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

• US 8,710,826 B2
• Filed: 10/27/2011
• Issued: 04/29/2014
• 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: Gauglitz, Karl, Bocek, Thomas M., Rutzer, Mark, Matthews, Lorin W., Reagor, William E., Erbstoeszer, Blair
• Primary Examiner(s): DOUGHERTY, SEAN PATRICK
• Application Number:
  US13/282,968
• Publication Number:
  US 20120046707A1
• Time in Patent Office: 915 Days
• Field of Search: 324/200, 324/207.13, 324/207.16, 324/244, 340/539.12, 340/551, 600/12, 600/14, 600/411, 600/423, 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. A system, comprising:

a stimulation lead having a proximal portion and a distal portion;

at least one electrode along the distal portion of the at least one lead; and

an implantable medical device, coupled to the proximal portion of the lead, the implantable device including;

a stimulator adapted to generate stimulation signals with appropriate signal characteristics to provide therapy; and

a controller to communicate with the stimulator, the controller being adapted to deliver an electrical signal to the at least one electrode to provide therapy;

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; and

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 to the controller when the impedance changes by a prescribed amount.

2. The system of claim 1, wherein the impedance measuring circuit is adapted to measure impedance of the sensing circuit at a frequency from 1 kHz to 1 MHz.

3. The system of claim 1, wherein the impedance measuring circuit is adapted to measure impedance of the sensing circuit at a frequency of 20 Hz.

4. The system of claim 1, wherein the impedance measuring circuit is adapted to measure impedance of the sensing circuit once every 30 seconds.

5. The system of claim 1, wherein the inductor includes a ferrite drum core inductor.

6. The system of claim 1, wherein the inductor includes a gapped toroidal inductor.

7. The system of claim 1, wherein the inductor includes a tape wound core inductor.

8. The system of claim 1, wherein the inductor includes an isotropic material.

9. The system of claim 1, wherein the inductor is sized to fit within a housing of an implantable medical device.

10. The system of claim 1, wherein the inductor has maximum dimensions of 0.4 inches in width, 0.4 inches in length and 0.3 inches in height.

11. A system, comprising:

an inductor including saturable magnetics, such that the inductor is configured to saturate as a result of a prescribed magnetic field intensity of a magnetic resonance imaging device independent of direction;

a measuring circuit electrically connected to the inductor, the measuring circuit configured to programmably detect a parameter indicative of impedance of a circuit including the inductor and to provide a signal when the detected parameter changes by a prescribed amount; and

a controller configured to receive the signal and modify the system in response to the signal.

12. The system of claim 11, wherein the parameter indicative of impedance includes phase shift.

13. The system of claim 11, wherein the controller is configured to change a lead configuration in response to the signal.

14. The system of claim 11, further comprising an implantable medical device.

15. The system of claim 14, wherein the controller is configured to change the device to a pacing mode in response to the signal.

16. The system of claim 14, wherein the controller is configured to change the device to a tachy mode in response to the signal.

17. The system of claim 14, wherein the controller is configured to change the device to a protect mode in response to the signal.

18. The system of claim 14, wherein the implantable medical device includes a cardiac resynchronization therapy (CRT) device.

19. The system of claim 14, wherein the implantable medical device includes a defibrillator.

20. The system of claim 14, wherein the implantable medical device includes a neural stimulator.