Abstract:
A device and a method for working in the presence of electromagnetic fields, in particular fields occurring in magnetic resonance tomography (referred to below as “MRT” or “MRI”) imaging devices. More precisely, the invention relates to a medical device (MD) in which an electrode is in contact with bodily tissue, and for detection of electromagnetic interference fields the input characteristic of the MD is automatically modified by a switching device in such a way that the influences of the electromagnetic interference fields are minimized.

Description:
This application claims the benefit of U.S. Provisional Patent Application 61/288,857, filed on Dec. 22, 2009, which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to a device and a method for working in the presence of electromagnetic fields, in particular fields occurring in magnetic resonance tomography imaging devices. (“MRT” or “MRI” stand for magnetic resonance tomography and magnetic resonance imaging respectively, wherein these two acronyms are used interchangeably herein). 
     2. Description of the Related Art 
     Although MRI testing is becoming increasingly important in diagnostic medicine, it is contraindicated for some patients. Such contraindication may result from an active implanted medical device (also referred to below as “implant” or “IMD”). Besides MRI testing, however, other technical applications pose a risk to the user of medical devices or implantable medical devices, particularly when such applications generate strong electromagnetic interference fields (electromagnetic interference (EMI)) in their surroundings. 
     In order to still allow MRI testing or working in the surroundings of electromagnetic interference fields, various approaches are known which relate either to performing the MRI testing or to the implantable medical device. 
     Among others, technologies based on conventional processes for identifying magnetic fields are known for detecting magnetic fields. US 2008/0154342 describes a method which uses a giant magnetoresistance (GMR) sensor to detect problematic magnetic fields from MRT devices. However, these technological approaches are not very specific, and create increased energy requirements which result in a shorter operating period for equivalent energy reserves. 
     A further technological approach is the use of optical signal transmission instead of typical electrode lines based on electrical signal transmission. Use of these optical lines prevents the injection of electromagnetic interference fields from an MRI device into the electrode lines, but the overall system is more complex since on the one hand the electrical signals must first be converted to optical signals, and on the basis of the signals the stimulation pulses must be generated from the optical signals at the stimulation site, and on the other hand signals that are measured at the stimulation site must likewise be converted. As a rule, such higher-complexity systems also increase the energy requirements for an implant. Such a system based on optical signal transmission is described in US 2005/0090886 and U.S. Pat. No. 7,450,996. 
     In addition, a system is known from US 2002/0133204 and U.S. Pat. No. 6,901,292 which by selection of certain protective devices provides various modes for operating an electrical implant. A disadvantage of this system is that for high RF fields it also allows heating of the electrodes, since the focus of the cited documents is on electromagnetic compatibility with metal detectors and electronic security units. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of one or more embodiments of the invention is to provide a device and a method for medical devices and implantable medical devices which eliminate the disadvantages of the prior art and allow safe operation in the presence of electromagnetic interference fields. The object is achieved by use of a medical device (MD) having the features as claimed herein. 
     The medical device (MD) comprises a unit for detecting electromagnetic interference, having at least one sensor and/or indicator for electromagnetic interference fields and a switching device, at least one electrode, at least one control unit which is connected or connectable to the unit for detecting electromagnetic interference, and at least one detection unit and/or at least one stimulation unit which are respectively connected or connectable to the at least one control unit and the at least one electrode, the at least one electrode being in contact with bodily tissue, and for detection of electromagnetic interference fields by the unit for detecting electromagnetic interference the input characteristic of the at least one electrode for the electronics system of the MD or IMD is automatically modified by means of the at least one switching device in such a way that the influences of the electromagnetic interference fields are minimized. 
     For example, the input characteristic is modified using at least one EMI capacitor and/or a resonant circuit and/or other suitable components. An EMI capacitor is a capacitor for reducing the influences of electromagnetic interference radiation. The unit for detecting electromagnetic interference may be composed of a sensor and/or indicator for electromagnetic fields and/or a combination of multiple sensors and/or indicators for electromagnetic fields. Examples of such sensors and indicators, without being limited thereto, include GMR sensors, MagFET sensors, Hall sensors, electro-optical converters, monitoring of battery voltages during capacitor charging processes, detection of RF fields, detection of magnetic gradient fields, detection of currents induced by electromagnetic fields, and detection of specific vibrations, or components designed as sensors for detection of vibrations induced by Lorentz forces. 
     The MD is preferably an implantable medical device (IMD). 
     It is also preferred that the at least one electrode is situated in a tissue pocket beneath the skin and/or in a cardiac chamber and/or in a coronary vessel such as the coronary sinus. 
     The MD is particularly preferably an external cardiac pacemaker and/or defibrillator and/or an implantable cardiac pacemaker and/or an implantable cardioverter/defibrillator (ICD) and/or an implantable cardiac resynchronization therapy (CRT) device and/or a neurostimulator and/or a device for monitoring physiological functions such as, but not limited to, a device for monitoring cardiac function. 
     It is also preferred that the input characteristic of the MD may be modified by means of a switching device having one or more switching elements. 
     It is further preferred that the switching device has a linear or nonlinear characteristic curve, wherein the characteristic curve properties are implemented by a specialized component and/or material. Use is made, for example, of the characteristic curves for a voltage-controlled capacitor, a voltage-controlled resistor (FET transistor, for example), or a PIN diode. Also used is the characteristic curve for the material composite which implements a GMR sensor. A further assembly for the switching objective referenced here is a multiaxial sensitive Reed switch, for example three Reed switches orthogonally oriented in three spatial directions. 
     It is also preferred that the switching device is switched using one or more EMI capacitors  310  in series, or that the switching device is switched in parallel using an existing EMI capacitor circuit. 
     It is also preferred that the input characteristic may be modified using components having voltage-dependent capacitance. In particular, the EMI capacitors themselves may be designed as voltage-controlled capacitors. 
     It is also preferred that the switching device is designed in such a way that the EMI capacitors are reversed in polarity using a frequency which may be modified and/or predetermined, and/or which is automatically matched to the measured field intensity parameters, the polarity reversals being carried out only for specific field parameters detected by the unit for detecting electromagnetic interference, or carried out continuously. 
     It is particularly preferred that the switching is carried out using a so-called H-circuit. 
     It is further preferred that the polarity reversal frequencies of the individual switching elements of the switching device are different and/or time-invariant and/or time-variant. 
     It is also preferred that the polarity reversal frequency is determined taking into account the polarity of the charge flowing through the electrode line(s). 
     The charge determination may be based, for example, on the current integral and/or calculated from a voltage measurement of the known capacitance and the electrode impedance as determined by measurement, in each case at the same polarity. 
     It is also preferred that the switching element of the switching device is a modifiable resistor, modifiable capacitor, semiconductor, or a combination thereof. 
     The semiconductor switching element is particularly preferably a transistor, thyristor, diode for alternating current (diac), triode for alternating current (triac) switch, and/or a diode. 
     It is also preferred that the switching of the switching device or the changing of the operating point on the characteristic curve is triggered electrically, optically, acoustically, magnetically, or thermally. 
     It is also preferred in special cases that the connection of the input lines via the EMI capacitors has to be ensured only when the interference has a given frequency or lies in a given frequency range. In this respect the switching device may be regarded as a frequency-dependent switch, i.e., as a device whose impedance at a given frequency decreases by more than 3 dB only in the surroundings such as, but not limited to, a bandwidth &lt;10 MHz. This may be realized using an LC series circuit. 
     It is also preferred that the switching device functions depending on the frequency and is designed as an LC series circuit, the resonance of the LC series circuit being settable to the interference frequency to be diverted. For example, the setting may be made by making use of the EMI capacitance, which is present anyway. The resonance is settable to the interference frequency to be diverted, and may, for example, be set by programmed switching of the L and/or C components. 
     It is further preferred that a communication unit is also present which is designed to establish a unidirectional or bidirectional connection with an MRI device that is present. 
     It is particularly preferred that the unidirectional or bidirectional connection is established via telemetry and/or RF communication, and/or on the basis of RFID and/or the induction of targeted electromagnetic interference. Also possible are connections based on other, nonwired processes such as, but not limited to, mobile wireless processes, and data transmission standards such as WLAN, Bluetooth, and/or Certified Wireless USB. 
     It is also particularly preferred that the MRI device is able to trigger and/or control switching of the switching device and/or change the operating point on the characteristic curve of the switching device by means of the communication unit of the MD or IMD. 
     It is also preferred that the unit for identifying electromagnetic interference for the detection of electromagnetic fields includes at least one of the following sensors or indicators: GMR sensor, MagFET sensor, Hall sensor, electro-optical converter as indicator, monitoring of battery voltages during capacitor charging processes as indicator, detection of RF fields as indicator, detection of magnetic gradient fields as indicator, detection of currents induced by electromagnetic fields as indicator, and detection of specific vibrations, or components designed as sensors for detection of vibrations induced by Lorentz forces, as indicator. 
     It is further preferred that, in addition to modifying the input characteristic, at least one of the following measures is initiated for detecting electromagnetic interference fields: changing to an MRI safe state, remaining for a prolonged period of time in an MRI safe state or a state that is insensitive to electromagnetic interference fields, synchronization of electrical measurements (impedance measurements, for example) using field intensity minimum values occurring with periodic or pulsed electromagnetic fields, or synchronization of a stimulation using these same minimum values, and emission of electromagnetic pulses for signaling that a medical device, in particular an implant, is present in the electromagnetic field, in particular for signaling to an MRI device, with the possibility of thus transmitting information as well as the interference and displaying same on the MRI screen. 
     It is further preferred that a position sensor is used for plausibility checking, and a positive MRI identification is made only when the position sensor reports a prone posture and/or another presettable posture. 
     The position sensor is particularly preferably self-calibrating, the calibration taking place under presettable boundary conditions such as, but not limited to, times and/or heart rates and/or respiratory rate and/or hemodynamic parameters and/or activity (motion sensor). 
     Also preferred is a method for producing an MRI safe state of an MD by use of one of the MDs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Several aspects of the invention are illustrated in  FIGS. 1 through 8 . 
         FIG. 1  shows a schematic illustration of the sequence of an MRI test; 
         FIG. 2  shows a block diagram of an EMI protective device switched according to the invention; 
         FIG. 3  shows a more detailed block diagram of an EMI protective device switched according to the invention; 
         FIG. 4  shows a block diagram of a variant of an EMI protective device switched according to the invention; 
         FIG. 5  shows a block diagram of a variant of an EMI protective device switched according to the invention; 
         FIG. 6  shows a block diagram of a variant of an EMI protective device switched according to the invention; 
         FIG. 7  shows a block diagram of a variant of an EMI protective device switched according to the invention, with controllable capacitors; and 
         FIG. 8  shows a block diagram of a variant of an EMI protective device switched according to the invention by means of an H-circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  describes the prior art, in which the ICD patient  100  receives follow-up care from a cardiologist before the planned MRT test, and the ICD is switched off  110 . After a time delay of hours to days the MRT test is performed by a radiologist  120 . After a further delay the patient is once again under the care of the cardiologist  130 , and the ICD is switched back on. During the entire time from 110 to 130 the patient is without protection of the implanted defibrillator, and is essentially without rhythm monitoring. This residual risk is accepted in return for the benefits of the MRT test. 
       FIG. 2  shows a block diagram of the approach according to the invention, in which the incoming electrode lines ( 206  right ventricular,  205  right atrial, and  207  shock line for emitting high-energy pulses) are connected to the input protection circuit  210 , and the characteristic of the input protection circuit  210  may be modified by a unit for detecting electromagnetic interference  230 . The input characteristics of the electrodes for the electronics system of the MD or IMD together with the switching device  240  are collectively referred to as an input protection circuit  210 . The signals from the electrode lines  205 ,  206 ,  207  which may be modified or manipulated by the input protection circuit  210  are relayed to the implant electronics system  220 , which may include a communications unit for external communications into and out of the IMD, for example to an MRI unit. For the invention it is irrelevant, as is clearly evident to one skilled in the art, whether the implant has one or several electrode lines  205 ,  206 ,  207 . Variants are also conceivable in which one, several, or all electrodes are designed only as areas on the exterior of the implant. 
     As a result of the invention, precautions for necessary MRT safety, or for general safety in intense RF fields, are taken only when this boundary condition is present, and in the function mode of the implant, which is necessary the vast majority of the time, the settings may be optimized in favor of the signal quality. The approach according to the invention also takes into consideration that excessively high EMI capacitors  310  increase the hazard potential of stimulation from gradient fields in the MRT. 
     In general terms, this means that when a strong electromagnetic field is detected, in particular RF fields which occur in the surroundings of MRT scanners which represent a hazard potential for the patient and the implant, the EMI protective circuit of the implant is automatically reconfigured. In the simplest case, for this purpose the EMI capacitor values are increased, or first of all the EMI capacitors  310  are switched on or an existing protective circuit is switched in parallel. To this end their common contact point, for example on the implant housing, may be disconnected by a switch, thus allowing the high-resolution, broadband signal recording, in particular also the impedance determination for a hemodynamic sensor, in everyday operation of the implant. Particularly high EMI capacitors are necessary for minimizing the interactions with RF fields. However, this increases the risk that gradient fields may induce currents, capable of stimulation, which would find a lower-impedance path over the high EMI capacitors. According to the invention this risk is minimized by repetitively changing the polarity of the capacitors, so that during a polarity it is not possible for sufficient charge to flow in one direction to depolarize the cardiac muscle tissue. This allows the requirements for high interference immunity and very high signal recording quality to be combined in an electronic implant. This approach also minimizes the risk of inadvertent, and thus potentially arrhythmogenic, stimulation by gradient fields. 
     In another exemplary embodiment shown in  FIG. 3 , EMI capacitors  310  are decoupled from the housing by a switching device  240  as a function of the measurements of the unit for detecting electromagnetic interference  230 . The EMI capacitors  310  and the switching device  240  are part of the input protective device, and the EMI capacitors  310  are switched via a common switching device  240 . 
     The design variant in  FIG. 4  has a design analogous to that of  FIG. 3 , except that each EMI capacitor has its own switching device  240 , which in each case may be switched separately from the unit for detecting electromagnetic interference  230 . 
     In special cases, for the approach according to the invention it is sufficient to ensure the connection of the input lines via the EMI capacitors only when the interference has a given frequency or lies in a given frequency range. In this respect the switching device may be regarded as a frequency-dependent switch, i.e., as a device whose impedance at a given frequency decreases by more than 3 dB only in the surroundings (for example, a bandwidth &lt;10 MHz). This may be realized using an LC series circuit, as shown in  FIG. 5 . The inductance  321  is matched to the EMI capacitor  320  in such a way that the resonance occurs at the frequency (deviation in terms of the referenced bandwidth) at which the input line is to be switched at the lowest possible impedance from the implant housing. The losses from components L and C are taken into account in their selection so that the quality of the resonant circuit Q is &gt;10. 
     To allow this approach to be used within the same implant for various interference frequencies, the approach according to the invention provides that the C or L (or both) may be switched over. The switching may occur as a function of the interference field detection unit (which determines the frequency of the interference and selects the matching LC combination). Alternatively, the switching may be performed by the external programming device. 
       FIG. 6  shows only the example of the switchable inductance  322 . To adjust (reduce) the quality, the inductance may be shunted via a corresponding resistor. The device is dimensioned (L, C, including the losses thereof, and optionally the shunt resistor) so that during resonance the input lines are thus coupled to the housing at an impedance of &lt;200 ohm (per line). 
       FIG. 7  shows an exemplary embodiment in which instead of the EMI capacitors  310  controllable EMI capacitors  311  are used, which in each case are controlled directly by the unit for detecting electromagnetic interference  230 , so that a switching device  240  may be dispensed with. The circuit illustrated in  FIG. 7  is a so-called H-circuit, which preferably may be used for polarity reversal. 
       FIG. 8  shows an input protection circuit  210  analogous to that in  FIG. 3 , wherein the switched EMI capacitor  410  is switched by an H-circuit, thus allowing periodic polarity reversal. The H-circuit for the switched EMI capacitor  410  is controlled by the unit for detecting electromagnetic interference  230 . Only one switched EMI capacitor  410  in an H-circuit is illustrated in  FIG. 8 , the dashed lines indicating an analogous design for the other electrode lines  206  and  207 . In one variant of this exemplary embodiment, all electrode lines  205 ,  206 , and  207  are connected to an EMI capacitor  410  in an H-circuit. 
     It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.