ELECTROSURGICAL INSTRUMENT WITH IMPEDANCE TRANSFORMER FOR DELIVERING MICROWAVE ENERGY

Abstract

An electrosurgical instrument for delivering microwave energy having a predetermined frequency into biological tissue in contact with the instrument, wherein the instrument comprises a first coaxial transmission line having a second coaxial transmission line connected to the distal end thereof, the second coaxial transmission line having a length and a characteristic impedance that are configured to match the impedance of the first coaxial transmission line to the load impedance at the distal end of the distal coaxial transmission line when the instrument is in contact with the tissue, at the operating frequency.

Claims

1.-22. (canceled)

23. An electrosurgical instrument configured for delivering microwave frequency energy having a predetermined operating frequency into tissue having a predetermined characteristic impedance in contact with a distal end of the instrument, the instrument comprising: a proximal coaxial transmission line for conveying microwave frequency energy comprising a first inner conductor, a first outer conductor formed coaxially with the first inner conductor, and a first dielectric layer separating the first inner conductor and the first outer conductor; a distal coaxial transmission line for conveying microwave frequency energy comprising a second inner conductor connected to the first inner conductor, a second outer conductor formed coaxially with the second inner conductor and connected to the first outer conductor, and a second dielectric layer separating the second inner conductor and the second outer conductor; wherein a ratio of an inner diameter of the second outer conductor to the outer diameter of the second inner conductor is such that a characteristic impedance of the distal coaxial transmission line is intermediate between a characteristic impedance of the proximal coaxial transmission line and a load impedance at the distal end of the distal coaxial transmission line when the distal end of the instrument is in contact with the tissue; wherein a length of the distal coaxial transmission line is such that the distal coaxial transmission line is an impedance transformer that improves the impedance match between the proximal coaxial transmission line and the load impedance at the distal end of the distal coaxial transmission line when the distal end of instrument is in contact with the tissue, at the predetermined operating frequency; and wherein the electrosurgical instrument comprises a further distal coaxial transmission line comprising a third inner conductor connected to the second inner conductor, a third outer conductor formed coaxially with the third inner conductor and connected to the second outer conductor, and a third dielectric layer separating the third inner conductor and the third outer conductor.

24. The electrosurgical instrument according to claim 23, wherein a length of the distal coaxial transmission line is substantially equal to (2n+1)λ/4, where λ is the wavelength in the distal coaxial transmission line of microwave frequency energy having the predetermined operating frequency and n is an integer greater than or equal to 0.

25. The electrosurgical instrument according to claim 23, wherein the electrosurgical instrument is for coagulating tissue.

26. The electrosurgical instrument according to claim 23, wherein the ratio of the inner diameter of the second outer conductor to the outer diameter of the second inner conductor is such that a characteristic impedance of the distal coaxial transmission line is substantially equal to √{square root over (Z.sub.inZ.sub.L)}, where Z.sub.in is the characteristic impedance of the proximal coaxial transmission line and Z.sub.L is the load impedance at the distal end of the distal coaxial transmission line when the distal end of the instrument is in contact with the tissue.

27. The electrosurgical instrument according to claim 23, wherein a characteristic impedance of the further distal coaxial transmission line is the same as a characteristic impedance of the proximal coaxial transmission line.

28. The electrosurgical instrument according to claim 23, wherein a length of the further distal coaxial transmission line is such that the further distal coaxial transmission line substantially cancels out a reactive part of the predetermined characteristic impedance of the tissue at the predetermined operating frequency.

29. The electrosurgical instrument according to claim 23, wherein the further distal coaxial transmission line is rigid.

30. The electrosurgical instrument according to claim 23, wherein the electrosurgical instrument comprises: an open-circuited or short-circuited stub connected in parallel to the further distal coaxial transmission line.

31. The electrosurgical instrument according to claim 30, wherein a characteristic impedance of the stub is the same as a characteristic impedance of the further distal coaxial transmission line.

32. The electrosurgical instrument according to claim 30, wherein a characteristic impedance of the stub is not the same as a characteristic impedance of the further distal coaxial transmission line.

33. The electrosurgical instrument according to claim 30, wherein the electrosurgical instrument comprises a plurality of the open-circuited or short-circuited stubs connected in parallel to the further distal coaxial transmission line.

34. The electrosurgical instrument according to claim 23, wherein a separation between the outer diameter of the second inner conductor and the inner diameter of the second outer conductor is less than a separation between an outer diameter of the first inner conductor and an inner diameter of the first outer conductor.

35. The electrosurgical instrument according to claim 23, wherein the second dielectric layer has a higher relative permittivity than the first dielectric layer.

36. The electrosurgical instrument according to claim 23, wherein the distal coaxial transmission line is rigid.

37. The electrosurgical instrument according to claim 23, wherein the distal coaxial transmission is tapered from a wider proximal end thereof to a narrower distal end thereof.

38. The electrosurgical instrument according to claim 23, wherein the proximal coaxial transmission line is a coaxial cable.

39. The electrosurgical instrument according to claim 23, wherein the electrosurgical instrument comprises a plurality of the distal coaxial transmission lines for improving the impedance match between the proximal coaxial transmission line and the tissue at the predetermined operating frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0107] Embodiments of the present invention will now be discussed, by way of example only, with reference to the accompanying Figures, in which:

[0108] FIG. 1 shows a computer simulation of an electrosurgical instrument according to an embodiment of the present invention being used to coagulate liver tissue;

[0109] FIG. 2 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 1;

[0110] FIG. 3 shows a further computer simulation of an electrosurgical instrument according to an embodiment of the present invention being used to coagulate liver tissue with the instrument at an angle to the normal of the liver tissue surface;

[0111] FIG. 4 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the further computer simulation shown in FIG. 3 for three different angles of the instrument;

[0112] FIG. 5 shows a computer simulation of an electrosurgical instrument according to a further embodiment of the present invention being used to coagulate liver tissue;

[0113] FIG. 6 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 5

[0114] FIG. 7 shows a computer simulation of an electrosurgical instrument according to a further embodiment of the present invention being used to coagulate liver tissue, wherein the first and second coaxial transmission lines are narrower than in the embodiment of FIG. 1;

[0115] FIG. 8 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 7;

[0116] FIG. 9 shows a computer simulation of an electrosurgical instrument according to a further embodiment of the present invention being used to coagulate liver tissue, wherein the second coaxial transmission line is narrower than in the embodiment of FIG. 1;

[0117] FIG. 10 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 9;

[0118] FIG. 11 is a schematic illustration of an electrosurgical instrument according to a further embodiment of the present invention.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

[0119] As discussed above, the inventors have realised that an advantageous way to achieve controlled delivery of microwave frequency radiation into tissue in a localised area would be to couple microwave frequency energy directly to the tissue from an exposed end of a coaxial transmission line (e.g. a coaxial cable) by pressing the exposed end of the coaxial transmission line against the tissue.

[0120] However, the inventors have realised that biological tissue in contact with an exposed end of a coaxial transmission line would present a low impedance to the microwave frequency energy relative to the impedance of the coaxial transmission line, and that there would therefore be a significant impedance mismatch between the coaxial transmission line and the biological tissue.

[0121] The inventors have realised that this problem can be overcome by providing an impedance transformer at the distal end of the coaxial transmission line in order to better match the impedance of the coaxial transmission line to the impedance of the tissue, so that the microwave frequency energy is more effectively coupled/delivered to the tissue with less significant reflection of the energy.

[0122] The present inventors have realised this can be achieved in practice, while still achieving the advantages of coupling the energy directly to the tissue from an exposed end of a coaxial transmission line, by providing the impedance transformer in the form of a further coaxial transmission line connected to the distal end of the first coaxial transmission line, wherein the further coaxial transmission line has a length and characteristic impedance that are configured to better match the impedance of the first coaxial transmission line to the load impedance at the distal end of the further coaxial transmission line.

[0123] Therefore, in a first embodiment of the present invention illustrated in FIG. 1, there is provided an electrosurgical instrument 1 comprising a first (proximal) coaxial transmission line 3 for conveying microwave frequency energy from a proximal (rear) end thereof to a distal (front) end thereof. Furthermore, there is provided a second (distal) coaxial transmission line 5 for conveying microwave frequency energy from a proximal (rear) end thereof to a distal (front) end thereof. The second coaxial transmission line 5 is connected at the proximal end thereof to the distal end of the first coaxial transmission line 3, so that microwave frequency energy can be conveyed directly from the first coaxial transmission line 3 to the second coaxial transmission line 5.

[0124] The first and second coaxial transmission lines 3, 5 are both symmetrical around respective central axes thereof. Furthermore, the first and second coaxial transmission lines 3, 5 are aligned with each other end to end so that their central axes lie on the same line.

[0125] The first coaxial transmission line 3 comprises a cylindrical first inner conductor 7, a tubular first outer conductor 9 and a tubular first dielectric layer 11 separating the first inner conductor 7 and the first outer conductor 9. The first dielectric layer 11 is provided directly on an external surface of the first inner conductor 7, and the first outer conductor 9 is provided directly on an external surface of the first dielectric layer 11.

[0126] In this embodiment the first coaxial transmission line is 50 Ohm Sucoform 86 coaxial cable. The first inner conductor 7 has a diameter of 0.53 mm, the first outer conductor 9 has an inner diameter of 1.65 mm (and therefore the first dielectric layer 11 has an outer diameter of 1.65 mm), and the first outer conductor 9 has an outer diameter of 2.1 mm. Of course, in other embodiments a different type of coaxial cable with different dimensions and properties, or a different type of coaxial transmission line, may be used instead.

[0127] In this embodiment the first dielectric layer 11 is made from PTFE having a relative permittivity of 2.1. Of course, in other embodiments a different dielectric material may be used.

[0128] In this embodiment the first inner conductor 7 is a metal wire. Specifically, the first inner conductor 7 is a steel wire plated with copper and silver. The first outer conductor 9 is a metal braid. Specifically, the first outer conductor 9 is a braid formed from copper wire plated with tin. Of course, in other embodiments different materials may be used for the first inner and outer conductors 7, 9.

[0129] The length of the first coaxial transmission line 3 is not critical to the operation of the instrument 1 described below and can be chosen based on the particular environment in which the instrument 1 is intended to be used.

[0130] The second coaxial transmission line 5 comprises a cylindrical second inner conductor 13, a tubular second outer conductor 15 and a tubular second dielectric layer 17 separating the second inner conductor 13 and the second outer conductor 15. The second dielectric layer 17 is provided directly on an external surface of the second inner conductor 13, and the second outer conductor 15 is provided directly on an outer surface of the second dielectric layer 17.

[0131] In this embodiment the second inner conductor 13 has a diameter of 1.2 mm, meaning the second inner conductor 13 is wider than the first inner conductor 7, and the second outer conductor 15 has an inner diameter of 1.8 mm (and the second dielectric layer 17 therefore has an outer diameter of 1.8 mm). The outer diameter of the second outer conductor 15 is wider than the outer diameter of the first outer conductor 9. Of course, in other embodiments the second inner and outer conductors 13, 15 may have different dimensions. In this embodiment, the outer diameter of the second outer conductor 15 is 2.5 mm.

[0132] In this embodiment the second dielectric layer 17 is a glass ceramic dielectric. Specifically, the second dielectric layer 17 is MACOR® and has a relative permittivity of 5.67 (which value may be frequency dependent, and thus depend on the specific frequency of microwave radiation with which the electrosurgical instrument is being used). Of course, in other embodiments a different dielectric material may be used instead.

[0133] The second inner conductor 13 may comprise a solid cylinder of stainless steel. The outer surface of the second inner conductor 13 may be coated, for example plated, with silver. Of course, other materials may be used for the second inner conductor 13.

[0134] The second outer conductor 15 may comprise a hollow tube of stainless steel. The inner surface of the second outer conductor 15 may be coated, for example plated, with silver. Of course, other materials may be used for the second outer conductor 15.

[0135] The second inner conductor 13 is connected to the distal end of the first inner conductor 7 with their central axes aligned. The second outer conductor 15 is also connected to the distal end of the first outer conductor 9 with their central axes aligned. Thus, microwave frequency energy can be conveyed from the first coaxial transmission line 3 to the second coaxial transmission line 5. The first and second coaxial transmission lines 3, 5 may therefore have overlapping central axes.

[0136] In this embodiment the second dielectric layer is thinner than the first dielectric layer.

[0137] The second inner conductor 13, second outer conductor 13 and second dielectric layer 17 are exposed at a planer distal end face of the second coaxial transmission line 5. The planar distal end face of the second coaxial transmission line 5 can be pressed against tissue in order to deliver microwave frequency energy into the tissue, as described further below.

[0138] The electrosurgical instrument 1 shown in FIG. 1 can be configured to coagulate a particular type of tissue having a particular impedance using microwave energy having a particular frequency. This is achieved by configuring the length of the second coaxial transmission line 5 and the ratio of the inner diameter of the second outer conductor 15 to the diameter of the second inner conductor 13 so that the second coaxial transmission line 5 functions as an impedance transformer that better matches the characteristic impedance of the first coaxial transmission line 3 to the characteristic impedance of the tissue at the particular frequency.

[0139] The theory of impedance transformers is well known and understood in the technical field and therefore a detailed description is not repeated here.

[0140] Preferably, the second coaxial transmission line 5 is configured to exactly match the impedance of the first coaxial transmission line 3 to the impedance of the tissue, so that the maximum amount of microwave energy is delivered to the tissue. Of course, acceptable performance of the electrosurgical instrument 1 can be achieved without exactly matching the impedances, because a minor impedance mismatch (for example up to 10%, or up to 20%) may lead to reflection of only a minor amount of microwave energy away from the tissue.

[0141] There are two requirements for the second coaxial transmission line 5 to function as an impedance transformer that better matches the characteristic impedance of the first coaxial transmission line 3 to the characteristic impedance of the liver tissue at the predetermined operating frequency. Firstly, the length of the second coaxial transmission line 5 must be such that the second coaxial transmission line 5 is an impedance transformer that improves the impedance match between the first coaxial transmission line 3 and the tissue at the predetermined operating frequency. For example, the second coaxial transmission line may have a length that is substantially equal to (2n+1)λ/4. Secondly, the impedance of the second coaxial transmission line 5 must be intermediate between the impedance of the first coaxial transmission line 3 and the impedance of the tissue being coagulated. For optimal impedance matching, the impedance of the second coaxial transmission line 5 must be substantially equal to √{square root over (Z.sub.inZ.sub.L)}, where Z.sub.in is the characteristic impedance of the first coaxial transmission line 3 and Z.sub.L is the predetermined characteristic impedance of the tissue.

[0142] In the dielectric material of the second dielectric layer 17 the microwave frequency energy travels as a speed v, where:

[00001]$\begin{array}{cc}v=\frac{c}{\sqrt{{\mu }_r{\mathrm{.Math.}}_r^{\prime }}}& \left(1\right)\end{array}$

where c is the speed of light, μ.sub.r is the relative permeability of the dielectric material and ε.sub.r is the relative permittivity (the dielectric constant) of the dielectric material.

[0143] Assuming the dielectric material is non-magnetic and therefore has a relative permeability of 1, the microwave frequency energy travels at a speed in the second dielectric layer 17:

[00002]$\begin{array}{cc}v=\frac{c}{\sqrt{{\mathrm{.Math.}}_r}}.& \left(2\right)\end{array}$

[0144] The wavelength λ of the microwave frequency energy in the second dielectric layer 17 is therefore given by:

[00003]$\begin{array}{cc}\lambda =\frac{c}{f\sqrt{{\mathrm{.Math.}}_r^{\prime }}}& \left(3\right)\end{array}$

where f is the frequency of the microwave frequency energy.

[0145] Thus, using equation (3) the wavelength in the second coaxial transmission line 5 of microwave energy having the desired operating frequency can be determined based on the relative permittivity (dielectric constant) of the dielectric material, which can be looked up, calculated of found by experimentation. The length of the second coaxial transmission line 5 that is equal to (2n+1)λ/4 can then easily be determined, and the length of the second coaxial transmission line 5 in the electrosurgical instrument 1 can be set to be substantially equal to the calculated length.

[0146] Alternatively, as discussed above, the optimal length of the second coaxial transmission line 5 may be different to this, because it may also be affected by the specific geometry of the second coaxial transmission line 5. Thus, the optimal length may be calculated based on the geometry of the second coaxial transmission line 5, possibly in addition to using the wavelength in the second coaxial transmission line 5 calculated as described above. Alternatively, the optimal length may be determined based on simulation or experimentation. The optimal length is the length that minimises the return loss, i.e. minimises the amount or proportion of the reflected microwave frequency energy, which corresponds to maximising the impedance match between the first coaxial transmission line 3 and the tissue. Of course, the actual length of the second coaxial transmission line does not have to be the exact optimal length, because other similar lengths may also give acceptable (non-optimal) performance.

[0147] The impedance of a coaxial cable is given by equation (4).

[00004]$\begin{array}{cc}{Z}_0=60\sqrt{\frac{{\mu }_r}{{\mathrm{.Math.}}_r}}\ln \left(\frac{D}{d}\right)& \left(4\right)\end{array}$

where μ.sub.r is the relative permeability of the dielectric material, ε.sub.r is the relative permittivity (the dielectric constant) of the dielectric material, D is the inner diameter of the outer conductor and d is the outer diameter of the inner conductor. Assuming the dielectric material is non-magnetic and therefore has a relative permeability of 1, the impedance of the coaxial transmission line is given by equation (5).

[00005]$\begin{array}{cc}{Z}_0=\frac{60}{\sqrt{{\mathrm{.Math.}}_r}}\ln \left(\frac{D}{d}\right)& \left(5\right)\end{array}$

[0148] According to equation (5), the impedance of the coaxial cable is determined solely by the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor, for a particular dielectric material with a particular relative permittivity. Thus, using equation (5) the necessary ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor to provide a coaxial cable having a particular impedance can be calculated.

[0149] Thus, equation (5) can be used to calculate the ratio of the inner diameter of the second outer conductor 15 to the outer diameter of the second inner conductor 13 that results in the characteristic impedance of the second coaxial transmission line 5 being intermediate between the characteristic impedance of the first coaxial transmission line 3 and the tissue, and the ratio in the electrosurgical instrument can be set to be the calculated value.

[0150] Preferably, a ratio is calculated that results in the characteristic impedance of the second coaxial transmission line 5 being substantially equal to √{square root over (Z.sub.inZ.sub.L)}, where Z.sub.in is the characteristic impedance of the first coaxial transmission line and Z.sub.L is the predetermined characteristic impedance of the tissue, because this provides exact impedance matching between the first coaxial transmission line and the tissue and therefore maximises the amount of microwave energy delivered to the tissue.

[0151] Appropriate specific diameters of the second inner and outer conductors may be determined based on a number of variables, including the corresponding diameters in the first coaxial transmission line 1, the geometry of the tissue to which the microwave frequency energy is being delivered, and by the frequency of the microwave frequency energy.

[0152] In the computer simulation illustrated in FIG. 1, the electrosurgical instrument 1 is configured for coagulating liver tissue using microwave frequency energy having a characteristic frequency of 5.8 GHz. Of course, in other embodiments the desired operating frequency may be different, and/or the desired tissue to be coagulated (and the corresponding characteristic impedance) may be different and/or the instrument may be for achieving an effect other than coagulation, such as ablation.

[0153] In this computer simulation the 50 ohm Sucoform 86 cable was modelled as being 10 mm long, for simplicity.

[0154] Simulations were carried out using CST Microwave Studio over a bandwidth from 3.3 GHz to 8.3 GHz, with a centre frequency of 5.8 GHz. A liver load was modelled placed directly against the open circuit end of the distal coaxial transmission line. The impedance of the liver load was modelled as being 58+j10.6 Ohms. This corresponds to modelling the liver load as having a dielectric constant of approximately 38 compared to the dielectric constant of 5.67 for the MACOR dielectric in the second coaxial transmission line 5.

[0155] Initially a second inner conductor 13 diameter of 1 mm and a second dielectric layer 17 outer diameter of 1.65 mm were selected for the second coaxial transmission line 5, which had an outer diameter of 2.5 mm. Using these parameters it was found by performing various simulations that the ideal length of the second coaxial transmission line 5 for maximising the amount of microwave energy delivered to the liver tissue was close to 9 mm.

[0156] With the length of the second coaxial transmission line 5 set at 9 mm the outer diameters of the second inner conductor 13 and the second dielectric layer 17 were varied and it was found that a second inner conductor 13 outer diameter of 1.2 mm and a second dielectric layer 17 outer diameter of 1.8 mm gave a good match to the liver tissue at an operating frequency of 5.8 GHz with a reasonable bandwidth.

[0157] The performance of the electrosurgical instrument was checked in the simulations to determine the pattern of absorption in the tissue and the level of radiation in unwanted directions, and the performance was found to be acceptable.

[0158] FIG. 1 shows a computer simulation of an electrosurgical instrument 1 according to an embodiment of the present invention being used to coagulate liver tissue in contact with the distal end of the instrument 1. The length and diameter parameters of the second coaxial transmission line 5 were set to the optimal parameters determined above.

[0159] It can be seen in FIG. 1 that the power absorption pattern 19 in the liver tissue 21 in the simulation is localised in an area of the liver tissue 21 directly in contact with the distal end of the distal coaxial transmission line 5, and is circularly symmetrical about the shared central axis of the first and second coaxial transmission lines.

[0160] This result demonstrates that with the present invention it is possible to achieve controlled localised delivery of power into tissue in contact with the electrosurgical instrument in order to cause controlled localised coagulation in that area of tissue.

[0161] FIG. 2 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 1. As well known in the technical field, the S-parameter is a measure of the return loss of microwave energy due to impedance mismatch, and as such the S-parameter is indicative of the degree of impedance mismatch. The S-parameter can be defined by equation (6).

P.sub.I=SP.sub.R   (6)

where P.sub.I is the outgoing power in the instrument towards the tissue, P.sub.R is the reflected power away from the tissue and S is the S-parameter.

[0162] As shown in FIG. 2, in the simulation results there is a very good impedance match for the desired operating frequency of 5.8 GHz, meaning very little microwave energy was reflected away from the liver tissue at this frequency of microwave energy in the simulations. This demonstrates that configuring the length and diameter parameters of the distal coaxial transmission line of the electrosurgical instrument as discussed above can maximise the amount of microwave energy that is delivered by the electrosurgical instrument into the desired localised area of tissue in contact with the distal end of the distal coaxial transmission line.

[0163] While such a good match may be difficult to achieve in practice, these results also illustrate that an acceptable match of the impedances may be achievable over a range of different tissues with different (but similar) relative permittivities. For example, a poorer but still acceptable S-parameter of −15 dB may be achieved with a different type of tissue having a slightly higher or lower impedance than liver tissue.

[0164] Further simulations were carried out to determine the effect of tilting the instrument so that only the corner of the distal end of the distal coaxial transmission line was in contact with the tissue. It was found that for angles greater than 1 degree between the distal end of the distal coaxial transmission line and the surface of the tissue the match was poor. The simulated absorption pattern for 5 degrees tilt is shown in FIG. 3 and the S-parameter (return loss) for 1, 2 and 5 degrees tilt is shown in FIG. 4.

[0165] As shown in FIG. 3, when the electrosurgical instrument is tilted so that only part of the electrosurgical instrument contacts the tissue, the microwave power is delivered into an even smaller localised area of tissue where the contact is made. As shown in FIG. 3, the power delivery pattern 22 is smaller and more localised than the power delivery pattern 19 in FIG. 1.

[0166] For 5 degrees tilt the power radiated, i.e. that left the electrosurgical instrument but did not enter the intended tissue, was −23.18 dB compared to the input power, i.e. about 0.5%. The return loss was −0.84 dB, i.e. only about 17.5% of the power left the applied end of the electrosurgical instrument, and about 97% of this power was absorbed in the intended tissue.

[0167] As shown in FIG. 4, increasing the amount of the angle/tilt between the central axis of the instrument and the normal to the tissue surface significantly increases the return loss, meaning more of the microwave power is reflected and less of the microwave power is delivered to the tissue. The electrosurgical instrument will therefore be less efficient at these higher angles of tilt, but the microwave energy will be localised into a smaller area.

[0168] FIG. 5 shows a computer simulation result for an electrosurgical instrument 23 according to a second embodiment of the present invention. The electrosurgical instrument 23 according to the second embodiment differs from the electrosurgical instrument 1 according to the first embodiment because in the second embodiment the second coaxial transmission line 25 tapers from a wider proximal end 27 to a narrower distal end 29. In this embodiment the distal end 29 has approximately half the diameter of the proximal end 27.

[0169] Specifically, in this embodiment at the distal end 29 of the second coaxial transmission line the diameter of the second inner conductor is 0.6 mm, the diameter of the second dielectric layer is 0.9 mm and the outer diameter of the second outer conductor is 1.25 mm.

[0170] In this embodiment the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor is kept constant along the length of the distal coaxial transmission line, so that the impedance of the distal coaxial transmission line is constant along its length. Furthermore, the proportions of the exposed distal end of the distal coaxial transmission line that is in contact with the tissue is the same as in the embodiment of FIG. 1.

[0171] As shown in FIG. 5, an advantage of the second coaxial transmission line 25 being tapered in this manner is that the microwave energy is delivered to a smaller area of the tissue at the distal end of the second coaxial transmission line 25. This can be seen by comparing the power absorption pattern 19 in FIG. 1 with the power absorption pattern 31 in FIG. 5. Thus, the microwave energy delivery is more localised in the second embodiment. This may be particularly useful, for example, when it is desirable to focus the energy delivery into a specific area or type of tissue, for example into a blood vessel or lumen.

[0172] FIG. 6 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 5. In this simulation the length of the second coaxial transmission line 25 was lengthened from the 9 mm of the first embodiment to 10 mm so that the lowest return loss was obtained at the desired operating frequency of 5.8 GHz. The change in length of the second coaxial transmission line was necessary because the wavelength of the microwave frequency energy is longer with this tapered geometry of the second coaxial transmission line (this is an example in which the optimal length of the second coaxial transmission line is not an odd multiple of λ/4.

[0173] It is apparent from FIG. 6 that the return loss is more significant than with the embodiment illustrated in FIG. 1 (compare FIGS. 6 and 2). This may be because the geometry (e.g. length and ratio of the diameters) of the second coaxial transmission line was not fully optimised for the liver load tissue. Therefore, it may be possible to achieve a lower return loss with this geometry of the second coaxial transmission line by further optimising the geometry of the second coaxial transmission line.

[0174] However, the performance of the instrument 25 is still good with this return loss so the instrument 25 can be successfully used to coagulate the tissue, and as discussed above this embodiment has an advantage that the power is delivered into a smaller volume of tissue.

[0175] A further advantage of the tapered nature of the second coaxial transmission line 25 in this embodiment is that it is possible to press the distal end of the coaxial transmission line 25 further into the tissue than with the first embodiment, because the distal tip is narrower in this embodiment.

[0176] The other features of the second embodiment are the same as in the first embodiment and description thereof is not repeated here for conciseness.

[0177] Delivery of the microwave frequency energy into a more localised area of the tissue may also be achieved by maintaining the cylindrical shape of the second coaxial transmission line shown in FIG. 1 but using a narrower coaxial transmission line. This similarly results in a narrower distal end of the second coaxial transmission line for delivering the microwave frequency energy into a more localised area of tissue.

[0178] FIG. 7 shows a computer simulation of an electrosurgical instrument according to a further embodiment of the present invention being used to coagulate liver tissue, wherein the first and second coaxial transmission lines are narrower than in the embodiment of FIG. 1. The configuration of the instrument shown in FIG. 7 is similar to that shown in FIG. 1 apart from the dimensions of the first and second coaxial transmission lines. Only the differences from the embodiment shown in FIG. 1 will be discussed here.

[0179] In the embodiment of FIG. 7, the first coaxial transmission line is Sucoform 47 coaxial cable. Sucoform 47 coaxial cable comprises the same materials as Sucoform 86 coaxial cable discussed above, but different dimensions. Specifically, in Sucoform 47 cable the inner conductor has an outer diameter of 0.31 mm, the PTFE dielectric layer has an outer diameter of 0.94 mm and the outer conductor has an outer diameter of 1.20 mm. Sucoform 47 cable has a characteristic impedance of 50 Ohms.

[0180] A potential disadvantage of using Sucoform 47 as the first coaxial transmission line relative to the wider Sucoform 86 cable is that the Sucoform 47 cable has higher losses, so the efficiency of the instrument will be lower. However, an advantage of using Sucoform 47 is that the relative proportions of the first and second coaxial transmission lines at the junction between them are similar to the previously described embodiments, despite the diameter of the second coaxial transmission line being narrower.

[0181] The second coaxial transmission line is a cylindrical transmission line that is narrower than in FIG. 1. Specifically, the second inner conductor has an outer diameter of 0.702 mm, the second dielectric layer has an outer diameter of 1.053 mm and the second outer conductor has an outer diameter of 1.462 mm.

[0182] As shown in FIG. 7, the narrower nature of the second coaxial transmission line 5 in this embodiment results in more localised delivery of the microwave frequency energy into the liver tissue (compare FIGS. 1 and 7, taking into account the different scales).

[0183] FIG. 8 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 7. As can be seen by comparing FIGS. 6 and 8, the return loss for this embodiment is comparable to the return loss of the tapered embodiment illustrated in FIG. 5.

[0184] FIG. 9 shows a computer simulation of an electrosurgical instrument according to a further embodiment of the present invention being used to coagulate liver tissue, wherein the second coaxial transmission line is narrower than in the embodiment of FIG. 1. FIG. 10 is a plot of the magnitude of the S-parameter (return loss) against the frequency of the microwave radiation for the computer simulation shown in FIG. 9.

[0185] The configuration of the instrument shown in FIG. 9 is similar to that shown in FIG. 1 apart from the dimensions of the second coaxial transmission line. Only the differences from the embodiment shown in FIG. 1 will be discussed here.

[0186] The first coaxial transmission line in the embodiment shown in FIG. 9 is Sucoform 86 coaxial cable, as in the embodiment of FIG. 1. However, the second coaxial transmission line is narrower, and has the same dimensions as in the embodiment of FIG. 7. In other words, the second inner conductor has an outer diameter of 0.702 mm, the second dielectric layer has an outer diameter of 1.053 mm and the second outer conductor has an outer diameter of 1.462 mm. As shown in FIG. 9, more localised delivery of the microwave frequency energy into the tissue can be achieved with the embodiment of FIG. 9 than with the embodiment of FIG. 1, because of the narrower distal tip of the second coaxial transmission line in FIG. 9.

[0187] FIG. 10 shows that the return loss is acceptable when feeding the second coaxial transmission line using the wider Sucoform 86 cable instead of the narrower Sucofrom 47 cable.

[0188] An advantage of using the wider Sucoform 86 coaxial cable as the first coaxial transmission line is that the power loss is less in this cable. Therefore, the efficiency of the instrument will be greater when Sucoform 86 cable is used as the first coaxial transmission line.

[0189] The dimensions of the distal tip of the second coaxial feed cables in FIGS. 1, 5, 7 and 9 are set out in Table 1 below, for ease of reference.

TABLE-US-00001 TABLE 1 Inner conductor Dielectric Outer conductor Embodiment diameter layer diameter diameter FIG. 1  1.2 mm  1.8 mm  2.5 mm FIG. 5  0.6 mm  0.9 mm  1.25 mm FIGS. 7 and 9 0.702 mm 1.053 mm 1.462 mm

[0190] Of course, in alternative embodiments the dimensions may be different to those given in Table 1.

[0191] In summary, all embodiments of the present invention provide an electrosurgical instrument that efficiently couples microwave energy into a localised area of tissue directly in contact with the electrosurgical instrument.

[0192] The delivery of the microwave energy to the tissue is significantly improved where good contact is made between the distal end of the electrosurgical instrument (for example the distal end of the second coaxial transmission line) and the tissue. Where the electrosurgical instrument is at an angle so that there is an air gap between at least part of the distal end of the electrosurgical instrument and the tissue the return loss can be poor, particularly for angles of greater than 1 degree.

[0193] In all embodiments, very little power is radiated in unwanted directions (in other words in any direction other than into the area of tissue in contact with the probe), regardless of the return loss.

[0194] In all embodiments the electrosurgical instrument can be used to contact the tissue at an angle to coagulate a very small piece of tissue close to the edge of the distal end of the second coaxial transmission line.

[0195] In addition, a more concentrated region of heating of the tissue may also be achieved by using a tapered second coaxial transmission line to deliver the microwave energy into a smaller area of tissue at the distal end of the second coaxial transmission line.

[0196] More concentrated delivery of microwave frequency energy into the tissue can also be achieved by using a narrower cylindrical second coaxial transmission line. In order to reduce power losses, the narrower second coaxial transmission line may be fed by a wider first coaxial transmission line.

[0197] FIG. 11 is a schematic illustration of a further embodiment of the present invention. As with the previously described embodiments, in this embodiment the electrosurgical instrument comprises the first and second coaxial transmission lines 3, 5. The configuration of the first and second coaxial transmission lines 3, 5 shown in FIG. 11 is an example configuration only, and the first and second coaxial transmission lines may instead have another configuration (shape, size, etc.), for example one of the other example configurations of the other embodiments described above and illustrated in FIGS. 1 to 10.

[0198] In this embodiment the electrosurgical instrument comprises a third coaxial transmission line 33. The third coaxial transmission line 33 comprises a third inner conductor that is connected to the second inner conductor, a third outer conductor that is coaxial with the third inner conductor and connected to the second outer conductor, and a third layer of dielectric material separating the third inner and outer conductors.

[0199] The third coaxial transmission line 33 is axially aligned with (coaxial with) the first and second coaxial transmission lines 3, 5.

[0200] The third inner conductor, third outer conductor and third layer of dielectric material are exposed at a planar distal end face of the third coaxial transmission line 33. In use, as illustrated in FIG. 11, the distal end face of the third coaxial transmission line 33 can be pressed against tissue to deliver microwave frequency energy into the tissue, as described in more detail below. Thus, in this embodiment the distal end of the third coaxial transmission line 33 forms the distal end of the instrument, and not the distal end of the second coaxial transmission line 5 as in the previously described embodiments.

[0201] The third coaxial transmission line 33 is configured to remove a reactive part (imaginary component) of the impedance of the tissue 35. Once the reactive part of the impedance of the tissue 35 has been removed, the second coaxial transmission line 5 matches the subsequent purely real impedance to the impedance of the first coaxial transmission line 3. Thus, the effects of impedance mismatch of both the real and reactive (imaginary) parts of the impedance of the tissue 35 are taken into account and addressed in this embodiment.

[0202] In this embodiment the third coaxial transmission line 33 has the same impedance as the first coaxial transmission line 3 (for example 50 Ohm). Indeed, in this embodiment the third coaxial transmission line 33 is a same type of coaxial cable as the first coaxial transmission line 3.

[0203] An appropriate length for the third coaxial transmission line 33 to cancel out/remove the reactive part of the impedance of the tissue 35 can be determined mathematically, for example using a Smith Chart, based on variables such as the impedances of the tissue and the first coaxial transmission line and the frequency of the microwave frequency radiation. If the calculated length of the third coaxial transmission line is too short to be practical, a multiple of λ/2 can be added to the calculated length to determine a more practically appropriate length. The appropriate length of the third coaxial transmission line may alternatively be determined by computer simulation/modelling, or by experimentation.

[0204] As an example, the appropriate length of the third coaxial transmission line may be determined using the following steps, assuming the tissue impedance (load) is (10-j10)Ω and the impedance of the first coaxial transmission line is 50Ω:

[0205] (1) Normalise the tissue impedance to the impedance of the first coaxial transmission line:

[00006]$Z_N=\frac{\left(10-j10\right)\Omega }{50\Omega }=\left(0.2-j0.2\right)\Omega ;$

[0206] (2) Plot the normalised impedance on the Smith Chart and draw the VSWR circle;

[0207] (3) Rotate the normalised load to the real r axis and note the movement in wavelengths from the load towards the generator. This removes the reactive (imaginary) component of the impedance of the load;

[0208] (4) If the Δλ is too short to realise practically, add

[00007]$\frac{n\lambda }{2}$

to this length until a practical length is achieved.

[0209] Once the reactive part of the impedance of the tissue has been cancelled out, the subsequently purely real impedance can be matched to the impedance of the first coaxial transmission line by an appropriately configured second coaxial transmission line. Specifically, the Smith Chart can be used to find the value from the real r axis where the load has a purely real value only (which may be to the left or the right of the centre), and this value can then be normalised by multiplying by the impedance of the first coaxial transmission line to find the real impedance to be matched to the impedance of the first coaxial transmission line.

[0210] The following is a simple numerical example relating to the embodiment of FIG. 11, assuming the tissue impedance (load) is (10 -j10)Ω and the impedance of the first coaxial transmission line is 50Ω.

[0211] Carrying out the above steps using a Smith Chart results in the desired length of the third coaxial transmission line being determined to be 0.033λ and the value from the real r axis where the load has a purely real value only to be r.sub.new=0.19. The normalised load to be matched to the impedance of the first coaxial transmission line 3 by the second coaxial transmission line 5 is then determined to be Z.sub.new=0.19×50Ω=9.5Ω. The optimal impedance of the second coaxial transmission line 5 can thus be determined to be:

Z.sub.T=√{square root over (Z.sub.inZ.sub.L)}=√{square root over (50×0.9)}=21.8Ω

[0212] Appropriate diameters for the second inner and outer conductors can then be determined based on this value and equation (6).

[0213] Assuming a microwave frequency of 5.8 GHz and that the second dielectric material is PTFE with a relative permittivity of 1.5, the optimal length of the second coaxial transmission line 5, i.e.

[00008]$\frac{\lambda }{4},$

can be determined using equation (3) to be 10.56 mm. If this length is too short to he practical, it can be increased by adding a multiple of

[00009]$\frac{\lambda }{2}$

to the length, for to arrive at a length of 31.68 mm.

[0214] As mentioned above, the desired length of the third coaxial transmission line 33 in this example is 0.033Ω, which corresponds to a length of 1.39 mm. Again, if this length is too short to be practical, it can be increased by adding a multiple of

[00010]$\frac{\lambda }{2}$

to the length, for to arrive at a length of 22.51 mm.

[0215] The following is a simple numerical example illustrating the need for impedance matching in the embodiments of the present invention. This example assumes a tissue impedance of (12-j15)Ω and a single coaxial transmission line with an impedance of 50Ω pressed against the tissue.

[0216] Without any impedance matching the amount of power that would be reflected would be:

[00011]$\Gamma =\frac{Z_L-Z_0}{Z_L+Z_0}=0.63-j0.37$$.Math.\Gamma .Math.=0.731$

[0217] The proportion of power delivered to the tissue load is then given by:

P=(1-0.731.sup.2)=0.466

[0218] Thus, with no impedance matching only 47% of the power will be delivered to the load, which means that the electrosurgical instrument would be relatively inefficient. The delivered power can be significantly improved by performing impedance matching as in the embodiments of the present invention described above.

[0219] In one practical embodiment, the dielectric material in the second and/or third coaxial transmission line may be air, or another gas. In this case, a piece of material may be positioned over the end of the coaxial transmission line, for example a piece of Kepton tape or a mica window.