Patent Description:
Ablation therapy is a process by which target tissue of a patient is partially or completely damaged. Ablation may be achieved by a variety of techniques, such as chemical ablation, cryoablation, laser ablation, or radio frequency (RF) ablation. RF ablation, which utilizes RF signals in the frequency range of <NUM> kilohertz (KHz) to <NUM>, can be used to treat various conditions afflicting the human anatomy. RF nerve ablation may be used, for example, to treat pain, such as osteoarthritic pain of the spine. RF ablation therapy reduces pain through the destruction of nerves using RF energy. RF ablation therapy may also be used, for example, to treat cardiac arrhythmias or hypertension.

The amount, or intensity, of the RF energy can be tuned for a particular cannula size, target tissue temperatures, and dwell time. For example, certain anatomical targets generally require a relatively large cannula and relatively large amounts of RF energy to create a relatively large lesion. Conversely, other anatomical targets may require less RF energy and a smaller cannula to limit collateral damage, i.e., to surrounding tissue. RF ablation generally involves the application of RF energy to target tissue by one or more electrodes connected to an RF signal generator, or an ablation generator. When the target tissue is ablated, or at least subjected to ablative energy generated by the RF generator, lesions form in the tissue. The one or more electrodes may be incorporated, for example, onto a catheter that can be navigated to the target tissue or, alternatively, onto respective needles that can each be inserted into corresponding target tissue. The electrodes deliver current to the target tissue to generate an electric field that heats the surrounding tissue and, with enough current, damages the tissue.

RF ablation operates by delivering current to target tissue, and that current must have a return path from the patient to electrical ground either through the signal generator or to ground directly. The ground path may be provided, for example, by another electrode on a catheter. More often the ground path is provided by a cutaneous patch electrode, or "ground pad," which may be applied, for example, to the thigh of the patient. The objective of the ground pad is to provide a good electrical connection between its one or more contacts and the patient. An improperly applied or loose ground pad, or a damaged contact, can result in increased impedance in the ground path at the application site, which can further result in heating at the application site and undesired burns or lesions. Some ground pads incorporate redundant contacts to ensure at least one contact makes a good electrical connection. However, single-contact ground pads are more common given their simplicity and lower cost. Accordingly, it is critical to ascertain whether the ground pad is properly attached with a good electrical connection when performing electric field-based ablation therapy, such as RF ablation.

<CIT> discloses a radiofrequency ablation device for reducing the incidence of skin burns. <CIT> discloses an apparatus and method for measuring contact quality of a negative plate of a high-frequency electrotome device. <CIT> discloses an electrosurgical system for measuring contact quality of a return pad.

Aspects and/or embodiments and/or examples disclosed in the following description, but that are not covered by the appended claims, are considered as not being part of the present invention and are disclosed by way of example only. The present disclosure is directed to systems and methods that provide detection of ground pad placement in time-multiplexed electric field-based ablation systems, such as RF ablation systems.

The present disclosure is further directed to an RF ablation system including a plurality of electrodes, a ground pad, and a signal generator. The electrodes are configured to be positioned at respective tissue sites within a patient's body, and the ground pad is positioned on the patient's body to provide a ground path. The signal generator is coupled to the ground pad and the electrodes via corresponding channels that include a selected channel and a plurality of unselected channels. The signal generator commutates switching circuits for the corresponding channels to close the selected channel and to open the plurality of unselected channels, and measures a first impedance over the selected channel. The signal generator commutates the switching circuits to close the selected channel and the plurality of unselected channels, and then measures a second impedance over the selected channel. The signal generator computes a difference between the first and second impedances, and determines the ground pad has at least a poor electrical connection to the patient's body when the difference exceeds a threshold.

The present disclosure is further directed to a non-therapeutic method of detecting placement of a ground pad for a radio frequency (RF) ablation system. The method includes controlling switching circuits of a signal generator for corresponding channels to close a selected channel and to open a plurality of unselected channels. The method includes measuring a first impedance over the selected channel based on a non-therapeutic RF signal applied through an electrode and a patient's body, and through a ground path established by a ground pad. The method includes controlling the switching circuits to close the selected channel and the plurality of unselected channels. The method includes measuring a second impedance over the selected channel. The method includes computing a difference between the first impedance and the second impedance. The method includes determining the ground pad has at least a poor electrical connection to the patient's body when the difference exceeds a threshold.

The present disclosure is further directed a signal generator for radio frequency (RF) ablation therapy. The signal generator includes a ground terminal, a plurality of channels, a plurality of switching circuits, and a microcontroller. The ground terminal is configured to be coupled to a ground pad for application to a patient's body. The plurality of channels is configured to be coupled to corresponding electrodes for placement in the patient's body to form corresponding RF circuits. The plurality of channels include a selected channel and a plurality of unselected channels. The plurality of switching circuits correspond to the plurality of channels and each is configured to open and close the corresponding RF circuits. The microcontroller is configured to commutate the plurality of switching circuits to close the selected channel and to open the plurality of unselected channels. The microcontroller is configured to compute a first impedance over the selected channel. The microcontroller is configured to commutate the plurality of switching circuits to close the selected channel and the plurality of unselected channels. The microcontroller is configured to compute a second impedance over the selected channel. The microcontroller is configured to compute a difference between the first impedance and the second impedance, and determine the ground pad has at least a poor electrical connection to the patient's body when the difference exceeds a threshold.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

RF ablation systems often utilize multiple electrodes to deliver ablation energy to multiple tissue sites in the anatomy of the patient. The ablation signal generator generally includes multiple channels for generating the respective RF signals to be delivered through the electrodes. The voltage, current, frequency, and duration of the application of the signals, in combination with the electrical load of the tissue itself, dictate the effect of the ablative energy on the target tissue, as well as surrounding tissue. The basic waveform of each RF signal is a high frequency (e.g., <NUM> to <NUM>) sinusoidal voltage (or current) waveform, and that waveform is applied, or pulsed, for a pulse duration, or pulse width. For example, the RF signal may be applied for a duration of <NUM>, with another duration between pulses, referred to as inter-pulse delay. The pulse width and inter-pulse delay are adjustable to treat a specific patient and specific target tissue. Generally, an operating range exists for the combination of RF signal properties that will deliver, at a lower bound, minimum ablative energy to form the desired lesions and, at an upper bound, maximum ablative energy to avoid damaging the wrong tissue. When treating with multiple ablation electrodes simultaneously, these boundaries apply in the aggregate. For example, there is an aggregate current limit for ablative energy through the multiple electrodes at a given instant in time. Moreover, appropriate timing of the RF pulses (e.g., pulse width and interpulse delay) can improve tissue selectivity, reduce musculoskeletal stimulation, and avoid certain side effects of treatment, such as burns and gas buildup.

Electric field-based ablation systems, such as RF ablation systems, require a ground path be established from the patient to allow current to conduct through the patient's tissue with a relatively low-impedance return path. The ground path could be made, for example, directly from the patient to an Earth ground. Alternatively, the ground path can be provided through the ablation signal generator itself. In either case, the ground path is often established using a ground pad attached to the skin of the patient, for example, on the patient's thigh. Because the RF signals can damage tissue at the application site of the ground pad if there is an improperly applied, or loose fitting, ground pad, or a damaged contact within the ground pad, it is important to ascertain whether the ground pad is attached with a good electrical connection before delivering ablative energy to the patient.

A common method of detecting whether a ground pad is properly attached with a good electrical connection is to measure and evaluate the impedance of the RF circuit formed by the electrode and its lead, the patient's body, and the ground pad. Many signal generators can make these measurements and evaluations before beginning ablation therapy. For example, a calibrated low-power signal is transmitted from a channel of the signal generator, the current conducts from the corresponding electrode, through the patient's body, and returns via the ground path. The current and voltage are measured, and the impedance is computed. Generally, if the ground pad is properly attached with a good electrical connection, the impedance of that connection should be very low (e.g., no more than <NUM> ohm). If the electrical connection at the ground pad is open or poor, the impedance at that connection should be very high, like an open circuit (e.g., above <NUM>,<NUM> ohm). The ablation load in the patient's tissue will depend on the location of the ablation electrode. For example, blood is a relatively good conductor, while other tissue is of higher impedance. Consequently, depending on the location of the ablation electrode, the ablation load may range from about <NUM> ohm to several kiloohm. Accordingly, the signal generator can detect the large impedance (e.g., above <NUM>,<NUM> ohm) when the ground pad is improperly applied or has a poor electrical connection.

At least some RF ablation systems utilize time-multiplexing of the numerous signal generator channels, which enables delivery of maximum power through each electrode, i.e., maximum power through an enabled electrode, and ideally no power through disabled electrodes. The energization strategy for such systems then cycles through each channel periodically, enabling application of the RF signal to the enabled channel for a selected pulse width, after which it is disabled and another channel is enabled for the selected pulse width. Pulse width in such an energization strategy may vary for each patient, as will the inter-pulse delay. For example, in one embodiment, the time-multiplexing may apply the RF signal from a first channel to a first electrode for a <NUM> pulse width, and then it is disabled while the signal generator cycles through each other channel for their corresponding pulse widths, which may be the same or different than the <NUM> pulse width for the first channel.

Time-multiplexing of the signal generator channels can be accomplished, for example, by introducing an RF switching circuit in series with each channel's output lead. Each RF switching circuit may include one or more semiconductor switching devices that interrupt the RF signal generated by the signal generator. Such semiconductor switching devices may include a metal-oxide semiconductor field-effect transistors (MOSFET), an insulated gate bipolar transistor (IGBT), or other solid state device. In an "ideal circuit," when an RF switching circuit is open, its impedance would be infinite. In practice, or in "non-ideal circuits," the RF switching circuit, particularly at high frequencies, such as RF, a capacitive leakage path exists through the device when it is open.

The leakage path through opened channels generally disrupts the assessment of the ground pad. More specifically, when measuring the impedance over a single channel, instead of measuring the series impedance of that channel to ground (the ideal circuit), the signal generator drives the calibrated low-power signal over multiple parallel paths to ground through the leakage paths of each open channel in addition to the series impedance of the closed channel. When the ground pad has a good electrical connection, the signal generator detects the intended impedance to ground through the ground pad (e.g., about <NUM> ohm) in parallel with each leakage path through the opened channels, each of which includes an ablation load and the impedance of the leakage path through the switching circuit. The relatively high-impedance leakage paths have little impact on the total impedance detected by the signal generator when the ground pad is properly applied and a good electrical connection to ground exists. However, when the ground pad is improperly applied or the electrical connection to ground is open or poor, the detected impedance will be that of the multiple parallel leakage paths through the opened channels, each of which is in the range of about <NUM> ohm to <NUM>,<NUM> ohm, depending on the type and condition of the target tissue, i.e., the ablation load. Consequently, even when the ground pad is improperly applied, has a poor electrical connection, or open, the impedance to ground is well below the threshold for determining the ground pad is not in place. For example, the detected impedance may be several hundred ohm or even several thousand ohm, while the threshold for detection may be <NUM>,<NUM> ohm.

The disclosed systems and methods provide detection of ground pad placement in time-multiplexed electric field-based ablation systems, such as RF ablation systems. The calibrated low-power signal is transmitted by the signal generator over a selected channel with all other unselected channels open, i.e., the respective switching circuits are open, and the impedance is measured. The calibrated low-power signal is transmitted again with all channels closed, i.e., all of the respective switching circuits are closed, and the impedance is measured again. The two impedance measurements are compared, and if the difference is above a certain threshold (e.g., <NUM> ohm), then the signal generator determines the ground pad is not properly applied, is missing entirely, or at least has a poor electrical connection. For example, when the ground pad is in place, the difference between the impedance measurements may be up to several tens of ohms or possibly up to <NUM> ohms, because the unselected channels are either relatively high-impedance leakage paths in parallel to the primary ground path through the ground pad, which is a low impedance (e.g., at most <NUM> ohm), or low-impedance closed circuits in parallel to the primary ground path through the ground pad. And when the ground pad is improperly applied, open, or at least has a poor electrical connection, the difference between the impedance measurements may be several hundred ohms, because the unselected channels are the only ground path, or are approximately the lowest impedance ground path. Consequently, when the unselected channels are open, there is no lower impedance (e.g., less than <NUM> ohm) ground path than the parallel leakage paths, and the parallel impedance may be equivalent to several hundred ohms or more. And when the unselected channels are closed, they provide multiple parallel low-impedance ground paths, which may be equivalent to several tens of ohms or up to <NUM> ohms. Thus, the difference between the impedance measurements will be on the order of several hundreds of ohms, versus tens of ohms, or up to <NUM> ohms, when the ground pad is in place.

The disclosed systems and methods are generally embodied in an electric field-based ablation system, such as an RF ablation system. <FIG> illustrates an example embodiment of a system <NUM> for RF ablation therapy. Certain embodiments, such as system <NUM>, include an electrode assembly <NUM> disposed at the distal end, for example, of an ablation lead <NUM> and an ablation needle <NUM>. As used herein, "proximal" refers to a direction toward the end of the ablation lead <NUM> near the clinician and "distal" refers to a direction away from the clinician and (generally) inside the body of a patient <NUM>. In alternative embodiments, system <NUM> may include a plurality of needles having one or more electrodes at their respective distal ends (as shown in <FIG>). Electrode assembly <NUM> includes one or more individual, electrically-isolated electrode elements. In some embodiments, each electrode element, also referred to herein as an ablation electrode, is individually wired such that it can be selectively paired or combined with any other electrode element to act as a bipolar or a multi-polar electrode.

Electrode assembly <NUM> includes a plurality of electrodes configured to be used as briefly outlined above and as described in greater detail below. Electrode assembly <NUM> is incorporated as part of ablation lead <NUM> used for RF ablation. System <NUM> introduces a modulated electric field into tissue <NUM> in a body of a patient <NUM>. In the illustrative embodiment, tissue <NUM> comprises nerve tissue for the treatment of pain, e.g., chronic spinal pain. It should be understood, however, that embodiments may be used to diagnose or treat a variety of other body tissues.

System <NUM> enables RF ablation therapy to form lesions on target tissue <NUM>. System <NUM> utilizes electric current in the form of an RF alternating current (AC) signal delivered by multiple electrodes on electrode assembly <NUM>, or respective electrodes on a plurality of ablation needles (as shown in <FIG>, for example). In alternative embodiments, system <NUM> may be used to perform electroporation therapy in which electric current is delivered as a pulsed electric field in the form of short-duration direct current (DC) pulses between closely spaced electrodes on electrode assembly <NUM>. Pulse widths of these DC signals may be on the order of one nanosecond to several milliseconds, and the DC pulses may be repeated to form a pulse train. When a strong electric field is applied to tissue in vivo, the cells in the tissue are subjected to a trans-membrane potential that opens the pores on the cell wall, hence the term electroporation. Electroporation may be reversible (i.e., the temporally-opened pores will reseal) or irreversible (i.e., the pores will remain open). For example, in the field of gene therapy, reversible electroporation (i.e., temporarily open pores) is used to transfect high molecular weight therapeutic vectors into the cells. In other therapeutic applications, a suitably configured pulse train alone may be used to cause cell destruction, for instance by causing irreversible electroporation (IRE).

It should be understood that while the energization strategies for ablation are described as involving AC and/or DC waveforms, embodiments may use variations of AC or DC pulses and remain within the scope of the invention. For example, exponentially-decaying pulses, exponentially-increasing pulses, and combinations thereof may be used. Moreover, while certain embodiments of system <NUM> are described herein with respect to RF ablation therapy, it should be understood that system <NUM> may be used, additionally or alternatively, for other forms of electric field-based ablation therapy.

System <NUM> further includes a ground pad <NUM> that provides a ground path, for example, for RF signals transmitted by a signal generator <NUM> through electrode assembly <NUM> and into the body of the patient <NUM>. In the illustrated embodiment, ground pad <NUM> is a cutaneous patch electrode. Likewise, system <NUM> may include additional return electrodes that may also be cutaneous patch electrodes. Although at least some ground pads may include two or more ground contacts, the disclosed systems and methods generally utilize a single contact in ground pad <NUM>. In alternative embodiments, the systems and methods described herein may be expanded and modified to operate with dual contact ground pads, or ground pads having two or more ground contacts. In certain embodiments, return electrodes may be any other type of electrode suitable for use as a return electrode, or ground path, including, for example, one or more catheter electrodes. Return electrodes that are catheter electrodes may be part of another electrode assembly (not shown) or part of a separate catheter (not shown). In some embodiments, for example, system <NUM> includes a bipolar catheter electrode assembly that includes a plurality of electrode pairs, where each electrode pair includes two electrodes with one electrode functioning as the return electrode.

System <NUM> may further include a computer system <NUM> (e.g., including an electronic control unit and memory) Computer system <NUM> may further include conventional interface components, such as various user input/output mechanisms and a display, among other components.

System <NUM> may include a suitable detector and tissue sensing circuit integrated with signal generator <NUM> or computer system <NUM> that identify which electrodes of electrode assembly <NUM> have characteristics (e.g., electrical characteristics such as impedance, phase angle, reactance, etc.) indicative of contact with tissue <NUM>. Signal generator <NUM> or computer system <NUM> may then select which electrodes or electrode pairs of electrode assembly <NUM> to energize based on the electrodes identified as being in contact with tissue <NUM>. Suitable components and methods for identifying electrodes in contact with tissue are described, for example, in <CIT>.

It should be understood that electrode assembly <NUM> is not limited to the specific constructions shown and described herein, and may include any other suitable electrode assembly and have any other suitable construction that enables system <NUM> to function as described herein.

Signal generator <NUM> is a high-frequency signal generator configured to generate AC signals at an RF frequency in the range of about <NUM> kilohertz (KHz) and <NUM>. For example, in certain embodiments, signal generator <NUM> generates AC signals at about <NUM>. In alternative embodiments, signal generator <NUM> can generate various RF frequencies in the RF range. In some embodiments, signal generator <NUM> is configured to output energy in waveforms at selectable energy levels, such as fifty joules, one-hundred joules, two-hundred joules, and the like. Other embodiments may have more or fewer energy settings, and the values of the available settings may be the same or different. In alternative embodiments, signal generator <NUM> is a monophasic or biphasic signal generator configured to generate a series of DC pulses. For example, in some embodiments, signal generator <NUM> outputs or generates a pulse having a magnitudes between about <NUM> V and about <NUM> kV. Other embodiments may output or generate any other suitable voltage.

System <NUM> may include a variable impedance. The variable impedance may be used to change one or more characteristics, such as amplitude, duration, pulse shape, and the like, of an output of signal generator <NUM>. Although described as a separate component, the variable impedance may be integrated with ablation lead <NUM> or signal generator <NUM>. The variable impedance may include one or more impedance elements, such as resistors, capacitors, or inductors connected in series, parallel, or combinations of series and/or parallel. The variable impedance is may be connected in series with ablation lead <NUM>. Alternatively, the impedance elements of the variable impedance ay be connected in parallel with ablation lead <NUM> or in a combination of series and parallel. Moreover, in other embodiments, the impedance elements of the variable impedance are connected in series and/or parallel with ground pad <NUM>. Some embodiments include more than one variable impedance, each of which may include one or more impedance elements. In such embodiments, each variable impedance may be connected to a different electrode or group of electrodes to allow the impedance through each electrode or group of electrodes to be independently varied. In other embodiments, the impedance of system <NUM> may not need to be varied and the variable impedance may be omitted.

<FIG> is a schematic diagram of an example ablation needle <NUM> for use with RF ablation system <NUM> shown in <FIG>. Ablation needle <NUM> includes an elongate body <NUM> having a distal end <NUM> for insertion into a patient's body, and a proximal end <NUM> configured to remain exterior to the body. An electrode <NUM> is positioned at distal end <NUM> such that, upon insertion, electrode <NUM> can be positioned at a tissue site for delivering ablative energy. Generally, elongate body <NUM> is electrically isolated from electrode <NUM> at distal end <NUM>. To enable proper positioning of distal end <NUM>, elongate body <NUM> may have any length <NUM> and diameter <NUM> suitable for reaching a given region of target tissue in a given patient's body. For example, a physician may utilize an ablation needle <NUM> having one length <NUM> and diameter <NUM> for performing renal denervation in a first patient, and a different length <NUM> and diameter <NUM> for performing the same procedure in a second patient. Likewise, electrode <NUM> may have various lengths <NUM> and diameters <NUM>. Selection of length <NUM> and diameter <NUM> of electrode <NUM> may be based on various factors, including, for example, patient anatomy, type of target tissue, and the size of the lesion proscribed for the ablation therapy. The size of electrode <NUM> generally limits the amount of power that can be delivered through ablation needle <NUM>, and also the surface area of tissue contact that can be achieved for ablation therapy. For example, a large electrode <NUM> may contact a greater tissue area, which generally increases the ablation load. The larger electrode <NUM> can also deliver more ablative energy and produce a larger lesion, but generally with less precision than a smaller electrode <NUM>, and potentially requiring more total current to achieve ablative energy levels in the target tissue. In some RF ablation therapies, a smaller electrode <NUM> is desired to deliver more concentrated RF energy to a smaller region of tissue.

Proximal end <NUM> of elongate body <NUM> includes a connector assembly <NUM> that provides electrical coupling of ablation needle <NUM> to a lead wire <NUM>. Lead wire <NUM> couples, directly or indirectly, to a channel of an ablation signal generator, such as signal generator <NUM> shown in <FIG>. Connector assembly <NUM> may include a lead connector <NUM> and a needle connector <NUM> to enable mechanical and electrical separation of ablation needle <NUM> from lead wire <NUM> for easier insertion and removal from the patient's body. In alternative embodiments, connector assembly <NUM> may be fixed such that lead wire <NUM> and ablation needle <NUM> cannot be separated without destroying ablation needle <NUM>, or at least not without significant effort or rendering ablation needle <NUM> permanently inoperable for therapeutic use.

<FIG> is a cross-sectional illustration of ablation needle <NUM> placed proximate a disc <NUM> of a patient's spine shown adjacent to a vertebra <NUM>. Generally, disc <NUM> includes a nucleus <NUM>, an annular fibrosis <NUM>, and a thin layer <NUM> defining an annular nuclear interface, or transitional, zone. Ablation needle <NUM> is illustrated positioned in the disc annulus, or annular fibrosis <NUM> for RF ablation therapy via an introducer needle <NUM>. Alternatively, ablation needle <NUM> may be positioned in disc nucleus <NUM>.

<FIG> is a schematic diagram of signal generator <NUM> shown in <FIG>. Signal generator <NUM> includes a plurality of channels <NUM> for delivering ablative energy to the body of a patient <NUM> through, for example, ablation lead <NUM> or a plurality of ablation needles <NUM>. <FIG> illustrates a single channel <NUM> for clarity, however signal generator <NUM> generally includes two or more channels. The plurality of channels <NUM> are time-multiplexed such that only a single channel <NUM> is active, or selected, at a given moment in time. Signal generator <NUM> includes a microcontroller <NUM> that controls each channel <NUM>. For example, microcontroller <NUM> may include a plurality of digital output channels that each produce logic level DC signals to control respective switching circuits. More specifically, each switching circuit <NUM> includes one or more solid state switching device <NUM>, such as a metal-oxide semiconductor field-effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), or other suitable semiconductor device for making and breaking an RF circuit formed by signal generator <NUM>, an electrode <NUM> for that channel <NUM>, target tissue <NUM> in the body <NUM> of the patient, and ground pad <NUM>. Microcontroller <NUM> is galvanically isolated from each switching circuit <NUM> by a galvanic isolator <NUM>. The logic level DC signals pass through galvanic isolator <NUM> to a gate driver <NUM> that supplies suitable voltage and current to operate the gate of solid state switching device <NUM>. Each switching circuit <NUM> may also include a gate impedance <NUM>.

Signal generator <NUM> includes an RF source <NUM> that generates the RF signals that carry the ablative energy through solid state switching device <NUM> to electrodes <NUM>, and through the body of the patient <NUM> to ground pad <NUM>. In certain embodiments, signal generator <NUM> includes a plurality of RF sources <NUM> for driving the plurality of channels <NUM>. RF source <NUM> may be controlled, for example, by microcontroller <NUM>. Control of RF source <NUM> may include frequency, voltage amplitude, current amplitude, or phase. Microcontroller <NUM> also controls RF source <NUM> to detect proper application and electrical connection of ground pad <NUM>, including, for example, the generation of a calibrated low-power signal that is transmitted through a selected channel <NUM> of the plurality of channels <NUM>. The transmission through the selected channel <NUM> is controlled by microcontroller <NUM> and its control of switching circuits <NUM> for each channel <NUM>. The low-power signal may be, for example, a <NUM> V RF signal applied for <NUM>. The voltage, current, frequency, and duration can be adjusted for a specific patient by controlling both RF source <NUM> and the switching circuit <NUM> for the selected channel <NUM> accordingly.

Signal generator <NUM> also includes a current sensing circuit <NUM> and a voltage sensing circuit <NUM>. Current sensing circuit <NUM> is coupled to the plurality of channels <NUM> to enable sensing, or measuring, of current through each channel <NUM>. Current sensing circuit <NUM> may include, for example, a shunt resistor or a hall effect sensor. Voltage sensing circuit <NUM> is coupled to the plurality of channels <NUM> to enable sensing, or measuring, of a root mean squared (RMS) voltage applied to each channel <NUM>. Voltage sensing circuit <NUM> may include, for example, a voltage divider circuit for detecting the voltage applied to a given channel <NUM>.

Signal generator <NUM> also includes a ground reference, or simply "ground," <NUM> to which RF source <NUM>, current sensing circuit <NUM>, and voltage sensing circuit <NUM> are referenced. Moreover, ground pad <NUM> provides a ground path back to the same ground <NUM> to close the RF circuits formed by each channel <NUM>.

<FIG> is a circuit diagram of an RF circuit <NUM> illustrating the impedances and ground paths in a time-multiplexed RF ablation system. RF circuit <NUM> includes RF source <NUM> of signal generator <NUM> and a plurality of channels <NUM>, including a selected channel <NUM> and unselected channels <NUM>, <NUM>, and <NUM>. Each of the channels <NUM> (e.g., as shown in <FIG>) is configured to supply ablative energy to target tissue <NUM> through a switching circuit <NUM> and, more specifically through solid state switching device <NUM>. Accordingly, the RF circuit for each channel is composed of an ablation load <NUM> representing the impedance of tissue <NUM> at which a given electrode <NUM> is targeted, or positioned, and a switch impedance <NUM> representing the impedance of the solid state switching devices <NUM> in each switching circuit <NUM>. RF circuit <NUM> also includes ground pad <NUM>, which provides a ground path back to signal generator <NUM>.

Each of the impedance components of RF circuit <NUM> has an impedance that varies depending on, for example, the positioning of the corresponding electrode <NUM> for a given channel <NUM> (i.e., the type and condition of the tissue at that site), the state of solid state switch devices <NUM>, or the state of ground pad <NUM>, i.e., the condition of its electrical connection to the body of the patient <NUM>.

The impedance of ground pad <NUM> may be relatively low when it is properly applied, exhibiting a contact impedance of about <NUM> ohm or less. Conversely, when ground pad <NUM> is improperly applied, open, or has a poor electrical connection, the contact impedance is generally in excess of several thousand ohms, for example, above <NUM>,<NUM> ohms.

The switch impedances <NUM> of solid state switch devices <NUM> may be very low when the solid state switch device <NUM> is closed. For example, when closed, the switch impedance <NUM> may be <NUM> ohm or less. Conversely, when solid state switch devices <NUM> are open, switch impedance <NUM> is relatively high, often several hundred ohms. Notably, when open, switch impedance <NUM> is not so high as to function as an open circuit at high frequencies, e.g., RF frequencies in the range of <NUM> to <NUM>, due to the capacitive leakage current conducted through the solid state switch device <NUM>. For example, in one embodiment, switch impedance <NUM> is about <NUM> ohms in the open state.

The ablation loads <NUM> of the tissue <NUM> of the patient vary based on the location, type of tissue, and the condition of the tissue. For example, the ablation load <NUM> may be as low as several tens of ohms (e.g., blood), or <NUM>,<NUM> or more ohms for other tissue.

<FIG> is a flow diagram of an example method <NUM> of detecting placement of ground pad <NUM> for RF ablation system <NUM> shown in <FIG>. The ground pad is a cutaneous patch having a single electrical contact for establishing a ground path from a patient's body and, more specifically, a return path back to signal generator <NUM> for creating the necessary electric fields in the tissue of the patient. A plurality of electrodes is placed at respective tissue sites in the patient's body. The electrodes may be integrated, for example, onto electrode assembly <NUM> of ablation lead <NUM> (shown in <FIG>). Alternatively, each electrode may be disposed on a distal end of a corresponding ablation needle <NUM> (shown in <FIG>). Once the electrode assembly <NUM> or ablation needles <NUM> are positioned, they are connected to corresponding channels <NUM> of signal generator <NUM>.

Signal generator <NUM> generally will detect whether ground pad <NUM> is properly placed before enabling ablative energy to be generated. Microcontroller <NUM> controls switching circuits <NUM> to commutate <NUM> corresponding channels <NUM> to close selected channel <NUM> and to open a plurality of unselected channels, e.g., unselected channels <NUM>, <NUM>, and <NUM>. A first impedance is then measured <NUM> over selected channel <NUM>, through an electrode positioned in the patient's body, and through a ground path established by ground pad <NUM>. Microcontroller <NUM> controls RF source <NUM> to transmit a low-power RF signal over selected channel <NUM>. The low-power RF signal is generally generated at the therapeutic ablation frequency (e.g., <NUM>) and supplied to selected channel <NUM> through its corresponding switching circuit <NUM> for a set duration. The duration of the low-power RF signal is generally brief enough to avoid accumulating enough RF energy in the tissue <NUM> to potentially burn, damage, or ablate the tissue <NUM>. For example, the pulse duration may be in the range of <NUM> to several tens of ms, e.g., up to <NUM>. In one embodiment, for example, the low-power RF signal is "pulsed" for <NUM>.

The current conducted through selected channel <NUM> is measured by current sensing circuit <NUM>, and an RMS voltage applied to selected channel <NUM> is measured by voltage sensing circuit <NUM>. The first impedance is then computed based on the measured current and measured RMS voltage.

Switching circuits <NUM> are then commutated <NUM> to close selected channel <NUM> and the plurality of unselected channels <NUM>, <NUM>, and <NUM>. A second impedance is then measured <NUM> over selected channel <NUM> in the same manner as the first impedance. Microcontroller <NUM> then computes <NUM> a difference between the first impedance and the second impedance, and determines <NUM> the ground pad <NUM> is either properly placed, or is not properly placed or at least has a poor electrical connection to the patient's body. When the difference exceeds a threshold (e.g., <NUM> ohms), microcontroller <NUM> determines ground pad <NUM> is not properly placed, open, or at least has a poor electrical connection. If the difference does not exceed the threshold, then ground pad <NUM> is in place with a good electrical connection.

When ground pad <NUM> is properly placed, microcontroller <NUM> controls switching circuits <NUM> to commutate to close selected channel <NUM> and to open the unselected channels <NUM>, <NUM>, and <NUM>. Microcontroller <NUM> then enables transmission of an RF ablation signal over selected channel <NUM>. Microcontroller <NUM> controls switching circuits <NUM> to time-multiplex the RF ablation signal over the plurality of channels <NUM>.

Although certain steps of the example method are numbered, such numbering does not indicate that the steps must be performed in the order listed. Thus, particular steps need not be performed in the exact order they are presented, unless the description thereof specifically require such order. The steps may be performed in the order listed, or in another suitable order.

Although the embodiments and examples disclosed herein have been described with reference to particular embodiments, it is to be understood that these embodiments and examples are merely illustrative of the principles and applications of the present disclosure. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and examples and that other arrangements can be devised without departing from the scope of the present disclosure as defined by the claims.

Some embodiments involve the use of one or more electronic processing or computing devices. As used herein, the term "microcontroller" and related terms, e.g., "processor," "computer", "processing device," "computing device," and "controller," are not limited to just those integrated circuits referred to in the art as a computer, but broadly refer to a processor, a processing device, a controller, a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a microcomputer, a programmable logic controller (PLC), a reduced instruction set computer (RISC) processor, a field programmable gate array (FPGA), a digital signal processing (DSP) device, an application specific integrated circuit (ASIC), and other programmable circuits or processing devices capable of executing the functions described herein, and these terms may be used interchangeably herein. These processing devices are generally "configured" to execute functions by programming or being programmed, or by the loading or other provisioning of instructions for execution. The above examples are not intended to limit in any way the definition or meaning of the terms processor, processing device, and related terms.

In the embodiments described herein, memory may include, but is not limited to, a non-transitory computer-readable medium, such as flash memory, a random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and non-volatile RAM (NVRAM). As used herein, the term "non-transitory computer-readable media" is intended to be representative of any tangible, computer-readable media, including, without limitation, non-transitory computer storage devices, including, without limitation, volatile and non-volatile media, and removable and non-removable media such as a firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal. Alternatively, a floppy disk, a compact disc - read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD), or any other computer-based device implemented in any method or technology for short-term and long-term storage of information, such as, computer-readable instructions, data structures, program modules and sub-modules, or other data may also be used. Therefore, the methods described herein may be encoded as executable instructions, e.g., "software" and "firmware," embodied in a non-transitory computer-readable medium. Further, as used herein, the terms "software" and "firmware" are interchangeable, and include any computer program stored in memory for execution by personal computers, workstations, clients and servers. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein.

Claim 1:
A radio frequency, RF, ablation system (<NUM>) comprising:
a plurality of electrodes (<NUM>, <NUM>) configured to be positioned at respective tissue sites within a patient's body;
a ground pad (<NUM>) configured to be positioned on the patient's body to provide a ground path from the patient's body; and
a signal generator (<NUM>) coupled to the ground pad and further coupled to the plurality of electrodes via corresponding channels (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the corresponding channels including a selected channel (<NUM>) and a plurality of unselected channels (<NUM>, <NUM>, <NUM>), the signal generator configured to:
control switching circuits (<NUM>) for the corresponding channels to close the selected channel and to open the plurality of unselected channels;
measure a first impedance over the selected channel;
control the switching circuits to close the selected channel and the plurality of unselected channels;
measure a second impedance over the selected channel;
compute a difference between the first impedance and the second impedance; and
determine the ground pad has at least a poor electrical connection to the patient's body when the difference exceeds a threshold.