# US 2014/0194729 A1 · Systems and methods for sensing external magnetic fields in implantable medical devices > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2014/0194729 A1 - Document ID: US20140194729A1 - Status: Published application - Kind: A1 - Application number: 14/204,559 - Filed: 2014-03-11 - Published: 2014-07-10 - Earliest claimed: 2006-07-26 - Assignees: Cardiac Pacemakers, Inc. - Applicants: Cardiac Pacemakers, Inc. - Inventors: Karl Gauglitz; Thomas M. Bocek; Mark Rutzer; Lorin W. Matthews; William E. Reagor; Blair Erbstoeszer - Web page: https://patentgrep.com/patent/US20140194729A1 ## CPC - G01R 33/287: {involving active visualization of interventional instruments, e.g. using active tracking RF coils or coils for intentionally creating magnetic field inhomogeneities} - A61N 1/3605: {Implantable neurostimulators for stimulating central or peripheral nerve system} - A61N 1/3718: {Monitoring of or protection against external electromagnetic fields or currents} - A61N 1/3956: {Implantable devices for applying electric shocks to the heart, e.g. for cardioversion} - 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)} - 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)} - G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12) - A61N: ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY (measurement of bioelectric currents A61B; surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body A61B18/00; anaesthetic apparatus in general A61M; incandescent lamps H01K; infrared radiators for heating H05B) - G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02) ## 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 ### Claim 1 An omnidirectional magnetic resonance imaging (MRI) detection apparatus to communicate a presence of an MRI device to an implantable medical device (IMD), the apparatus comprising: an inductor having a magnetic core that saturates in the presence of a magnetic field having a flux density of the MRI device independent of direction, wherein the inductor is sized to fit within a housing of the IMD; and a control module connected to the inductor, the control module adapted to monitor the inductor and to send a signal to the IMD when the inductor saturates. ### Claim 2 (depends on 1) The apparatus of claim 1, wherein the control module is configured to measure impedance of a circuit including the inductor. ### Claim 3 (depends on 2) The apparatus of claim 2, wherein the control module is adapted to measure impedance of a circuit including the inductor at a frequency from 1 kHz to 1 MHz. ### Claim 4 (depends on 2) The apparatus of claim 2, wherein the control module is adapted to measure impedance of a circuit including the inductor at a frequency of 20 Hz. ### Claim 5 (depends on 2) The apparatus of claim 2, wherein the control module is adapted to measure impedance of a circuit including the inductor once every 30 seconds. ### Claim 6 (depends on 1) The apparatus of claim 1, wherein the inductor includes a ferrite drum core inductor. ### Claim 7 (depends on 1) The apparatus of claim 1, wherein the inductor includes a gapped toroidal inductor. ### Claim 8 (depends on 1) The apparatus of claim 1, wherein the inductor includes a tape wound core inductor. ### Claim 9 (depends on 1) The apparatus of claim 1, wherein the inductor includes an isotropic material. ### Claim 10 (depends on 1) The apparatus 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. ### Claim 11 An system for MRI device detection, comprising: an inductor including saturable magnetics, such that the inductor is configured to saturate as a result of a prescribed magnetic field intensity of the MRI device independent of direction; a transthoracic impedance measurement module (TIMM) electrically connected to the inductor, the TIMM configured to measure 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. ### Claim 12 (depends on 11) The system of claim 11, wherein the system is implantable. ### Claim 13 (depends on 11) The system of claim 11, wherein the controller is configured to change a lead configuration in response to the signal. ### Claim 14 (depends on 11) The system of claim 11, further comprising an implantable medical device (IMD). ### Claim 15 (depends on 14) The system of claim 14, wherein the controller is configured to change the IMD to a pacing mode in response to the signal. ### Claim 16 (depends on 14) The system of claim 14, wherein the controller is configured to change the IMD to a tachy mode in response to the signal. ### Claim 17 (depends on 14) The system of claim 14, wherein the controller is configured to change the IMD to a protect mode in response to the signal. ### Claim 18 (depends on 14) The system of claim 14, wherein the IMD includes a cardiac resynchronization therapy (CRT) device. ### Claim 19 (depends on 14) The system of claim 14, wherein the IMD includes a defibrillator. ### Claim 20 (depends on 14) The system of claim 14, wherein the IMD includes a neural stimulator.