Systems and methods for sensing external magnetic fields in implantable medical devices

• US 8,049,489 B2
• Filed: 07/26/2006
• Issued: 11/01/2011
• Est. Priority Date: 07/26/2006
• Status: Active Grant

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Matthews, Lorin W., Rutzer, Mark, Reagor, William E., Gauglitz, Karl, Erbstoeszer, Blair, Bocek, Thomas M.
• Primary Examiner(s): Hindenburg; Max
• Assistant Examiner(s): DOUGHERTY, SEAN PATRICK
• Application Number:
  US11/460,081
• Publication Number:
  US 20080071168A1
• Time in Patent Office: 1,924 Days
• Field of Search: 600/423, 600/547, 600/12, 600/14, 600/411, 600/420, 324200-207, 324/13, 324/207.16, 324/244, 340/539.12, 340/551, 607/116
• US Class Current: 324/207.16
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: A61N 1/08 Arrangements or circuits for monitoring, protecting, controlling or indicating for external stimulators A61N1/3603; for implantable neurostimulators A61N1/36128; for heart stimulators A61N1/37; for defibrillators A61N1/3925
• CPC Class Codes: A61N 1/3605 Implantable neurostimulators for stimulating central or peripheral nerve system
• CPC Class Codes: A61N 1/3718 Monitoring of or protection against external electromagnetic fields or currents
• CPC Class Codes: A61N 1/3956 Implantable devices for applying electric shocks to the heart, e.g. for cardioversion
• CPC Class Codes: G01R 33/287 involving active visualization of interventional instruments, e.g. using active tracking RF coils or coils for intentionally creating magnetic field inhomogeneities
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26

Abstract

Systems and methods for sensing external magnetic fields in implantable medical devices are provided. One aspect of this disclosure relates to an apparatus for sensing magnetic fields. An apparatus embodiment includes a sensing circuit with at least one inductor having a magnetic core that saturates in the presence of a magnetic field having a prescribed flux density. The apparatus embodiment also includes an impedance measuring circuit connected to the sensing circuit. The impedance measuring circuit is adapted to measure impedance of the sensing circuit and to provide a signal when the impedance changes by a prescribed amount. According to an embodiment, the sensing circuit includes a resistor-inductor-capacitor (RLC) circuit. The impedance measuring circuit includes a transthoracic impedance measurement module (TIMM), according to an embodiment. Other aspects and embodiments are provided herein.

Claims

1. An apparatus, comprising:

a sensing circuit including at least one inductor having a magnetic core that saturates in the presence of a magnetic field having a flux density of a magnetic resonance imaging device, the magnetic field having a prescribed flux density; and

an impedance measuring circuit connected to the sensing circuit, the impedance measuring circuit adapted to measure impedance of the sensing circuit at programmed intervals and to provide a signal when the impedance changes by a prescribed amount.

2. The apparatus of claim 1, wherein the impedance measuring circuit includes a transthoracic impedance measurement module (TIMM), the TIMM configured to measure impedance of the sensing circuit.

3. The apparatus of claim 1, wherein the impedance measuring circuit delivers a current pulse to the sensing circuit to measure impedance.

4. The apparatus of claim 3, wherein the impedance measuring circuit delivers a current pulse to the sensing circuit periodically.

5. The apparatus of claim 1, wherein the sensing circuit includes a resistor-inductor-capacitor (RLC) circuit.

6. The apparatus of claim 1, wherein the impedance measuring circuit provides the signal to an implantable medical device.

7. The apparatus of claim 6, wherein the implantable medical device modifies device behavior after receiving the signal.

8. An apparatus, comprising:

means for sensing the presence of a magnetic field from a magnetic resonance imaging device, wherein the means for sensing includes an isotropic material such that the means for sensing is adapted to saturate as a result of magnetic field intensity independent of direction; and

means for measuring impedance connected to the sensing means, including means for signaling when the measured impedance of the sensing means changes by a prescribed amount, wherein the means for measuring is adapted to measure impedance of the means for sensing at programmed intervals.

9. The apparatus of claim 8, wherein the means for sensing includes an inductor.

10. The apparatus of claim 9, wherein the inductor includes a magnetic core.

11. The apparatus of claim 10, wherein the magnetic core saturates at a selected magnetic flux density.

12. The apparatus of claim 11, wherein the selected magnetic flux density includes a flux density of a Magnetic Resonance Imaging (MRI) machine.

13. The apparatus of claim 10, wherein the prescribed amount includes a difference in impedance caused by saturation of the magnetic core.

14. The apparatus of claim 8, wherein the means for sensing includes an RLC circuit.

15. The apparatus of claim 8, wherein the means for measuring includes a TIMM, the TIMM configured to measure impedance of the sensing means.

16. The apparatus of claim 8, wherein the means for measuring includes a controller.

17. An implantable medical device, comprising:

a sensing circuit including at least one inductor having a magnetic core that saturates in the presence of a magnetic field having a prescribed flux density of a magnetic resonance imaging device;

an impedance measuring circuit connected to the sensing circuit, the impedance measuring circuit adapted to periodically measure impedance of the sensing circuit and to provide a signal when the impedance changes by a prescribed amount; and

a controller adapted to receive the signal and modify device behavior while in the presence of the magnetic field.

18. The device of claim 17, wherein the impedance measuring circuit includes a transthoracic impedance measurement module (TIMM), the TIMM configured to measure impedance of the sensing circuit.

19. The device of claim 17, wherein the sensing circuit includes a resistor-inductor-capacitor (RLC) circuit.

20. The device of claim 17, wherein the sensing circuit includes multiple inductors oriented in more than one direction.

21. The device of claim 17, wherein the prescribed flux density includes flux density of a Magnetic Resonance Imaging (MRI) machine.