US20140042154A1 - Microwave Application - Google Patents

Abstract

A microwave applicator comprising a coaxial electrical input (4) and a waveguide (1) filled with dielectric (2), an inner conductor (7) of the coaxial input extending longitudinally within one end of the waveguide to launch microwaves in the TM₀₁mode to travel to the distal end face (8) of the waveguide so that microwaves are transmitted when the distal end face is contacted by the biological tissue to be treated.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application claims priority under 35 U.S.C. §119(e) to U.S. Non-Provisional Application No. 10/530,823, filed Nov. 18, 2005, filed as application No. PCT/GB2003/004082 on Sep. 15, 2003, and U.S. Non-Provisional Application No. 12/206,090, filed Sep. 8, 2008, all of which are incorporated herein by reference.
TECHNICAL FIELD
• This invention relates to a microwave applicator suitable for heating biological tissue and a method of heat treating surface tissue.
• The applicants have previously proposed a microwave applicator for surgical use comprising a waveguide of reduced diameter by virtue of containing a dielectric of high permittivity. A coaxial electrical input generates microwaves in the TE11 mode within the dielectric and these radiate from the distal end face of the waveguide.
DISCLOSURE OF THE INVENTION
• According to a first aspect, the present invention, consists in a microwave applicator comprising a coaxial electrical input and a waveguide filled with dielectric, a central conductor of the coaxial input extending longitudinally within one end of the waveguide to launch microwaves preferably in the TM01 mode, to travel to the distal end face of the waveguide so that microwaves are transmitted from the distal end face when in contact with the biological tissue to be treated.
• The TM01 mode is preferred because it has a field pattern that is a good match with the coaxial input, better than the fundamental TE11 mode more commonly used The TM01 also produces a simple transition between the coaxial input and the waveguide. The central conductor is preferably coaxially aligned within a circular waveguide and extends a short way within the waveguide to match the general dimensions of the waveguide, especially its length and diameter, and the permittivity of the dielectric and frequency of the electrical input.
• The distal end face of the waveguide is preferably flat and radiates microwave energy with parallel wavefronts that advance into the biological tissue in contact with the distal end face and have minimum lateral spreading. The depth of penetration of the microwaves is dependent upon the frequency and electrical input power, but typically only a small distance of penetration is required for local heat treatment of tissue in microsurgery. In an alternative embodiment, the distal end face may be slightly domed and centred on the axis of the waveguide instead of being flat.
• Another particularly important feature of the invention is the ability to make use of resonance in the waveguide so that reflections from the transition at the input end, and from the distal end face caused by the change in dielectric at each, are out of phase and therefore enhance forwards transmission when the distal end face is in contact with the biological tissue, and are in phase and therefore enhance reflection to the coaxial input when the distal end face is out of contact with the biological tissue. Therefore, microwave energy is only transmitted to any appreciable extent from the distal end face when in contact with the biological tissue to be treated, and this is a key safety feature in the mode of operation
• According to a second aspect, the invention consists in a microwave applicator comprising a waveguide, a coaxial electrical input with a central conductor extending longitudinally within one end of the waveguide to launch microwaves preferably in the TM01 mode, that travel to the distal end of the waveguide and are transmitted into biological tissue to be treated, a diaphragm of low loss dielectric material being provided within the waveguide so as to extend laterally of the waveguide to reflect the microwaves travelling along it, the longitudinal location of the diaphragm being selected in relation to the ends of the waveguide so that the coherent addition of the reflected waves from the waveguide junction and the diaphragm combine to create a wave which is of correct magnitude and phase to cancel the reflection from the coaxial waveguide junction.
• Preferably, the thickness of the diaphragm and the permittivity of the dielectric material from which the diaphragm is made are selected to determine the magnitude of the rearward reflection of microwaves from the diaphragm for optimum cancellation of the rearward reflection in the coaxial input.
• Preferably, the waveguide is air-filled, and the distal end of the waveguide is adapted to contact (or nearly contact) the surface tissue to be treated.
• The microwave applicator of the first or second aspect of the invention can be used in a method of heat treating surface tissue, the distal end face of the waveguide being brought into contact with the surface tissue for treatment.
DESCRIPTION OF THE DRAWINGS
• The invention will now be described by way of example with reference to the accompanying drawings, in which:
• FIG. 1 is an axial section of a flexible laparoscopic microwave applicator according to a first embodiment of the invention;
• FIG. 2 shows graphs of reflection coefficient against microwave frequency for the applicator ofFIG. 1 when the distal end is in air (graph S) and when in contact with biological tissue (graph T);
• FIG. 3 is an axial section of a microwave applicator according to a second embodiment of the invention;
• FIG. 4 is an axial section of a microwave applicator according to a third embodiment of the invention;
• FIG. 5 is an axial section of a microwave applicator according to a fourth embodiment of the invention;
• FIG. 6 is an axial section of a microwave applicator according to a fifth embodiment of the invention; and
• FIG. 7 is a schematic drawing of a microwave applicator in use for medical treatment according to the invention.
EMBODIMENTS OF THE INVENTION
• The microwave applicator ofFIG. 1 consists of awaveguide1 formed by a cylindrical body of dielectric2 covered in an outer layer ofaluminium foil3, and a flexible coaxial cableinput power supply4 which is connected to the waveguide at one end so that theouter conductor5 of the coaxial cable is electrically connected via acopper ferrule6 to thealuminium tape3, and theinner conductor7 of the coaxial cable extends axially ashort distance1 into thedielectric body2. Thedistal end face8 of thewaveguide1 is flat and is covered by a layer of a non-stick polymer coating such as FEP or paralene.
• Thewaveguide1 preferably operates in the TM₀₁mode.
• Typically, for this microwave applicator to operate at a frequency of 9.2 GHz the dimensions of the waveguide are as follows; overall length L=12.9 mm, diameter D=5.2 mm, I=5.9 mm; and the permittivity of the dielectric body k=25. The dielectric is typically, Hik 500 f dielectric material sold by Emerson & Cummings.
• The performance of the microwave applicator ofFIG. 1 at different frequencies is shown by the graphs ofFIG. 2, in which Graph S shows the variation of reflection coefficient S₁₁when thedistal end face8 of the applicator is in air, and in which Graph T shows the variation of reflection coefficient S₁₁when thedistal end face8 of the applicator is in contact with biological tissue to be treated. A drop in value of S₁₁inFIG. 2 is indicative of a good microwave match, which is clearly demonstrated in Graph T at the designed operating frequency of 9.2 GHz. Under these conditions, the applicator is transmitting maximum microwave energy into the biological tissue, but if the contact with the tissue is broken and the distal end face is in air or gas such as CO₂, then the transmitted energy falls immediately to a much lower level as the energy is reflected back to the coaxial cableinput power supply4.
• It will be appreciated that a microwave applicator as shown inFIG. 1 can be relatively inexpensive to manufacture, and can therefore be sold as a disposable product for microsurgery.
• The microwave applicator ofFIG. 1, being of reduced diameter of 5.2 mm, is suitable for insertion through a Trocar in laproscopic surgery to produce a local heating effect in a controlled manner adjacent the distal end face when in contact with biological material. For example, such an applicator may be used to destroy small surface tumours, for the treatment of ovarian cancer, or the treatment of endometriosis, or any surface lesions.
• The second embodiment of the invention shown inFIG. 3 is similar in configuration to that ofFIG. 1, and the same reference numbers are used for equivalent components. However, thewaveguide1 comprises a rigid aluminiumcylindrical wall13, typically 2 mm thick, and thedielectric body2 is composed of a hard ceramic material, such as stabilised zirconia (TECHNOX® 2000) which has a permittivity K=25. This dielectric material gives the applicator an ability to handle higher power levels, typically, up to 200 watts, compared with a power level of, say, 45 watts for the applicator ofFIG. 1. Thewaveguide1 preferably operates in the TM₀₁mode.
• The applicator ofFIG. 3 is designed to operate at a lower frequency of 2.45 GHz, and also has an increased length L=50 mm and diameter D=20 mm. It will be appreciated that the diameter D is determined by the frequency of operation and permittivity K, and is selected to allow treatment of an appropriate size area of tissue, the increased area compared withFIG. 1 being balanced by the increased power to provide an appropriate power density at thedistal end8 for the treatment intended.
• Theprojection1 of thecentral conductor7 extends 25 mm into thedielectric body2, and anair gap9 is provided between theouter conductor5 and dielectric of thecoaxial cable input4 and thewaveguide1 to allow the dielectric filling of the coaxial cable to expand.
• The applicator ofFIG. 3 with a largerdistal end8 is more suitable for the treatment of larger surface breaking tumours, for example, primary and secondary tumours on the liver.
• In an alternative embodiment of the invention, suitable for treating smaller liver tumours, the same configuration as that ofFIG. 3 is used but, the stabilised zirconia dielectric is replaced by alumina having a permittivity K=10, and the dimensions are as follows: L=18 mm; D=10 min; I=11 mm; and the operating frequency is 9.2 GHz, This applicator will treat a small area of tissue than that ofFIG. 3, but will cause less collateral damage.
• The microwave applicator shown inFIG. 4 consists of an air-filledwaveguide11 formed by an aluminiumcylindrical wall13 with aninput connection14 for a flexible coaxial cable input power supply which is connected to the waveguide at one end. The outer surface of thewall13 is covered by aUPVC sleeve16. The outer conductor of the coaxial cable is electrically connected to thewall13 of the waveguide via a brass fitting17, and the inner conductor of the coaxial cable is connected to aconductor15 which extends axially ashort distance1 into thewaveguide11. APTFE bush18 separates and forms an electrical insulator between theconductor15 andfitting17. APerspex diaphragm20 is located laterally within the waveguide near the open end within a rebatedsection21 which spaces it a distance w away from theopen end18. Thediaphragm20 has a thickness t. Thewaveguide11 preferably operates in the TM₀₁mode.
• The operating frequency of the applicator is 7 GHz and the dimensions are typically L=108 mm; D=42 mm; I=13 mm; w=27.7 mm; and t=4.3 mm. These dimensions are selected in connection with the operating frequency and permittivity of the diaphragm so that when in use with the open end of the waveguide in contact with surface tissue to be treated, the rearward reflections of microwaves from the tissue and the transition between thecoaxial cable14 andwaveguide11 are substantially cancelled out by the reflections from the diaphragm which reduces reflections within the coaxial cable. In this balance situation, the majority of the microwave energy is then transmitted to the tissue being treated. In particular, the thickness t of thediaphragm20 and the permittivity k of the material of which it is composed will determine the size of rearward reflection of microwaves from it. The location of thediaphragm20 relative to the ends of thewaveguide11 will determine the relative phases of the rearward reflections of the microwaves.
• A microwave applicator such as that ofFIG. 4 would be suitable for skin treatments such as the treatment of psoriasis, especially because the end of the probe has minimal contact with the tissue being treated. Preferably, the edge of thewaveguide11 at the open end may be coated or fitted with some other protection (22) such as silicone rubber, for engagement with the tissue.
• An alternative embodiment of the invention is illustrated inFIG. 5 in which the microwave applicator consists of a waveguide31 supported at one end of a rigid tube32 having ahandle33 andinput power connection34 at its other end. The applicator is held and manipulated by thehandle33 and the waveguide31 forms a treatment head. Acoaxial cable35 extends through the tube32 and connects theinput power connection34 to the waveguide31.
• The waveguide31 consists of acylindrical aluminium sleeve36 which is connected to analuminium end cap37 at the input end. Analuminium support bush38 is connected at one end to acentral bore39 in theend cap37, and is connected at its other end to the tube32 so that thecoaxial cable35 can pass through it. The Outer conductor40 of thecoaxial cable35 is exposed and electrically connected to theend cap37, and theinner conductor41 of thecoaxial cable35 is exposed to project axially into the waveguide31. Acylindrical body42 of dielectric comprising TECHNOX® 2000 ceramic is fitted within thesleeve36 and is formed with acentral bore43 to receive theinner conductor41. Anair gap44 is formed between the end of the dielectric filling45 of the coaxial cable and thedielectric body42 of the waveguide.
• Thehandle33 is machined from aluminium and has aconcentric recess45 at its input end to receive theinput power connection34. Acentral bore46 in the handle accommodates thecoaxial cable35.
• The waveguide31 preferably operates in the TM₀₁mode.
• Typically, the applicator is designed to operate at 2.45 GHz and the dimensions of the waveguide comprise asleeve36 of length L=50 mm and inner diameter D=20 mm, and aninner conductor41 with an exposed length I=11.8 mm and anair gap44 oflength 2 mm. The tube32 is typically stainless steel and has a diameter of 8 mm and length 250 mm or other appropriate length to reach tissue being treated. The tube32 may be bent laterally at one ormore locations46 along its length to improve the ease with which the radiation emitting end face47 of the treatment head can be engaged in surface-to-surface contact with tissue to be treated.
• Another embodiment of the invention is illustrated inFIG. 6, which like that ofFIG. 5, consists of awaveguide51 forming a treatment head at one end of arigid tube52, and a handle and input power connection (not shown) at the other end of thetube52. Thewaveguide51 consists of a machined aluminium component consisting of acylindrical sleeve54 andend wall55 having anaperture56 to receive thetube52 andcoaxial cable57. A cylindrical body of dielectric58 comprising Hik 500 f is fitted inside thesleeve54 and is formed with acentral bore59 to receive theinner conductor60 of thecoaxial cable57. Theouter conductor61 of thecable57 is electrically connected to theend wall55 and anair gap62 is formed between the end of the dielectric filling63 of thecable57 and thedielectric body58.
• Typically, the applicator is designed to operate at 9.2 GHz and the waveguide dimensions comprise asleeve54 of length L=16 mm and inner diameter 10 mm, and aninner conductor60 with an exposed length of 13 mm, and anair gap62 of 2 mm.
• The microwave applicator ofFIGS. 5 and 6 can be used, like the applicators ofFIGS. 1 and 3, to treat tumours or other surface lesions, or for the treatment of endometriosis. The treatment head is inserted through an opening in a patient's body, either through a Trocar or directly through a larger incision. The applicator is then manipulated to bring the end face of the treatment head into face-to-face contact with the tissue to be treated adjacent the tumour or lesion. Microwave radiation emitted from the end face of the treatment head is then transmitted into the tumour or lesion and surrounding tissue and causes it to be ablated. This process can be repeated as necessary to ablate multiple tumours or lesions at different locations before the treatment is completed.
• A microwave applicator according to the invention can also be used to stop the flow of blood from bleeding tissue by bringing the end face of the treatment head into contact with the bleeding tissue and transmitting radiation to ablate the tissue.
• Although the methods of medical treatment described above use the microwave applicators illustrated it should be appreciated that the invention also relates generally to the method of treating tumours or lesions on or within the surface of tissue, especially the liver, using any form of microwave applicator having a treatment head with a radiation emitting surface which can engage the tissue in face-to-face contact so as to transmit radiation into the tissue.
• FIG. 7 illustrates medical treatment of surface tumours on a patient'sliver70. An incision is made across the patient's abdomen and the abdominal wall is held inclamps71 to create an opening into the abdominal cavity. The liver is then freed from the surrounding tissue so that it can be accessed for treatment. The condition of the liver is assessed and decisions are made as to the treatment to be given, whether to remove part of the liver and/or to ablate part of the liver. In the drawing, the liver shown hasmultiple surface tumours72 which require ablation. Amicrowave applicator73, preferably such as illustrated in any ofFIGS. 1 to 6, is manipulated to align the end face of the treatment head with a tumour, and to bring the end face into contact with the tumour. The power supply to the applicator is then switched on so that it generates microwaves that irradiate the tumour, causing it to be ablated. The applicator is then lifted away from the liver surface either before or after the power has been reduced or switched-off. The whole process is then repeated multiple times to ablate each of the surface tumours in turn.
• Real time intra-operative ultrasound scanning is used to assess the location and extent of the tumours within or on the surface of the liver. Also, the extent of the ablation achieved when treating individual tumour is assessed using intra-operative ultrasound scanning. The ablated tissue appears as a “thermal cloud” emanating from the end face of the applicator when viewed in the scan output. The treatment is therefore continued until the “thermal cloud” encompasses the whole of the tumour and a safety margin around it.
• A larger tumour or tumours may require removal of part of the liver. This is done by cutting the liver, which causes bleeding. The microwave applicator is then used to stem the bleeding by bringing the end face of the treatment head into contact with the bleeding surface and turning on the power to the applicator to ablate the bleeding surface. This procedure is then repeated over the whole of the bleeding surface.
• Once treatment of the liver is completed, the abdomen is washed with saline solution, and the abdominal wall is sutured.

Claims (18)

1. A microwave applicator comprising a coaxial electrical input and a waveguide filled with dielectric, an inner conductor of the coaxial input extending longitudinally within one end of the waveguide to launch microwaves in the TM01 mode to travel to the distal end face of the waveguide so that microwaves are transmitted when the distal end face is contacted by biological tissue to be treated.

2. A microwave applicator as claimed inclaim 1 in which the inner conductor is axially aligned with the waveguide.

3. A microwave applicator as claimed inclaim 1 in which the waveguide is a circular waveguide.

4. A microwave applicator as claimed inclaim 1 in which the distal end face is substantially flat and normal to the axis of the waveguide.

5. A microwave applicator as claimed inclaim 1 in which the distal end face is fiat or slightly domed and centred on the axis of the waveguide.

6. A microwave applicator as claimed inclaim 1 in which the distal end face has a polymer coating.

7. A microwave applicator as claimed inclaim 1 in which the length and diameter of the waveguide, the length of the inner conductor within the waveguide, and the permittivity of the dielectric material are selected so that at the designed operating frequency, the waveguide is in resonance,

8. A microwave applicator as claimed inclaim 1 in which the waveguide is adapted so that in operation., when the distal end face is in contact with biological tissue to be treated, forwards transmission from the distal end face is enhanced by the relative phase of reflections from the distal end face and the input to the waveguide; and when the distal end face is in air or gas, reflections to the input are enhanced by the relative phase of reflections from the distal end face and the input to the waveguide.

9. A microwave applicator comprising a waveguide, a coaxial electrical input with an inner conductor extending longitudinally within one end of the waveguide to launch microwaves in the TM01 mode that travel to the distal end of the waveguide and are transmitted into biological tissue to be treated, a diaphragm of low loss dielectric material being provided within the waveguide so as to extend laterally of the waveguide to reflect the microwaves travelling along it, the longitudinal location of the diaphragm being selected in relation to the ends of the waveguide so that the phase of reflections from the diaphragm and said ends serve to reduce or cancel rearward reflections in the coaxial input.

10. A microwave applicator as claimed inclaim 9 in which the thickness of the diaphragm, and the permittivity of the dielectric material from which it is made are selected to determine the magnitude of the rearward reflection of microwaves from the diaphragm for optimum cancellation of the rearward reflection in the coaxial input.

11. A microwave applicator as claimed inclaim 9 which is air-filled.

12. A method of heat treating surface tissue using the microwave applicator ofclaim 1 in which the end face of the waveguide is brought into contact with the surface tissue.

13. A method as claimed inclaim 12 in which the surface tissue is internal tissue and the applicator is inserted into a body for treatment.

14. A method as claimed inclaim 13 in which the insertion of the applicator is via a Trocar.

15. A method as claimed inclaim 12 in which the surface tissue is the external skin of the body.

16. A method of treating a liver in a body comprising providing a microwave applicator having a treatment head at one end capable of emitting microwave radiation from an emitting face, inserting the microwave applicator through an incision into the body positioning the head of the microwave applicator in contact with a surface of the liver with the emitting face adjacent to a region to be treated, and powering the microwave applicator so that the emitting face emits microwave radiation that heats said region to be treated.

17. A method of treating biological tissue to stop bleeding comprising providing a microwave applicator having a treatment head at one end capable of emitting microwave radiation from an emitting face, positioning the head of the microwave applicator in contact with a surface of the biological tissue to be treated with the emitting face adjacent to bleeding tissue to be treated, and powering the microwave applicator so that the emitting face emits radiation that heats the bleeding tissue to be treated.

18. A method of treating a skin condition such as psoriasis using a microwave applicator having a microwave emitting window which is brought into contact with, or into close proximity of, skin to be treated and is powered so as to emit microwave radiation and irradiate the skin to be treated.

Images