a
variable spaced pitch.
In the specific construction shown, the center portion 24 of the winding
includes a narrowly spaced center subportion 27 and more widely spaced end
subportions 28 either side. The end portions 26 again comprise a winding
of a narrow spaced pitch. In one preferred construction, the end portions
26 each have a length of 0.472 inch and are wound to a pitch of 0.0169
inch. The center end subportions 28 are each,.689 inch in length and are
wound to a pitch of 0.0276 inch each. The center subportion 27 is
identical to the end portions 26 having a length of 0.472 inch and a pitch
of 0.0169 inch.
The effective heating pattern of the FIG. 2 probe is approximately 8 cm or
slightly greater than 3 inches. The heating pattern also has uniform
radial depth along substantially its entire length. Depending on the power
level of the microwave energy supplied to the probe, the uniform heating
pattern may extend radially for a centimeter or more. The axial extension
of the heating pattern toward the proximal end of the probe over a short
portion of the braided outer conductor 13 allows a thermocouple to be
located at that point. In this manner, the thermocouple or other heat
sensor connections will not interfere with the operation of the probe and
yet are attached in an area where the measured temperature is
representative of that effectively applied by the probe.
Referring now to FIGS. 4 and 5, there is shown an alternate embodiment of
the probe which is of a rigid or semi-rigid construction and intended for
direct insertion into body tissue without the use of a catheter. The probe
30 is of a basic coaxial construction, including an axial inner conductive
member 31, a dielectric material 32 surrounding the inner conductor, and a
conductive outer shell or layer 33. To provide the desired pattern of
heating to extend beyond the tip of the probe, the outer conductive layer
33 is suitably formed to a point and makes conductive contact 35 with the
inner conductor 31 at the tip of the probe.
To provide the axially varying open area in the outer conductive layer 33,
a series of transverse slots 34 is cut into the outer layer 33 and
underlying dielectric material 32 on diametrically opposite sides of the
probe 30. The slots 34 are of maximum length and depth in the center
portion of the axially disposed slot pattern and become progressively
smaller in both axial directions therefrom. The deepest centrally located
slot or slots may have a maximum depth of approximately 1/3 the diameter
of the probe and the receding depths of the slots in opposite axial
directions should preferably fit a smooth curve, as shown by the dashed
line 36 in FIG. 5.
The rigid probe 30, in a preferred embodiment, has an outer diameter of
0.086 inch. The slot pattern comprises nine slots 34 spaced at 0.25 inch
with the small slot nearest the tip spaced 0.375 inch therefrom. The
foregoing dimensions, however, may be varied over fairly broad ranges. In
order to provide a smooth outer surface on the probe and to better match
the probe impedance to the tissue in which it is intended to be used, the
slots are filled with a material having a high dielectric constant, such
as titanium dioxide.
The dashed line 37 in FIG. 4 is representative of the uniform heating
pattern attained with each embodiment of the hyperthermia probe of the
subject invention. Each embodiment described hereinabove includes an
axially symmetrical pattern around the opening provided in the outer
conductor. Such symmetry, though desirable, is not necessary and
variations including asymmetrical patterns in the openings may also be
used to provide the desired heating pattern.
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