Patent Description:
Tissue ablation may be used to treat a variety of clinical disorders and several ablation techniques have been developed, including cryoablation, microwave ablation, radio frequency (RF) ablation, and ultrasound ablation. Numerous treatment schemes affect the nerve using RF power applied by a catheter contacting the inside wall of the artery.

Such techniques are typically performed by a clinician who introduces a catheter having an ablative tip to the target tissue via the venous vasculature, positions the ablative tip adjacent to what the clinician believes to be an appropriate region based on tactile feedback, mapping electrocardiogram (ECG) signals, anatomy, and/or fluoroscopic imaging, actuates flow of an irrigant to cool the surface of the selected region, and then actuates the ablative tip for a period of time believed sufficient to destroy tissue in the selected region.

Although commercially available ablative tips may include thermocouples for providing temperature feedback via a digital display, such thermocouples typically do not provide meaningful temperature feedback during irrigated ablation. For example, the thermocouple only measures surface temperature, whereas the heating or cooling of the tissue that results in tissue ablation may occur at some depth below the tissue surface. Moreover, for procedures in which the surface of the tissue is cooled with an irrigant, the thermocouple will measure the temperature of the irrigant, thus further obscuring any useful information about the temperature of the tissue, particularly at depth. As such, the clinician has no useful feedback regarding the temperature of the tissue as it is being ablated or whether the time period of the ablation is sufficient.

Accordingly, it may only be revealed after the procedure is completed, that the targeted aberrant pathway was not adequately interrupted. In such a circumstance, the clinician may not know whether the procedure failed because the incorrect region of tissue was ablated, because the ablative tip was not actuated for a sufficient period of time to destroy the target tissue, because the ablative tip was not touching or insufficiently touching the tissue, because the power of the ablative energy was insufficient, or some combination of the above. Upon repeating the ablation procedure so as to again attempt to ablate the target tissue, the clinician may have as little feedback as during the first procedure, and thus potentially may again fail to destroy the aberrant pathway. Additionally, there may be some risk that the clinician would retreat a previously ablated region of the target tissue and not only ablate the target tissue, but damage adjacent tissues.

In some circumstances, to avoid having to repeat the ablation procedure as such, the clinician may ablate a series of regions of the target tissue along which the target tissue is believed to lie, so as to improve the chance of successful ablation. However, there is again insufficient feedback to assist the clinician in determining whether any of those ablated regions are sufficiently destroyed.

<CIT>describes a hyperthermia treatment apparatus in which a microwave source is used to deposit energy in living tissue to effect hyperthermia. The apparatus includes a radiometer for measuring temperature at depth within the tissue, and includes a controller that feeds back a control signal from the radiometer, corresponding to the measured temperature, to control the application of energy from the microwave source.

<CIT> describes an integrated heating and sensing catheter apparatus for treating arrhythmias, tumors and like, having a diplexer that permits near simultaneous heating and temperature measurement. This patent too describes that temperature measured by the radiometer may be used to control the application of energy, e.g., to maintain a selected heating profile.

Despite the promise of precise temperature measurement sensitivity and control offered by the use of radiometry, there have been few successful commercial medical applications of this technology. One drawback of previously-known systems has been an inability to obtain highly reproducible results due to slight variations in the construction of the microwave antenna used in the radiometer, which can lead to significant differences in measured temperature from one catheter to another. Problems also have arisen with respect to orienting the radiometer antenna on the catheter to adequately capture the radiant energy emitted by the tissue, and with respect to shielding high frequency microwave components in the surgical environment so as to prevent interference between the radiometer components and other devices in the surgical field.

Acceptance of microwave-based hyperthermia treatments and temperature measurement techniques also has been impeded by the capital costs associated with implementing radiometric temperature control schemes. Radiofrequency ablation techniques have developed a substantial following in the medical community, even though such systems can have severe limitations, such as the inability to accurately measure tissue temperature at depth, e.g., where irrigation is employed. However, the widespread acceptance of RF ablation systems, extensive knowledge base of the medical community with such systems, and the significant cost required to changeover to, and train for, newer technologies has dramatically retarded the widespread adoption of radiometry.

<CIT> and <CIT>, describe systems for radiometrically measuring temperature during ablation.

In view of the foregoing, it would be desirable to provide systems and methods that permit a high degree of radiometric measurement of temperature at depth in tissue to achieve accurate temperature measurement with microwave heating.

It would further be desirable to provide systems and methods for calibration of such microwave heating and radiometric measurement systems.

In addition, it would be desirable to provide an ablation system having feedback mechanisms for detecting and/or preventing overheating of target tissue during an ablation procedure to improve efficacy and safety of the ablation system.

While there is a breadth of energy based devices to treat a range of conditions, giving promise of improved outcomes, lower risks and shortened recovery times, there remains significant opportunity to exploit capabilities of distinct technologies to deliver optimal therapy to drive outcome and improve risk profiles.

<CIT>) relates to systems and methods for microwave ablation of target tissue and measuring temperature of the target tissue during ablation.

The invention relates to a system as defined in the appended claims. Embodiments, examples or aspects in the following disclosure, in particular methods, which do not fall under the scope of the claims are presented for illustration purposes only and do not form part of the invention.

The present invention provides ablation systems for ablating target tissue as well as sensing parameters (e.g., temperature) during ablation. In a preferred embodiment, the ablation systems utilize microwave energy for ablation. For example, the system for ablating target tissue within a patient may include a catheter having a proximal region and a distal region, and a main antenna disposed at the distal region of the catheter. The main antenna may both emit energy to ablate the target tissue and measure a radiometer temperature generated as a result of the energy emission. The system further includes a reference termination disposed at the distal region of the catheter for measuring a reference temperature at the distal region. The system is designed for safe and efficacious energy delivery into tissue by, for example, emitting energy in a controlled, repeatable manner that allows for feedback and energy emission titration based on sensed parameters (e.g., tissue temperature) measured during ablation. The system may include a cooling sleeve disposed over at least the distal region of the catheter. The cooling sleeve may be coupled to a source of coolant and to permit the coolant to flow over the main antenna and the reference termination, thereby cooling the main antenna and the reference termination during pre-ablation calibration and during an ablation procedure. In this manner, ex vivo calibration prior to the in vivo ablation procedure closely aligns with the ablation procedure to ensure accurate sensing of parameters such as target tissue during ablation.

Additionally, the system further may include a processor operatively coupled to the main antenna and the reference termination. The processor may cause the main catheter to emit energy and measure radiometer temperature and the reference termination to measure reference temperature in an interleaving manner. For example, processor may cause the main catheter to emit energy for a first time period, and to cause the main catheter to measure radiometer temperature and the reference termination to measure reference temperature in an alternating manner for a second time period. The first time period may be at least <NUM>% of a sum of the first and second time periods. Moreover, the processor may be programmed to cause the main catheter to measure radiometer temperature and the reference termination to measure reference temperature in an alternating manner via a switch electrically coupled to the main antenna and the reference termination.

The processor may be programmed to calculate a target tissue temperature based on the measured radiometer temperature and the measured reference temperature. Moreover, the processor may be programmed to estimate a volume of an ablation lesion created by the energy emission during the ablation procedure based on the target tissue temperature. For example, the ablation lesion volume may be estimated based on at least one of an average target tissue temperature or an area under a plotted curve of the target tissue temperature. Further, the processor further may permit titration of the energy emission based on the volume of the ablation lesion. In addition, the processor may modulate the energy emission such that the calculated target tissue temperature is maintained within a predetermined threshold.

In accordance with another aspect of the present invention, the processor may be programmed to perform a reference termination calibration to account for heating of the reference termination during energy emission via the main antenna and a radiometer calibration to account for heating of an environment adjacent the target tissue during energy emission via the main antenna. In addition, the processor may be programmed to calculate a target tissue temperature based on the measured radiometer temperature and the measured reference temperature while accounting for heating of the reference termination and the environment adjacent the target tissue during energy emission via the main antenna.

For example, the reference termination calibration may include measuring output voltage resulting from energy emission generated by the reference termination for varying levels of energy emitted by the main antenna while the main antenna and reference termination are in a constant temperature bath providing high fluid flow across the main antenna such that a temperature of an environment adjacent the main antenna remains constant, and comparing the measured voltage with the varying levels of energy emission to account for an effect of energy emission on the reference termination during energy emission.

Moreover, the radiometer calibration may include measuring first and second temperatures in response to impingement of the main antenna with first and second noise levels, respectively, while the main antenna and the reference termination are in a constant temperature bath and comparing the first and second temperatures with the first and second noise levels to account for an effect of energy emission on the environment adjacent the target tissue during energy emission. Alternatively, the radiometer calibration may include measuring a first output voltage and a first temperature in response to a first radiometer signal while the main antenna and reference termination are in a first bath having a first temperature, measuring a second output voltage and a second temperature in response to a second radiometer signal while the main antenna and reference termination are in a second bath having a second temperature different from the first temperature, and comparing the first and second output voltages with the first and second temperatures to account for an effect of energy emission on the environment adjacent the target tissue during energy emission.

In accordance with yet another aspect of the present invention, the processor may be programmed to calculate a target tissue temperature based on the measured radiometer temperature and the measured reference temperature, and to monitor the target tissue temperature to predict and/or detect a pop, e.g., a rapid target tissue temperature rise followed by a sudden target tissue temperature drop, within the target tissue temperature. Accordingly, the processor may generate an alert if the pop is detected. Moreover, the processor may be programmed to automatically modulate the energy emission via the main antenna to reduce at least one of the target tissue temperature or a rate of increase of the target tissue temperature if the pop is predicted. In addition, the system further may include a display operatively coupled to the processor, such that the processor causes the display to display the pop within the target tissue temperature.

In accordance with another aspect of the present invention, an alternative system for ablating target tissue within a patient is provided. The system may include a catheter having a proximal region and a distal region, and a main antenna having a monopole. The main antenna may be disposed at the distal region of the catheter and may emit energy to ablate the target tissue and measure a radiometer temperature generated as a result of the energy emission. In addition, the system may include a reference termination disposed at the distal region of the catheter, such that the reference termination may measure a reference temperature at the distal region. Moreover, the system may include a processor operatively coupled to the main antenna and the reference termination, the processor configured to: cause the main catheter to measure radiometer temperature and the reference termination to measure reference temperature in an alternating manner via a switch electrically coupled to the main antenna and the reference termination; and calculate a target tissue temperature based on the measured radiometer temperature and the measured reference temperature.

The monopole may include a proximal radiating element and a distal radiating element, such that a proximal end of the proximal radiating element has a short designed to defeat a choke action of the proximal radiating element. Accordingly, the switch may be disposed between the proximal radiating element and the distal radiating element. Alternatively, the switch may be disposed within a proximal region of the proximal radiating element, wherein the proximal region is proximal to a junction between the proximal radiating element and the distal radiating element.

The switch may include first and second switching diodes. Moreover, the switch further may include a third switching diode which improves isolation of the reference termination from the radiometer temperature during ablation of the target tissue. Additionally, the switch may include a fourth switching diode which improves isolation of the reference termination from the radiometer temperature during measurement of the reference temperature. The second switching diode and the fourth switching diode may be in series with the main antenna, and separated by a microstrip transmission line. The system further may include a switch module sized and shaped to house the switch. The switch module may include proximal and distal coaxial connectors structured to be removeably coupled to a coaxial cable of the catheter.

In view of the foregoing, it would be desirable to provide systems and methods for treating living tissue that employs a radiometry system, e.g., a microwave radiometry system, for temperature measurement and control. In accordance with one aspect of the invention, systems are provided for radiometrically measuring temperature during microwave ablation, i.e., calculating temperature based on signal(s) from a radiometer. In a microwave ablation system, the antenna determines how the ablation signal power is distributed within the target tissue. This can be quantified as power loss density. In a radiometric sensing system, the antenna works exactly in reverse where power loss density becomes the power source density. The total received power is the sum of all the power sources in the measurement volume. The relative received magnitude of the power sources is the same as the relative dissipation magnitudes of the power losses for the transmitting or ablation case.

Unlike standard thermocouple techniques used in existing commercial ablation systems, a radiometer may provide useful information about tissue temperature at depth-where the tissue ablation occurs-and thus provide feedback to the clinician about the extent of tissue damage as the clinician ablates a selected region of the target tissue. Specifically, the present disclosure overcomes the drawbacks of previously-known systems by providing improved systems and methods for microwave ablation of target tissue, and measuring temperature of the target tissue during ablation. Moreover, the present disclosure provides improved systems and methods for calibrating the ablation system to account for effects of energy emission on the reference termination and the environment adjacent the antenna, estimating ablation lesion volume, and detecting and/or predicting a pop condition indicative of undesirable heating and/or movement of the ablation system, thereby improving safety and efficacy of the system. The novel inventions described herein may have broad application to catheter/probe-based therapies, including but not limited to targets in the vascular system and soft tissue targets in liver, kidney, prostate and lung. For example, the principles of the present invention described herein may be incorporated into known ablation systems such as NeuWave™ Microwave Ablation System (available by Ethicon, part of Johnson & Johnson, Bridgewater, New Jersey and Cincinnati, Ohio).

Microwave heating to target tissue and microwave radiometry as a means of monitoring the temperature of the heated tissue ensures that the desired temperatures are delivered to adequately treat the target tissue and achieve therapeutic goals, and are described in <CIT>, previously cited. Specifically, heating and temperature sensing is accomplished with a catheter using a single antenna which is shared for both functions. The microwave heating may be directed toward the target tissue. A radiometer, operating at the same frequency and time sharing the antenna with the microwave generator, senses the microwave emissions from the region surrounding the antenna and converts these to tissue temperature. In this case, the volume of tissue being monitored includes, e.g., tumorous lung tissue. An algorithm relates the temperature at the target region to the volume temperature reading.

However, there are obstacles to achieving accurate temperature measurement using radiometry with microwave heating. These result from the dissipative losses in the relatively long coaxial cable between the radiometer and the antenna. The usual approach uses a Dicke radiometer which compares the unknown temperature of the target tissue being heated to an internal reference of known temperature in the radiometer. The radiometer output voltage is: <MAT>.

Where Slope is the volts per degree sensitivity and Offset is the sum of all the fixed errors. These constants are determined by calibration using a hot and cold input termination.

<FIG> illustrates a simplified block diagram of such a system having a Dicke radiometer. As shown in <FIG>, an input switch, e.g., Dicke switch <NUM>, is used that selects either antenna input <NUM> or an internal reference input, e.g., reference temperature termination <NUM>. The approach is popular because everything in the measurement path behind Dicke switch <NUM> is common to both the target measurement from antenna input <NUM> and the reference measurement from reference temperature termination <NUM>, and most of the possible measurement errors drop out of the calculations.

The problem with antenna catheters is the dissipative loss in the coaxial cable running the length of the catheter. The emissions resulting from the cable losses are indistinguishable from the emissions received by the antenna. The radiometer measures the antenna temperature combined with the cable temperature. The problem is aggravated by the desire for small diameter catheters requiring high loss, small diameter coaxial cables, and heating of the coaxial cable caused by dissipation of some of the generator power.

A solution is disclosed in the block diagram of <FIG>. As illustrated in <FIG>, Dicke switch <NUM> and reference termination <NUM> have been moved out to the end of the coaxial cable, e.g., short flexible cable <NUM> at the distal end of the main catheter cable, near the connection to antenna <NUM>. Now the coaxial cable is part of both the target measurement from antenna <NUM> and the reference measurement from reference termination <NUM>, and heat dissipating therefrom drops out of the temperature calculation. However, the scheme is subject to some error resulting from heating of the reference due to its proximity to the heating cable.

To overcome the drawbacks of previously-known radiometry systems, the present invention integrates the Dicke switch radiometer function integrated into the antenna. For example, referring now to <FIG>, a block diagram illustrating microwave heating and temperature sensing system <NUM> constructed in accordance with the principles of the present invention is provided. As shown in <FIG>, generator <NUM> supplies ablative energy to switching antenna <NUM> through Transmit/Receive (T/R) switch <NUM> followed by antenna switch bias diplexer <NUM>. Generator <NUM> may be any previously-known commercially available ablation energy generator, e.g., a microwave energy generator, thereby enabling radiometric techniques to be employed with reduced capital outlay.

Further, radiometer <NUM> receives temperature measurements from switching antenna <NUM> via cable <NUM>, e.g., coaxial cable. Switching antenna <NUM> includes a main antenna having one or more microwave radiating elements for emitting microwave energy and for measuring temperature of tissue adjacent the main antenna, and a reference termination for measuring a reference temperature. In addition, switching antenna <NUM> includes a switching network, e.g., a Dicke switch, integrated therein for detecting the volumetric temperature of tissue subjected to ablation. The switching network selects between the signals indicative of measured radiometer temperature from the main antenna of switching antenna <NUM>, e.g., the temperature of the tissue adjacent the main antenna during the ablation procedure, and signals indicative of the measured reference temperature from the reference termination of switching antenna <NUM>. As the switching network is integrated within switching antenna <NUM>, and sufficiently far from the connection point of cable <NUM> and switching antenna <NUM>, heating of the reference termination by cable <NUM> is avoided.

Switch <NUM> and antenna switch bias diplexer <NUM> may be disposed within handle <NUM>, along with radiometer <NUM> for receiving temperature measurements from switching antenna <NUM> depending the state of switch <NUM>. For example, switch <NUM> may be in an ablation state such that microwave power may be transmitted from generator <NUM> to switching antenna <NUM>, or switch <NUM> may be in a measurement state such that radiometer <NUM> may receive temperature measurement from switching antenna <NUM>, e.g., from the main antenna and/or the reference termination. Accordingly, switch bias diplexer <NUM> may be in a main antenna state such that radiometer <NUM> may receive temperature measurement from the main antenna, or switch bias diplexer <NUM> may be in a reference termination state such that radiometer <NUM> may receive temperature measurement from the reference termination. Handle <NUM> may be reusable, while cable <NUM> and switching antenna <NUM> may be disposable.

System <NUM> further includes controller <NUM> coupled to generator <NUM> and switching antenna <NUM> via, e.g., handle <NUM> and cable <NUM>, to coordinate signals therebetween. Controller <NUM> thereby provides generator <NUM> with the information required for operation, transmits ablative energy to switching antenna <NUM> under the control of the clinician, and may display via a temperature display the temperature at depth of tissue as it is being ablated, for use by the clinician. The displayed temperature may be calculated based on signal(s) measured by switching antenna <NUM> using computer algorithms. Thus, controller <NUM> includes a processor having memory for storing instructions to be executed by controller <NUM>. The processor may comprise one or more commercially available microcontroller units that may include a programmable microprocessor, volatile memory, nonvolatile memory such as EEPROM for storing programming, and nonvolatile storage, e.g., Flash memory, for storing firmware. The memory of the processor stores program instructions that, when executed by the processor, cause the processor and the functional components of system <NUM> to provide the functionality ascribed to them herein. The processor is configured to be programmable such that programming data is stored in the memory of the processor or accessible via a network. As will be readily understood to one skilled in the art, while <FIG> is illustrated to show one controller, the processor may include multiple processors utilized in a single location/housing or multiple locations/housings. Further, the reusable equipment in <FIG> may be housed in a common housing or separate housings.

The processor may direct switch <NUM> to move between the ablation state and the measurement state as described above. For example, the processor may cause the main antenna of switching antenna <NUM> to emit microwave energy when switch <NUM> is in the ablation state, and may cause the radiometer <NUM> to receive signals indicative of temperature measurement from switching antenna <NUM>, e.g., from the main antenna and/or the reference termination, when switch <NUM> is in the measurement state. In addition, the processor may direct switch bias diplexer <NUM> to move between the main antenna state and the reference termination state as described above. For example, the processor may receive signals indicative of measured radiometer temperature from the main antenna of switching antenna <NUM>, e.g., the temperature of the tissue adjacent switching antenna <NUM> during the ablation procedure, when switch bias diplexer <NUM> is in the main antenna state, and signals indicative of the measured reference temperature from the reference termination of switching antenna <NUM> when switch bias diplexer <NUM> is in the reference termination state. Accordingly, the processor can calculate the volumetric temperature of the tissue subject to ablation based on the signals. Moreover, the processor may modulate the level of energy emitted via main antenna <NUM> based on the calculated volumetric temperature of the tissue subject to ablation continuously as part of a feedback loop to ensure that the temperature of the target tissue is maintained within a predetermined threshold.

In accordance with one aspect of the present invention, the processor directs switch <NUM> to be positioned in the ablation state for a majority of an ablation period, e.g., more than <NUM>%, more than <NUM>%, more than <NUM>%, or preferably more than <NUM>%, to maximize the power dissipated. Accordingly, the processor may direct switch <NUM> to be positioned in the measurement state for the remainder of the ablation period, e.g., less than <NUM>%, less than <NUM>%, less than <NUM>%, or preferably less than <NUM>%, respectively. Moreover, during the ablation period when switch <NUM> is in the measurement state, the processor may direct switch bias diplexer <NUM> to alternate between being positioned in the main antenna state and the reference termination state.

For example, in a one second cycle, the processor may direct switch <NUM> to be positioned in the ablation state for <NUM> milliseconds such that the main antenna emits microwave energy to the target tissue for <NUM> milliseconds, and then direct switch <NUM> to be positioned in the measurement state for <NUM> milliseconds. During the <NUM> milliseconds that switch <NUM> is in the measurement state, the processor may direct switch bias diplexer <NUM> to alternate between the main antenna state and the reference termination state every, e.g., <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> milliseconds. As will be understood by a person having ordinary skill in the art, the processor may direct switch <NUM> to be positioned in the ablation state for more or less than <NUM> milliseconds, and the processor may direct switch bias diplexer <NUM> to alternate every time period that include any time less than <NUM> millisecond or more than <NUM> milliseconds. Moreover, at least one of the switching components, e.g., switch <NUM> and switch bias diplexer <NUM>, may be integrated in switching antenna <NUM> as described in further detail below.

The microwave power propagates from generator <NUM> down cable <NUM> in the catheter to switching antenna <NUM> at the catheter tip. The microwave power radiates outward from the main antenna of switching antenna <NUM> into the target tissue (e.g., target lung tissue such as a tumor). In other examples, such as where the ablation system is used for denervation, an introducer device may be used to deliver the catheter within the body lumen, and a spacer device may be used to ensure that switching antenna <NUM> is deployed in the approximate center of the body lumen. The volume of blood flowing through the body lumen at body temperature may cool the surface of the body lumen in immediate contact with the blood. In addition to, or alternatively, coolant from outside the body, introduced through a coolant lumen of the catheter may be used to cool the surface of the surface of the body lumen. Tissue beyond the lumen wall, that does not experience this cooling, heats up. Sufficient microwave power is supplied to heat the target tissue (e.g., nerve area) to a temperature that destroys the target tissue.

A computer simulation of the temperature field created by microwave heating is shown in <FIG> illustrates a cut through the switching antenna and surrounding tissue. The effect is symmetrical around the antenna so just one half of the cut plane is shown. The temperature along a radial line through the peak temperature shows the temperature within the target tissue. The temperature rises inside the tissue near the tissue surface and reaches a maximum at a depth near the target tissue. <FIG> also illustrates the microwave power loss density pattern perceived by the switching antenna. Since the switching antenna and frequency are common to both the generator and radiometer, the patterns produced for both functions are coincident and the radiometer optimally monitors the heated region.

Referring now to <FIG>, switching antenna <NUM> of microwave ablation system <NUM> is provided. Switching antenna <NUM> includes main antenna <NUM> that is used for both microwave heating and temperature sensing, and reference termination <NUM> for measuring a reference temperature, e.g., temperature adjacent switching antenna <NUM>. For example, main antenna <NUM> of switching antenna <NUM> includes one or more microwave radiating elements, e.g., first microwave radiating element 44a and second microwave radiating element 44b, that are designed to receive power from generator <NUM> via cable <NUM>, and to emit microwave energy into the surrounding target tissue at a level sufficient to ablate the target tissue.

Main antenna <NUM> of switching antenna <NUM> further includes means for detecting microwave emissions from the region surrounding the antenna, e.g., one or more circuits formed by microwave radiating elements 44a, 44b, and converts these to temperature of the tissue adjacent switching antenna <NUM>, i.e., radiometer temperature. Switching antenna <NUM> further includes reference termination <NUM> for measuring a reference temperature. In addition, switching antenna <NUM> integrates switching network <NUM>, e.g., a Dicke switch, disposed between the dipole halves of microwave radiating elements 44a, 44b of main antenna <NUM> of switching antenna <NUM>. As described in detail above, the processor may direct switching network <NUM> to alternate between permitting microwave energy emission via main antenna <NUM> and permitting temperature measurement via main antenna <NUM> or reference termination <NUM>.

The volume temperature output will be the difference between the radiometer temperature, e.g., the temperature of the tissue heated surrounding main antenna <NUM>, and the reference temperature measured by reference termination <NUM>. The volume temperature output may be calculated based on signals indicative of the measured radiometer temperature from microwave radiating elements 44a, 44b of main antenna <NUM> and the signals indicative of the measured reference temperature from reference termination <NUM> using algorithms, such as those described in <CIT> and<CIT>.

Specifically, all of the switching components, e.g., switching diodes 46a, 46b, and reference termination <NUM> are located at the junction of the two antenna dipole halves. The junction between the two antenna dipole halves may have a length of, e.g., no more than <NUM>, and preferably no more than <NUM>. Accordingly, the integrated antenna/switch configuration of microwave ablation system <NUM> is physically shorter and more flexible. Switching diodes 46a, 46b are actuated by biasing switching diodes 46a, 46b ON or OFF, and are switched to the same state in unison. Accordingly, only a single bias source is required and may be operatively coupled to switching diodes 46a, 46b via conductors of cable <NUM>. Switching diodes 46a, 46b may be, e.g., microwave PIN diodes, and are biased with a small forward current in the ON state or back biased with a negative voltage in the OFF state.

In addition, microwave choke arrangement <NUM> is provided to minimize fold back of the radiating pattern of microwave energy from microwave radiating elements 44a, 44b onto the coaxial catheter shaft. The choke is formed by connecting the proximal dipole half, e.g., microwave radiating element 44a, to cable <NUM> at the feed point of main antenna <NUM>. A coaxial structure is formed between microwave radiating element 44a and cable <NUM> which results in the open circuit choke between main antenna <NUM> and cable <NUM>.

Input from main antenna <NUM> or from reference termination <NUM> is selected by reversing the polarity of the bias current applied to center conductor <NUM> of cable <NUM>. The series-connected switching diodes 46a, 46b are either a small resistance that passes the microwave signal or a small capacitance blocking the signal depending on the bias polarity. Resistors, e.g., bias components <NUM>, return the bias current through outer conductor <NUM> of cable <NUM>. A bias current diplexer supplies the bias to the proximal end of the catheter outside the body.

The chip level switching components (diodes, resistors and capacitor) are very small and reside on a ceramic card in the short space between the dipole halves of microwave radiating elements 44a, 44b. Cable <NUM> and the antenna structures are formed of flexible materials that may navigate through tight passages. The only rigid section may be switching network <NUM>, which is no longer than about <NUM>.

System <NUM> is suitable for applications such as ablation of lung tissue where reference termination <NUM> must establish a reference temperature. For this reason, reference termination <NUM> is located on the proximal side of the antenna structure so that a temperature sensor does not have to cross the feed point of main antenna <NUM> which may disrupt the antenna radiating pattern. A thermocouple circuit formed by outer conductor <NUM> and a very thin dissimilar metal wire terminating near the reference resistor of reference termination <NUM> may be used for this purpose.

As illustrated in <FIG>, microwave radiating elements 44a, 44b are a basic dipole that receives power from generator <NUM> via cable <NUM>. As shown in <FIG>, microwave radiating elements 44a, 44b may have a cylindrical shape. As will be understood by a person having ordinary skill in the art, microwave radiating elements 44a, 44b may have other shapes including a spiral winding. Within each of microwave radiating elements 44a, 44b is a balun transformer. The balun transformer transforms a single ended transmission line system to a balance system as shown in <FIG>, which illustrates balun transformer 54a.

Referring now to <FIG>, alternative exemplary microwave ablation system <NUM> is provided. Microwave ablation system <NUM> is constructed similarly to microwave ablation system <NUM> of <FIG> wherein like components are identified by like-primed reference numbers. For example, cable <NUM>' corresponds with cable <NUM>, switching antenna <NUM>' corresponds with switching antenna <NUM>, main antenna <NUM>' corresponds with main antenna <NUM>, microwave radiating elements 44a', 44b' correspond with microwave radiating elements 44a, 44b, switching diodes 46a', 46b' correspond with switching diodes 46a, 46b, and reference termination <NUM>' corresponds with reference termination <NUM>. As shown in <FIG>, within each of microwave radiating elements 44a, 44b is balun transformer 54a, 54b, respectively.

Microwave ablation system <NUM> differs from microwave ablation system <NUM> in that reference termination <NUM>' is disposed distal to second microwave radiating element 44b'. Specifically, switching antenna <NUM>' integrates a switching network, e.g., a Dicke switch including switching diodes 46a', 46b', into main antenna <NUM>' which allows reference termination <NUM>' to protrude out from the distal end of main antenna <NUM>'. Accordingly, system <NUM> may be used in applications such as renal denervation where reference termination <NUM>' may be maintained at body temperature by blood flow.

The structure of main antenna <NUM>' is unique in that it integrates a radiometer Dicke switch function into a flexible remote antenna and provides for radiometer reference termination <NUM>' to protrude from main antenna <NUM>' into a stable temperature region, e.g., path of blood flow. The volume temperature output will be the difference between the radiometer temperature, e.g., the temperature of the tissue heated surrounding main antenna <NUM>' and the reference temperature, e.g., known stable body temperature provided by blood flow over reference termination <NUM>', e.g., in the renal artery. The volume temperature output may be calculated based on signals indicative of the measured radiometer temperature from microwave radiating elements 44a', 44b' of main antenna <NUM>' and the signals indicative of the measured reference temperature from reference termination <NUM>' using algorithms, such as those described in <CIT> and <CIT>.

As illustrated in <FIG>, microwave radiating elements 44a', 44b' include two back to back balun transformers 54a, 54b. As shown in <FIG>, two switching diodes, e.g., switching diodes 46a', 46b', are integrated within microwave radiating elements 44a', 44b' of main antenna <NUM>'. Switching diode 46a' is positioned between balun transformers 54a, 54b, and switching diode 46b' is positioned distal to balun transformer 54b, e.g., between balun transformer 54b and reference termination <NUM>' (not shown). When switching diodes 46a', 46b' are closed, the single ended input is transformed to the balanced output that connects to microwave radiating elements 44a', 44b'. Balun transformer 54a is shorted at the distal end of main antenna <NUM>', and therefore, transforms to an open circuit at the balanced output. When switching diodes 46a', 46b' are open as shown in <FIG>, the transformation is not made and the structure becomes a straight through transmission line path to the distal end of main antenna <NUM>' where the reference termination, e.g., reference termination <NUM>', is located as illustrated in <FIG>.

<FIG> illustrates switching antenna <NUM>' having back to back balun transformers 54a, 54b, with switching diodes 46a', 46b' integrated therein, and reference termination <NUM>' having bias blocking capacitor <NUM> and reference termination resistor <NUM>. As further shown in <FIG>, connection 62a connects to microwave radiating element 44a', and connection 62b connects to microwave radiating element 44b'. Switching diodes 46a', 46b' are actuated by biasing switching diodes 46a', 46b' ON or OFF, and are switched to the same state in unison. Accordingly, only a single bias source is required and may be operatively coupled to switching diodes 46a', 46b' via conductors of cable <NUM>.

Switching diodes 46a', 46b' may be, e.g., microwave PIN diodes, and are biased with a small forward current in the ON state or back biased with a negative voltage in the OFF state. Bias blocking capacitor <NUM> prevents bias current from dissipating in reference termination resistor <NUM> of reference termination <NUM>'. Reference termination resistor <NUM> may be located any distance from balun transformers 54a, 54b of microwave radiating elements 44a', 44b' to minimize heating of reference termination <NUM>' as long as the connecting transmission line is of the same characteristic impedance as the resistor value of reference termination resistor <NUM>.

Referring now to <FIG>, antenna power loss density patterns for both switch positions of switching diodes 46a', 46b', e.g., ON and OFF, is provided. For example, <FIG> illustrates power dissipation in the tissue during operation of switching antenna <NUM>' when switching diodes 46a', 46b' are biased ON. As shown in <FIG>, a volume of tissue at a predetermined depth within the target tissue, e.g., where the target tissue to be ablated is located, is heated to the desired temperature sufficient for ablation. <FIG> illustrates power dissipation in the tissue when switching diodes 46a', 46b' are biased OFF, and thus no dissipation is shown indicating that switching antenna <NUM>' is detecting only reference termination <NUM>'.

To overcome the challenge of constructing the balun structure and mounting the switching diodes in a flexible, small diameter catheter, a three conductor transmission line structure is used to form balun transformers 54a, 54b as shown in <FIG>. As illustrated in <FIG>, thin, flexible dielectric substrate <NUM> includes center conductor <NUM> printed on the top surface of substrate <NUM>, and two split ground conductors 68a, 68b printed on the bottom surface of substrate <NUM>. Substrate <NUM> may be, for example, at most <NUM>" thick, and preferably up to <NUM> inches thick. In addition, the dielectric constant of substrate <NUM> is relatively high, e.g., on the order of at least <NUM>. Transmission line impedance is a function of widths of the conductors and the size of the gap between split ground conductors 68a, 68b.

Switching antenna <NUM>' may need to flex during delivery to the target tissue site, e.g., to make the turn from the femoral artery into the renal artery. To keep the geometry of switching antenna <NUM>' small, unpackaged diodes are used and are encapsulated to prevent damage as main antenna <NUM>' flexes. For example, <FIG> illustrates diode chip <NUM> and ribbon connection <NUM> positioned on top side circuit trace <NUM>, and encapsulant <NUM>. In addition, <FIG> illustrates connection 62a which connects to microwave radiating element 44a', and connection 62b which connects to microwave radiating element 44b'.

In an embodiment where main antenna <NUM>' is stiff in one plane of the substrate, main antenna <NUM>' has flexibility in at least one plane such that it may navigate, e.g., the bends in the arteries of the patient. For example, main antenna <NUM>' may be relatively stiff in the plane of substrate <NUM> but may curl in the plane perpendicular to substrate <NUM>. This is judged to be adequate flexibility requiring only that the catheter be twisted to orient it with the direction of the required bend. Thus, the structure of main antenna <NUM>' allows main antenna <NUM>' to be flexible in at least one plane, and preferably in both planes. A foam dielectric may be used to fill the regions above and below substrate <NUM> under microwave radiating elements 44a', 44b'. A braided metal shield layer may also be used to cover balun transformers 54a, 54b under microwave radiating elements 44a', 44b'.

Referring now to <FIG>, an exemplary ablation system having a coolant sleeve disposed thereon is provided. As shown in <FIG>, coolant sleeve <NUM> may be disposed over cable <NUM> and switching antenna <NUM>. Coolant sleeve <NUM> may include inner tube <NUM> having passageway <NUM> sized and shaped to surround cable <NUM> and switching antenna <NUM> and to permit a coolant to flow therethrough. Inner tube <NUM> may be coaxial with cable <NUM> and switching antenna <NUM>. In addition, coolant sleeve <NUM> may include outer tube <NUM> having passageway <NUM> in fluid communication with passageway <NUM> of inner tube <NUM> via junction cavity <NUM>, such that the coolant may flow through passageway <NUM>, junction cavity <NUM>, and out through passageway <NUM> in the direction of the arrows illustrated in <FIG>. As shown in <FIG>, outer tube <NUM> may also be coaxial with cable <NUM> and switching antenna <NUM>. Accordingly, the proximal end of coolant sleeve <NUM> may be fluidly coupled to a source of coolant. As the coolant flows over switching antenna <NUM>, the coolant cools the surface of switching antenna <NUM> and prevents switching antenna <NUM> from heating up beyond a predetermined amount. Coolant sleeve <NUM> may allow for closed-loop cooling such that the coolant is maintained within coolant sleeve <NUM> and is not expelled into the body of the patient. As described in further detail below, coolant sleeve <NUM> further may be used to prevent reference termination <NUM> from heating up beyond a predetermined amount during pre-ablation calibration. The coolant further may be used to cool the surface of the tissue being ablated, thereby allowing for energy to be deposited deeper into the target tissue. Accordingly, peak temperatures are achieved at depth in the tissue rather than at the surface as shown in <FIG>.

Referring now to <FIG>, exemplary method <NUM> for ablating target tissue in accordance with the principles of the present disclosure is provided. During pre-ablation, at step <NUM>, the processor of the system may perform a reference termination calibration to account for the effect of microwave energy emission on the reference termination. For example, there is a temperature offset between main antenna <NUM>, e.g., the reference temperature sensor (thermocouple) on the outside of switching antenna <NUM>, and reference termination <NUM>, e.g., the microwave reference termination within switching antenna <NUM>. The offset is a function of thermal resistance between main antenna <NUM> and reference termination <NUM>. Heating of reference termination <NUM> is caused by dissipation of a small amount of the applied microwave ablation power in main antenna <NUM>. Calibration of reference termination <NUM> involves using radiometer <NUM> to measure the temperature rise of reference termination <NUM> while holding the tissue adjacent switching antenna <NUM> being measured at a constant temperature.

<FIG> illustrates exemplary method step <NUM> for performing a reference termination calibration. At step <NUM>, switching antenna <NUM> is positioned in a constant temperature bath, which acts as the dissipating tissue. In effect, the normal temperature measurement is conducted backwards where the known reference is the water bath seen by main antenna <NUM> and the unknown reference is reference termination <NUM>. At step <NUM>, a high, circulating fluid flow may be provided in the bath such that the environment around switching antenna <NUM> does not heat as the high fluid flow removes all heat generated by the microwave energy emitted from main antenna <NUM>. At step <NUM>, varying levels of microwave energy are emitted via main antenna <NUM>, which causes reference termination <NUM> to heat slightly due to the microwave energy going through the cable/circuit, and voltage resulting from energy emission generated by reference termination <NUM> is measured for each of the varying levels of energy emission. As described above, the environment around switching antenna <NUM> does not heat as the varying levels of microwave energy are emitted due to the high flow bath across main antenna <NUM>. At step <NUM>, the measured voltages are compared with the varying levels of energy emission to account for the effect of energy emission on reference termination <NUM> during energy emission via main antenna <NUM>. Specifically, comparison of the temperature of reference termination <NUM> relative to the external temperature sensor reveals a linear relationship with applied microwave power whose slope is the thermal resistance. This thermal resistance constant is multiplied by the applied power level to find the temperature of reference termination <NUM> during ablations.

Referring again to <FIG>, during pre-ablation, at step <NUM>, the processor of the system may perform a radiometer calibration to account for the effect of microwave energy emission on the environment adjacent the target tissue during energy emission via main antenna <NUM>. Radiometer calibration provides the ability to determine the sensed microwave energy by radiometer <NUM> versus the temperature of the environment adjacent the target tissue during energy emission.

Referring now to <FIG>, exemplary method <NUM> for performing a reference termination calibration is provided. At step <NUM>, switching antenna <NUM> is positioned in a constant temperature bath. At step <NUM>, using a known microwave noise source that is already calibrated to temperature, main antenna <NUM> is impinged with a first noise level to create a first known temperature, and at step <NUM>, the first temperature is measured. At step <NUM>, using the known microwave noise source that is already calibrated to temperature, main antenna <NUM> is impinged with a second noise level different from the first noise level to create a second known temperature, and at step <NUM>, the second temperature is measured. Accordingly, during this radiometer calibration, reference termination <NUM> need not be cooled. At step <NUM>, the first and second measured temperatures are compared with the first and second noise levels to calibrate out the effect of energy emission via main antenna <NUM> on the environment adjacent switching antenna <NUM>. Moreover, first and second output voltages resulting from energy emission generated by reference termination <NUM> may be recorded while measuring the first and second temperatures such that the temperature difference between the first and second temperatures divided by the voltage difference provides the degrees per volt sensitivity of radiometer <NUM>.

Referring now to <FIG>, alternative exemplary method <NUM>' for performing a reference termination calibration is provided. At step <NUM>, switching antenna <NUM> is positioned in a first bath having a first known temperature. At step <NUM>, a radiometer signal is applied to main antenna <NUM>, and at step <NUM>, a first output voltage resulting from energy emission generated by reference termination <NUM> responsive to the application of the radiometer signal is measured. At step <NUM>, switching antenna <NUM> is positioned in a second bath having a second known temperature different from the first temperature. At step <NUM>, the radiometer signal is again applied to main antenna <NUM>, and at step <NUM>, a first output voltage resulting from energy emission generated by reference termination <NUM> responsive to the application of the radiometer signal is measured. As described above, a coolant may be permitted to flow across switching antenna <NUM>, thereby cooling switching antenna <NUM>. Accordingly, the temperature of reference termination <NUM> does not change when placed in the two different baths having different temperatures, and the only rise in temperature is that of the unknown environment adjacent switching antenna <NUM>. At step <NUM>, the first and second measure output voltages are compared with the first and second known temperatures to calibrate out the effect of energy emission via main antenna <NUM> on the environment adjacent switching antenna <NUM>. As will be understood by a person having ordinary skill in the art, a user may use radiometer calibration method <NUM> or <NUM>', and further may perform reference termination calibration step <NUM> and radiometer calibration method <NUM>, <NUM>' in any preferred order.

Referring again to <FIG>, at step <NUM>, switching antenna <NUM> is positioned adjacent target tissue, e.g., lung tissue. At step <NUM>, as described above, the process switches between permitting main antenna <NUM> to emit microwave energy and permitting main antenna <NUM> to measure radiometer temperature generated as a result of energy emission by main antenna <NUM> via the switching network of switching antenna <NUM> in an interleaving manner. At step <NUM>, the processor switches between permitting main antenna <NUM> to measure radiometer temperature and permitting reference termination <NUM> to measure the reference temperature. For example, as described above, in one ablation cycle (which may be repeated as desired) main antenna <NUM> may emit microwave energy for over <NUM>% of the ablation cycle to maximize power dissipation, and main antenna <NUM> and reference termination <NUM> may alternate and measure radiometer temperature and reference temperature, respectively, for the remainder of the ablation period of the ablation cycle. At step <NUM>, the processor may calculate target tissue temperature based on the measure radiometer temperature and the measured reference temperature using the calibrated values described above to account for the effect of energy emission on reference termination <NUM> and for the effect of microwave energy emission on the environment adjacent the target tissue during energy emission via main antenna <NUM>. For example, <FIG> is a chart illustrating the measured target tissue temperature during a period of microwave ablation.

Referring again to <FIG>, at step <NUM>, the processor may estimate the volume of the ablation lesion resulting from the microwave energy emission via main antenna <NUM> during the ablation procedure based on the target tissue temperature. Specifically, the volume of the ablation lesion created by energy emission during an ablation procedure may be estimated based on at least one of an average target tissue temperature or an area under a plotted curve of the target tissue temperature. For example, <FIG> illustrates a graph plotting average target tissue temperature versus the estimated ablation lesion volume, and <FIG> illustrates a graph plotting the radiometer area under a plotted curve of target tissue temperature versus the estimated ablation lesion volume. Moreover, the estimated ablation lesion volume may be used to permit titration of the energy emission to achieved desired therapeutic goals.

Referring now to <FIG>, a pop condition, e.g., a steam pop, may be detected and/or predicted via algorithms programmed into the processor described above. As shown in <FIG>, pop condition <NUM> is indicative of a rapid target tissue temperature rise followed by a sudden target tissue temperature drop. The sudden drop may be indicative of the switching antenna being moved out of location, and therefore no longer heating the target tissue. Accordingly, as the processor monitors the target tissue temperature in real time, the processor may be able to detect when the target tissue temperature is raising too quickly, or otherwise in a manner outside a predetermined threshold, and predict that a pop condition will be observed. When pop condition <NUM> is detected or predicted to occur, the processor may automatically cut off heating and/or generate an alert to alert the user that there is an issue.

Additionally or alternatively, the processor may be programmed to automatically modulate the energy emission via main antenna <NUM> in response to detection or prediction of a pop condition to thereby prevent over heating of the target tissue and/or other issues. For example, the energy emission via the main antenna may be modulated to reduce at least one of the target tissue temperature or a rate of increase of the target tissue temperature if the pop is predicted. Detecting and prediction of pop conditions improves the safety and efficacy of the ablation systems described herein. Moreover, the processor may be coupled to a display for displaying the monitoring of the target tissue temperature such that a user may visualize the pop condition within the target tissue temperature. In addition, the temperature may be controlled to a set temperature point by modulating the power to achieve a constant temperature.

Clinical testing results discussed below confirm efficacy of the microwave heating and measurement systems described herein. For example, <FIG> illustrates data indicative of microwave ablation-induced thermal lesions in homogenous tissue.

<FIG> illustrate results of heat sink testing using an ablation system in accordance with the principles of the present invention. For example, a glass tube was positioned in the field of the ablation to draw away heat from the zone of heating. This would mimic a blood vessel. As shown in <FIG>, a lower radiometer area under the curve ("Rad AUC") is achieved when the flow in the tube (or heat sink) is on compared to when it is off. This correlates to a smaller lesion size. Accordingly, the volume of the lesion may be determined even when there is a heat sink, e.g., a blood vessel, pulling heat away from the ablation zone. Known methods only permit the user to control power and set the level of power and time, which does not allow the user to determine if the heat is effectively heating and destroying tissue. For example, the user cannot know if there are ten vessels pulling heat away (and making smaller lesions) or zero vessels pulling heat away. In accordance with the principles of the present invention, the user may more precisely predict lesion size in the presence of heat sinks such as blood vessels.

<FIG> illustrates results of heat sink testing on cow liver using an ablation procedure conducted in accordance with the principles of the present invention. Specifically, a more extreme example of a heat sink was simulated. The antenna was positioned just below the surface of the tissue and the tissue was positioned in a water bath. Accordingly, on one side, the antenna was seeing all tissue, and on the other side, the antenna was seeing less tissue and mostly water/saline. In addition, flow in the water was created to pull away the heat in the water as an extreme way of heat sinking. Here again, the radiometer could detect when there was a lot of heat sink with the antenna near the surface and the water pulling heat away (low AUC) and when there was no heat sink (fully embedded in tissue), and thus a higher heating and higher AUC.

<FIG> illustrates results of lung ablation testing on cow liver using an ablation procedure conducted in accordance with the principles of the present invention. Compared to liver tissue discussed above, which is very homogenous, liver tissue is non-homogenous, e.g., having air pockets and connective tissue, etc. As shown in <FIG>, microwave ablation systems constructed in accordance with the principles of the present invention may further be used on non-homogenous tissue to predict ablation lesion volume with a strong AUC correlation to lesion volume. Moreover, <FIG> illustrates data indicative of radiometer AUC resulting from lung ablation testing, and <FIG> illustrates data indicative of delivered energy resulting from lung ablation testing. Specifically, <FIG> illustrates regression plots of radiometer AUC versus diameter, length, and volume (from left to right), and <FIG> illustrates regression plots of delivered microwave energy versus diameter, length, and volume (from left to right).

Referring now to <FIG>, a basic dipole of the microwave radiating elements of a switching antenna of an exemplary microwave ablation system is provided. Specifically, <FIG> illustrates a switching antenna constructed similar to switching antenna <NUM> of <FIG> with the switching network omitted for clarity. As shown in <FIG>, the switching antenna includes microwave radiating elements 44a, 44b forming two dipole halves of the switching antenna. As described above, microwave choke arrangement <NUM> at the proximal end of microwave radiating element 44a minimizes fold back of the radiating field pattern of microwave energy from microwave radiating elements 44a, 44b onto the coaxial catheter shaft. The choke is formed by connecting the proximal dipole half, e.g., microwave radiating element 44a, to cable <NUM> at the feed point of the switching antenna. A coaxial structure is formed between microwave radiating element 44a and cable <NUM> which results in the open circuit choke between the switching antenna and cable <NUM>.

As shown in <FIG>, and according to the present invention, the basic dipole of the switching antenna of <FIG> may be converted into a monopole by shorting the proximal end of microwave radiating element 44a" to defeat the choke action of microwave radiating element 44a". Accordingly, microwave radiating elements 44a" and 44b" may form a monopole. The monopole may have a similar diameter as cable <NUM>" as shown in <FIG>, thereby providing an overall smaller diameter switching antenna. The radiation fold back pattern of the monopole may be tolerated as the smaller diameter device may be required for applications described herein.

Referring now to <FIG>, the switching antenna of <FIG> is illustrated with switching network <NUM>" depicted. As shown in <FIG>, switching network <NUM>" may be positioned at a junction between microwave radiating elements 44a", 44b". As shown in <FIG>, switching network <NUM>" may be constructed similar to switching network <NUM> of <FIG>, except that switching network <NUM>" may further include third switching diode 46c in addition to first switching diode 46a" and second switching diode 46b", as well as an additional bias component <NUM>". Third switching diode 46c may improve isolation of reference termination <NUM>" from the radiometer temperature, e.g., heating of tissue due to ablation, during ablation of the target tissue.

Referring now to <FIG>, another alternative switching network is provided. Switching network <NUM>‴ includes fourth switching diode 46d, in addition to first switching diode 46a‴, second switching diode 46b‴, and third switching diode 46c'. Fourth switching diode 46d may improve isolation of reference termination <NUM>‴ from the radiometer temperature during measurement of the reference temperature. As shown in <FIG>, fourth switching diode 46d and second switching diode 46b‴ may be in series with the main antenna, e.g., microwave radiating element 44b‴, and separated by microstrip transmission line <NUM> on the switching network substrate. Microstrip transmission line <NUM> may improve the isolation achieved by the two switching diodes 46b‴, 46d, which may be especially useful for applications using higher ablation frequencies. As will be understood by a person having ordinary skill in the art, switching network <NUM>‴ may replace switching network <NUM>" in the switching antenna of <FIG>.

Referring now to <FIG>, the switching antenna of <FIG> is illustrated with switching network <NUM>" pushed back from the monopole tip to accommodate a smaller diameter coaxial cable, e.g., cable <NUM>", at or near the main antenna, e.g., microwave radiating element 44b". As shown in <FIG>, switching network <NUM>" may be pushed back into the distal region of cable <NUM>", proximal to the proximal end of microwave radiating element 44b". Alternatively, as shown in <FIG>, switching network <NUM>" may be pushed further back into the distal region of cable <NUM>", e.g., at a point along cable <NUM>" where cable <NUM>" transitions from smaller diameter coaxial cable portion 20a" to larger diameter coaxial cable portion 20b". Specifically, as switching network <NUM>" may be disposed within larger diameter coaxial cable portion 20b" of cable <NUM>", further away from microwave radiating element 44b", cable <NUM>" may include smaller diameter coaxial cable portion 20a" extending between microwave radiating element 44b" and switching network <NUM>".

Switching network <NUM>" may be disposed in switch module <NUM>, which may be structured to be removeably coupled to a coaxial cable of a target device. As shown in <FIG>, switch module <NUM> may be removably coupled to a distal end of larger diameter coaxial cable portion 20b" of cable <NUM>" via proximal connector <NUM> and to a proximal end of smaller diameter coaxial cable portion 20a" of cable <NUM>" via distal connector <NUM>, thereby providing an electrical connection between the generator and microwave radiating element 44b". As will be understood by a person having ordinary skill in the art, though <FIG> depicts cable portion 20b" having a larger diameter than cable portion 20a", cable portions 20a" and 20b" may have the same diameter, such that cable <NUM>" may have a uniform diameter throughout.

Referring now to <FIG>, an exemplary switch module is provided. As shown in <FIG>, switch module <NUM> may include proximal connector 132a which may be electrically coupled with proximal connector <NUM>, and distal connector 132b which may be electrically coupled with distal connector <NUM>. For example, proximal connector 132a may have a lumen sized and shaped to receive a portion of proximal connector <NUM> such that proximal connector 132a and proximal connector <NUM> may be releasably engaged, and distal connector 132b may have a lumen sized and shaped to receive a portion of distal connector <NUM> such that distal connector 132b and distal connector <NUM> may be releasably engaged. Accordingly, switch module <NUM> may easily be integrated with an existing target device.

As shown in <FIG>, switching network <NUM>" may be disposed on substrate <NUM>", which may be disposed within switch module <NUM>. For example, as shown in <FIG>, switch module <NUM> may include conductor <NUM>, e.g., the center conductor of coaxial cable <NUM>", to provide an electrical connection between cable <NUM>" and substrate <NUM>" within switch module <NUM>. Moreover, as shown in <FIG>, switch module <NUM> further may include ledge <NUM> to provide support to substrate <NUM>" within switch module <NUM>.

Referring now to <FIG>, switching network <NUM>" of <FIG> is illustrated on substrate <NUM>". As shown in <FIG>, first switching diode 46a" and second switching diode 46b" may be disposed on a first side of substrate <NUM>" (left photo), and third switching diode 46c may be disposed on an opposite side of substrate <NUM>" (right photo).

Claim 1:
A system for ablating target tissue within a patient, the system comprising:
a catheter (<NUM>, <NUM>', <NUM>") having a proximal region and a distal region;
a main antenna (<NUM>, <NUM>') comprising a monopole, the main antenna disposed at the distal region of the catheter and configured to emit energy to ablate the target tissue and to measure a radiometer temperature generated as a result of the energy emission;
a reference termination (<NUM>, <NUM>', <NUM>") disposed at the distal region of the catheter, the reference termination configured to measure a reference temperature at the distal region; and
a processor (<NUM>) operatively coupled to the main antenna and the reference termination, the processor configured to:
cause the main catheter to measure radiometer temperature and the reference termination to measure reference temperature in an alternating manner via a switch (<NUM>, <NUM>', <NUM>") electrically coupled to the main antenna and the reference termination; and
calculate a target tissue temperature based on the measured radiometer temperature and the measured reference temperature,
wherein the monopole comprises a proximal radiating element (44a, 44a', 44a") and a distal radiating element (44b, 44b', 44b"), a proximal end of the proximal radiating element comprising a short configured to defeat a choke action of the proximal radiating element.