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

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.

1 . A method, comprising:
 sensing a magnetic field using a circuit having an inductor with a magnetic core that saturates in the presence of a magnetic field having a prescribed flux density;   measuring impedance of the circuit; and   sending a signal to an implantable medical device when the measured impedance indicates the inductor is saturated.   
     
     
         2 . The method of  claim 1 , further comprising:
 modifying behavior of the implantable medical device upon receipt of the signal.   
     
     
         3 . The method of  claim 2 , wherein modifying behavior includes changing to a pacing mode. 
     
     
         4 . The method of  claim 2 , wherein modifying behavior includes changing to a tachy mode. 
     
     
         5 . The method of  claim 2 , wherein modifying behavior includes changing lead configuration. 
     
     
         6 . The method of  claim 2 , wherein modifying behavior includes changing to a protect mode. 
     
     
         7 . The method of  claim 1 , wherein measuring impedance of the circuit includes measuring impedance of the circuit by providing a current pulse to the circuit and measuring voltage across the circuit. 
     
     
         8 . A system, comprising:
 a sensing circuit including an inductor with a magnetic core that saturates in the presence of a magnetic field having a prescribed flux density; and   a measuring circuit configured to measure impedance of the sensing circuit; and   a control circuit configured to provide a signal to an implantable medical device when the measured impedance indicates the inductor is saturated.   
     
     
         9 . The system of  claim 8 , wherein the prescribed flux density includes a flux density of a Magnetic Resonance Imaging (MRI) machine. 
     
     
         10 . The system of  claim 8 , wherein the measuring circuit includes a transthoracic impedance measurement module (TIMM) configured to measure impedance of the sensing circuit. 
     
     
         11 . The system of  claim 8 , wherein the measuring circuit includes a controller. 
     
     
         12 . The system of  claim 8 , wherein the implantable medical device is configured to modify device behavior upon receipt of the signal. 
     
     
         13 . The system of  claim 12 , wherein the implantable medical device is configured to change to a pacing mode upon receipt of the signal. 
     
     
         14 . The system of  claim 12 , wherein the implantable medical device is configured to change to a tachy mode upon receipt of the signal. 
     
     
         15 . The system of  claim 12 , wherein the implantable medical device is configured to change lead configuration upon receipt of the signal. 
     
     
         16 . The system of  claim 12 , wherein the implantable medical device is configured to change to a protect mode upon receipt of the signal. 
     
     
         17 . A method of detecting a magnetic resonance imaging (MRI) device, the method comprising:
 using a sensing circuit including multiple inductors, each of the multiple inductors oriented in a different direction and having a magnetic core that saturates in the presence of a magnetic field having a flux density of the MRI device independent of direction, to sense the presence of a magnetic field near an IMD; and   using a control module connected to the sensing circuit to monitor the inductors and to send a signal to the IMD when at least one of the inductors saturates.   
     
     
         18 . The method of  claim 17 , comprising using the control module to measure impedance of a circuit including the inductors. 
     
     
         19 . The method of  claim 18 , comprising using the control module to measure impedance of a circuit including the inductors at a frequency from 1 kHz to 1 MHz. 
     
     
         20 . The method of  claim 18 , comprising using the control module to measure impedance of a circuit including the inductors at a frequency of 20 Hz.