U.S. patent application number 11/513885 was filed with the patent office on 2006-12-28 for radiation applicator.
Invention is credited to Nigel Cronin.
Application Number | 20060293651 11/513885 |
Document ID | / |
Family ID | 10848507 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060293651 |
Kind Code |
A1 |
Cronin; Nigel |
December 28, 2006 |
Radiation applicator
Abstract
This invention provides an elongate microwave radiator for
insertion into a living body to treat tissue at a predetermined
operating frequency. The radiator defines a monopole antenna at its
tip. The monopole antenna includes a dielectric material
surrounding the monopole. The dielectric material is configured to
act as a resonator at the predetermined operating frequency, and
encompasses generally the whole of a near-field radiation emitted
by the monopole. In an illustrative embodiment, the dielectric
material extends from the antenna a distance determined in
accordance with the wavelength of the radiation in the
dielectric.
Inventors: |
Cronin; Nigel; (Bath,
GB) |
Correspondence
Address: |
CESARI AND MCKENNA, LLP
88 BLACK FALCON AVENUE
BOSTON
MA
02210
US
|
Family ID: |
10848507 |
Appl. No.: |
11/513885 |
Filed: |
August 31, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09914375 |
Jan 15, 2002 |
7118590 |
|
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11513885 |
Aug 31, 2006 |
|
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Current U.S.
Class: |
606/33 ;
607/101 |
Current CPC
Class: |
A61N 5/04 20130101; A61B
18/1492 20130101; A61N 1/04 20130101; A61B 18/14 20130101; A61B
2018/00577 20130101; A61B 18/18 20130101; A61B 2018/1861 20130101;
A61B 18/1815 20130101 |
Class at
Publication: |
606/033 ;
607/101 |
International
Class: |
A61B 18/18 20060101
A61B018/18 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2000 |
GB |
GB 00/00682 |
Claims
1. An elongate microwave radiator for insertion into a living body
to treat tissue at a predetermined operating frequency, the
radiator comprising a monopole antenna at its tip, the monopole
antenna comprising: a monopole; and a dielectric material
surrounding the monopole, the dielectric material being configured
to act as a resonator at the predetermined operating frequency, and
encompassing generally the whole of a near-field radiation emitted
by the monopole.
2. The radiator as claimed in claim 1 in which the dielectric
material extends from the antenna a distance determined in
accordance with the wavelength of the radiation in the
dielectric.
3. The radiator as claimed in claim 1 in which the dielectric
material extends from the antenna a distance determined in
accordance with the major dimension (L) of the antenna in the
dielectric.
4. The radiator as claimed in claim 1 in which the dielectric
material extends from the antenna a distance at least substantially
equal to 2 L.sup.2/.lamda., where L is the major dimension of the
antenna and .lamda. is the wavelength of the radiation in the
dielectric.
5. The radiator as claimed in claim 1 in which the dielectric
material comprises a substantially cylindrical portion with the
antenna extending axially at its centre a distance L.
6. The radiator as claimed in claim 2 in which the dielectric
material extends from the antenna a distance substantially equal to
half the wavelength of the radiation in the dielectric.
7. The radiator as claimed in claim 1 in which the monopole
comprises a coaxial conductor with a central conductor that
projects beyond outer screening of the coaxial conductor at the
distal end to form the antenna.
8. The radiator as claimed in claim 7 in which the antenna has a
length substantially equal to half the wavelength of the radiation
in the dielectric.
9. The radiator as claimed in claim 7 including a transformer
operatively connected between the coaxial conductor and the
dielectric material to reduce reflection of radiation back into the
coaxial conductor at the boundary with the dielectric material.
10. The radiator as claimed in claim 9 in which the transformer
includes a space within the coaxial conductor into which packing of
the coaxial conductor can expand.
11. An elongate radiator for insertion into a living body to treat
biological tissue at a predetermined operating frequency, the
radiator comprising a monopole antenna at a tip thereof, the
monopole antenna comprising: a monopole; and dielectric material
surrounding and extending beyond the monopole, the dielectric
material terminating in a rounded tip portion and being condo to
act as a resonator at the predetermined operating frequency to
thereby enhance transmission of radiation in a forward direction
therefrom.
12. The radiator as claimed in claim 11 in which the tip portion is
substantially hemispherical.
13. A method of coupling radiation into biological material, the
radiation being generated by an applicator comprising a monopole
antenna including a monopole surrounded by a dielectric body of the
monopole antenna, the method comprising the steps of: configuring
the dielectric body of the monopole antenna to act as a resonator;
and selecting the dielectric constant of the dielectric body in
accordance with the wavelength of the radiation in the dielectric
so that generally the whole of the near-field of the radiation is
encompassed by the dielectric body.
14. The method as claimed in claim 13 in which the dielectric body
extends from the monopole antenna a distance at least substantially
equal to 2 L.sup.2/.lamda., where L is the major dimension of the
antenna and .lamda. is the wavelength of the radiation in the
dielectric.
15. The method as claimed in claim 13 in which the major dimension
of the monopole antenna is its length, which is substantially equal
to half a wavelength of the radiation in the dielectric.
16. The method as claimed in claim 13 in which the dielectric body
is located in relation to the biological material so that the
far-field radiation lies within the biological material.
17. The method as claimed in claim 13 in which the dielectric
constant of the dielectric body is high, but is lower than that of
the biological material.
18. The method as claimed in claim 13 in which the dielectric
constant of the dielectric body varies, and is higher at its core
than its outer periphery, and the dielectric constant at its outer
periphery is lower than that of the surrounding biological matter.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/914,375, filed on Jan. 15, 2002 entitled RADIATION
APPLICATOR by Nigel Cronin.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to microwave radiators and, in
particular, to microwave ablation devices.
[0004] 2. Background Information
[0005] A known microwave radiator, used for microwave ablation of
tissue, compromises a microwave generator operatively coupled to an
elongated waveguide for conveying the microwaves to the ablation
site. The waveguide is sufficiently thin to be inserted into the
body and contains a core dielectric material which enables
efficient transmission of microwaves through the waveguide. At the
emission end of the waveguide, the dielectric core protrudes and
provides a radiating tip for coupling microwaves into surrounding
tissue. An object of the inventor is to provide an improved
radiation applicator.
[0006] According to one aspect, the invention includes an elongate
microwave radiator for insertion into a living body to treat
biological tissue at a predetermined operating frequency, the
radiator comprising a monopole at its tip and dielectric material
surrounding the monopole; characterized in that said dielectric
material is adapted so that it acts as a resonator at said
predetermined operating frequency, and encompasses generally the
whole of the near-field radiation emitted by the monopole.
[0007] The invention is based on an appreciation of the fact that a
monopole antenna generates a near-field, and that the near-field
contains large field amplitudes which exist quasi-statically in the
local region of the monopole and do not radiate energy. In a normal
communications antenna, this local region is air-filled and these
near-field amplitudes have no effect except to contribute reactance
to the antenna impedance. However, in a medical application, if the
near-field region contains biological matter, which is highly
lossy, the near-field amplitudes will generate heat. Because of the
high amplitudes and small volume of the near-field region, much
heat can be generated in the near-field region, which reduces the
energy in the far-field. Field penetration is therefore reduced,
and local charring in the near-field region becomes a limiting
factor in the power that can be input to the antenna.
[0008] The dielectric body according to the invention serves to
provide a low loss environment to encompass the near field region
so that more power is transmitted to the biological matter in the
far-field region.
[0009] The extent of the near-field is determined by the wavelength
.lamda. of the radiation in the dielectric and the length L of the
monopole according to the relationship 2 L.sup.2/.lamda.. The
extent of the near-field therefore is proportional to .lamda., and
it is possible to reduce the extent of the near-field region by
increasing, the dielectric constant of the body to reduce the
wavelength of the radiation within it. The overall external
dimension of the device can therefore be reduced for insertion into
a living body. A higher dielectric constant will also accommodate
the use of lower frequency radiation, which would otherwise
increase the wavelength and the extent of the near-field; the lower
frequency radiation being beneficial in increasing radiation
penetration into the far-field.
[0010] A monopole antenna, for good impedance matching, has L
generally equal to .lamda./2. By substitution in the above
relationship, the extent of the near-field is then equal to
.lamda./2, and this determines the minimum extent of the dielectric
material. Furthermore, a .lamda./2 dimension for the dielectric
material is consistent with its operation as a resonator to ensure
that the radiator is effective in transmitting radiation at the
required power levels for the treatment of biological material.
[0011] In one embodiment of the invention, the dielectric body
comprises a cylindrical shape with the monopole extending axially
along its center. A radiator of this kind can be designed with a
minimum radius for insertion into biological matter such as a
liver, and will create an annular radiation field around it. A
pointed tip may be provided at the free end of the dielectric body
to assist penetration of biological matter.
[0012] As the dielectric constant is increased, it may exceed that
of the biological matter, which can lead to total internal
reflection of radiation within the dielectric and a consequent
reduction in transmitted radiation. In order to overcome this
problem, the dielectric body is formed so that the dielectric
constant at its core is higher than that at tits outer periphery,
the latter having a value intermediate that of the core and the
biological matter. Thus, the dielectric constant at the core may be
higher than that of the surrounding biological matter so as to help
reduce the overall diameter of the radiator. The different
dielectric constants may correspond to different layers of
dielectric, each with a different dielectric constant, or may
correspond to different levels in a dielectric in which the
dielectric constant varies throughout the depth.
[0013] According to another aspect, the invention includes an
elongate microwave radiator for insertion into a living body to
treat biological tissue at predetermined operating frequency, the
radiator comprising a monopole at its tip and dielectric material
surrounding and extending beyond the monopole; characterized in
that said dielectric material terminates in a rounded tip portion
and is adapted so that it acts as a resonator at said predetermined
operating frequency and enhances transmission of radiation in the
forward direction of insertion.
[0014] Preferably, the tip portion is generally hemispherical and
has a radius generally equal to half a wavelength of the
radiation.
[0015] The radiator may further comprise a coaxial conductor
(preferably packed with a dielectric) which supplies radiation to
the monopole antenna from a radiation generator. Preferably, the
monopole then comprises an exposed length of the central conductor
of the coaxial conductor at its distal end. Preferably, the exposed
length of the central conductor providing the monopole, is
generally half the wavelength of the radiation in the dielectric.
The coaxial conductor may be rigid or flexible cable.
[0016] Preferably, the dielectric material has a dielectric
constant, or relative permittivity, such that the length of the
monopole is reduced. Advantageously, there can be a transformer
between the coaxial conductor and the dielectric monopole to reduce
reflection of radiation back into the coaxial conductor from the
boundary between it and the dielectric material. Such a transformer
can advantageously contain a space into which the dielectric
packing of the coaxial conductor can expand.
[0017] According to yet another aspect, the invention includes
methods of coupling radiation into biological material using the
devices according to the invention.
[0018] According to yet another aspect, the invention consists in
methods of coupling radiation into biological material using the
devices according to the invention.
[0019] Further advantages and features of the invention will become
apparent to readers skilled in the art upon consideration of the
following description of embodiments of the invention, the
embodiments being described by way of example only, and with
reference to the accompanying figures in which:
[0020] FIG. 1 is shows a first embodiment of the radiation
applicator;
[0021] FIG. 2 is shows the tip section of the radiation applicator
of FIG. 1 in more detail;
[0022] FIG. 3 shows a second embodiment of the tip section of the
radiation applicator incorporating a transformer;
[0023] FIG. 4 shows a third embodiment of the radiation
applicator;
[0024] FIG. 5 shows the tip of the radiation applicator of FIG. 4;
and
[0025] FIG. 6 shows a side-elevation of a variation design of the
radiation applicator of FIG. 4.
[0026] FIG. 1 shows the general arrangement of the radiation system
100. A radiation generator 110, for example, a microwave generator,
produces radiation which is coupled into coaxial cable 120 which
transmits the radiation to a distal tip region 130 at which there
is an antenna for emitting the radiation into the material
surrounding the tip 130. In use, the coaxial cable 120 is
introduced into a living body and the tip 130 is positioned
adjacent a region which it is desired to irradiate. For example,
the device could be inserted into an artery to irradiate plaques on
the walls thereof or the device could be introduced into a uterus
to irradiate the endometrium. The supply of radiation is controlled
by a control device 140, often a foot pedal, which is used to
signal the microwave generator to begin, adjust or stop the supply
of radiation to the tip 130.
[0027] FIG. 2 shows the tip region 130 of the radiation applicator
of FIG. 1 in more detail. The tip region, generally indicated 200,
shows the distal end of the coaxial cable which comprises an outer
conductor 210 spaced from a core conductor 220. The space between
the conductors 210 and 220 is filled with dielectric material 230.
The antenna for emitting radiation conducted by the cable comprises
a length 240 of the core conductor of the coaxial cable extending
beyond the outer conductor 210 at the distal end of the coaxial
cable to form a monopole. To enhance the radiating qualities of the
monopole 240, it is preferred that its length is about one half of
a wavelength of the radiation dielectric. The monopole 240 is
enveloped by dielectric body 250 in which the wavelength of the
employed radiation is reduced below its free-space value hence
enabling the monopole to be shorter than might otherwise be
possible. The dielectric body 250 comprises a cylindrical portion
260 which envelops the monopole 240. The diameter of the
cylindrical portion 260 is generally equal to the wavelength of the
radiation in the dielectric at the operating frequency so that it
is tuned to act as a resonator to increase the power it radiates.
Also, the dielectric body comprises a hemispherical section 270
which supports partial internal reflection of the radiation from
the antenna in the forward direction as indicated by arrows 280 and
290. Preferably, the hemispherical section 270 is dimensioned so as
to provide a resonator which further enhances radiation from the
dielectric body in 250 in the forward direction. Resonance of
radiation partially reflected within the dielectric body 250 can be
encouraged by, for example, dimensioning the hemispherical section
270 to have a radius approximately equal to one half of a
wavelength of the radiation employed. It will be appreciated that
the dielectric body can have other dimensions and shapes provided
that they encourage forward propagation of the radiation by means
of internal reflection and/or resonance.
[0028] When this equipment is to be used for endometrial ablation
it is desirable to use radiation having a frequency around 9.2 GHz.
In free-space, the wavelength of such radiation is about 32 mm.
Forming the dielectric body from, for example, a material having a
dielectric constant .epsilon..sub.R=25 reduces the wavelength to
about 6 mm. Correspondingly, the diameter and overall length of the
dielectric body are then also about 6 mm.
[0029] FIG. 3 shows an alternative embodiment of the tip section of
the radiation applicator device, generally indicated 300. Here, in
order to reduce reflection of radiation from the coaxial cable at
the boundary between it and the dielectric body, a transformer 310
is incorporated between the coaxial cable and the dielectric body.
The transformer 310 comprises several sections (for example, three:
320, 330, 340) of cylindrical shape and of successively increasing
radius towards the dielectric body. Advantageously, at least the
section 320 of the transformer adjacent the coaxial cable does not
contain a solid filler material. This provides the benefit that,
when the device is heated, for example in manufacture or in use,
the dielectric material filling the space between the core and the
outer conductors of the coaxial cable can expand into the
transformer thus relieving otherwise deleterious pressures.
[0030] The near-field radiation generated by the applicator of
FIGS. 2 and 3 extends from the monopole 240 a distance determined
by the formula 2 L.sup.2/.lamda., where L is the length of the
monopole, and .lamda. is the wavelength of the radiation in the
dielectric body 250. However, the preferred value of L is
.lamda./2, and therefore the near-field radiation does not extend
into the region of radius .lamda./2 about the monopole. Therefore,
the near-field radiation does not extend into the more lossy
biological material that surrounds the applicators in use, and the
resulting detrimental affects of local charring and reduction of
radiation penetration are reduced or avoided. Instead, the
microwave power is emitted into the far-field to increase
penetration and power transfer.
[0031] FIG. 4 shows yet another embodiment of the invention in
which a generator 310 supplies microwave energy via a rigid coaxial
conductor 320 to a tip region at the distal end of the conductor.
Dielectric packing 330 is provided between the inner and outer
conductors of the coaxial conductor 320. As shown in more detail in
FIG. 5, a length of the inner conductor 340 at the tip is exposed
by removal of the outer conductor so as to form a monopole to emit
radiation. The monopole 340 is embedded axially in a cylindrical
body of dielectric 350 which has substantially the same outer
diameter as the coaxial conductor 320. A pointed metal tip 370 is
fixed to the end of the dielectric body 350 and serves to assist
penetration not biological matter, such as a liver to perform
ablation on a tumour. The monopole 340 preferably has a length of
substantially equal to half a wavelength of the radiation in the
dielectric, and the radius of the dielectric body 350 is also
preferably substantially equal to half a wavelength of the
radiation in the dielectric. The near-field radiation emitted by
the monopole will then lie within a region 2 L.sup.2/.lamda., which
is equal to a radius of half of the wavelength of the radiation in
the dielectric so that the near-field lies substantially totally
within the dielectric. The dielectric constant of the dielectric
body is selected to be high so as reduce losses within the
dielectric. The microwave energy is therefore emitted into the
far-field region in an annular pattern around the tip so as to
increase field penetration and power transfer. Typically, a
radiation applicator used with a generator operating at 10 GHz and
having a dielectric body with dielectric constant
.epsilon..sub.R=25, will have a dielectric body radius of 3 mm.
Because the radius of the dielectric body 350 is substantially
equal to half a wavelength, it is tuned to set as a resonator,
which increases the power it radiates.
[0032] In order to reduce the diameter of the tip of the
applicator, the dielectric body is made of a material with as high
a dielectric constant as possible, except that this is limited by
the dielectric constant of surrounding biological matter in which
the applicator is used. When the dielectric constant of the
dielectric body exceeds that of the biological matter, total
internal reflection can occur at the outer surface of the
dielectric body, and field penetration becomes evanescent and
localized. In order to overcome this limitation, the dielectric
body 350 may be formed with an inner core 360 composed of a
material with a high dielectric constant, and an outer layer 380
composed of a dielectric with a lower dielectric constant
intermediate that of the core and the surrounding biological
material so as to match the wave impedance of the radiation between
the core and the biological material. In order to achieve this, the
refractive index of the outer layer 380 and that of biological
material, and the outer layer thickness should be equal to a
quarter of the wavelength of the radiation in the outer layer.
Thus, the core radius would also be equal to a quarter of the
wavelength of the radiation in the core in order to produce an
overall nominal radius of half a wavelength at the tip.
[0033] In alternative embodiments of the invention, multiple outer
layers may be used to increase the band-width of the applicator
(i.e. the range of frequencies over which the applicator can be
used) by making the layers each with a suitable refractive index
and thickness. However, this will lead to an increase in the
overall diameter of the tip. In the limit, the dielectric body
could be made with continuously varying refractive index which
decreases towards its outer surface.
[0034] Al alternative technique to reduce the dielectric constant
of the outer layer 380 comprises forming indentations such as
grooves 390, shown in FIG. 6, in the outer surface so that the
average dielectric constant of the dielectric and the material in
the grooves is reduced. The grooves may run longitudinally or
circumferentially around the body 350.
[0035] It will be appreciated that the embodiment of FIGS. 2 and 3
can also be modified to incorporate an outer layer or layers of
different dielectric constant, such as shown in FIGS. 5 and 6, the
outer layer following the curve of the hemispherical tip.
[0036] Dielectric materials with a high dielectric constant that
are suitable include those such as TiO.sub.2 with permittivity of
100 and CaTiO.sub.2 with permittivity of 155. These dielectrics
would be suitable for use in the core 360 so as to reduce its
diameter. The outer layer(s) 370 could be made of a composite of
TiO.sub.2 and AlO.sub.2 having a permittivity between that of the
core and the biological material. Materials with even higher
permittivities may be used such as ferroelectric materials, an
example being Ba.sub.1-xSr.sub.x TiO.sub.3 (BST) which has a
permittivity of around 600.
[0037] Therefore, by suitable choice of dielectric(s) it is
possible to produce radiation applicators with a tip diameter as
low as 3 to 6 mm to allow their use in laparoscopic medical
procedures, or even below 3 mm to allow percutaneous medical
procedures.
[0038] Radiation applicators according to the invention can also be
used to measure the dielectric constant of biological material by
measuring the microwave radiation reflected back from the tip
through the coaxial conductor.
* * * * *