Abstract:
Testing a thermocouple-based RF ablation system is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency power output thereof in a predefined manner in response to predefined variations in a temperature signal from the simulator. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, and while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying the communicated temperature signals, and verifying that a variation in the power output of the ablator module in response to varying the temperature signals conforms to the predefined manner.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to tissue ablation systems. More particularly, this invention relates to simulated operation of an RF generator in tissue ablation system. 
     2. Description of the Related Art 
     Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. 
     Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. 
     U.S. Patent Application Publication No. 2009/0030411 by Werneth et al. describes an ablation catheter in which a thermocouple can be used to measure the temperature local to the thermocouple prior to, during or after the delivery of ablation energy. It is explained that when an ablation is performed, maintaining the tissue at a temperature below a threshold is required. Information recorded from the thermocouple is used to adjust energy delivery or to modify its frequency, based on temperature information analysis. 
     U.S. Patent Application Publication No. 2011/0218526 provides another example of a thermocouple in an ablation system, which electrodes may be electrically coupled to an output portion of an RF generator, and each thermocouple may be electrically coupled to a feedback portion of the RF generator. A processor accepts an input voltage and produces an output voltage, based on feedback signals from the thermocouples, and then adjusts a duty cycle modulator as well as an amplitude modulator according to the feedback signals. 
     SUMMARY OF THE INVENTION 
     There is provided according to embodiments of the invention an apparatus for testing a tissue ablation system, which includes emulation circuitry connectable to an ablator module being tested, the ablator module has an adjustable radiofrequency (RF) power output and a monitor display. The emulation circuitry includes a first arm of a first thermocouple metallic material linked to the power output of the ablator module, and a second arm of a second thermocouple metallic material connected to the monitor display. A return pathway extending from the first arm to the ablator module permits passage of RF current and blocks direct current (DC). An adjustable voltage source producing a DC potential is connected via an output circuit across the first arm and the second arm, the output circuit having a greater resistance to RF current than to direct current. 
     According to an aspect of the apparatus, the return pathway includes a DC blocking capacitor. 
     According to a further aspect of the apparatus, the output circuit includes a chain of resistors connected in series with an inductor. 
     According to one aspect of the apparatus, the inductor includes a plurality of ferrite inductors in connected in series with the adjustable voltage source. 
     According to yet another aspect of the apparatus, a value of the inductor is 1 mH. 
     There is further provided according to embodiments of the invention a method of testing a thermocouple-based RF ablation system, which is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency (RF) power output thereof in a predefined manner in response to predefined variations in a temperature signal. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, and while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying the communicated temperature signals, and verifying that a variation in the power output of the ablator module in response to varying the temperature signals conforms to the predefined manner. 
     There is further provided according to embodiments of the invention a method of testing a thermocouple-based RF ablation system, which is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency (RF) power output thereof in a predefined manner in response to predefined variations in a temperature signal and has a temperature display monitor. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying a potential of the communicated temperature signals in accordance with known temperature-dependent potentials of a thermocouple junction to represent respective temperatures, and calibrating the temperature display monitor to conform to the respective temperatures represented by of the communicated temperature signals. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a pictorial illustration of a system for performing diagnostic and therapeutic procedures on a heart of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention; 
         FIG. 2  is a schematic diagram of a thermocouple-based RF ablation system, in accordance with an embodiment of the invention; 
         FIG. 3  is a detailed electrical schematic of a temperature simulator, which simulates operation of the ablation system shown in  FIG. 2 , in accordance with an embodiment of the invention; 
         FIG. 4  is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system, in accordance with an embodiment of the invention; and 
         FIG. 5  is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system to calibrate an RF generator, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
     Aspects of the present invention may be embodied in software programming code, which is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known non-transitory media for use with a data processing system, such as a diskette, hard drive, electronic media or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to storage devices on other computer systems for use by users of such other systems. 
     The term “couple” or “coupled” is intended to mean either an indirect or direct connection. Thus, if a first device is coupled to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections, or via inductive or capacitive coupling. 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing diagnostic and therapeutic procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall at an ablation target site. Optionally, electrical activation maps may then be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
     Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias. 
     The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a positioning processor  22 , located in a console  24 . 
     Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24 , are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
     Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor such as thermocouples  31 , may be mounted on or near the ablation electrode  32  and optionally or near the sensing electrode  33 . The thermocouples  31  are connected to the electrode circuit as described in further detail below. 
     The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
     The positioning processor  22  is an element of a positioning subsystem in the system  10  that measures location and orientation coordinates of the catheter  14 . 
     In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils  28 . The positioning subsystem may employ impedance measurement, as taught, for example in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
     As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . Console  24  includes a processor, preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by the above-noted sensors and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14  and to analyze the electrical signals from the electrodes. 
     Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally-placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
     In order to accurately ablate tissue, for example according to known procedures in which tissue temperature is an important variable, it is desirable to understand and model the behavior of the ablation catheter in actual operation. This can be done, according to embodiments of the invention, using a jig, which behaves as a temperature simulator, and which is connected to an RF generator. The simulator operates while the RF generator is active by separating a relatively high power RF current from a low power DC current, using a capacitor to provide a preferred low-impedance path for the RF component. The DC component is a voltage of about 40 μV, approximating typical thermocouple junction voltages, and which is detected and quantitated. By externally controlling a DC source for the voltage, the detected voltage and therefore the simulated temperature is immediately affected, irrespective of the activity of the RF generator. 
     Embodiments of the present invention separate the two effects, i.e., the junction potential, V j , and the RF power, P, in a simulation/calibration jig. Positive and negative terminals of the jig are connected to the μV input of the generator and to ground, respectively. 
     Reference is now made to  FIG. 2 , which is a schematic diagram of a thermocouple-based RF ablation system  45 , in accordance with an embodiment of the invention. A tissue ablation module, which is realized as an RF generator module  47  includes an RF generator  43  and a display  49 , which presents a microvolt DC reading. 
     The generator module  47  is connectable to catheter  14  in actual ablation operation. The generator  43  is adjustable, and the direct current (DC) response of thermocouples  31  occurs during actual operation of the catheter  14 . The DC output of the thermocouples  31  (v j ) can thus be correlated with the power (P) produced by the RF generator  43  and measured using the display  49 . However, in this system, the accuracy of the monitored DC output and hence the temperature reading is affected by the presence of induced RF current in the thermocouple circuit. 
     Reference is now made to  FIG. 3 , which is a detailed electrical schematic of a jig  51 , which simulates operation of the ablation system  45  ( FIG. 2 ), in accordance with an embodiment of the invention. In the circuitry shown in  FIG. 3 , the flows of direct current and RF current are indicated by solid arrows and arrows drawn as broken lines, respectively. 
     As noted above, in actual operation of an ablation system, the RF power produced by the generator module  47  heats resistive tissue of a patient, causing ablation of part of the tissue. In the jig  51  the resistive tissue is represented by a load resistor  41 , which is connected to the RF power output of a generator  43 . The resistor  41  needs to be able to dissipate power on the order of 25 W. 
     A positive terminal  53  made of copper-constantan and a constantan wire  55  are thermocouple metallic elements of the sort that may implement a thermocouple in an ablation catheter. In such a catheter, the thermocouple may be in physical contact with the ablation electrode or more loosely coupled to the ablation electrode without actual physical contact. In the jig  51 , RF current is present at the positive terminal  53 . In the example of  FIG. 3 , the positive terminal  53  uses a copper conductor  57  (carrying the power) as one “arm” of the thermocouple—the other arm being the constantan wire  55 . The potential V j  generated at the positive terminal  53 , arising from the junction temperature, is typically of the order of microvolts. The potential may be fed back to the power generator via negative terminal  59 , which typically uses the measured potential to control the power delivered by the generator. Other thermocouple metallic elements and alloys may be substituted for copper and constantan in the positive terminal  53  and negative terminal  59 . 
     A generator suitable for use as the generator  43  is the nMARQ™ RF generator produced by Biosense Webster. This generator has a microvolt input  61  and the capability of displaying the power delivered by the electrode and the impedance “seen” by its output terminal. The generator also displays the temperature of the electrode, using the junction potential V j  described above that is received via the microvolt input  61  and shown on the display  49 . This temperature is generally not the actual temperature of the patient tissue or of the electrode-tissue interface. As stated in the manual “The temperature displayed on the nMARQ Multi-Channel RF Generator does not represent the temperature of the tissue nor the temperature of the interface between the electrode and the tissue.” The temperature registered by the thermocouple (and displayed by the nMARQ generator), and the temperature of the tissue are different because of the heating effect of the RF power. 
     As shown in the schematic, known value resistors  63 ,  65  are placed across the copper-constantan junction of the thermocouple. The resistors  63 ,  65  are part of a resistor chain that includes resistors  67 ,  69 . The resistors  63 ,  65 ,  67 ,  69  have values of 150-200 Ohms. The chain is connected to a variable DC source  71 , which combines with the DC voltage V j  and appears across positive terminal  53  and negative terminal  59 . The Model NIPCI-6073 data acquisition tool, available from National Instruments Corporation, 11500 N. Mopac Expwy, Austin, Tex. 78759-3504, is suitable for the source  71 . RF current in a portion of the circuitry delineated by a box  73  is largely eliminated by the presence of 1 mH ferrite inductors  75 , and also by connecting a resistor  77  in series with a 0.15 mF DC blocking capacitor  79  to provide a return path having low impedance to RF, and leading from the positive terminal  53  to ground. As a result, DC is blocked from the return path, but RF is permitted. At the same time, RF current is effectively blocked from a second circuit, which is a path formed by source  71 , inductors  75  and the resistor chain, and which has a greater resistance to RF current than to DC current. This is largely due to the reactance of the inductors  75  seen by the RF source. However, the combined DC output of the source  71  and the voltage V j  flows readily in the second circuit, and DC voltage appears at the negative terminal  59  and at the microvolt input  61  of the generator module  47 . 
     The effect is to separate the DC potential between the positive terminal  53  and the negative terminal  59  from the RF current produced by the generator module  47 . The separation of the DC potential and the RF power allows the jig to be used for two purposes: 
     (1) Simulating different values of the thermocouple potential V j  and the RF power P independently of each other by adjusting the outputs source  71  and the generator  43 . This type of simulation allows various ablation algorithms built into generators (such as the nMARQ RF generator) to be modified or evaluated. Such algorithms typically use values of the potential V j  to control the RF power P. The jig  51  allows simulations of scenarios, such as rapid temperature excursions, e.g., beyond safety limits, or very stable temperatures. When such scenarios occur, the response of the generator module  47  can be evaluated. 
     (2) Calibration of the value of the potential V j  for different values of the power P and other variables such as change of the power P with time. The electromotive force produced by copper-nickel alloys such as constantan as a function of temperature is well-known. In a calibration mode, any desired temperature can be simulated, and the readout of the generator module  47  may be adjusted to correct errors. This calibration can be elaborated to correct for errors that vary according to the levels of RF power being produced. Such calibrations are typically performed at the factory, but may be repeated by maintenance personnel, or even by an operator if desired. 
     While the generator  43  and the display  49  may be integral, as in  FIG. 3 , this is not essential, and they may be provided separately. The simulations and calibrations described above may be performed in any case. 
     The following procedures are explained for convenience with respect to the circuitry shown in  FIG. 3 , but they are not limited to the particular configuration shown therein. 
     Reference is now made to  FIG. 4 , which is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system, in accordance with an embodiment of the invention. At initial step  81  the jig  51  is connected to the generator module  47 , to the display  49  and the direct current source  71 . 
     Next, at step  83 , the power output of the generator module  47  and the source  71  are independently adjusted so as to simulate a sequence of events, which the generator module  47  is expected to recognize and to respond in accordance with its internal programming. 
     For example, the simulator may be adjusted such that the display  49  initially registers 38° C. and progresses to 44° C. while the power varies up to 25 W. 
     In an alternative testing sequence, the simulator may be adjusted such that the display  49  initially registers 38° C. and progresses to an upper temperature limit of 47° C., with oscillations of +/−2° C., during which the power may reach a target of 25 W and then drop, so as to maintain the temperature readings below 47° C. 
     In yet another alternative testing sequence, designed for testing safety of the ablator, the display  49  may initially be set to register 47° C. and progress to 80° C. It is expected that the generator module  47  will issue an alert indicating an abnormally high temperature and will produce control signals intended to reduce or discontinue power output in order to stop the ablation. 
     Next, at decision step  85 , it is determined if the generator module  47  has responded to the testing sequence as programmed. If the determination is affirmative, then control proceeds to final step  87  where a successful result is reported. 
     If the determination at decision step  85  is negative, then control proceeds to final step  89  where failure is reported. 
     Reference is now made to  FIG. 5 , which is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system to calibrate an RF generator having a microvolt input, in accordance with an embodiment of the invention. 
     At initial step  91 , the jig  51  is connected to the generator module  47 , to the display  49  and the direct current source  71 . 
     Next, at step  93 , the source  71  is adjusted to simulate a first temperature, e.g., 25° C. A bias control in the generator module  47  is adjusted such that the display  49  reads 25° C. The generator module  47  may be activated to produce power at an operational level to assure that the display  49  continues to read 25° C. 
     Next at step  95 , the source  71  is adjusted to simulate a second temperature, e.g., 75° C. A sensitivity control in the generator module  47  is adjusted such that the display  49  reads 48° C. The generator module  47  may be activated to produce power at an operational level to assure that the display  49  continues to read 75° C. 
     Steps  93 ,  95  may be iterated, varying the bias and sensitivity controls as necessary to improve the quality of the readings of the display  49 . 
     Next, at decision step  97 , it is determined if the readings of the display  49  are accurate within a defined tolerance limit. If the determination is affirmative, then control proceeds to final step  99  where a successful result is reported. 
     If the determination at decision step  97  is negative, then control proceeds to final step  101  where failure is reported. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.