# Patent

## Title
Model reference identification and cancellation of magnetically-induced voltages in a gradient magnetic field

## Abstract

Systems and methods of dynamically controlling an implanted medical device located within a patient's body in the presence of a gradient magnetic field or other external interference are disclosed. The system can include a reference model of the implanted medical device and of body tissue within the patient's body in the absence of a gradient magnetic field, and a control unit configured to dynamically control voltages or currents applied to a lead of the implanted medical device based on predicted parameters determined by the reference model.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/029,743, filed on Feb. 19, 2008, entitled “Model Reference Identification and Cancellation Of Magnetically-Induced Voltages In A Gradient Magnetic Field,” which is incorporated herein by reference in its entirety.


TECHNICAL FIELD
The present invention relates generally to implantable medical devices and the delivery of diagnostic and therapeutic treatments during medical procedures such as magnetic resonance imaging (MRI). More specifically, the present invention relates to the identification and cancellation of magnetically-induced voltages in a gradient magnetic field or other external noise.
BACKGROUND
Magnetic resonance imaging (MRI) is a non-invasive imaging method that utilizes nuclear magnetic resonance techniques to render images within a patient's body. Typically, MRI systems employ the use of a static magnetic coil having a magnetic field strength of between about 0.2 to 3 Teslas. During the procedure, the body tissue is briefly exposed to RF pulses of electromagnetic energy in a plane perpendicular to the magnetic field. The resultant electromagnetic energy from these pulses can be used to image the body tissue by measuring the relaxation properties of the excited atomic nuclei in the tissue. Pulsed gradient magnetic fields are used for spatial variation of static fields for image phase, frequency encoding, and slice selection within the body.
During imaging, the electromagnetic radiation produced by the MRI system may be picked up by implantable device leads used in implantable medical devices such as pacemakers or cardiac defibrillators. This energy may be transferred through the lead to the electrode in contact with the tissue, which may lead to elevated temperatures at the point of contact. The degree of tissue heating is typically related to factors such as the length of the lead, the conductivity or impedance of the lead, and the surface area of the lead electrodes. Exposure to a magnetic field such as a pulsed gradient magnetic field may also induce an undesired voltage in the lead.
SUMMARY
The present invention relates generally to the identification and cancellation of voltages induced on an implanted medical device located within a patient's body in the presence of a gradient magnetic field or other external noise. An illustrative system for identifying and cancelling magnetically-induced voltages on an implanted medical device having a lead implanted in or near the heart includes a reference model including one or more impedance parameters associated with the lead and of the body tissue within the patient's body, and a control unit adapted to control a voltage or current applied to the lead in the presence of a magnetic field. In some embodiments, the control unit is configured to dynamically control the voltage or current applied to the lead based on the one or more impedance parameters stored within the model. In use, and in some embodiments, the control unit is configured predict the variables (e.g., voltage or current) to be applied to the lead in the presence of the magnetic field in order to compensate for the effects of the field.
An illustrative method of dynamically controlling an implanted medical device located within a patient's body in the presence of a gradient magnetic field includes creating a model of the implanted lead and of body tissue within the body in the absence of a magnetic field, detecting the presence of a gradient magnetic field within the body, measuring the response of an excitation voltage or current applied to the lead in the presence of the gradient magnetic field, comparing the measured response against a modeled response obtained by the model in the absence of a gradient magnetic field, calculating the error between the measured response and the desired, modeled response, and modifying the voltage or current applied to the lead based on the calculated error to steer the measured response towards the desired response.
While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an illustrative medical device having a lead implanted within the body of a patient and subjected to a gradient magnetic field;
FIG. 2 is a block diagram showing several illustrative components of the pulse generator of FIG. 1;
FIG. 3 is a block diagram showing an illustrative model-based system for modeling a medical device implanted within the body;
FIG. 4 is a block diagram showing the model-based system of FIG. 3 in the presence of a gradient magnetic field; and
FIG. 5 is a flow chart showing an illustrative method of dynamically controlling an implanted medical device in the presence of a gradient magnetic field.



While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION
FIG. 1 is a schematic view of an illustrative medical device 12 equipped with a lead implanted within the body of a patient. In the illustrative embodiment depicted, the medical device 12 includes a pulse generator 14 implanted within the patient's body and a lead 16 (e.g., a unipolar or bipolar lead) placed at a location in or near the patient's heart 18. The heart 18 includes a right atrium 20, a right ventricle 22, a left atrium 24, and a left ventricle 26. The pulse generator 14 can be implanted subcutaneously within the body, typically at a location such as in the patient's chest or abdomen, although other implantation locations are possible.
A proximal portion 28 of the lead 16 can be coupled to or formed integrally with the pulse generator 14. A distal tip portion 30 of the lead 16, in turn, can be implanted at a desired location in or near the heart 18 such as the right ventricle 22, as shown. Although the illustrative embodiment depicts only a single lead 16 inserted into the patient's heart 18, in other embodiments multiple leads can be utilized so as to electrically stimulate other areas of the heart 18. In some embodiments, for example, the distal portion of a second lead (not shown) may be implanted in the right atrium 20. In addition, or in lieu, another lead may be implanted in or near the left side of the heart 18 (e.g., in the coronary veins) to stimulate the left side of the heart 18. Other types of leads such as epicardial leads may also be utilized in addition to, or in lieu of, the lead 16 depicted in FIG. 1.
During operation, the lead 16 can be configured to convey electrical signals from the pulse generator 14 to the heart 18. For example, in those embodiments where the pulse generator 14 is a pacemaker, the lead 16 can be used to deliver electrical therapeutic stimulus for pacing the heart 18. In those embodiments where the pulse generator 14 is an implantable cardiac defibrillator, the lead 16 can be utilized to deliver electric shocks to the heart 18 in response to an event such as a heart attack or ventricular tachycardia. In some embodiments, the pulse generator 14 includes both pacing and defibrillation capabilities.
When the pulse generator 14 is subjected to a gradient magnetic field, as shown generally by arrow “B” in FIG. 1, a magnetically-induced voltage (VEMF) is delivered to the lead 16 that interferes with the therapeutic electrical signals delivered by the lead 16. During an MRI procedure, for example, a rapidly changing magnetic field B produced by an energized MRI coil induces an electromotive force voltage on the lead 16 that combines with the excitation voltage normally generated by the pulse generator 14. The voltage VEMF produced can be determined from Faraday's Law as follows:




  
    V
    EMF
  
  =
  
    
      
        d
        B
      
      
        d
        t
      
    
    ⁢
    A
  




Thus, as can be understood from the above equation, the magnitude of the induced voltage VEMF is dependent on the time rate of change of the magnetic field





  
    d
    B
  
  
    d
    t
  




and the effective area A upon which the gradient magnetic field acts.

For a unipolar lead 16 such as that depicted in FIG. 1, the effective area A upon which the gradient magnetic field B acts is defined generally as the area that is bounded by the length of the lead 16 from the proximal end 28 to the distal tip 30 and the distance from the lead tip 30 to the pulse generator 14. For a bipolar lead configuration, the effective area A upon which the gradient magnetic field B acts is typically less than for a unipolar lead, and is defined generally by the area between the tips of the lead electrodes. In either configuration, the voltage VEMF appears as a voltage source in the circuit loop 32 formed by the pulse generator 14, the lead 16, and the tissue impedance between the pulse generator 14 and the lead tip 30. This voltage VEMF induces a current on the lead 16 along with the desired therapeutic stimulus current generated by the pulse generator 14. During operation, this voltage VEMF can result in inappropriate currents on the lead 16 that are then transmitted into the surrounding cardiac tissue. The induced voltage VEMF may also result in device inhibition or inappropriate detections leading to charge timeout faults.
FIG. 2 is a block diagram showing several illustrative components of the pulse generator 14 of FIG. 1. As shown in FIG. 2, the pulse generator 14 includes a control unit 34 adapted to run a control algorithm 36, which as discussed further herein, can be used to provide closed-loop control over the excitation voltage or current applied to the pulse generator lead 16 in the presence of a gradient magnetic field or other external noise. The control unit 34 can be coupled to other components of the pulse generator 14, including a timer circuit 38 for taking time and date measurements, an energy source 40 such as a rechargeable battery or power capacitor, and a storage memory 42 such as a flash memory or ferroelectric memory for storing data and commands used by the pulse generator 14.
The control unit 34 can further include control circuitry for controlling various other implantable medical devices coupled to the pulse generator 14, including one or more remote sensing devices 44 and/or therapy delivery devices 46. Examples of remote sensing 44 devices that can be coupled to the pulse generator 14 can include, but are not limited to, pressure sensors, accelerometers, pulmonary sound sensors, chemical sensors, and temperature sensors. In one embodiment, for example, the control unit 34 can be coupled to a magnetic sensor such as a reed switch or Hall-effect sensor that can be used to detect the presence of magnetic fields within the body.
In the illustrative embodiment of FIG. 2, the control algorithm 36 includes a lead and tissue reference model 48 used by the algorithm 36 to model one or more physical characteristics of the pulse generator lead 16 and the cardiac tissue adjacent to the lead 16. In some embodiments, for example, the reference model 48 includes an impedance parameter associated with the lead 16 as well as an impedance parameter associated with the cardiac tissue between the lead tip 30 and the pulse generator 14. In certain embodiments, the model parameters are stored as parameters within a look-up table or the like, and can be used by the control algorithm 36 to control the operation of the pulse generator 14, including the timing and magnitude of excitation voltage or current signals applied to the lead 16.
FIG. 3 is a block diagram showing an illustrative model-based system 50 for modeling an implanted medical device such as the pulse generator of FIG. 2. As shown in FIG. 3, an excitation voltage V0 generated by the pulse generator 14 is applied to block 52, which represents the actual load characteristics of the lead 16 and the cardiac tissue. In the illustrative embodiment of FIG. 1, for example, the lead and tissue block 52 may represent the impedance of the lead 16 and the impedance of the cardiac tissue between the pulse generator 14 and the lead tip 30. In those embodiments in which a bipolar lead is used, the lead and tissue block 52 may represent the impedance of the cardiac tissue between the exposed portions of the electrodes.
In the absence of a gradient magnetic field, an accurate reference model 48 of the lead and cardiac tissue is developed based on one or more measured responses 54 from the lead and tissue 52. In some embodiments, for example, the reference model 48 may be generated by comparing via a comparator 58 a measured electrical current 54 from the implanted lead and body tissue 52 against a predicted electrical current 56 generated by the model 48 in the absence of a magnetic field. Measurement of the electrical current 54 within the lead 16 can be taken, for example, by a current sensor or by sensing a voltage drop across a reference resistor in series with the lead 16 and the body tissue.
Based on the responses 54,56 received from both the measured (i.e., actual) parameters and the model-based parameters, the comparator 58 outputs an error signal 60 indicating the error in the model's predicted response. This error signal 60 is then fed to the control unit 34, which updates one or more parameters in the model 48 to better simulate the actual lead and tissue impedance 52, thus reducing the model error. If, for example, the model 48 approximates a lead and tissue impedance that is greater than the actual, measured impedance, the control unit 34 may decrease the modeled impedance within the model 48 to minimize the error, similar to that performed by a closed-loop feedback controller such as a proportional-integral-derivative (PID) controller. Other suitable model parameter identification techniques including, but not limited to, least squares estimation (LS), maximum likelihood estimation (MLE), and best linear unbiased estimates (BLUE) may also be used to accurately calculate the model parameters.
The process of updating the model 48 to better approximate the actual lead and body tissue characteristics can be performed continuously, at predetermined time periods, or in response to a control signal from another device. In some embodiments, for example, the process of updating the model 48 can be accomplished initially when the lead is implanted within the body, and then subsequently at predetermined time intervals (e.g., every five minutes) until a static magnetic field is detected within the body, indicating the presence of a gradient magnetic field. In certain embodiments, model parameters such as the impedance of the lead 16 and the surrounding cardiac tissue, for example, can be updated by the control unit 34 by continuously or periodically measuring the voltage across a reference resistor coupled to the lead 16, and then comparing the measured voltage against a predicted current outputted by the model 48.
FIG. 4 is a block diagram showing the model-based system 50 of FIG. 3 in the presence of a gradient magnetic field. In some embodiments, the presence of a static magnetic field within the body during an MRI procedure can be detected by a magnetic sensor such as a Hall effect sensor, reed switch, or the like. From this sensed static magnetic field, the control unit 34 may then deduce that a gradient magnetic field is present within the body, activating an MRI mode within the control unit 34 to compensate for the induced EMF voltage VEMF at the pulse generator 14. Other techniques can also be used to determine the presence of the gradient magnetic field to activate the MRI mode within the control unit 34. In some embodiments, for example, a signal received from an external device in communication within the pulse generator 14 (e.g., from the MRI device) may signal the presence of the gradient magnetic field, causing the control unit 34 to enter into the MRI mode of operation.
In the presence of a gradient magnetic field, a magnetically-induced voltage VEMF produces a current that is transmitted through the lead and body tissue 52, which affects the measured response 54. In some cases, for example, the presence of the voltage VEMF may result in a greater amount of current within the lead than is desired for performing the therapy. This increase in response 54 is then compared against the response 56 predicted by the model 48 in the absence of the magnetic field to produce an error signal 60 that is fed to the control unit 34. In those embodiments where the actual and modeled responses 54,56 are currents, the error signal 60 may represent, for example, the magnitude of the difference between the actual current outputted by the lead and the predicted current outputted by the model 48.
Based on the error signal 60, the control unit 34 seeks to minimize the error between the actual lead and body tissue response 54 and the predicted (i.e., desired) response 56 generated by the model 48 in the absence of the magnetic field. In some embodiments, the control unit 34 then feeds a compensation voltage signal 62 to an adder 64, which is then added to the excitation voltage V0 applied to the leads as voltage signal 66. In this manner, the control unit 34 dynamically modifies the voltage signal 66 applied to the lead and body tissue 52 so that the actual response 54 from the system 50 is similar to the modeled response 56 in the absence of the magnetic field. The output from the system 50 can be continuously compared with the measured responses 54 from the actual lead and body tissue 52, allowing the control unit 34 to maintain the error minimization between the model 48 and the lead and body tissue 52.
FIG. 5 is a flow chart showing an illustrative method 68 of dynamically controlling an implanted medical device in the presence of an external interference such as a gradient magnetic field. The method 68 begins generally at block 70 with the step of measuring the response of an excitation voltage or current applied to a lead implanted at a location within the body prior to the presence of a gradient magnetic field. In some embodiments, for example, measuring the response of the excitation voltage applied to the lead can include measuring the electrical current in the lead as a result of an excitation voltage generated by the pulse generator. Measurement of the electrical current within the lead can be accomplished, for example, by sensing the voltage across a reference resistor coupled to the lead, using a bridge circuit to sense the current within the lead, and/or by other suitable technique. From these measurements, the impedance characteristics of the implanted lead and body tissue are then determined.
From the measured response to the excitation voltage, one or more characteristics of the lead and body tissue are then analyzed to create a model of the lead and body tissue (block 72). In some embodiments, for example, the impedance characteristics of the lead and the body tissue are modeled based on one or more measured responses to an excitation voltage applied to the lead. The model may include, for example, an impedance parameter associated with the lead and an impedance parameter associated with the patient's heart.
Once a model is created representing the impedance of the implanted lead and body tissue, an anticipated response from the model resulting from the applied excitation voltage can then be compared against the actual, measured response produced by the excitation voltage to generate an error signal (block 74). The error signal can then be used by the control unit to adjust the model parameters to better approximate the actual, measured response from the implanted lead and body tissue in order to minimize the error signal (block 76). In some embodiments, for example, the model parameters can be continually or periodically adjusted by the pulse generator to minimize the error produced by the model, or to compensate for any changes to the lead and tissue impedance that occur over time.
The pulse generator can be configured to detect the presence of a magnetic field within the body (block 78), and then initiate a control algorithm within the control unit that dynamically modifies the voltage applied to the lead based on the parameters stored within the model (block 80). In some embodiments, for example, the pulse generator includes a sensor such as a Hall-effect sensor or reed-switch that can detect a static magnetic field or other external noise within the body produced by an MRI device, indicating the presence of a gradient magnetic field. The signal from the sensor can then be fed to the control unit, prompting the control unit to modify the voltage applied to the lead based on the model. In other embodiments, the pulse generator can be put into an MRI mode via a signal received from an external device (e.g., the MRI device), from another external device, or from another device implanted within the body.
Upon the detection of the magnetic field, the control unit can be configured to compare the actual response of the system in the presence of the magnetic field with the predicted response outputted by the model and determined in the absence of the magnetic field (block 82). Based on this comparison, an error signal is created which is then fed to the control unit for use in modifying the excitation voltage or current applied to the lead. In some embodiments, for example, the error signal may represent the magnitude of the difference between the current that is presently being applied to the lead and the current predicted by the model in the absence of the magnetic field. Based on the error signal, the control unit then seeks to minimize the error signal by dynamically modifying the voltage or current applied to the lead (block 84).
Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.

## Claims

1. A method of dynamically controlling an implanted medical device located within a patient's body in the presence of a magnetic resonance imaging (MRI) field, the method comprising:
detecting the presence of an MRI field within the body;
measuring the response of an excitation voltage or current applied to an implantable lead in the presence of the MRI field;
comparing the measured response obtained in the presence of the MRI field with a predicted response outputted by a lead and tissue reference model, the model including one or more model parameters stored within a memory unit for modeling the lead and body tissue within the body; and
dynamically modifying a voltage or current applied to the lead based at least in part on the predicted response outputted by the model.

2. The method of claim 1, wherein the lead and tissue reference model is formed by:
measuring the response of an excitation voltage or current applied to the lead in the absence of an MRI field;
comparing the measured response in the absence of the MRI field to a predicted response generated by the model and outputting an error signal; and
adjusting one or more model parameters of the model to minimize the error signal.

3. The method of claim 1, wherein the implantable lead is a bipolar lead including a plurality of lead electrodes electrically coupled to a pulse generator, and wherein a the lead and tissue reference model comprises a model of the lead implanted within the heart and of the cardiac tissue disposed between the lead electrodes.

4. The method of claim 3, wherein the model includes one or more impedance parameters associated with the implanted lead and the cardiac tissue between the lead electrodes.

5. The method of claim 1, wherein the implantable lead is a unipolar lead including an electrode electrically coupled to a pulse generator, and wherein the lead and tissue reference model comprises a model of the lead implanted within the heart and of the body tissue disposed between the electrode and the pulse generator.

6. The method of claim 5, wherein the model includes one or more impedance parameters associated with the implanted lead and the body tissue between the electrode and the pulse generator.

7. The method of claim 1, wherein the lead and tissue reference model is formed in the absence of the MRI field.

8. The method of claim 1, wherein measuring the response of the excitation voltage or current includes measuring the current in the implanted lead.

9. The method of claim 1, wherein dynamically modifying the voltage or current applied to the lead based on the predicted response outputted by the model includes minimizing an error signal generated by comparing the measured response of the lead in the presence of the MRI field to a predicted response from the model determined in the absence of the MRI field.

10. The method of claim 9, wherein dynamically modifying the voltage or current applied to the lead based on the predicted response outputted by the model is performed by a control unit including a closed-loop feedback controller.

11. A method of dynamically controlling an implanted medical device located within a patient's body in the presence of a gradient magnetic resonance imaging (MRI) field, the method comprising:
measuring the response of an excitation voltage or current applied to a lead implanted in or near the heart in the absence of a gradient MRI field;
creating a model of the implanted lead and of body tissue within the body, the model including one or more impedance parameters stored within a memory unit for modeling the implanted lead and the body tissue within the body;
comparing the measured response in the absence of the gradient MRI field to an anticipated response generated by the model and outputting an error signal;
adjusting one or more of the model parameters to minimize the error signal;
detecting the presence of a gradient MRI field within the body;
measuring the response of an excitation voltage or current applied to the lead in the presence of the gradient MRI field;
comparing the measured response obtained in the presence of the gradient MRI field with a predicted response outputted by the model; and
modifying a voltage or current applied to the lead based at least in part on the predicted response from the model.

12. The method of claim 11, wherein the lead is a bipolar lead including a plurality of lead electrodes electrically coupled to a pulse generator, and wherein creating a model of the implanted lead and of body tissue includes creating a model of the lead implanted within the heart and of the cardiac tissue disposed between the lead electrodes.

13. The method of claim 12, wherein the one or more impedance parameters includes an impedance parameter associated with the implanted lead and an impedance parameter associated with the cardiac tissue between the lead electrodes.

14. The method of claim 11, wherein the lead is a unipolar lead including an electrode electrically coupled to a pulse generator, and wherein creating a model of the implanted lead and of body tissue includes creating a model of the lead implanted within the heart and of the body tissue disposed between the electrode and the pulse generator.

15. The method of claim 14, wherein the one or more impedance parameters includes an impedance parameter associated with the implanted lead and an impedance parameter associated with the body tissue between the electrode and the pulse generator.

16. The method of claim 11, wherein creating a model of the lead is performed in the absence of the gradient MRI field.

17. The method of claim 11, wherein measuring the response of the excitation voltage or current includes measuring the current in the implanted lead.

18. The method of claim 11, wherein dynamically modifying the voltage or current applied to the lead based on the predicted response outputted by the model includes minimizing an error signal generated by comparing the measured response of the lead in the presence of the gradient MRI field to a predicted response outputted from the model.

19. The method of claim 18, wherein dynamically modifying the voltage or current applied to the lead based on the predicted response outputted by the model is performed by a control unit including a closed-loop feedback controller.