Abstract:
A method and system for monitoring a microwave tissue ablation process, particularly the determination of an ablation size estimate during microwave ablation of such tissue. The method includes applying a correction value to the determined temperature value, so as to provide a corrected temperature value

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to systems and methods for monitoring a microwave tissue ablation process, particularly the determination of an ablation size estimate during microwave ablation of such tissue. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the treatment of tumours, for example tumours caused by a disease such as cancer, it is known to use microwave ablation techniques to ablate the tumour. Such microwave ablation techniques typically ablate the targeted tissue by delivering a controlled amount of microwave energy into the tumour. 
         [0003]    Minimally-Invasive techniques for delivering such microwave energy have been shown to be effective in the treatment of tumours. In a minimally-invasive technique, a microwave emitter is inserted directly into a point of treatment, either using a normal body orifice or via percutaneous insertion. Such minimally-invasive procedures and devices provide a means of treating tumours in patients who either cannot undergo other forms of treatment (e.g. radiotherapy, surgical resection, chemotherapy) or where ablation is preferred as a therapy. 
         [0004]    One example type of commonly used microwave antenna assembly includes a dipole antenna, which consists of a coaxial construction having an inner conductor and an outer conductor with a dielectric junction (feed point) separating a portion of the inner conductor. The inner conductor may be coupled to a portion corresponding to a first dipole radiating portion, and a portion of the outer conductor may be coupled to a second dipole radiating portion. The dipole radiating portions may be configured such that one radiating portion is located proximally of the dielectric junction, and the other portion is located distally of the dielectric junction. 
         [0005]    Careful monitoring of the temperature of the tissue surrounding the tumour can significantly improve the outcome of ablation treatment. Existing designs of microwave ablation apparatus make use of thermocouples or other temperature sensors mounted on the probe; they can be used to estimate the progress of the ablated region. However, the measurement of temperature of the tumour and or the surrounding healthy tissue of the treatment region can be improved. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the present invention, there is provided a method of monitoring a microwave tissue ablation process, the method comprising providing a power source operable to provide a controlled microwave energy output, providing a microwave probe having a body and an antenna extending through the body and operable to deliver microwave energy from the power source out of the microwave probe, providing a thermocouple on the body of the microwave probe, the thermocouple being separated from the body by a layer of thermally insulating material, the thermocouple being operable to generate an output voltage in dependence upon a temperature experienced by the thermocouple, supplying a controlled microwave energy output from the power source to the antenna, thereby causing microwave energy to radiate from the antenna to form an ablation zone, detecting an output voltage generated by the thermocouple in response to a temperature experienced by the thermocouple, determining a temperature value relating to the detected output voltage, and applying a predetermined correction value to the determined temperature value, so as to provide a corrected temperature value the correction value relating to a total amount of energy supplied to the antenna from the start of such ablation to the determination of the temperature value. 
         [0007]    According to a second aspect of the present invention, there is provided a method of controlling a microwave probe during microwave ablation of tissue, the method comprising a method of monitoring tissue temperature according to the first aspect, and further comprising controlling the microwave energy supplied from the power source to the antenna such that the corrected temperature value reaches a predetermined target value. 
         [0008]    According to a third aspect of the present invention, there is provided a microwave ablation apparatus comprising a power source operable to provide a controlled microwave energy output, a microwave probe having a body and an antenna extending through the body and operable to deliver microwave energy from the power source out of the microwave probe, a thermocouple on the body of the microwave probe, the thermocouple being separated from the body by a layer of thermally insulating material, the thermocouple being operable to generate an output voltage in dependence upon a temperature experienced by the thermocouple, a detector operable to detect an output voltage generated by the thermocouple in response to a temperature experienced by the thermocouple, a calculation unit operable to determine a temperature value relating to the detected output voltage, a correction unit operable to apply a predetermined correction value to such a determined temperature value, and to provide a corrected temperature value, the correction value relating to a total amount of energy supplied to the antenna from the start of such ablation to the determination of the temperature value, and a controller operable to control microwave energy supplied to the antenna from the power source such that the corrected temperature value reaches a predetermined target value. 
         [0009]    In one example, the predetermined correction value also relates to the location of the thermocouple on the body of the microwave probe. 
         [0010]    In one example, the predetermined correction value also relates to time of application of the microwave energy. 
         [0011]    In one example, the predetermined correction value also relates to a temperature of a coolant fluid used inside the body of the microwave probe. 
         [0012]    In one example, the corrected temperature value is a predetermined multiple of the determined temperature value. 
         [0013]    One example method further comprises determining an ablation estimate from the corrected temperature value, the ablation estimate relating to a volume of tissue ablated due to radiation of microwave energy from the antenna. 
         [0014]    In one example, a plurality of such thermocouples are provided on the body of the microwave probe, the thermocouples being located at respective locations on the body, and a respective corrected temperature value is provided for each thermocouple, in dependence upon respective determined temperature values and correction values for each thermocouple. 
         [0015]    In one example, the supply of microwave energy from the power source is terminated when the corrected temperature value substantially reaches the predetermined target value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates of a microwave ablation apparatus; 
           [0017]      FIG. 2  illustrates a microwave probe of the apparatus of  FIG. 1 ; 
           [0018]      FIG. 3  illustrates an alternative microwave probe of the apparatus of  FIG. 1 ; 
           [0019]      FIG. 4  illustrates a controller of the apparatus of  FIG. 1 ; and 
           [0020]      FIG. 5  is a flowchart illustrating a method of use of the apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  is a schematic diagram illustrating a microwave ablation system  1  comprising a controller unit  2 , and a microwave power generator  4  which is connected to the controller via a control connection  6 . An ablation antenna assembly  8  is connected to the microwave power generator  4  via a power connection  10 . The antenna assembly includes a tip portion  12  which aids insertion of the antenna assembly into the tissue being treated, and enables a desired output pattern of microwave energy from the antenna assembly  8 . The controller unit  2  is operable to control the power generator  4  to supply the correct magnitude and frequency of microwave energy to the antenna assembly  8 . For example, the microwave energy may be delivered using the 911 MHz, or 2.45 GHz wavebands, or any suitable waveband up to 14 GHz. 
         [0022]      FIG. 2  is a cross-sectional view of part of an exemplary microwave probe assembly  20  embodying an aspect of the present invention. It will be readily appreciated that other constructions of probe assembly may embody aspects of the present invention. The microwave probe assembly  20  comprises a body  22 , which is preferably cylindrical in form. The body  22  extends from a first (proximal) end  20   1  to a second (distal) end  20   2 , and defines a longitudinal axis of the assembly. The body  22  defines an inner volume, in which most of the other components of the assembly are housed. The body  22  provides the assembly with the necessary rigidity for insertion into the tissue being treated. The body  22  is preferably of a rigid material, such as a composite material (for example glass fibre, carbon fibre, aramid fibre), stainless steel, other biocompatible metals (e.g. titanium) or combinations of, and is typically 1.5 to 3 mm mm in diameter. 
         [0023]    An applicator tip  24  is attached to the second end  20   2  of the body, to close off the inner volume at the second end. The applicator tip is preferably a faceted trocar and has a relatively sharp distal end point. The applicator tip  24  is designed to be suitable for insertion into the tissue being treated, and partly to affect the transmission pattern for microwave energy into that tissue. It also forms a water tight seal to the internal volume of the body  22 , if required when the applicator uses an internal cooling fluid. 
         [0024]    A coaxial conductor assembly  26  extends along the inner volume of the body  22  from the first end  20   1  towards the second end  20   2 . The coaxial conductor assembly  26  is connectable, at a proximal end thereof, to the microwave energy generator  4  of  FIG. 1 . The coaxial conductor assembly  26  extends substantially along the longitudinal axis of the body  22 , and comprises an inner conductor  28 . The inner conductor  28  is of an electrically conductive material such as copper. Surrounding the inner conductor  28  is a dielectric layer  30  which extends along the inner conductor  28 , radially outwardly thereof. The dielectric layer  30  is of any appropriate dielectric material. Surrounding the dielectric layer  30 , is an outer conductor  32 , which is of an electrically conductive material such as copper. The outer conductor  32  extends along the dielectric layer  30 , radially outwardly thereof. Typically, the inner conductor  28  is a wire having a circular cross section, such that the dielectric layer  30  is a cylinder of dielectric material surrounding an outer surface of the inner conductor  28 . The outer conductor  32  is then formed by a cylinder of electrically conductive material surrounding an outer surface of the dielectric layer  30 . 
         [0025]    The inner conductor  28  defines a signal feed-point  34  at its distal end (that is, the end towards the second end  20   2  of the body  22 ). A dipole tip portion  36  extends longitudinally from the distal end of the coaxial conductor assembly  36  into a reception aperture in the applicator tip  24 . The reception aperture  38  is located centrally with respect to the longitudinal axis of the assembly within the applicator tip  24 . The reception aperture  38  is designed so as to locate centrally the dipole tip portion  36  into the tip  24 . The tip material is chosen for it mechanical and electrical properties, which have to be considered in the design. 
         [0026]    The dielectric layer  30  extends along the complete length of the inner conductor  28  to the distal end thereof adjacent the dipole tip portion  36 . The outer conductor  32  stops short of the distal end of the inner conductor  28  and dielectric layer  32 , and so is spaced apart longitudinally from the signal feed-point  34  and dipole tip portion  36 . 
         [0027]    In one example constructions, a dielectric fluid  46  may be provided within the inner volume of the body  20  in order to provide a key functional element to the microwave design and also provide a cooling fluid for the antenna assembly. This fluid will typically be isotonic saline or deionised water. 
         [0028]    In a first embodiment of an aspect of the present invention, a first thermocouple  40  is located on an outer surface  23  of the body  22 . The first thermocouple  40  is not shown to scale in  FIG. 2 , but has been enlarged in order to show the details of the embodiment. The exact position of the first thermocouple  40  on the outer surface  23  of the body  22  is chosen in order to provide the most appropriate temperature readings, as will be discussed below. 
         [0029]    A first layer  42  of thermally insulating material is provided between the first thermocouple  40  and the outer surface  23  of the body  22 . This first layer  42  serves to insulate the first thermocouple  40  from the heat of the body  22 , and so aids more accurate measurement of temperature outside of the body  22 . This accuracy is improved by virtue of insulating the first thermocouple  40  from the temperature of the body  22  and from the effects of the cooling fluid  46  used within the body  22 . 
         [0030]    The first thermocouple  40  is connected to the controller  2  of  FIG. 1  by a signal connection  44 . The signal connection  44  transfers the output of the first thermocouple  40  to the controller  2 . As is well known, the output of a thermocouple is a voltage, the magnitude of which is dependent upon the temperature being experienced by the thermocouple. In the present case, the first thermocouple  40  supplies a first output signal, via the signal connection  44 , to the controller  2 . The first output signal relates to the temperature outside of the body  22 , with reduced effect of the temperature within the body  22 . As will be described below, the controller  2  receives and processes the first output signal. 
         [0031]    A second embodiment of an aspect of the present invention is illustrated in  FIG. 3 , which shows a microwave probe of the same basic structure to that of the embodiment of  FIG. 2 . The microwave probe  20  of  FIG. 3  differs from that of FIG.  2  by virtue of the provision of a second thermocouple  50  in addition to the first thermocouple  40  of  FIG. 2 . The second thermocouple  50  is provided at position on the outer surface  23  of the body  22  of the probe  20  different to that of the first thermocouple  40 . The relative positions of the first and second thermocouples are chosen so as to provide the lost useful temperature measurements relating to the tissue being ablated. 
         [0032]    A second layer  52  of thermally insulating material is provided between the second thermocouple  50  and the outer surface  23  of the body  22 . This second layer  52  serves to insulate the second thermocouple  50  from the heat of the body  22 , and so aids more accurate measurement of temperature outside of the body  22 . This accuracy is improved by virtue of insulating the second thermocouple  50  from the temperature of the body  22  and from the effects of the cooling fluid  46  used within the body  22 . 
         [0033]    The second thermocouple  50  is connected to the controller  2  of  FIG. 1  by a signal connection  54 . The signal connection  54  transfers the output of the thermocouple  40  to the controller  2 . In the present case, the second thermocouple  50  supplies a second output signal, via the signal connection  54 , to the controller  2 . The second output signal relates to the temperature outside of the body  22 , with reduced effect of the temperature within the body  22 . As will be described below, the controller  2  receives and processes the second thermocouple output signal. 
         [0034]      FIG. 4  illustrates schematically the controller  2  of  FIG. 1 , and  FIG. 5  illustrates operation of the system using such a controller. The first step ( 100 ) is to provide the power supply, controller, and microwave probe assembly. Microwave energy is then supplied ( 102 ) to the microwave probe. 
         [0035]    The controller  2  comprises a detector  60  connected to receive the first output signal from the first thermocouple  40 , and the second output signal from the second thermocouple  50 , if provided. The detector  60  is operable to receive (step  104 ) the output signal(s) and provide (step  106 ) a detected output signal as an output to a calculation unit  62 . 
         [0036]    The calculation unit  62  is operable to determine (step  108 ) a temperature value that corresponds to each received detected output signal. The calculated temperature value is then provided to a correction unit  64  which is operable to apply (step  110 ) a predetermined correction factor to received calculated temperature values in order to provide a more accurate measurement of temperature of the zone being ablated outside of the microwave probe assembly. 
         [0037]    The correction unit  64  operates to apply a mathematical correction factor to the calculated temperature values. In an embodiment of the present invention, the correction factor relates to the total amount of microwave energy supplied to the antenna at the time of determination of the calculated temperature value. In addition, the correction factor may include a simple multiple factor, and/or may be based on time of application of microwave energy, or on the temperature of the coolant used in the probe assembly. Other additional bases for the correction factor may include one or more of the following considerations: the nature of the tumour i.e. the primary cancer type, the host organ, the patient&#39;s physical condition (e.g. blood flow, body temperature etc), previous medical cases derived from a user database. 
         [0038]    These correction factors may be used singly, or in an appropriate combination. Respective correction factors are provided for each thermocouple used in the probe, and are preferably characterised for different microwave power levels and application times. The correction factors are stored in a data storage unit  66  accessible by the correction unit  64 . The correction factors are preferably determined using experimental data obtained using comparison of thermocouple signals with measurements from other devices, such as fluoroptic probes, measuring the temperature of tissue during experimental ablation processes. The correction factors may be updated during operation of the system. 
         [0039]    The calculated temperature values are provided to a control unit  68  which is operable to control (step  112 ) supply of microwave energy to the microwave probe assembly by supplying control signals to the power supply via a control connection  70 . The control unit  68  controls the microwave power supply on the basis of the corrected temperature value. When a desired corrected temperature value is reached, the control unit operates to turn off the supply of microwave energy in order to stop the ablation process. 
         [0040]    The control unit is operable to determine an appropriate end point for the application of microwave energy to the tissue being ablated on the basis of the corrected temperature values. Such accurate determination of the temperature of the ablated tissue as treatment proceeds leads to more accurate and successful treatment by ablation of the appropriate tissue.