Abstract:
A magnetic field sensor device suitable for use in an implantable medical device (such as a pacemaker, cardioverter/defibrillator, or cardiac resynchronization therapy device) is able to detect magnetic fields, such as the fields generated by a Magnetic Resonance Imaging (MRI) device, over a wide measurement range and to discriminate between different field strengths. Multiple sensors provided within the magnetic field sensor device are optimally biased to provide a power saving solution which is accurate enough for medical devices applications. The output of the magnetic field sensor device can be used to switch the implantable medical device to different operational modes, e.g., between programmable and “MRI safe” modes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application 61/184,345 filed Jun. 5, 2009, the entirety of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an improved magnetic field detection device, and in particular to an improved magnetic field detection device for recognizing magnetic field strength over several magnitudes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Magnetic resonance imaging, further referred to as MRI, is of increasing relevance for modern diagnosis. Up to now active implanted medical devices (active IMDS), such as (but not restricted to) pacemakers, cardioverters/defibrillators, and cardiac resynchronization therapy devices, prevent effective utilization of MRI technology for diagnosis. Even the newly-presented so-called “MRI conditional” implants pose difficulties insofar as such devices have to be switched to a so-called “MRI safe” mode, which requires at least a consultation with a heart specialist before and after the MRI examination (if not requiring the presence of a heart specialist during the entire MRI examination). 
         [0004]    These problems can be overcome by a reliable detection of MRI fields and appropriate programming of the active IMDs, as described in EP 1 935 450 by Digby et al. or by Shankar et al. in U.S. Pat. No. 6,963,779. Wahlstrand et al. disclose in U.S. Pat. No. 5,438,990 the use of a so called MagFET, a magnetic field effect transistor or magnetic field sensitive MOSFET as a reed switch. U.S. Pat. No. 5,179,429 to Ristic et al. discloses a magnetic field sensor based on a bipolar transistor with split collector contacts. Also known in the art is the use of hall sensors replacing the reed switch and enabling the device to detect the strength of a magnetic field, as in U.S. Pat. No. 6,510,345 Van Bentem et al.. 
         [0005]    These solutions still have power consumption which is unsuitable for use in a medical device. Therefore, it is an object of the invention to provide a sensor device accurate enough to discriminate between different magnitudes of a magnetic field, while at the same time consuming less power than prior solutions. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides a sensor device able to detect magnetic fields over a wide measurement range, and to discriminate between different field strengths of the magnetic fields. For high field detection, the device includes at least a MagFET sensor or a magnetotransistor or a hall sensor, and for low field detection the device includes a reed is switch or a hall sensor or a GMR (giant magnetoresistance) sensor. Furthermore, the device generates different signals depending on the magnetic field strengths. For purposes of this document, the measurement of field strength below a predefined threshold, or below the sensitivity of the sensor for low magnetic fields, will be referred to as the detection or sensing of effectively no magnetic field. The phrase “detection of a magnetic field” should be understood as detecting a field, and in the case of using a MagFET sensor, a hall sensor, a magnetotransistor or a GMR sensor, should be understood as detecting and/or measuring the magnitude or field intensity of a magnetic field. 
         [0007]    “MagFET sensor” does not refer only to a sensor with one sensor element, but also refers to a sensor with two or more sensor elements. In particular, an array of three sensor elements is preferred because it can sense magnetic fields in all three spatial directions. 
         [0008]    The combination of a reed switch or a hall sensor or a GMR sensor for low field detection, and a MagFET sensor or hall sensor or a magnetotransistor for high field detection, allows a reduction of power consumption and an enhancement of measurement accuracy, because the different sensors can work, or more precisely measure, in a measurement range they are optimized for. The reed switch is a preferred element for low field detection for field strengths exceeding 1 mT, preferably in the range of 1 mT to 3 mT, while MagFET sensors or hall sensors or magnetotransistors are more suitable for high field detection in the order of a few tesla. 
         [0009]    Also preferred is that the magnetic field detection is carried out by a reed switch for low magnetic fields, by a MagFET sensor for out-of-plane components of high magnetic DC fields, and by a magnetotransistor for in-plane components of high magnetic DC fields. 
         [0010]    In another version of the invention, either the combination of a reed switch for low magnetic field detection and a MagFET for high magnetic field detection, or the combination of a reed switch for low magnetic field detection and a MagFET and a magnetotransistor for high magnetic field detection, is additionally combined with a high frequency coil or a communication antenna and/or a RF antenna to detect the presence of high frequency magnetic fields. High frequency magnetic fields in addition to high DC magnetic fields are a good and common indicator of the presence of a MRI apparatus. 
         [0011]    In a preferred version of the invention, the sensor for low field detection is a reed switch susceptible to field strengths exceeding 1 mT. 
         [0012]    In a further preferred version of the invention, the low field detection is sensitive to magnetic field strength between 1.0 mT and 2.5 mT, preferably between 1.5 mT and 2.0 mT, and the MagFET sensor and/or magnetotransistor for high field detection is sensitive to magnetic field strengths between 0.5 T and 7.0 T. 
         [0013]    It is also preferred that the different signals generated by the device include generating a first signal if detecting a low magnetic field between 0.5 mT and 2.0 mT, generating a second signal if detecting a high magnetic field exceeding 1.0 T, and generating a third signal for a magnetic field below 1.0 mT. The signal(s) may be transmitted to a unit controlling the mode of a device incorporating the magnetic field detection device. 
         [0014]    The invention also involves an implantable medical device with a magnetic field detection device including at least a MagFET sensor and/or a magnetotransistor for detecting high magnetic fields exceeding 0.5 T, and a reed switch or a hall sensor for detecting low magnetic fields exceeding 2.0 mT, preferably exceeding 1 mT. 
         [0015]    It is preferred to use a magnetotransistor to detect in-plane components of the static (DC) magnetic field, and a MagFET to detect the out-of-plane component of the static magnetic field. It is further preferred that both sensors are integrated in/on the same circuit board or integrated circuit so that they measure magnetic field components which are orthogonal to each other. 
         [0016]    In another variation of the invention, two or more MagFET sensors or magnetotransistor sensors are arranged on an ultra high density circuit board which is nonplanar in such a way that the two or more sensors measure different vectors or components of the magnetic field, preferably orthogonal components of the magnetic field. 
         [0017]    In a preferred version the magnetic field detection device includes means for generating a first signal if detecting a low magnetic field, and means for generating a second signal if detecting a high magnetic field. These signals may be, but are not restricted to, control signals to alter the operation mode of the implantable medical device. 
         [0018]    Also preferred is that the detection of a low magnetic field triggers switching of the implantable medical device to a first mode, and detection of a high magnetic field triggers switching to a second mode. The first mode may be a mode enabling programming of the implantable medical device, and the second mode may be a MRI safe mode. 
         [0019]    Also preferred is that the MRI safe mode includes switching to a 000, V00 or D00 mode and/or inhibiting the delivery of a high energy shock. In these modes, the first character stands for the pacing mode and the second character stands for the sensing mode (0 means no pacing/sensing, V means ventricular pacing/sensing, and D means dual pacing/sensing in the atrium and the ventricle), and the third character represents the reaction to the sensing (0 means no reaction). 
         [0020]    Furthermore, the invention includes a method for detecting magnetic fields over several orders of magnitude, characterized in that the low detectable magnetic field strength is measured by a reed switch or a hall sensor or a GMR sensor, and the high detectable magnetic field strength is measured by a MagFET sensor and/or a magnetotransistor, and different signals are generated depending on the measured magnetic field strength. The method preferably additionally includes combining the high magnetic field detection with a high frequency coil, a communication antenna, and/or a RF antenna to detect the presence of high frequency magnetic fields which are present during a MRI examination. 
         [0021]    Preferably, detection of a low magnetic field triggers switching of an implanted medical device to a first mode, and detection of a high magnetic field triggers switching of the device to a second mode. 
         [0022]    Also preferred is that the first mode is a mode enabling programming of the implantable medical device, and the second mode is a MRI safe mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Some possible exemplary aspects of the invention are illustrated in  FIGS. 1 to 7 , which show: 
           [0024]      FIG. 1  a schematic view of a multi-sensor system for detecting magnetic fields; 
           [0025]      FIG. 2  a perspective view of a N-channel MagFET; 
           [0026]      FIG. 3  a schematic representation of a split drain MagFET; 
           [0027]      FIG. 4   a  an exemplary flowchart of a magnetic detection device; 
           [0028]      FIG. 4   b  an exemplary flowchart of a magnetic detection device; 
           [0029]      FIG. 5  a schematic illustration of a MagFET in a magnetic sensor; 
           [0030]      FIG. 6  a cross section of a Lateral Magneto-transistor in CMOS technology; 
           [0031]      FIG. 7  a schematic representing a MagBJT circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 1  shows a schematic view of a multi-sensor system in accordance with the invention. The different sensors  110 ,  120  and  130  are controlled via a bias/sensor control  111 ,  121  and  131 , and are optimally biased for their working point. Therefore, energy consumption is minimized. The sensors are connected with the control block  140 , comprising a sensor control block and an analog conditioning circuit, which delivers the field measurement and/or a control signal based on the measured magnetic field. 
         [0033]    The assembly of a dual drain n-channel MagFET is schematically shown in a perspective view in  FIG. 2 . When operated in a magnetic field perpendicular to the channel or gate  210  of the electrical signal, such a field being identified with the arrow  250 , the currents measured between the source  220  and drain  230  or  240  differ from each other because of the force (identified by arrow  260 ) affecting the charge carriers. The difference in the current is a measurement of the magnetic field. The basic functions of a MagFET are well known by a person ordinary skilled in the art. 
         [0034]      FIG. 3  shows a schematic representation of a split drain n-channel MagFET, with the two different drains D 1  ( 310 ) and D 2  ( 311 ), a common source  300 , and gate  320 . 
         [0035]    The flow charts of  FIGS. 4   a - 4   b  show basic functions of an exemplary magnetic field detection device.  FIG. 4   a  illustrates operation where no reasonable magnetic field was detected during the previous measurement (where “reasonable” means there was no field detected exceeding a certain threshold, such as but not limited to 1.7 mT or 1.9 mT). After measuring of magnetic field activity at  400 , it is decided whether the detected field is equal to or exceeding 2 mT, and/or whether the detected field is exceeding 0.5 T, or whether no reasonable magnetic fields has been sensed. In the case at  411  in which the measured magnetic field is equal to or exceeding 2 mT (but not exceeding 0.5 T), the reed switch or low field sensor output is used to trigger a signal  421  for initiating a first operation mode such as (but not limited to) a so called magnet-mode or programming mode. A subsequent measurement  450  is then performed as per the flow chart  4   b.    
         [0036]    In the case at  412  in which the measured magnetic field is equal to or exceeds 2 mT and exceeds 0.5 T, the MagFET sensor or high field sensor output is used to trigger a signal  422  for initiating a second mode such as (but not limited to) a so called MRI-safe mode. The subsequent measurement  450  is then performed as per the flow chart  4   b.    
         [0037]    As per step  413 , if neither a magnetic field equal to nor exceeding 2 mT, nor exceeding 0.5 T, has been sensed, sensing will be continued with step  400 . 
         [0038]      FIG. 4   b  illustrates the operation of the device, if in the previous measurement a magnetic is field has been sensed which is equal to or exceeding a certain threshold (such as 2.0 mT) and/or is exceeding a second threshold (such as 0.5 T). In these cases, after measurement  450 , it is decided whether a field is equal to or exceeding 2 mT, and/or whether a field is exceeding 0.5 T, or whether no reasonable magnetic field has been sensed. In the case  461  in which the measured magnetic field is equal to or exceeding 2 mT, but not exceeding 0.5 T, the reed switch or low field sensor output is used to trigger a signal  471  for initiating a first operation mode such as (but not limited to) a so-called magnet-mode or programming mode. The subsequent measurement  450  is performed following the flow chart  4   b.    
         [0039]    In the case  462  in which the measured magnetic field is equal to or exceeding 2 mT and also exceeding 0.5 T, the MagFET sensor or high field sensor output is used to trigger a signal  472  for initiating a second mode such as (but not limited to) a so-called MRI-safe mode. The subsequent measurement  450  is performed following the flow chart  4   b.    
         [0040]    If neither a magnetic field equal nor exceeding 2 mT nor exceeding 0.5 T has been sensed, and a signal  463  for initiating switching to the default mode is triggered, sensing will be continued with step  400 . 
         [0041]      FIG. 5  shows a schematic illustration of a magnetic sensor utilizing a p-channel  513 ,  514  and/or a n-channel  511 ,  512  MagFET. The differences in the drain currents are evaluated by a comparator  540 . In  FIGS. 4   a - 4   b , under high field conditions (exceeding 0.5T), the reed switch may not necessarily be closed. Therefore, owing to lack of detection of the reed switch, it cannot be concluded that no magnetic field is present. 
         [0042]      FIG. 6  shows a cross-section of a lateral magnetotransistor in CMOS technology, as is appropriate to combine with a MagFET to improve sensitivity. The assumed magnetic field, B vector, is represented at  601 . The magnetotransistor is a bipolar transistor implemented on a semiconductor surface whose structure and operating conditions are appropriately selected and optimized to boost magnetic sensitivity of its collector current. 
         [0043]      FIG. 6  shows a typical lateral magnetotransistor manufactured in a p-well (base  602 ) wherein a voltage is applied to the two base contacts, B+( 622 ) and B−( 621 ), to accelerate the minority carrier injection into the base  602  region. The two n+ regions  603  and  604 , separated by the length L of the lateral base  602 , serve as the Emitter E ( 630 ) and Collector C ( 650 ) of the magnetotransistor. Assuming that the magnetotransistor is adequately forward-biased, in the absence of magnetic field the electrons are injected into the base region  602  by the emitter  630  and drift mainly along the base length and are collected by collector C ( 650 ), producing collector current I C  . Some electrons diffuse downwards and are collected by the secondary collector S ( 610 ), producing the substrate current I S . In the absence of a magnetic field and adequate bias conditions, the ratio of I C /I S  is constant. Applying a magnetic field B perpendicular to the magnetotransistor, not shown, causes the electrons to be deflected towards the substrate region and therefore very few electrons contribute to the collector current I C . This causes a very small change in the ratio of I C /I S . In the presence of a magnetic field B ( 601 ) in the plane of the magnetotransistor in  FIG. 6 , the electrons are deflected towards the device surface, causing the collector current I C , to increase. This causes an appreciable change in the ratio of I C /I S , which can be measured to get a measure of the applied magnetic field. 
         [0044]    A schematic representation of a magnetotransistor is shown in  FIG. 7 , indicating the collector and base currents with arrows. 
         [0045]    The scope of the invention is not limited to above-mentioned exemplary versions of the invention. A person skilled in the art will understand that the various versions of the methods and devices are related to each other, so that combinations of the versions are also encompassed within the scope of the invention. 
         [0046]    It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and versions are possible in light of the foregoing discussion. The disclosed examples and versions are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate versions as may come within the true scope of this invention.