Patent Publication Number: US-2023149071-A1

Title: Methods and apparatus for reducing leakage currents in cryo, radio-frequency, and pulsed-field ablation systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/279,346, filed 15 Nov. 2021, and entitled “METHODS OF CANCELLING LEAKAGE CURRENTS IN CRYO, RADIO FREQUENCY AND PULSED FIELD ABLATION SYSTEMS,” which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This disclosure relates to methods and systems associated with ablation catheters used with cryoablation, pulsed field ablation (PFA), and radio-frequency (RF) ablation generators and other medical devices. 
     BACKGROUND 
     Cardiac arrhythmias disrupt normal heart rhythm and reduce cardiac efficiency. These arrhythmias can be treated using cryoablation, PFA, and/or RF ablation therapy. The delivery of ablation therapy involves the use of a reliable, powerful, and precisely controlled source of electrical energy, e.g., in the form of a high-voltage pulse or RF generator. Electrical pulses or continuous wave (CW) sinusoids are delivered to the intended endocardial sites to perform reversible or irreversible electroporation in the case of PFA, and thermally induced necrosis via RF using an ablation-therapy delivery device. Reversible electroporation is used to reverse permeabilize cells to catalyze acceptance of genes or drugs, whereas irreversible electroporation is used to create permanent and lethal nanopores which can electrically isolate target areas of the myocardium and prevent arrhythmias, such as atrial fibrillation. The use of RF energy creates lesions via thermal necrosis which also isolates target areas of myocardium. 
     SUMMARY 
     Disclosed herein are, among other things, various aspects, features, and embodiments of methods and apparatus for monitoring and actively reducing leakage currents flowing on patient applied parts used in ablation therapy. In an example, a signal-processing circuit connected between a toroidal-coil sensor and a sleeve-capacitor coupler, both AC-coupled to the catheter cable, applies a Fourier transform and an energy minimization algorithm to the output of the toroidal-coil sensor to determine amplitudes and phases for frequency components of the signal applied to the sleeve-capacitor coupler. A corresponding current coupled through the sleeve-capacitor coupler into the catheter cable counteracts the leakage current to force the total non-therapy electrical current flowing on the patient applied parts to a level that is lower than a fixed threshold value, e.g., selected in accordance with an applicable standard. 
     One example provides a medical-treatment apparatus including a current sensor AC-coupled to wiring of a catheter cable. The apparatus also includes a signal-processing circuit configured to generate an output current based on spectral content of a first current sensed by the current sensor in the catheter cable. The apparatus additionally includes an AC-signal coupler connected to receive the output current from the signal-processing circuit and positioned along the catheter cable. The signal-processing circuit is configured to generate the output current such that a second current coupled by the AC-signal coupler into the wiring of the catheter cable in response to the output current counteracts the first current to force a combination of the first current and the second current to a level that is lower than a fixed threshold value. 
     Another example provides a medical-treatment method, comprising the step of sensing a first current in a catheter cable using a current sensor AC-coupled to wiring of the catheter cable. The medical-treatment method further comprises the step of determining spectral content of the first current by applying a Fourier transform to a digital signal generated by digitizing an output signal of the current sensor and the step of generating an output current based on the spectral content. The medical-treatment method further comprises the step of applying the output current to an AC-signal coupler to couple a second current into the wiring such that the second current counteracts the first current to force a combination of the first current and the second current to a level that is lower than a fixed threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of embodiments described herein, and the attendant advantages, aspects, and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
         FIG.  1    is an illustration of a catheter-lab environment according to various examples. 
         FIG.  2    is a block diagram illustrating an effective electrical circuit existing in the catheter-lab environment of  FIG.  1    according to various examples. 
         FIG.  3    is a block diagram illustrating a circuit portion of the effective electrical circuit of  FIG.  2    according to various examples. 
         FIG.  4    is a block diagram illustrating a leakage-current processing circuit that can be used in the circuit portion of  FIG.  3    according to various examples. 
         FIG.  5    is an illustration of a circuit assembly used in the circuit portion of  FIG.  3    according to various examples. 
         FIG.  6    is a flowchart of an example method of operating the circuit portion of  FIG.  3    according to various examples. 
     
    
    
     DETAILED DESCRIPTION 
     It is noted that some embodiments reside in combinations of apparatus components and processing steps related to metering and cancelling low levels of undesired alternating current passing into a patient from a cryo-ablation, PFA, or RF ablation system. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the pertinent art having the benefit of the description herein. 
     Cryoablation, PFA, and RF ablation methods are significant in power and energy. As a result, the catheter cable used to deliver PFA or RF energy to the heart chamber faces numerous constraints and design challenges that need to be addressed for reliable, safe transmission. An important design constraint enforced by the International Electrotechnical Commission (IEC), via its standard 60601-1 , Medical electrical equipment—Part  1 : General requirements for basic safety and essential performance , is to limit to relatively low levels the amount of tissue-inserted catheter non-therapy electrical current capable of presenting an electrical shock hazard. According to the IEC 60601-1 standard, Section 8.7.3, Table 3, no more than a 10 μA current under normal conditions or no more than a 50 μA current for a single fault condition (such as an inadvertent alternating current (AC) mains connection to a patient) is allowed to drain from or source into a type CF (cardiac floating) applied part (e.g., an ablation catheter) in tissue contact with the patient. This undesirable leakage current can be reduced to within acceptable limits by eliminating extraneous conduction paths between the patient applied part (AP) catheter and surroundings. Nearly all modern medical electrical equipment found in a catheter lab has means for isolating the operator and patient from the AC power mains and earth surroundings. Yet, because the isolating devices are applied between the AC power mains and circuit loads, these measures only create a barrier against current flowing back towards the power source but may still be ineffective towards preventing errant currents from flowing into isolated patient connections that distribute capacitance between the catheter and surroundings. 
       FIG.  1    illustrates a catheter-lab environment  100  having an ablation therapy system  110  for treating a patient  102  with PFA or RF ablation. Example electric fields E corresponding to the catheter capacitive leakage paths occurring along a catheter cable  111  (or more generally, a patient therapy cable  111 ) are represented by curved dashed lines in  FIG.  1   . Those electric field lines in  FIG.  1    approximately represent distributed capacitance between the catheter cable  111  and various items that are at earth or “ground” potential, which creates an undesired leakage-current return-path circuit in the catheter-lab environment  100 . Such items typically include but are not limited to the cryoablation, RF and/or PFA generator, the electrophysiological recorder and related equipment, the floor, and the AC mains wiring. To counteract undesired leakage currents coupling to the catheter cable  111  through the leakage-current return-path circuit, in some examples, the ablation therapy system  110  includes a leakage-current processing circuit  120 , as indicated in the expansion block diagram of the ablation therapy system  110  shown in  FIG.  1   . The leakage-current processing circuit  120  is described in more detail below, e.g., in reference to  FIG.  4   . 
       FIG.  2    is a block diagram illustrating an effective electrical circuit  200  existing in the environment  100  according to various examples. More specifically, the circuit  200  may exist when the ablation therapy system  110  is applied to treat a heart  214  of the patient  102 . The circuit  200  includes an ablation therapy generator  220  comprising the leakage-current processing circuit  120 , an electronic-controller unit  222 , and an isolated power supply  224  connected to an AC mains power outlet  226 . The controller unit  222  is connected to receive a therapy supply voltage from the power supply  224  that operates to convert the AC mains power of the outlet  226  to a direct current (DC) therapy supply voltage, which is then used to deliver treatment, via the catheter cable  111 , to the heart  214 . A capacitance  215  schematically represents the distributed capacitance between the catheter cable  111  and a chassis  216  of the ablation therapy generator  220 . In some examples, a cable  234  is connected to the generator  220  to carry electrogram (EGM), electromyogram (EMG), or electrocardiogram (ECG) signals to the patient  102 . A capacitance  217  schematically represents the distributed capacitance between the cable  234  and the chassis  216 . The chassis  216  is typically electrically grounded to provide electrical shielding to various components of the generator  220  and, in some examples, to the ablation therapy system  110 . In addition to the treatment signals from the controller  222 , the catheter cable  111  typically carries a leakage current  218  AC-coupled thereto through the distributed capacitance  215  as indicated above. In operation, the leakage-current processing circuit  120  is used to counteract the leakage current  218  as described in more detail below. 
     In some examples, the circuit  200  further includes an electrical connection to the heart  214  comprising a tissue impedance  236  in electrical series with a voltage source  238 . The tissue impedance  236  is about the amount that a typical human body would resist according to the IEC 60601-1 standard, Section 8.7. The voltage source  238  typically is an inadvertent or accidental connection to the mains AC voltage in the environment  100 . In some examples, the voltage source  238  is an electrical terminal connected to another AC mains power outlet analogous to the outlet  226 . 
     Although the leakage-current processing circuit  120  is shown in  FIG.  2    as being internal to the ablation therapy generator  220 , various embodiments and examples are not so limited. In some examples, the leakage-current processing circuit  120  is placed in a suitable location external to the ablation therapy generator  220 . In some other examples, the leakage-current processing circuit  120  has several components thereof placed at different respective locations. 
     The following numerical estimates illustrate certain parameters of the circuit  200  according to some nonlimiting examples. When the leakage mechanism does not contain other resistive or inductive leakage paths, compliance with the above-mentioned standard 60601-1 is achieved with the total distributed capacitance  215  that is low enough to present a sufficiently high impedance Z at the AC mains frequency (e.g., 50 Hz or 60 Hz) to limit the leakage current  218  to less than 50 μA for a single fault, mains applied part (MAP) condition, e.g., according to Eq. (1). 
     
       
         
           
             
               
                 
                   
                     Z 
                       
                     &gt; 
                       
                     
                       
                         252 
                         ⁢ 
                              
                         VAC 
                       
                       
                         50 
                         ⁢ 
                             
                         μA 
                       
                     
                   
                   = 
                     
                   
                     5. 
                          
                     MΩ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Herein, 252VAC is approximately the nominal line voltage of 230VAC plus a 10% tolerance. Thus, a limitation on the corresponding distributed capacitance  215  (C 215 ) between the catheter cable  111  and the surroundings (including the chassis  216 ) passing a current at 50 Hz can be estimated using Eq. (2): 
     
       
         
           
             
               
                 
                   
                     
                       C 
                       
                         2 
                         ⁢ 
                         1 
                         ⁢ 
                         5 
                       
                     
                     &lt; 
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                            
                         50 
                         ⁢ 
                             
                         Hz 
                             
                         5.04 
                             
                         MΩ 
                       
                     
                   
                   = 
                   
                     632 
                     ⁢ 
                         
                     pF 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The amount of capacitance in accordance with Eq. (2) can thus be used as a design constraint for RF and PFA ablation systems. For example, a capacitance greater than 632 pF will likely displace a MAP current exceeding 50 micro-Amperes (μA). Although the above example satisfies the IEC 60601-1, Section 8.7.3, leakage-current limits in some geographies, such as Australia—New Zealand (ANZ), China, and Europe, an evaluation or regulatory agency in some other geographies, such as in countries that provide similar line voltage at a higher frequency, is unlikely to certify leakage-current compliance for the corresponding ablation therapy system. For example, South Korea&#39;s standard AC mains power is 220VAC+10% at 60 Hz. In the case of South Korea, the distributed capacitance  215  needs to be less than 548 pF to achieve compliance. 
     Some conventional methods directed at reducing the leakage currents  218  use external means of isolating the entire ablation system from the AC mains lines. Such methods typically rely on the use of isolation transformers and uninterruptible power supplies (UPSs). While the corresponding systems can typically provide adequate leakage-current reduction, such systems typically have technical limitations. For example, one of such technical limitations is that connections to the ablation generating equipment that provide an earth or ground potential may need to be avoided. Otherwise, the earth-connected circuit can provide an undesired leakage path. Yet, without the protective earth ground, a system operator (such as a practitioner  104 ,  FIG.  1   ) may be exposed to an electrical-shock hazard in at least some situations, e.g., when the “floating” equipment cabinet contacts a high voltage node or the AC mains voltage. Also, equipment that is not earth grounded may induce noise capable of distorting cardio and other physiological signals, such as EGM, EMG, and ECG signals (also see  234 , in  FIG.  2   ). Therefore, maintaining a good signal-to-noise ratio (SNR) and sufficient signal integrity is an expected feature of the ablation equipment. For example, a low noise rendering of an EGM displayed on an EP recorder after or during an ablation-therapy procedure typically informs the attending electrophysiologist (e.g.,  104 ,  FIG.  1   ) of acute therapy effectiveness and, as such, is an important expectation. Any possible impairment of such signals contrary to this expectation provides a motivation for finding a technical solution directed at inhibiting leakage currents, such as the leakage current  218 . 
     The IEC 60601-1 standard allows up to twice the single accessory MAP limit, or 100 μA, in the case of multiple patient connections. Yet, an ablation system making other patient connections, such as those to a second energy-delivery catheter, a coronary sinus (CS) catheter, an EP recorder, and/or surface ECG leads may increase the corresponding distributed capacitance to an amount that causes the multi-accessory 100 μA MAP limit to be exceeded. Where difficulty is encountered in reducing to a relatively low level the ablation-system capacitance possessing only a single catheter cable  111 , it may be even more difficult to reduce the distributed capacitance with multiple patient connections. 
     The above indicated and possibly some other related problems in the state of the art can beneficially be addressed using at least some embodiments disclosed herein. For example, some embodiments implement an active-correction approach, using which the MAP leakage-current goals can be met under a variety of usage scenarios. 
       FIG.  3    is a block diagram illustrating a circuit portion  300  of the effective electrical circuit  200  according to various examples. The circuit portion  300  includes the leakage-current processing circuit  120  connected to a current sensor  358  and a current coupler  356  as indicated in  FIG.  3   . In operation, the sensor  358  provides a measure of a leakage current  318  flowing toward the patient  102  along the catheter cable  111 . The leakage current  318  is typically induced through a variety of leakage and accidental paths collectively represented in the circuit portion  300  by the series including a current source  346  and an impedance  348 . The current source  346  is an effective leakage source representing a combination of different leakage and accidental sources the composition of which depends on the specific example. In some examples, the current source  346  includes one or more distributed sources capacitively coupled to the catheter cable  111  through the capacitance  215  and one or more lumped sources, such as the voltage source  238 . The impedance  348  is an effective impedance that includes an effective resistance  350  and an effective capacitance  352 . In some examples, the capacitance  215  represents a portion of the capacitance  352 . 
     In response to the measure of the leakage current  318  provided by the current sensor  358 , the leakage-current processing circuit  120  generates an output current which is directed through a resistance  355  to the current coupler  356 . In response to the output current, the current coupler  356  applies to the catheter cable  111  a correction current  304 . Signal processing implemented in the leakage-current processing circuit  120  causes the correction current  304  to be such that the combination of the currents  304  and  318  is smaller than one or more pertinent thresholds specified in the IEC 60601-1 standard. Such combination current is typically the non-therapy current that acts on the patient  102  through an effective patient resistance  354  and a patient applied part  306  of the catheter connected to the catheter cable  111 . In various examples, the correction current  304  counteracts the leakage current  318 , thereby reducing, canceling, or minimizing the effective non-therapy current i patient  acting on the patient  102 . In some examples, the correction current  304  cancels the leakage current  318  in accordance with Eq. (3): 
         i   patient   =i   318   +i   304 ≈0  μA   (3)
 
     where i 318  and i 304  denote the currents  318  and  304 , respectively. Example signal processing implemented in the leakage-current processing circuit  120  and capable of achieving the result expressed by Eq. (3) is described in more detail below in reference to  FIG.  4   . 
     In one specific example corresponding to  FIG.  3   , various pertinent elements of the circuit portion  300  have the following characteristics. The output of the leakage-current processing circuit  120  is at 477 V, has the frequency of 60 Hz, and the relative phase of 164 degrees. The leakage-current source  346  is at the effective voltage of 311 V, the frequency of 60 Hz, and the relative phase of 0 degrees. The resistances  350 ,  354 , and  355  are 1 MΩ, 1 kΩ, and 100 kΩ, respectively. The capacitances  352  and  356  are 800 pF and 500 pF, respectively. For this specific example, by applying Kirchoff&#39;s Current Law at Node A indicated in  FIG.  3   , the resulting patient current applying the numerical values in Eq. (3) are as follows: 
     
       
         
           
             
               
                 
                   
                     i 
                     patient 
                   
                   = 
                   
                     
                       
                         
                           311 
                           / 
                           0 
                           ⁢ 
                           ° 
                         
                         
                           
                             3.47 
                             e 
                             ⁢ 
                             6 
                             / 
                           
                           - 
                           
                             73.2 
                             ° 
                           
                         
                       
                       + 
                       
                         
                           477 
                           / 
                           164 
                           ⁢ 
                           ° 
                         
                         
                           
                             5.31 
                             e 
                             ⁢ 
                             6 
                             / 
                           
                           - 
                           
                             88.9 
                             ° 
                           
                         
                       
                     
                     ≈ 
                     
                       0.4 
                           
                       µA 
                       ⁢ 
                           
                       peak 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The values shown in Eq. (4) are in phasor form. Without the correction applied by the circuit  120 , the leakage current magnitude is 89 μA peak. With the correction implemented in the circuit portion  300 , the leakage current magnitude is reduced to 0.4 μA peak (or 0.28 μA root mean square (RMS)), which is well below the 50 μA RMS limit imposed by the IEC 60601-1 standard, Section 8.7. 
       FIG.  4    is a block diagram illustrating the leakage-current processing circuit  120  according to various examples. In operation, the leakage current  318  is sensed by the current sensor  358  (see  FIG.  3   ). In some examples, the current sensor  358  provides a voltage V s  in proportion to the leakage current&#39;s spectrum, whether this be at an AC line frequency, such as 50 Hz or 60 Hz, or at a power-supply switching frequency, such as in the range between 50 kHz and 500 kHz. The voltage V s  is amplified by an amplifier  460  and converted into digital form using an analog-to-digital (A/D) converter  462 . A resulting digital signal  463  is Fourier-transformed and inputted into an energy minimization algorithm run on an electronic processor (e.g., a microprocessor)  464 . In some examples, the electronic processor  464  is implemented using a computer. 
     The Fourier transform performed by the processor  464  operates to reveal the spectral content of the leakage current  318 , including amplitudes and relative phases of various frequency components thereof. The energy minimization algorithm run by the processor  464  serves to determine the spectral content for the correction current  304  capable of minimizing the energy of the current i patient , e.g., in accordance with Eq. (3). In various examples, different respective energy minimization algorithms, e.g., selected from the group consisting of the Nelder-Mead or simplex search algorithm, the Newton-Raphson algorithm, the conjugate gradient algorithm, and the steepest descent algorithm, are used. In a representative example, the used algorithm attempts to cancel the leakage current  318  by performing a search for appropriate parameters of the correction current  304  over three degrees of freedom: amplitude, frequency, and phase. 
     A control signal  465  from the processor  464  is applied to a direct digital synthesizer (DDS)  466 , which is thereby configured to synthesize sinusoids at frequencies determined by the processor  464 . The DDS  466  is capable of variously changing amplitudes and phases of individual sinusoids in response to the control signal  465 . In some examples, the DDS  446  generates (e.g., via an amplifier) an adjustable output amplitude in the range between about 10V and about 1 kV and imposes an adjustable phase in the range between −180° and +180° for each frequency component it generates. An output signal  467  generated by the DDS  466  in this manner is filtered by a lowpass filter  468  and amplified by a linear amplifier  470 . An earth reference coupling transformer  472  then operates to direct a resulting amplified, filtered signal  473  through the resistance  355  to the current coupler  356 . In some examples, to accomplish the earth return reference, one end of the isolation transformer&#39;s output winding is connected to the equipment&#39;s chassis ground (e.g.,  216 ,  FIG.  2   ). 
     In typical examples, leakage current frequencies correspond to an AC line frequency, which is 50 Hz in ANZ, China, and Europe and 60 Hz in Canada, Japan, South Korea, and the United States. In some examples, once revealed by the Fourier transform performed by the processor  464 , the correction frequencies are fixed, and only the amplitudes and phases thereof are varied. In other examples, other leakage current frequencies are present, such as the frequencies from switching power supplies (generally in the 50 kHz to 500 kHz range). Yet, the latter frequency components typically fall below the IEC 60601-1, Section 8.7, 50 μA threshold, and the processor  464  recognizes these components as having lower amplitudes than the offending, non-compliant AC line frequency component(s). Accordingly, in some examples, when a leakage component frequency is of a relatively low level, the algorithm is configured to ignore such components. 
     In some examples, the algorithm implemented by the processor  464  maps samples into successive correction attempts which are applied as data to the DDS  466 . Following the DDS  466 , the lowpass filter  468  removes quantization noise from the synthesized tones. In accordance with IEC specifications, such filtering prevents or reduces RF emissions radiating from the catheter cable  111 . The signals passed by the lowpass filter  468  are applied to the amplifier  470 , which can be implemented using a high voltage, low wattage, linear amplifier. Despite a high voltage output, the power of the amplifier  470  can be relatively low. More specifically, in some examples, 1 W of power is sufficient to provide an amplified output current of 1 mA at 1 kV. 
     In some examples, the current coupler  356  is implemented using a sleeve capacitor (also see  FIG.  5   ). Given that sufficient adjustment variability (amplitude, frequency, and phase) is available with respect to the correction current  304 , the sleeve capacitor  356  does not need to have a specific capacitance. Rather, it is sufficient that the sleeve capacitor  356  provides adequate coupling compatible with the dynamic-range limits of the amplifier  470 . The closed-form Eq. (5) can be used to estimate the capacitance of the sleeve capacitor  356  as follows: 
     
       
         
           
             
               
                 
                   
                     
                       capacitance 
                       sleeve 
                     
                     ( 
                     pF 
                     ) 
                   
                   = 
                   
                     
                       
                         length 
                         in 
                       
                       ⁢ 
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         ϵ 
                         r 
                       
                       ⁢ 
                       
                         
                           ϵ 
                           o 
                         
                         ( 
                         
                           0.0254 
                           
                             m 
                             in 
                           
                         
                         ) 
                       
                     
                     
                       log 
                       ⁡ 
                       ( 
                       
                         b 
                         a 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Eq. (5) gives an estimated capacitance of 500 pF for the sleeve capacitor  356  with the following values and constants:
         the inside conductor outer diameter, a=0.5″;   the outside conductor inner diameter, b=0.508″;   the dielectric permittivity ε r =3.3 for Polyethylene Terephthalate (PET) insulation, 0.004″ thick;   the dielectric permittivity of the free space ε o =8.854 pF/m; and   length=1.7 inches.
 
Other capacitance values of the sleeve capacitor  356  can be obtained, e.g., using other suitable values of the parameters a, b, ε r , and length.
       

       FIG.  5    is an illustration of a circuit assembly  500 , including a portion of the catheter cable  111 , the sleeve capacitor  356 , and the current sensor  358 , according to various examples. In the example shown, the current sensor  358  is implemented using a toroidal coil having the catheter cable  111  threaded through the coil&#39;s center opening as indicated in  FIG.  5   . In operation, the undesired current i patient  (see Eq. (3)) carried by a wiring bundle  502  of the catheter cable  111  generates, in the toroidal coil  358 , a corresponding electromagnetic field (EMF) potential or voltage V s  which is applied to the leakage-current processing circuit  120  as explained above (also see  FIG.  4   ). In response to the voltage V s , the leakage-current processing circuit  120  operates to adjust the current  304  coupled into the wiring bundle  502  of the catheter cable  111  by the sleeve capacitor  356  to substantially null the undesired current i patient . In various examples, the toroidal coil  358  and the sleeve capacitor  356  need not be co-located along the catheter cable  111 . For example, it may be advantageous to place the toroidal coil  358  relatively close to the ablation generator&#39;s front panel and to place the sleeve capacitor  356  around the input AC mains line cord as it enters the equipment rear. Different embodiments permit different respective choices for locating and placing the toroidal coil  358  and the sleeve capacitor  356 . 
       FIG.  6    is a flowchart of an example method  600  of operating the circuit portion  300  according to various examples. The method  600  includes sensing the leakage current  318  along the catheter cable  111  (in block  610 ). In some examples, the sensing is performed using the toroidal coil  358 . The method  600  also includes determining spectral content of the sensed leakage current (in block  612 ). In some examples, the frequency content determination is performed using the processor  464  as described in reference to  FIG.  4   . The method  600  further includes generating the correction current  304  to drive the sensed leakage current toward zero (in block  614 ), the correction current  304  being generated based on the amplitude, phase, and frequency content of the sensed leakage current. In some examples, the generation of the correction current  304  is performed using the leakage-current processing circuit  120 . The method  600  also includes coupling the correction current  304  into the catheter cable  111  to counteract the sensed leakage current (in block  616 ). In some examples, the coupling is performed using the sleeve capacitor  356 . 
     According to one aspect, a method of counteracting the leakage current  318  for an ablation catheter is provided. The method includes sensing the leakage current  318  along the catheter cable  111 . The method also includes determining an amplitude, phase, and frequency content of the sensed leakage current, generating a correction current  304  to drive the sensed leakage current toward zero, the correction current  304  being generated based on the amplitude, phase, and frequency content of the leakage current  318 , and coupling the correction current  304  into the catheter cable  111  to counteract the sensed leakage current. 
     According to another aspect, a leakage-current limiter (e.g., part of  300 ,  FIG.  3   ) for an ablation catheter is provided. The leakage-current limiter includes the current sensor  358  configured to sense the leakage current  318  along the cable  111  of the catheter. The leakage-current limiter also includes processing circuitry (e.g.,  120 ,  FIG.  3   ) configured to: determine spectral content of the sensed leakage current; and generate the correction current  304  to drive the sensed leakage current toward zero, the correction current  304  being based on the spectral content of the sensed leakage current. The leakage-current limiter also includes a capacitive structure (e.g.,  356 ,  FIGS.  3 ,  5   ) configured to couple the correction current  304  into the catheter cable  111  to counteract the sensed leakage current. 
     In some examples, the leakage current sensor  358  includes a toroidal coil configured to be positioned around the catheter cable  111 . In some examples, the capacitive structure comprises a sleeve capacitor  356  configured to be positioned around the catheter cable  111  to induce currents therein to counteract the sensed leakage current. In some examples, the sleeve capacitor  356  is positioned near one end of the catheter cable  111 , and the toroidal coil  358  is positioned near the opposite end of the catheter cable  111 . In some examples, the processing circuitry includes a fast Fourier transform (FFT) circuit configured to determine spectral content of the sensed leakage current, including determination of relative amplitudes and phases of various frequency components of the sensed leakage current. In some examples, the leakage-current limiter includes a direct digital synthesizer operatively coupled to the FFT circuit and configured to, for each of a plurality of frequency components identified by the Fourier transform of the sensed leakage current, generate a corresponding frequency component having an inverted phase. In some examples, the leakage-current limiter further includes a filter configured to filter out the digitization noise from the output of the direct digital synthesizer. In some examples, the processing circuitry (e.g., the electronic processor  464 ,  FIG.  4   ) is configured to run an energy reduction algorithm to determine the spectral content for the correction current  304 . In some examples, the energy reduction algorithm is configured to drive a difference between the sensed leakage current and the correction current  304  toward zero. 
     All terms used in the claims are intended to be given their broadest reasonable constructions and their ordinary meanings as understood by those knowledgeable in the technologies described herein unless an explicit indication to the contrary is made herein. In particular, use of the singular articles such as “a,” “the,” “said,” etc. should be read to recite one or more of the indicated elements unless a claim recites an explicit limitation to the contrary. 
     The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments incorporate more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in fewer than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter. 
     Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value or range. 
     As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, a data processing system, a computer program product and/or a computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining various software and hardware aspects and being generally referred to herein as a “circuit,” a “block,” or a “module.” Any process, step, action and/or functionality described herein may be performed by, and/or associated with, a corresponding module, which may be implemented in software and/or firmware and/or hardware. Furthermore, an example embodiment may take the form of a computer program product on a non-transitory computer usable storage medium having computer program code embodied in the medium that can be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, magnetic storage devices, and other suitable storage devices. 
     Some embodiments are described herein with reference to flowchart illustrations and/or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable memory or storage medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in a different (e.g., reverse) order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows or bidirectionally. 
     Computer program code for carrying out operations of the concepts described herein may be written in an object-oriented programming language such as Python, Java®, or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the “C” programming language. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user&#39;s computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     Many different embodiments have been disclosed herein, in connection with the above description and the drawings. Accordingly, it is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and applications other than the examples provided would be apparent to a person of ordinary skill in the pertinent art upon reading the above description. The scope should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the technologies discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In sum, it should be understood that the application is capable of modification and variation. 
     It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
     Unless otherwise specified herein, the use of the ordinal adjectives “first,” “second,” “third,” etc., to refer to an object of a plurality of like objects merely indicates that different instances of such like objects are being referred to, and is not intended to imply that the like objects so referred-to have to be in a corresponding order or sequence, either temporally, spatially, in ranking, or in any other manner. 
     Unless otherwise specified herein, in addition to its plain meaning, the conjunction “if” may also or alternatively be construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” which construal may depend on the corresponding specific context. For example, the phrase “if it is determined” or “if [a stated condition] is detected” may be construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event].” 
     Also, for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements. 
     As used herein in reference to an element and a standard, the term compatible means that the element communicates with other elements in a manner wholly or partially specified by the standard and would be recognized by other elements as sufficiently capable of communicating with the other elements in the manner specified by the standard. The compatible element does not need to operate internally in a manner specified by the standard. 
     The functions of the various elements shown in the figures, including any functional blocks labeled as “processors” and/or “controllers,” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and nonvolatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
     As used in this application, the terms “circuit,” “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry); (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions); and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.” This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. 
     It should be appreciated by those of ordinary skill in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.