Patent Description:
Catheters are used to perform a variety of tasks within human bodies and other bodies including the delivery of medicine and fluids, the removal of bodily fluids, and the transport of surgical tools and instruments. In the diagnosis and treatment of atrial fibrillation, for example, catheters may be used to deliver electrodes to the heart for electrophysiological mapping of the surface of the heart and/or to deliver ablative energy to cardiac tissue, among other tasks. In order to properly administer treatment, the position and orientation of a catheter inside the body must be continuously monitored. One known technique for determining the position and orientation of a catheter within a body is by tracking a plurality of sensors on the catheter using a position sensing and navigation system (sometimes called a localization system). In one exemplary system offered for sale by Abbot Laboratories, under the trademark ENSITE™ VELOCITY™, the sensors comprise electrodes. Such a localization system may be referred to as an electric field-based or impedance-based localization system. In such a system, excitation of pairs of electrodes on the outer surface of the body generates electrical fields within the body. Voltage measurements of catheter electrodes can then be used to determine the position and orientation of the catheter electrodes within a coordinate system of the localization system.

Another technique for determining the position and orientation of a catheter inside the body utilizes magnetic sensors and a magnetic field-based localization system. In such a system, a magnetic field generator may create a magnetic field within the body and to control the strength, orientation, and frequency of the field. The magnetic field(s) are generated by coils of the magnetic field generator and current or voltage measurements for one or more magnetic position sensors (e.g., magnetic field sensors) associated with the catheter are obtained. The measured currents or voltages are proportional to the distance of the sensors from the coils thereby allowing for determining a position of the sensors within a coordinate system of the magnetic field-based localization system.

A combined localization system combines an impedance-based system with a magnetic field-based system. Such a system is sold by Abbott Laboratories under the trademark Ensite Precision™. In such a system, locations of electrodes may be identified in an impedance-based coordinate system in conjunction with identifying the locations of one or more magnetic sensors in a magnetic-based coordinate system. In an embodiment, at least a portion of the electrodes and magnetic sensors may be co-located to define fiducial pairs. This co-location allows for determining a transformation (e.g., transformation matrix) between the coordinate systems. The transformation may be applied to the locations of any electrode to register these locations in the generally more accurate magnetic-based coordinate system once the transformation is determined. Accordingly, the impedance-based electrodes can be identified in the coordinate system of the magnetic field-based localization system thereby increasing the positioning accuracy for the electrodes.

The combined medical positioning system, while providing improved accuracy, has a number of shortcomings. For instance, impedance-based medical localization systems are subject to various types of interference that can impact the accuracy of position measurements. For example, the level of electrical impedance in the patient body is not necessarily constant. Further, the impedance-based system and the magnetic-field system are not, from an electrical standpoint, totally independent. Specifically, operation of the magnetic field-based system can induce noise into the measurements of the impedance-based system. To counteract such induced noise, prior systems have temporally alternated the operation and data collection of the two systems. However, such alternating operation is not feasible for an impedance-based localization system that operates continuously.

<CIT> relates to medical device navigation, in particular for registering a first medical device navigation system coordinate system with a second medical device navigation system coordinate system.

<CIT> relates to electrical impedance-based measurement of electrodes of a medical device to determine contact between tissue and the electrodes of the medical device.

<CIT> relates to medical instruments navigable within the body of a patient using externally applied magnetic fields.

<CIT> relates to systems and apparatus for generating three dimensional organ geometries and tracking medical devices in vivo and methods of real-time tracking of same.

In an aspect, a position sensing and navigation system is provided for measuring responses of electrodes and, in further arrangements, magnetic sensors of a medical device (e.g., catheter). The system includes an impedance-based localization system. Such an impedance-based localization system may include a signal generator configured to generate a plurality of unique drive signals. Each of the unique drive signals may have a unique modulation frequency that is a harmonic of a common base frequency. The signal generator may further be configured to continuously and simultaneously apply each of the plurality of drive signals across an individual pair of electrodes (e.g., surface patch electrodes) to generate an electric field. The impedance-based localization system may also include a measurement circuit for measuring responses (e.g., composite responses to the continuously applied drive signals) of one or more electrodes of a medical device disposed within the electric field. A demodulator is configured simultaneously demodulate the response signal(s) (e.g., composite response signal(s)) for each unique modulation frequency. The demodulator outputs a demodulated data stream that is received by a filter (e.g., down-sampling filter, decimating filter, etc.). The filter outputs impedance-based values proportional to the location of the one or more electrodes for each unique frequency. The impedance-based values may be utilized to render an image of the medical device and/or its surroundings (e.g., heart chamber). To reduce noise in the impedance-based values, a controller may pause input (e.g., skip data input) into the filter during the operation of an electrical noise generating device (e.g., magnetic localization system) such that portion of the demodulated data stream corrupted by electrical noise is discarded. In an embodiment, the input into the filter is paused for a time period that is an integer multiple of a thtr common base period corresponding to the common base frequency (i.e., of the unique modulation frequencies/drive signals). Use of such a time period allows resuming processing of the demodulated data stream without frequency interruption.

In a further arrangement, the system may include a magnetic localization system for use in identifying the location of one or more magnetic sensors of the medical device. The magnetic localization system may include a magnetic field generator for generating a magnetic field. A measurement circuit may measure the magnetic response of one or more magnetic sensors to the magnetic field to generate location information that may be utilized to, for example, render an image of the medical device and/or its surroundings (e.g., heart chamber). The magnetic localization system may be configured to operate (e.g., under control of a controller) while the input to the decimating filter of the impedance-based localization system is paused.

In an arrangement, the system utilizes digital signal processing where the drive signals are digital drive signals. In such an arrangement, the digital drive signals are converted to analog signals by one or more digital-to-analog (DAC) converters prior application across the individual pairs of electrodes. Likewise, one or more analog-to-digital (ADC) converters may convert analog responses of the electrodes to digital response signals. Is such an arrangement, the time period associated with the pause into the decimating filer is measured in discrete ADC samples.

In another arrangement, the system is configured to detect anomalous events that corrupt a measured response signal and discard the data acquired during the anomalous event to prevent corruption in the impedance-based values. In such an arrangement, the system includes a detector configured to analyze digital samples of the response signal prior to entry of the digital samples into the demodulator. The detector may analyze the digital samples on a sample-to-sample based. For instance, the detector may compare the slew rate of the signals to a predetermined threshold to determine the existence of an anomalous event in the response signal. If an anomalous event is detected, the data input into the filter may be paused for a time period (e.g., an integer multiple of the common base frequency) to effectively discard corrupted data. To prevent entry of corrupted data into the filter, the system may further include a buffer that stores a predetermined set of digital sample prior to their entry into the demodulator. Such a buffer may allow identifying an anomalous event and provide enough time to pause/skip input of the demodulated data stream into the filter.

In another aspect, method for use in sensing the position of elongated medical device within a body of a patient is provided. Generally, the method is used with an impedance-based localization system to reduce noise in impedance measurements caused by electric noise generating devices and/or anomalous events. The method includes generating a plurality of unique drive signals each having a unique modulation frequency that is a harmonic of a common base frequency. The drive signals are each simultaneously and continuously applied across a corresponding pair of electrodes (e.g., surface patch electrodes) to generate an electric field. A composite response signal is measured for one or more electrodes of a medical device disposed within the electric field. The composite response signal(s) is synchronously demodulated to generate a demodulated data stream. The demodulated data stream is paused during electric noise generating events during which the data of the data stream is discarded. The remaining demodulated data stream is filtered (e.g., down-sampled, decimated etc.) to output impedance-based values proportional to the location of the one or more electrodes for each unique drive signal. The impedance-based values may be utilized to render an image of the medical device and/or its surroundings (e.g., heart chamber).

The demodulated data stream may be paused at any time. However, each pause will be for a time period that is an integer multiple of the of the common base frequency (i.e., of the unique modulation frequencies/drive signals). Use of such a time period allows resuming processing (e.g., filtering) of the demodulated data stream without frequency interruption. The demodulated data stream may be paused on a predetermined schedule (e.g., duty cycle) to allow the operation of another noise generating device. For example, on a <NUM>:<NUM> duty cycle, the impedance-based localization system may operate half of the time while another noise generating device (e.g., magnetic localization system) operates the other half of the time. Other duty cycles (e.g., <NUM>:<NUM>; <NUM>:<NUM>, <NUM>:<NUM> etc.) are possible. However, all cycles will have a time period that is an integer multiple of the common base frequency of the drive signals. In a further arrangement, the demodulated data stream may be paused for a time period upon identifying an anomalous event.

The foregoing and other aspects, features, details, utilities and advantages of the present disclosure will be apparent from reading the following description and claims, and from reviewing the accompanying drawings.

Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, <FIG> illustrates one exemplary embodiment of a position sensing and navigation system <NUM> (e.g., localization system) for use in navigating an elongated medical device within a body of a patient and generating an image of the device within the body of the patient. As illustrated, the localization system is a combined localization system that acquires impedance measurement as well as magnetic measurements. In this embodiment, the system <NUM> includes, among other components, a model construction system <NUM> and processor apparatus attached to an elongate medical device <NUM>. In this embodiment, the elongated medical device is a catheter <NUM>. The processing apparatus <NUM> may take the form of an Electronic Control Unit (ECU), for example, that is configured to generate and render an image of catheter <NUM> and output the image of the catheter to a display <NUM>. The system <NUM> may further include a user input device (not shown). Although the system is described in terms of rendering a catheter, it should be understood that various elongate medical devices (e.g., introducer sheaths, pacing leads, etc.) could be rendered using the system.

As illustrated in <FIG> and <FIG>, the catheter <NUM> is configured to be inserted into a patient's body <NUM>, and more particularly, into the patient's heart <NUM>. The catheter <NUM> may include a cable connector or interface <NUM>, a handle <NUM>, a shaft <NUM> having a proximal end <NUM> and a distal end <NUM> (as used herein, "proximal" refers to a direction toward the portion of the catheter <NUM> near the clinician, and "distal" refers to a direction away from the clinician and (generally) inside the body of a patient. The connector <NUM> provides mechanical, fluid, and electrical connection(s) for cables, such as, for example, cables <NUM>, <NUM> extending to the ECU and/or other components of system <NUM> (e.g., a visualization, navigation, and/or mapping system, ablation generator, irrigation source, etc.). The handle <NUM>, which is disposed at the proximal end <NUM> of the shaft <NUM>, provides a location for the clinician to hold the catheter <NUM> and may further provide means for steering or guiding the shaft <NUM> within the body <NUM> of the patient. The catheter <NUM> may comprise an electrophysiological (EP) catheter for use in gathering EP data associated with the heart <NUM> to enable generation of an image of the geometry of the heart surface and related EP data. The catheter <NUM> may also allow removal of bodily fluids or injection of fluids and medicine into the body and may further provide a means for transporting surgical tools or instruments within a body including those used for pacing or tissue ablation. Although the catheter <NUM> is described as an EP catheter in an embodiment, it should be understood that the system can be used with a variety of different types of catheters including, for example, intracardiac echocardiography (ICE) catheters and ablation catheters using a wide variety of ablative energies (e.g., radiofrequency, cryogenic, ultrasound, laser or other light, etc.).

As best shown in <FIG>, the catheter <NUM> may include a plurality of electrodes <NUM> such as distal tip electrode <NUM><NUM>, proximal ring electrode <NUM><NUM>, and intermediate ring electrodes <NUM><NUM> (hereafter '<NUM>' unless specifically referenced). The electrodes <NUM> are provided to generate information regarding the position of catheter <NUM> and therefore may function as position sensors. The electrodes <NUM> may also provide information regarding the geometry of the heart <NUM>. The catheter <NUM> may also include one or more magnetic position sensor(s) <NUM>. The magnetic position sensor(s) <NUM> are also provided for use in determining the position of the catheter <NUM> within a body. In the illustrated embodiment, the magnetic sensor <NUM> is disposed within the shaft of the catheter and is formed of a coil. However, it should be understood that the magnetic sensor(s) sensors may take other forms. That is the magnetic sensor(s) may, for example, comprise any conventional position sensors for detecting changes in magnetic fields including Hall effect sensors, magnetoresistive sensors and sensors made from magnetoresistive materials, piezoelectric materials and the like. The catheter <NUM> may further include other conventional components such as, for example and without limitation, a temperature sensor, additional sensors or electrodes, ablation elements (e.g., ablation tip electrodes for delivering RF ablative energy, high intensity focused ultrasound ablation elements, etc.), and corresponding conductors or leads.

Referring again to <FIG>, the system further includes a plurality of patch electrodes <NUM>, a multiplex switch <NUM>, and a signal generator <NUM> (e.g., frequency source) that, in conjunction with the processor <NUM>, collectively define an impedance-based localization system. Other components are possible. The processing apparatus <NUM> may include a programmable microprocessor or microcontroller, or may include an application specific integrated circuit (ASIC). Further, the processing apparatus <NUM> may include a central processing unit (CPU) and an input/output (I/O) interface through which the processing apparatus <NUM> may receive a plurality of input signals including, for example, signals generated by patch electrodes <NUM> and the position sensors <NUM> (e.g., catheter electrodes). Further the processing apparatus may generate a plurality of output signals including, for example, those used to control and/or provide data to, for example, display device <NUM> and switch <NUM>. The processing apparatus <NUM> may be configured to perform various functions, such as those described in greater detail below, with appropriate programming instructions or code (i.e., software). Accordingly, the processing apparatus <NUM> is programmed with one or more computer programs encoded on a computer storage medium for performing the functionality described herein.

With the exception of reference patch electrode <NUM>B called a "belly patch electrode," the patch electrodes <NUM> are provided to generate electrical signals used, for example, in determining the position and orientation of the catheter <NUM> within a three-dimensional coordinate system and in generating EP data regarding the heart <NUM>. In one embodiment, patch electrodes <NUM> are placed orthogonally on the surface of the body <NUM> and are used to create axes-specific electric fields within body <NUM>. For instance, in one embodiment, patch electrodes <NUM>X1, <NUM>X2 may be placed along a first (x) axis. Patch electrodes <NUM>Y1, <NUM>Y2 may be placed along a second (y) axis, and patch electrodes <NUM>Z1, <NUM>Z2 may be placed along a third (z) axis. In addition, a reference electrode (e.g., <NUM>B) is attached to body <NUM>. Each of patch electrodes <NUM> may be coupled to multiplex switch <NUM>. In this embodiment, the processing apparatus <NUM> is configured, through appropriate software, to provide control signals to the switch <NUM> to thereby sequentially couple pairs of electrodes <NUM> to the signal generator <NUM>. Excitation of each pair of electrodes <NUM> generates an electric field within the body <NUM> and within an area of interest such as the heart <NUM>. Voltage levels at non-excited electrodes <NUM>, which are referenced to the belly patch electrode <NUM>B, are filtered and converted and provided to the processing apparatus <NUM> for use as reference values.

Electrodes <NUM> on the catheter <NUM> are disposed within electrical fields created in body <NUM> (e.g., within the heart <NUM>) by exciting the patch electrodes <NUM>. These electrodes <NUM> experience voltages that are dependent on the location between the patch electrodes <NUM> and the position of the electrodes <NUM> relative to the surface of the heart <NUM>. Voltage measurement comparisons (e.g., impedance responses) can be used to determine the position of the electrodes <NUM> within the heart <NUM>. Movement of the electrodes <NUM> within the heart <NUM> (e.g., within a heart chamber) produces information regarding the geometry of the heart <NUM>, EP data as well as location information for the catheter. Though discussed with respect to an orthogonal arrangement of patch electrodes <NUM>, the present disclosure is not meant to be so limited. Rather, in other embodiments, non-orthogonal arrangements (e.g., arrangements of non-orthogonal dipoles) may be utilized to determine the location coordinates (e.g., positions) of the electrodes <NUM>.

The system <NUM> determines the position and orientation of position sensors such as the electrodes <NUM> on an elongate medical device such as the catheter <NUM>. The model construction system <NUM> uses this position and orientation data to generate an image of the catheter <NUM> within the heart <NUM>. More particularly, the processing apparatus <NUM> of the model construction system <NUM> is configured to acquire measured data points (e.g., impedance responses) collected using the position sensors (i.e., electrodes <NUM>), where the measured data points corresponding to respective positions of electrodes <NUM>. In this embodiment, the model construction system <NUM> acquires the measured data points by activating electrodes <NUM> as described above. Generally, the model construction system <NUM> is configured to describe the measured data points as deviations from a parametric form (e.g., a curve, in the case of a one-dimensional catheter <NUM>, or a plane, in the case of a two-dimensional catheter <NUM>) and generate an image of the catheter using such deviations. Stated otherwise, the model construction system utilizes the measured data points with a mathematical model that describes a particular catheter supporting the electrodes to generate an image of that catheter based on the positions of the data points. One exemplary model construction system is set forth in <CIT> entitled "Methods and Systems for Generating Smoothed Images of an Elongate Medical Device".

As further shown in <FIG>, the system <NUM> may further incorporate a magnetic field-based localization system to determine the position and orientation of a catheter and/or similar medical devices within a body. In such a system, a magnetic field generator <NUM> may be employed having three orthogonally arranged coils, arranged to create a magnetic field within the body and to control the strength, orientation, and frequency of the field. Alternately, the magnetic field generator <NUM> may have more than three coils, and such coils may by arranged in pseudo-random orientations. The magnetic field generator <NUM> may be located above or below the patient (e.g., under a patient table) or in another appropriate location. Magnetic fields are generated by coils of the magnetic field generator and current or voltage measurements for one or more magnetic position sensors <NUM> (e.g., magnetic field sensors) associated with the catheter <NUM> are obtained. The measured currents or voltages of the sensors <NUM> are proportional to the distance of the sensors from the coils thereby allowing a position of the sensors within a coordinate system of the system. The positions of the sensors may be utilized by the model construction system to generate an image of the medical device on a display relative to, for example only, a cardiac model or geometry. Exemplary embodiments of magnetic field-based medical positioning systems are set forth in co-owned <CIT> and <CIT>.

When utilizing a dual electric field-based system (e.g., impedance-based system) and magnetic field-based system, the system <NUM> may utilize, for example, the EnSite Precision™ system commercially available from Abbott Laboratories, and generally shown with reference to <CIT> entitled "Method and Apparatus for Catheter Navigation and Location and Mapping in the Heart". In other embodiments, however, the system <NUM> may comprise other types of systems, such as, for example and without limitation: a magnetic-field based system such as the Carto™ system available from Biosense Webster, and as generally shown with reference to one or more of <CIT> entitled "Intrabody Measurement," <CIT> entitled "Medical Diagnosis, Treatment and Imaging Systems," and <CIT> entitled "System and Method for Determining the Location and Orientation of an Invasive Medical Instrument,", or the gMPS system from MediGuide Ltd. , and as generally shown with reference to one or more of <CIT> entitled "Medical Positioning System," <CIT> entitled "System for Determining the Position and Orientation of a Catheter," and <CIT> entitled "Medical Imaging and Navigation System,"; a combination electric field-based and magnetic field-based system such as the Carto <NUM>™ System also available from Biosense Webster; as well as other impedance-based localization systems, acoustic or ultrasound-based systems, and commonly available fluoroscopic, computed tomography (CT), and magnetic resonance imaging (MRI)-based systems.

In summary, the electrodes <NUM> and/or magnetic sensors <NUM> of the catheter <NUM> are electrically coupled to the processing apparatus <NUM> and are configured to serve a position sensing function. The electrodes <NUM> and/or magnetic sensor(s) <NUM> are placed within electric and/or magnetic fields created in the body <NUM> (e.g., within the heart) by sequentially exciting the patch electrodes <NUM> and/or operating the magnetic field generator <NUM>. Using various known algorithms, the processing apparatus <NUM> may then determine the location (position and orientation) of each electrode <NUM> and/or magnetic sensor <NUM> and record it as a measured data point corresponding to a respective position of each sensor in a memory or storage device, such as a memory <NUM>, associated with or accessible by the processing apparatus <NUM>. These data points may then be utilized by the model construction system to generate an image of the catheter and/or to generate a map of an interior patient cavity (e.g., heart chamber).

The impedance-based localization system provides the ability to simultaneously locate a relatively large number of electrodes and has found widespread acceptance and use in the industry. However, because impedance-based systems employ electrical current flow in the human body, these systems can be subject to measurement inaccuracies and efforts have been made to combine impedance-based localization systems with magnetic-based localization systems to improve overall accuracy. However, operation of the magnetic-based localization systems (e.g., magnetic field generator) can induce electrical interference that affect impedance measurements. Accordingly, prior systems have typically interleaved the operation of the impedance-based systems and the magnetic-based system to reduce such interference. By way of example, prior systems sequentially couple the patch electrodes <NUM> to the signal generator <NUM> for impedance measurement. In such an arrangement, the magnetic-based system may operate between the sequential operation of the patch electrodes minimizing electrical interference. While such operation is effective in reducing electrical interference in the impedance measurements, such operation is only possible if operation of the patch electrodes is discontinuous.

Aspects of the present disclosure are further based on the recognition that simultaneously and continuously application of separate unique frequency drive signals (e.g., localization frequencies or modulation frequencies) to the patch electrodes results in location impedance values (e.g., measured impedance values of catheter electrodes in response to being driven by the external patch electrodes) having lower noise levels. In such an arrangement, the patch electrodes are driven continuously rather than being sequentially coupled to a signal generator for impedance location measurements. Use of such continuous unique modulation frequencies not only allows for achieving low noise impedance values but also minimizes crosstalk between channels.

To achieve such low noise impedance location values, continuous unique frequency drive signals (e.g., modulation frequencies) may be utilized. For example, three continuous unique drive frequencies may be utilized where one unique frequency is applied to each patient axis (e.g., <NUM>X1 - <NUM>X2; <NUM>Y1- <NUM>Y2 and <NUM>Z1- <NUM>Z2). The signal processing system uses modulation frequencies that are orthogonal. That is, modulation frequencies that are multiples of a common base frequency. This provides modulation frequencies that are close in value (expressed in cycles per second or Hertz) and that do not interfere with one another. The period of the base frequency determines a time interval in which all the modulation frequencies are periodic. For example, if the base frequency is <NUM>, the base period is <NUM>/<NUM> seconds or <NUM> milliseconds (ms). At a point in time, any of the modulation frequencies will return to the same point cycle every <NUM>. For example, frequencies of <NUM>, <NUM> and <NUM> all have a common base frequency of <NUM> and any <NUM> interval will contain exactly <NUM>, <NUM> or <NUM> cycles, respectively. Such an interval may start at any arbitrary point and will repeat an identical point in each of the modulation frequencies, as discussed herein.

A system that simultaneously and continuously applies separate unique frequency drive signals system utilizes synchronous excitation and synchronous demodulation along with a filtering method that is compatible with orthogonal modulation frequencies. In such a system, a demodulator multiplies the sensor signal (e.g. composite measured signal of each catheter electrode in response to the multiple modulation signals/frequencies) received from an analog-to-digital converter (ADC) by each of the modulation frequencies, on a sample by sample basis. A filter essentially averages the demodulator outputs, one for each modulation frequency, over the base period and provides a result at a rate substantially reduced (down sampled or decimated) from the original A/D sample rate. In an embodiment, the filter is a cascade integrator-comb (CIC) filter. The output rate is the same as base frequency, in this example <NUM> times per second. The result is a set of values proportional to the location of the electrode in each axis (e.g., an X, Y, Z coordinate).

Synchronous demodulation allows the responses to the unique modulation frequencies to be detected independent of each other while minimizing crosstalk. As previously noted, synchronous demodulation includes multiplying the measured and digitized response signal (which is a composite of multiple modulation frequencies) by a replica of each drive signal of exactly the same frequency and a known phase offset. The resultant signal is then low-pass filtered and decimated to (in this example) <NUM> samples per second. The sampling rate of the analog-to digital converter (ADC) is not critical and in fact need not meet the traditional Nyquist sampling rate. However, the amplifying circuit must have adequate bandwidth to pass the signal to the ADC. By calibrating the system and compensating for expected phase delay between drive signal and received signal, quadrature demodulation may occur. Thus, a real component for resistive impedance and an imaginary component for reactive impedance may be found. This is commonly known as complex impedance. Synchronous demodulation also allows for signal extraction with very low current levels though higher current levels provide better signal-to-noise ratio.

<FIG> is a diagrammatic depiction of an embodiment of a combined localization system <NUM> that is configured to synchronously excite electrodes at unique modulation frequencies (e.g., patch electrodes <NUM> and/or catheter electrodes <NUM>) and synchronously demodulate responses of such electrodes. The system is similar to the system described in <FIG> and similar components utilize common reference numbers. In addition to the components described in relation to <FIG>, the system <NUM> includes an analog-to-digital converter (A-to-D) <NUM>, a filter <NUM> (e.g., bandpass filter), a digital to analog converter <NUM>, a filter <NUM> (e.g., bandpass filter), a multifrequency signal circuit or signal generator <NUM> and a demodulator circuit or demodulator <NUM>. Additional circuitry and/or components may be included as discussed below in <FIG> and <FIG>. Again, the system <NUM> may be electronically and/or mechanically coupled with an elongate medical device such as catheter <NUM>. The converters <NUM>, <NUM>, signal generator <NUM> and demodulator <NUM> allow the system to simultaneously apply unique drive/excitation signals to the patch electrodes and measure the responses of the catheter electrodes. More specifically, the signal generator <NUM> outputs multiple orthogonal excitation signals (e.g., modulation or localization frequencies) for use in assessing locations and/or impedances of one or more electrodes. More specifically, the signal generator <NUM> may generate a plurality of excitation or drive signals each having unique modulation frequencies.

The system <NUM> again includes a memory <NUM> and a processor <NUM>. The memory <NUM> may be configured to store data respective of the elongate medical device or catheter <NUM>, the patient, and/or other data (e.g., calibration data). Such data may be known before a medical procedure (medical device specific data, number of catheter electrodes, etc.), or may be determined and stored during a procedure. The memory may also be configured to store instructions that, when executed by the processor <NUM>, cause the ECU to perform one or more methods, steps, functions, or algorithms described herein. For example, but without limitation, the memory may include data and instructions for determining locations and/or impedances respective of one or more electrodes <NUM> on the elongate medical device <NUM>.

<FIG> illustrates one embodiment of the signal source <NUM> (e.g., current source) that provides an excitation signal for one pair of patch electrodes. In the present embodiment, the signal source <NUM> includes a field programmable gate array (FPGA) <NUM>. However, it will be appreciated that other circuitry, including without limitation, application specific integrated chips, Altera Cyclone series or Xilinx Spartan series may be utilized. In the present embodiment, the FPGA <NUM> includes a numerically controlled oscillator (NCO) <NUM>. The NCO <NUM> is a digital signal generator which creates a synchronous (i.e. clocked), discrete-time, discrete-valued representation of a waveform, usually sinusoidal. The NCO <NUM> is programmable to provide a waveform having a desired frequency, amplitude and/or phase.

In the present embodiment, the NCO <NUM> creates a sinusoidal waveform of a desired frequency (e.g., modulation frequency) based on an input (e.g., single fixed-frequency reference) provided from a microprocessor and/or control logic <NUM>. In the present embodiment a microprocessor/control logic <NUM> is incorporated in the FPGA provides the inputs to the NCO <NUM>. However, it will be appreciated that the NCO inputs may be provided by, for example, the processor <NUM> of the ECU. In any arrangement, the NCO <NUM> generates a digital waveform output having a desired frequency. The output of the NCO is received by a digital to analog converter (DAC) <NUM>, which converts the received digital signal to a corresponding analog signal. A bandpass filter <NUM> is utilized to smooth the converted analog signal. A differential driver (e.g., op amp) <NUM> receives the smoothed analog signal from the bandpass filter <NUM> and sends the same signal as a differential pair of signals, each in its own conductor to an isolation transformer <NUM>. Provided that impedances in the differential signaling circuit (e.g., differential driver and isolation transformer) are equal, external electromagnetic interference tends to affect both conductors identically. As the receiving circuit (isolation transformer) only detects the difference between the conductors, the technique resists electromagnetic noise compared to a one conductor arrangement. The isolation transformer <NUM> transfers AC current of the signals originating from the source <NUM> to the patch electrodes (e.g., <NUM>X1 - <NUM>X2). The isolation transformer <NUM> blocks transmission of DC components in the signals from passing to the patch electrodes while allowing AC components in signals to pass. The dual output from the isolation transformer <NUM> is received by AC coupler <NUM> (e.g., capacitor) that further limits low frequency current from passing to the patch electrodes. The AC coupler outputs the signals to the patch electrodes.

<FIG> illustrates one embodiment of a signal measuring circuit (e.g., signal sampler) and a synchronous demodulation circuit. Initially, a response signal (e.g., composite response to the multiple unique modulation frequencies) from one of the catheter electrodes (e.g., intracardiac electrode) is received at a filter <NUM> (e.g., buffer amplifier) that transfers a current from the electrode, which has a low output impedance level, to an analog to digital converter (ADC) <NUM>, which typically has a high input impedance level. The buffer amplifier prevents the ADC from loading the current of electrode circuit and interfering with its desired operation. The ADC <NUM> samples the received analog signal at a known sampling rate (e.g., <NUM>/s) and converts the analog response signal to a digital response signal. In the present embodiment, an output of the ADC passes through a digital isolator <NUM>, which transfers the digital response signal to the control system (e.g., ECU) while isolating the control system from the medical device.

The digital response signal passes to a synchronous demodulator circuit <NUM> which, in the present embodiment, is defined in the same FPGA utilized for the signal source <NUM>. As noted, synchronous demodulation consists of multiplying a digitized response signal by a replica of a drive signal of exactly the same frequency and a known phase offset. That is, a demodulation signal having the same frequency as the drive signal (e.g., modulation frequency) and a known phase offset from the drive signal is generated and multiplied with the digitized response signal. Generating the demodulation signal(s) using the same FPGA <NUM> that generates the drive signal(s) simplifies the demodulation process. However, it will be appreciated that this is not a requirement and that the synchronous demodulator circuit and the signal source may be separate and/or formed of different software and/or hardware components. In any arrangement, the synchronous demodulation circuit must be able to replicate the drive signal for a given unique frequency.

In the illustrated embodiment, the digital response signal is split as it is received by the synchronous demodulator circuit <NUM>. A numerically controlled oscillator (NCO) <NUM> generates sine and cosine representations of the drive signal (e.g., same frequency different phase) based on an input provided from the microprocessor and/or control logic <NUM>. The split digital response signals are multiplied point-by-point by the sine and cosine signals in sine and cosine multipliers 134A, 134B (hereafter <NUM> unless specifically referenced). That is, the digital response signals are processed by synchronous multipliers or demodulators. This yields a real (sine) and an imaginary (cosine) channel. The sine and cosine channels are filtered and decimated by low pass decimating filters 138A, 138B, (hereafter <NUM> unless specifically referenced) which in the present embodiment are formed of cascaded integrator-comb (CIC) filters. Following the example above, where the drive signal is a harmonic of a <NUM> base frequency, the channels/signals are decimated to <NUM> samples per second such that each decimated signal has an integer number of cycles. The decimated signals then pass through a gain and offset calibration 142A, 142B to compensate for expected phase delays between the source signal and the response signal. The signals may then be combined. Thus, a real component of resistive impedance and an imaginary component of reactive impedance may be found. This information may then be transmitted, for example, via an output port <NUM> to, for example, the processor of the ECU. This process is performed (e.g., simultaneously) for each of the three drive frequencies. The ECU may then use this information (impedance location measurements) to generate values proportional to the location of the electrode in each axis (e.g., an X, Y, Z coordinate). Though discussed in relation to determining impedance location measurements, it will be appreciated that the system <NUM> may also be used to synchronously excite and synchronously demodulate pairs of intracardiac electrodes (e.g., bi-polar electrodes) of the catheter <NUM>.

The system <NUM> described in <FIG> allows for continuously exciting the patch electrodes <NUM> and/or intracardiac electrodes <NUM> of a catheter to acquire low noise impedance responses. However, operation of the magnetic field-based localization system <NUM> introduces noise into the acquired impedance responses. That is, operation of the magnetic field-based localization system affects the impedance responses acquired by the impedance-based localization system, which continuously excites (e.g., applies drive signals) the patch electrodes and/or intracardiac electrodes. This is graphically illustrated in <FIG> and <FIG>. <FIG> illustrates impedance location values (e.g., ohms vertical axis) monitored over time (horizontal axis) of four catheter electrodes in a saline tank where the impedance values are acquired by an impedance-based localization system that continuously drives patch electrodes. As illustrated, the impedance location values over time or "location traces" 150a-d identify the position of the four catheter electrodes in a vertical axis. In this example, the catheter electrodes move from a stationary position upward approximately four inches and then back to the stationary position. As shown, each of the traces 150a-d is a substantially smooth line corresponding with low noise. <FIG> illustrates the same movement of the catheter electrodes and corresponding location traces 152a-d obtained by the same impedance-based localization during the operation of a magnetic field-based localization system. As shown, each of the traces <NUM> is distorted as noise from the operation of the magnetic field-based localization system corrupts the impedance location values obtained by the impedance-based localization system. Rendering an image of, for example, a catheter using such data would result in a shaky image requiring significant display filtering. Further, due to the continuous application of drive signals to the patch electrodes, there is no opportunity to operate the magnetic field-based localization system between application of electrode drive signals unlike prior systems, which sequentially drive the electrodes.

To counter the effects of the operation of a magnetic field-based localization system on impedance responses acquired by an impedance-based localization system that operates continuously, the combined system operates in a skipping mode. During the skipping mode, the decimating filter(s) (e.g., CIC filter) are starved of noisy data in an incoming data stream (i.e., demodulated data stream) for a predetermined time period. During this time period, the magnetic field-based localization system energizes its magnetic emitters (e.g., coils) and collects data for a location determination. The coils deenergize shortly before the end of the time period at which time the decimating filter resumes processing of the demodulated data stream. While similar in theory to temporally alternating the operation of both localization systems, the impedance-based localization system never ceases operation and continually drives, for example, the patch electrodes. Further, to allow the decimating filter(s) to properly resume operation, the incoming demodulated data stream must be at the exact location in its cycle before the incoming demodulated data stream was paused. Otherwise, the decimating filter will encounter a frequency discontinuity corrupting its output. Utilization of orthogonal frequencies for drive signals provides a means to stop and start the demodulated data stream at the exact same location in its cycle.

<FIG> illustrates three orthogonal drive signals, which are applied to the three pairs of surface patch electrodes (e.g., <NUM>X1 - <NUM>X2; <NUM>Y1- <NUM>Y2 and <NUM>Z1- <NUM>Z2) for electrode localization. In the illustrated embodiment, the first signal <NUM> has a frequency of <NUM>, the second signal <NUM> has a frequency of <NUM> and the third signal <NUM> has a frequency of <NUM>. As previously noted, the frequencies are multiples of a common base frequency of <NUM> and the period of the base frequency determines a time interval in which all the 'modulation frequencies' are periodic. When the base frequency is <NUM>, the base period is <NUM>/<NUM> seconds or <NUM> milliseconds (ms). At point in time, any of the modulation frequencies will return to the same point cycle every <NUM>. For example, the frequencies of <NUM>, <NUM> and <NUM> all have a common base frequency of <NUM> and any <NUM> interval will contain exactly <NUM>, <NUM> or <NUM> cycles, respectively. Accordingly, if a demodulation data stream into the decimating filter is paused for a time period that is an integer multiple of the base period, resumption of the demodulation data stream into the decimating filter will occur at the exact same location in all of the modulation frequencies.

<FIG> also collectively illustrate an exemplary demodulated data stream of a response signal including information of the three drive signals <NUM>, <NUM> and <NUM> as are applied to the patch electrodes. That is, these figures can be used to visualize a response signal from a catheter electrode that is measured in response to the drive signals and converted from an analog signal to a digital signal by and analog to digital converter (ADC). The digital signal is demodulated, which forms the demodulation data stream into the decimating filter(s) (e.g., 138A, 138B). e.g., <FIG>. In the illustrated embodiment, the sample rate of the ADC is <NUM>/s such that <NUM> ADC samples corresponds to a <NUM> base period. As shown in <FIG>, the demodulated data stream into the filter(s) is paused/suspended at ADC sample <NUM> at time <NUM>. <FIG> shows the resumption of the demodulated data stream at ADC sample <NUM> (<NUM> ADC samples after pausing) at time <NUM> exactly <NUM> (i.e., one base period) after the pause began. As shown, the right edge of the paused signals <NUM>-<NUM> of <FIG> match exactly with the left edge of the resumed signals <NUM>-<NUM> of <FIG>. Accordingly, upon resumption of the demodulated data stream into the filter, the filter sees no frequency discontinuity. There may, however, be some change in the amplitude in the resumed signals due to, for example, a change in electrode or catheter location, however, such a change is expected to be small due to the short pause interval.

The ability to resume processing of a multi-frequency response signal (e.g., demodulated data stream) at the same frequency point for all of the multiple frequencies allows for operating a noise generating device (e.g., magnetic field-based localization system) while processing of the response signal is paused. <FIG> illustrates pausing processing of response signals to allow for the operation of a noise generating device such as a magnetic field-based localization system. For simplicity, this example only illustrates two electrode drive signals (e.g., patch excitation signals), however, it will be appreciated that any number of drive signals are possible so long as the drive signals are orthogonal. As shown, the two drive signals <NUM>, <NUM> have different frequencies while having a common base frequency (i.e., orthogonal signals). As shown, each of the drive signals <NUM>, <NUM> is applied continuously. <FIG> also illustrates response signals (e.g., sensed signals) or demodulated data streams <NUM>, <NUM> generated in response to application of the drive signals <NUM>, <NUM>, respectively. These data streams <NUM>, <NUM> may represent the output of the demodulator(s) that will be received by the decimation filter(s). As shown, at a first point in time <NUM> (e.g., beginning of a base period), it may be desirable to, for example, obtain a magnetic field-based measurement of magnetic sensors or operate another noise generating device. Accordingly, the data streams <NUM>, <NUM> for each of the sensed signals or demodulated data streams may be paused at the beginning of the base period <NUM> and resumed at the end of the base period <NUM> or integer multiple of the base period (e.g., <NUM> for a base frequency of <NUM>). During this pause or skip, an electrical noise generating device such as a magnetic field generator may apply a signal <NUM> (e.g., magnetic field) after the beginning <NUM> of the base period and terminate the signal <NUM> a short time before the end of the base period. During the base period <NUM>, the demodulated data streams are paused, and the data associated with these signals is discarded. At the end of the base period <NUM>, the demodulated data streams resume entry into the filter at the exact frequency location in each sensed signal. Accordingly, the filter(s) sees an effective data stream 172A, 174A, from a frequency standpoint, that is continuous and without interruption. In effect, the pausing of the demodulated data streams into the filter(s) allows for interleaving the operation of a noise generating device such as a magnetic field-based localization system with a continuously operating impedance-based localization system without introducing noise into impedance-based measurements.

The pausing of the sensed signals allows the decimating filter to skip corrupted or noisy data. That is, the corrupted data is effectively discarded. This allows continued processing of the signals free of the noisy data. <FIG> illustrates a continuation of the graphically illustrated example of <FIG> where impedance location values for four electrodes are acquired during the operation of a magnetic field-based localization system. More specifically, <FIG> illustrates the location traces 154a-d that correspond to the location traces 152a-b of <FIG>. However, these location traces 154a-d are generated from data streams that are paused during the operation of the magnetic field-based localization system. That is, the decimating filter(s) skip the portions of the incoming data streams corresponding to the operation of the magnetic field-based localization system. As a result, the location traces 154a-d of <FIG> are significantly smoother (i.e., lower noise) than the location traces 152a-d of <FIG> and approaching the low noise level of location traces 150a-d generated free of any operation of a magnetic field-based localization system.

Of note, the pausing of the demodulated data streams during a noise or interference event (e.g., operation of a magnetic field generator) requires precise time intervals to allow corrupt (e.g., noisy) impedance data to be held off from the decimating filter and properly resumed. As disclosed herein, the precise time intervals are measured in terms of discrete ADC samples. For example, <NUM> ADC samples corresponds to <NUM> when a sampling rate is <NUM>/s. However, other means for precisely measuring the time intervals could be implemented though it is currently believed that ADC sample rate timing to be the most effective. Of further note, during a noise or interference event, while a data stream going into a decimation filter(s) is suspended, it is not necessary to suspend data (e.g., ADC output) from entering the demodulator(s) (e.g., synchronous multipliers <NUM>) as the demodulator(s) has no memory beyond a current sample. That is, the demodulator only provides the most recent product for the digital response signal multiplied by the corresponding values of from the reference signals (e.g., drive signals).

<FIG> illustrates one block diagram of one embodiment of a skip controller or skip control system <NUM> that may be implemented in the signal measuring circuit (e.g., signal sampler) and a synchronous demodulation circuit of <FIG> or in a similar circuit. The skip control system <NUM> permits interleaving response measurement of a continuously operating impedance-based localization system with another noise generating system or device such as a magnetic field-based localization system (e.g., external equipment). In this embodiment, the skip control system is used to interleave response measurements for a system using continuous orthogonal drive signals having a <NUM> base period. In the illustrated embodiment, a common clock <NUM> is connected to the ADC <NUM>, the sine and cosine generator <NUM> (e.g., numerically controlled oscillator) and an interleave counter <NUM>. The clock may utilize samples from the ADC <NUM> as a basis for timing. As illustrated, the ADC <NUM> receives an analog response signal from an electrode and converts the analog signal to a digital signal at a predetermined sampling rate (e.g., <NUM>/s) that is provided to the demodulator multiplier <NUM>. The demodulator multiplier demodulates the signal in accordance with the reference signal from the sine and cosine generator <NUM>. Optionally, a buffer filter <NUM> may be provided between the output of the ADC and the input of the demodulator. Functionality this filter <NUM> is further discussed in relation to <FIG> and <FIG>.

The interleave counter <NUM> monitors the time from the common clock and/or ADC. In the present embodiment, at every <NUM> period (e.g.. , <NUM> ADC samples for a sampling rate of <NUM>/s), the interleave counter generates a skip request which is provided to trigger or initiate operation of an external equipment system such as a magnetic field-based localization system. The skip request is also provided to a skip counter <NUM> disposed between the demodulator/multiplier <NUM> and the decimating filter <NUM>. In the present embodiment, the skip counter <NUM> begins counting a <NUM> period (e.g., <NUM> ADC samples for a sampling rate of <NUM>/s), upon receiving the skip request. During this <NUM> period, the skip counter (or the controller) discards the incoming demodulated data stream from the demodulator multiplier <NUM> preventing entry of this potentially noisy data into the filter <NUM>. During this <NUM> period, the external equipment may operate. For instance, the magnetic field-based localization system may energize magnetic coils to obtain magnetic measurements of one or more magnetic sensors. In such an arrangement, the external equipment is configured to start and complete its operation during the <NUM> skip period while the demodulated data stream is discarded. At the end of the <NUM> skip period, the demodulated data stream is provided to the filter <NUM> which processes the data stream (e.g., without frequency discontinuity) for the next <NUM> when the next skip request is received. In this embodiment, a <NUM>/<NUM> duty cycle or <NUM>: <NUM> interleave exists between the external equipment and the impedance-based system. However, it will be appreciated that other duty cycles are possible, for instance, the interleave counter period may be <NUM> providing <NUM> (one base period) the external equipment to operate and <NUM> (two base periods) for the impedance-based system to operate. Likewise, the skip counter <NUM> may have a <NUM> period providing the external equipment <NUM> to operate while the impedance-based system operates in the remaining <NUM>. Any combination is possible as long at the interleave counter and skip counter utilize integer multiples of the base period of the impedance drive signals associated with the incoming response signal.

Though primarily discussed above as allowing the interleaving of impedance-based measurements with magnetic field-based measurements, it will be appreciated that the interleaving process may be applied to other noise generating systems. For example, with minor changes, cardiac pacing pulses may also be rejected from the impedance processing system, minimizing the otherwise large disturbance caused by a pacing pulse. During cardiac procedures, it is common to use an external pacing system to stimulate the heart as part of diagnosis. The pacing pulse is generally at the rate of desired beats per minute, for example, <NUM> to <NUM> bpm. The pulse is of a short duration, usually under <NUM> milliseconds, but of a very large electrical amplitude. This short pulse disturbs the measured impedance responses for a very short time. The present disclosure may be modified to providing a "blanking" function that effectively removes the pacing pulse. That is, detecting the pacing pulse and keeping it out of the signal processing of the decimating filter, as described above, effectively suppresses the pacing pulse from the impedance data. Further, such a blanking function may be utilized to detect and suppress any anomalous event that results in a spike in a received response signal.

<FIG> illustrates one block diagram of one embodiment of a blanking controller or blanking control system <NUM>. As illustrated, the blanking control system <NUM> shares a number of common components with the skip control system <NUM> of <FIG>. Accordingly, like components utilize like reference numbers. As shown, the blanking control system incorporates an anomaly detector <NUM> connected to the output of the ADC <NUM>. The detection of an anomaly, such as a pacing pulse, is performed by measuring the sample-to-sample difference (slew rate) of the output of the ADC <NUM>. That is, adjacent samples of the ADC are compared and if a difference between the samples is greater than a predetermined threshold, the anomaly detector outputs a skip request to the skip counter <NUM>. The skip counter <NUM> begins counting a <NUM> period (e.g., one base period in the present example) upon receiving the skip request. During this <NUM> period, the skip counter discards the incoming demodulated data stream from the demodulator/multiplier <NUM> preventing entry of this potentially noisy data into the filter <NUM> and thereby blanking the anomalous event (e.g., presuming the anomalous event is shorter that <NUM> in duration).

In order to blank unknown anomalous events as they arrive in the measurement circuit, a buffer or delay filter <NUM> is disposed between the ADC <NUM> and the demodulator multiplier <NUM>. The delay filer <NUM> briefly stores a small set of ADC samples (e.g., in memory) to temporarily store data to allow the detector <NUM> time to analyze the samples and, when necessary, output a skip request. Only a small number of ADC samples are required, on the order of <NUM> to <NUM>. A small storage buffer of this many samples is used such that the samples are delayed by a corresponding number of sample periods. At a sampling rate <NUM>/s, this is a delay of only <NUM> to <NUM> microseconds, an imperceptible delay to a user. This short delay prevents the samples corrupted by the anomalous event (e.g., pacing pulse) reaching the decimation filter <NUM> before skipping (processing suspension) begins. If the anomalous event continues beyond <NUM> (e.g., one base period in the present example), the anomaly detector <NUM> may issue another one base period skip request. However, this is not a requirement. In any embodiment, the blanking control system <NUM> allows for removing unforeseen noise form the signals before they are processed.

<FIG> illustrates a combined control system <NUM> that combines the functionality of the skip control system <NUM> and the blanking control system <NUM>. As illustrated, the combined control system <NUM> is operative to utilize the interleave counter <NUM> to issue skip request and trigger external equipment. In addition, the Anomaly detector <NUM> may issue skip request upon identifying an incoming anomalous event, such as a pacing pulse.

<FIG> illustrates a process flow sheet illustrating one process <NUM> for interleaving impedance measurements with the operation of a noise generating device such as a magnetic field-based localization system. Initially, orthogonal drive signals are continuously and simultaneously applied <NUM> to patch electrodes of an impedance-based localization system to create an electric field. Once the electric field is generated, a response of an electrode (e.g., catheter electrode within the electric field) to all of the drive signals is measured <NUM>. Such measurement typically includes converting the measured response to a digital signal as discussed above. A demodulator synchronously demodulates <NUM> the measured response to generate a demodulated data stream <NUM>. Once demodulated, a decision may be made regarding if the demodulated data stream should be input into a filter (decimating filter). That is, a decision to pause <NUM> the data stream is made. Such a pause decision may be based on a duty cycle (e.g., timer, ADC cycles etc.) for operating the impedance-based localization system and an external noise generating device (e.g., magnetic field-based localization system), if the data stream is paused, a noise generating device may operate <NUM> for a time period that is an integer multiple of a base period of the orthogonal drive signals. If the data stream is not paused, the data stream enters the filter, which down-samples/filters <NUM> the data stream to generate impedance-based location value. If the process <NUM> continues <NUM>, the filter continues filtering <NUM> data stream continues being processed by the filter until the next pause decision.

Claim 1:
A position sensing and navigation system for use in navigating a medical device (<NUM>) within a body of a patient, comprising:
an impedance-based localization system having:
a signal generator (<NUM>) configured to continously and simultaneously apply each one of a plurality of drive signals across a corresponding one of a plurality of pairs of patch electrodes (<NUM>) to create an electric field, wherein the drive signals each have a unique frequency that is a harmonic of a common base frequency;
a demodulator (<NUM>) configured to synchronously demodulate a composite response signal of at least one electrode (<NUM>) of a medical device, disposed in the electric field, for the plurality of drive signals and output a demodulated data stream (<NUM>, <NUM>); and
a filter (<NUM>) configured to receive the demodulated data stream (<NUM>, <NUM>) and output impedance-based values proportional to the location of the electrode (<NUM>) for each unique frequency;
a magnetic field-based localization system having:
a magnetic field generator (<NUM>) for generating a magnetic field for use in acquiring a magnetic response from at least one magnetic sensor (<NUM>) of the medical device (<NUM>) disposed in the magnetic field; and
a controller configured to pause input of the demodulated data stream into the filter during operation of the magnetic field generator (<NUM>) for a time period that is an integer multiple of a common base period corresponding to the common base frequency