Abstract:
An implantable medical device configured to be compatible with the environment inside an Mill machine. The implantable medical device includes a housing constructed of an electrically conductive material and pulse generation circuitry within the housing for generating electrical voltage pulses. The implantable medical device further includes a first conductor that is configured to transmit the electrical voltage pulses from the pulse generation circuitry to a patient&#39;s cardiac tissue and a second conductor that is configured to provide an electrically conductive path from the patient&#39;s cardiac tissue back to the pulse generation circuitry. The implantable medical device further includes a selectively interruptible electrically conductive path connecting the pulse generation circuitry with the housing.

Description:
CLAIM OF PRIORITY 
       [0001]    This application is a continuation of U.S. application Ser. No. 15/153,189, filed May 12, 2016, which is a divisional application and claims benefit of priority of U.S. application Ser. No. 11/684,419, filed Mar. 9, 2007, now U.S. Pat. No. 9,345,888, entitled “MRI Compatible Implantable Medical Devices and Methods,” each of which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to implantable medical devices, and more particularly, to implantable medical devices that can be used during magnetic resonance imaging (MRI). 
       BACKGROUND OF THE INVENTION 
       [0003]    Many different types of medical devices can be implanted within patients to provide medical therapy. One common type of implanted medical device is a cardiac rhythm management device, such as a pacemaker or an implantable cardioverter defibrillator (ICD). Cardiac rhythm management devices can be used to provide medical therapy to patients who have a disorder related to cardiac rhythm. For example, a pacemaker may be used to treat a patient exhibiting bradycardia. Pacemakers are generally configured to provide a pacing pulse as necessary to control the heart rate. 
         [0004]    Magnetic resonance imaging (MRI) is a technique for visualizing body tissues of a patient for purposes of medical diagnosis and therapy. MRI relies on subjecting the body tissue of interest to a very strong uniform magnetic field, up to about 30,000 gauss, as well as a moderate strength but variable magnetic field of around 200 gauss. In the presence of these uniform and gradient magnetic fields, a radio frequency (RF) pulse is transmitted from a coil to the body tissue. Hydrogen atoms within the body tissue have a magnetic moment and tend to line up with the direction of the applied magnetic fields. Some of these hydrogen atoms will align facing one direction and others will align facing an opposite direction, such that most of the hydrogen atoms facing in alternating directions will tend to cancel each other out. However, a small percentage (but a significant absolute number) of hydrogen atoms will be unbalanced, or not cancelled out. The applied RF pulse tends to cause the unbalanced hydrogen protons to spin, or resonate, in a particular direction and at a particular frequency. When this RF pulse is turned off, the spinning hydrogen protons revert to their earlier, aligned position, and release their excess energy. The RF coil of the MRI machine is capable of detecting this released energy and transmitting a corresponding signal to a processor that in turn transforms the signal into an image of the body tissue. Because different tissues have different characteristic responses to the application of the RF pulse in the presence of the magnetic fields, these differences can be utilized to prepare an image showing areas of contrasting tissue types. 
         [0005]    MRI techniques have proven to be very effective at diagnosing certain medical conditions and therefore allowing for patients to receive timely, appropriate medical therapy. However, in many cases patients having an implanted medical device are contraindicated for MRI, and therefore may be unable to benefit from the full scope of diagnostic techniques available. One problem is that the MRI can induce a current within elements of the implanted medical device. A principle known as Faraday&#39;s law states that any change in a magnetic field around a conductive loop will cause a voltage to be induced in the conductive loop, and consequently, cause a current to flow in the conductive loop. In the case of a patient undergoing an MRI procedure, the time-varying magnetic field gradients of the MRI machine create the required changing magnetic field and elements of the implanted cardiac rhythm management device form a conductive loop, resulting in a current flow. Unfortunately, this current flow can interfere with the proper functioning of the medical device. 
         [0006]    For at least these reasons, implantable medical devices are needed that can be used during MRI examinations. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the invention relates to an implantable medical device that is configured for use within an MRI machine environment. The implantable medical device includes a housing constructed of an electrically conductive material and pulse generation circuitry within the housing for generating electrical voltage pulses. The implantable medical device further includes a first conductor that is configured to transmit the electrical voltage pulses from the pulse generation circuitry to a patient&#39;s cardiac tissue and a second conductor that is configured to provide an electrically conductive path from the patient&#39;s cardiac tissue back to the pulse generation circuitry. The implantable medical device further includes a selectively interruptible electrically conductive path connecting the pulse generation circuitry with the housing. 
         [0008]    Another aspect of the invention also relates to an implantable medical device. The implantable medical device includes a housing that is formed from an electrically conductive material and has pulse generation circuitry that is disposed within the housing for generating electrical voltage pulses. The implantable medical device also includes a diode that is disposed in series between the housing and the pulse generation circuitry, and also a switch that is disposed in series with the diode, where the switch is configured to selectively isolate the housing from the pulse generation circuitry. 
         [0009]    A further aspect of the invention relates to an implantable medical device having a housing constructed of an electrically conductive material and a pulse generator within the housing for generating electrical voltage pulses. These electrical voltage pulses can be characterized by a voltage relative to an electrical reference potential of the pulse generator. The implantable medical device further includes a first conductor that is configured to transmit electrical voltage pulses from the pulse generator to a patient&#39;s tissue, and a second conductor that is configured to transmit an electrical reference potential of the patient&#39;s tissue to the reference potential of the pulse generator. The implantable medical device further includes an electrically conductive path from the reference potential of the pulse generator to the housing, where the electrically conductive path includes a resistor. 
         [0010]    Yet another aspect of the invention relates to an implantable medical device including a housing comprising an electrically conductive material and pulse generation circuitry disposed within the housing for generating electrical voltage pulses. The implantable medical device also includes a resistor disposed in series between the housing and the pulse generation circuitry. 
         [0011]    A further aspect of the invention relates to an implantable medical device having a housing constructed of an electrically conductive material and having a pulse generator within the housing for generating electrical voltage pulses. These electrical voltage pulses can be characterized by a voltage relative to an electrical reference potential of the pulse generator. The implantable medical device further includes a first conductor that is configured to transmit electrical voltage pulses from the pulse generator to a patient&#39;s tissue and a second conductor that is configured to transmit an electrical reference potential of the patient&#39;s tissue to the reference potential of the pulse generator. The implantable medical device is configured so that there is no electrically conductive path between the housing and the pulse generator. 
         [0012]    Another aspect of the invention relates to an implantable medical device that includes a housing that is made from an electrically conductive material. The implantable medical device also includes pulse generation circuitry that is disposed within the housing for generating electrical voltage pulses. The pulse generation circuitry is configured to be electrically isolated from the housing at all times. 
         [0013]    The invention may be more completely understood by considering the detailed description of various embodiments of the invention that follows in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention may be more completely understood in connection with the following drawings, in which: 
           [0015]      FIG. 1  is a schematic view of a conductive loop formed in an implanted unipolar cardiac pacing device. 
           [0016]      FIG. 2  is a schematic view of a conductive loop formed in an implanted bipolar cardiac pacing device. 
           [0017]      FIG. 3  is a diagram of an idealized pacing pulse. 
           [0018]      FIG. 4  is a diagram of a pacing pulse affected by low frequency induced current. 
           [0019]      FIG. 5  is a schematic diagram of a cardiac pacing device in accordance with an embodiment of the present invention. 
           [0020]      FIG. 6  is a schematic diagram of a cardiac pacing device in accordance with an embodiment of the present invention. 
           [0021]      FIG. 7  is a schematic diagram of a cardiac pacing device in accordance with an embodiment of the present invention. 
       
    
    
       [0022]    While the invention may be modified in many ways, specifics have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives following within the scope and spirit of the invention as defined by the claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    A variety of implanted medical devices are used to administer medical therapy to patients. One example of such an implanted medical device is a cardiac rhythm management (CRM) device. An exemplary CRM device is a pacemaker, which consists generally of a pulse generator for generating a pacing pulse and one or more leads for delivering the pacing pulse to the cardiac tissue. Some pacemakers can be configured to sense the electrical activity of the patient&#39;s heart. In some pacing modes, if the pacemaker does not detect electrical activity above a certain trigger threshold within a certain time interval, the pacemaker will deliver a pacing pulse through the one or more leads to the cardiac tissue. This pacing pulse causes the heart to beat. 
         [0024]    Magnetic resonance imaging (MRI) is very powerful technique for non-invasively visualizing tissues of the body. Unfortunately, however, the time varying magnetic field gradients associated with MRI systems can interfere with the functioning of implanted medical devices, such as CRM devices. A principle known as Faraday&#39;s law states that a change in a magnetic field around a conductive loop will induce a voltage in the conductive loop, and consequently, cause a current to flow in the conductive loop. In the case of a patient undergoing an MRI procedure, the time varying magnetic field gradients of the MRI machine create the required changing magnetic field and the implanted pacemaker or other cardiac rhythm management device forms a conductive loop in which a current is induced. The induced currents can specifically include low frequency induced currents (LFICs) that can interfere with the functioning of the implanted medical device. For example, it is possible that LFIC could cause pacing of the heart by activating nerve or muscle cells within the heart. In this way, it may be possible for the MRI machine to inadvertently pace the patient&#39;s heart. The LFIC can also distort the waveshape of intended pacing pulses, possibly resulting in a diminished effectiveness of the pacing pulse. LFIC can further interfere with the pacemaker system&#39;s ability to properly sense cardiac activity, possibly resulting in inhibited pacing or rapid pacing. 
         [0025]    In a unipolar pacemaker system such as that depicted in  FIG. 1 , a loop  20  is formed from the pacemaker internal circuitry  22 , through the lead  24  to the electrode  26  in contact with cardiac tissue, and then through body tissue back to the pacemaker housing  28 . The area enclosed by this loop is significant and therefore a substantial amount of LFIC can be generated within this loop by the time varying magnetic field gradients of an MRI system. 
         [0026]    Conductive loops can also be created in the context of bipolar pacing systems.  FIG. 2  shows a simplified schematic diagram of some aspects of a typical bipolar pacemaker system. Bipolar pacemaker  54  includes a tip and ring electrode  32 , where the tip electrode  34  and ring electrode  36  are each implanted in cardiac tissue, but are separated by a relatively small distance from each other. Pacemaker  54  can include various circuitries, such as pulse generation circuitry, sensing circuitry, charging circuitry, control circuitry, and the like. Sensing circuitry, charging circuitry, and control circuitry (not shown in  FIG. 2 ) can be constructed according to principles known to those of skill in the art. In  FIG. 2 , pulse generator  38  includes pacing switch S p , pacing capacitor C p , recharging switch S r , and recharging capacitor C r . A pacing pulse is delivered when pacing switch S p  is closed. For a period of time after the pacing pulse, pacing switch S p  is opened and the recharging switch S r  is closed to recharge the pacing capacitor C p . When switch S p  is closed, it is called the pacing window, and when switch S r  is closed it is called the active recharge window. A housing  44  is provided that contains pulse generator  38 . The housing  44  can be constructed of a conductive material. 
         [0027]    As shown in  FIG. 2 , pulse generator  38  also includes switch S m  for switching between bipolar mode and unipolar mode. To select a unipolar mode of operation, switch S m  is configured to connect the pacemaker housing  44  to the pulse generator  38  circuitry. In the unipolar mode of operation, the tip electrode  34  generally serves as the cathode and the housing  44  itself serves as the anode. In  FIG. 2 , this occurs where switch S m  connects to terminal  2  of switch S m . To select a bipolar operation mode, switch S m  is configured to connect conductor  42  to the pulse generator  38  circuitry. In the bipolar mode of operation, the tip electrode  34  generally serves as the cathode and the ring electrode  36  generally serves as the anode. In the embodiment of  FIG. 2 , this occurs when switch S m  connects to terminal  1  of switch S m . 
         [0028]    In bipolar capable pacemakers, there is generally more than one conductive loop in which current can be induced. In bipolar mode, a first loop  46  is formed when either switch S p  or switch S r  is closed, the first loop  46  being formed either through switch S r  or capacitor C p  and switch S p , through capacitor C r , through first conductor  40  and tip electrode  34 , through cardiac tissue into ring electrode  36 , and through second conductor  42  to switch S m . However, first and second conductors  40 ,  42  are generally very close together, such as disposed together within one lead. Therefore, conductive loops that include both first conductor  40  and second conductor  42  generally enclose a very small area and therefore induced current in these loops is usually insignificant. 
         [0029]    However, conductive loops enclosing a relatively large area can also be formed by some bipolar pacemakers. Many bipolar pacemakers include an integrated circuit protection diode D 1 . Diode D 1  allows current to flow from the pacemaker housing  44  into the pulse generator circuitry to the reference potential (ground) of capacitor C p . This is useful to prevent the pacemaker ground from deviating from the pacemaker housing potential. However, this diode D 1  can facilitate the formation of conductive loops within the pacemaker. For example, when switch S p  is closed, loop  48  is formed passing through capacitor C p , switch S p , capacitor C r , conductor  40 , tip electrode  34 , tissue path  50 , back to housing  44  and through diode D 1 . When switch S r  is closed, loop  49  is formed passing through switch S r , capacitor C r , conductor  40 , tip electrode  34 , tissue path  50 , back to housing  44  and through diode D 1 . Loops  48  and  49  can be formed regardless of the position of switch S m . 
         [0030]    Furthermore, when switch S m  is in bipolar mode, another conductive loop  52  can be formed regardless of the positions of switches S r  and S p . Conductive loop  52  can be formed passing through second conductor  42 , electrode  36 , tissue path  50  to housing  44 , through diode D 1 , and back to second conductor  42  through switch S m . Loops  48 ,  49 , and  52  each enclose an area sufficiently large to make the generation of LFIC during MRI a concern. 
         [0031]    LFIC can have harmful effects on the patient. If the induced current is large enough, the current can cause activation of the heart muscle. The induced current can also cause distortion of a pacing pulse sent from the pacemaker through the leads to the heart. For example,  FIG. 3  shows an example of an idealized pacing pulse. At a first time T 1 , a pacing switch is closed causing a current pulse to be delivered through the leads for a period of time, until at time t 2  the pacing switch is opened and the current pulse diminishes. Also at time T 2 , a charging switch is closed to allow charging of a capacitor until time T 3  when the charging switch is opened.  FIG. 4  shows an example of how an idealized pacing pulse can be affected by the presence of LFIC. The current that is induced into the loop will add to or subtract from the voltage of the pacing pulse, resulting in a distorted pulse, such as that seen in  FIG. 4 . The LFIC may result in unreliable sensing of electrical activity in the heart, including both of the possibilities that a heart beat will not be captured and that the interference will cause a heartbeat to be captured when in fact one did not exist. In some cases, the induced distortion may cause the electrical pulse to be insufficient to capture the patient&#39;s heart. In some cases, the LFIC may be large enough in magnitude to capture the patient&#39;s heart at times other than during the pacing pulse. For example, by forward biasing diode D 1 , capture outside of the pace and active recharge window can be facilitated. In any case, the LFIC can interfere with the proper operation of the pacing device, possibly causing injury to the patient. 
         [0032]    An embodiment of an implantable medical device configured to minimize LFIC is shown in  FIG. 5 . In the embodiment of  FIG. 5 , the implantable medical device is a CRM device. Specifically, the CRM device of  FIG. 5  is a bipolar pacemaker  160  that also is capable of operating in a unipolar mode. Bipolar pacemaker  160  includes a tip and ring electrode  132 , having a tip electrode  134  and ring electrode  136 , a first conductor  140 , and a second conductor  142 . Pacemaker  160  includes pulse generator  138 , which includes pacing switch S p , pacing capacitor C p , recharging switch S r , and recharging capacitor C r . Pulse generator  138  may also include other components, including sensing circuitry, control circuitry, and charging circuitry. A housing  144  is provided that contains pulse generator  138  and can be constructed from a conductive material. In the embodiment of  FIG. 5 , pulse generator  138  also includes switch S m  for switching between bipolar pacing mode and unipolar pacing mode. Typically a medical provider such as a physician selects either bipolar mode or unipolar mode to optimize the performance of the pacemaker for an individual patient. Pacemaker  160  also includes an integrated circuit protection diode Di that allows current to flow through an electrically conductive path from the pacemaker housing  144  into the pulse generator circuitry to the reference potential (ground) of capacitor C p . 
         [0033]    Pacemaker  160  further includes switch S i  for controlling LFIC during MRI. Switch S i  can be in series with diode D 1 . Although switch S i  is shown in  FIG. 5  as being positioned between housing  144  and diode D 1 , switch S i  can also be positioned between diode D 1  and pulse generator  138 . Many embodiments of switch S i  are usable. In one embodiment, switch S i  is a transistor-type switch. In another embodiment, switch S i  is a relay-type switch. In some embodiments, switch S i  is configured to be opened when a patient is undergoing an MRI procedure and to be closed when a patient is not undergoing an MRI procedure. When switch S i  is closed, there is an electrically conductive path from housing  144  to pulse generator  138 . However, when switch S i  is open and switch S m  is in bipolar mode, there is no electrically conductive path from housing  144  to pulse generator  138 . Thus, when switch S i  is open and switch S m  is in bipolar mode, there are no conductive loops enclosing a large area, and therefore the formation of LFIC is minimized or reduced. 
         [0034]    Switch S i  may include associated control circuitry  162  for controlling the operation of switch S i . Many embodiments of control circuitry  162  are usable. In one embodiment, control circuitry  162  includes a sensor  164  for detecting the presence of a magnetic field associated with MRI. For example, the sensor  164  can be a magnetometer, a Hall-effect sensor, or a reed switch. Sensor  164  and control circuitry  162  are configured to detect the strong magnetic field associated with MRI, which can be on the order of 1,000 to 30,000 gauss, and to differentiate the MRI magnetic field from the earth&#39;s ambient magnetic field, which is generally less than 1 gauss. In another usable embodiment, control circuitry  162  includes a sensor for detecting the presence of low frequency induced current within loop  148 . For example, sensor can be a Hall effect sensor, or can be a sensor that measures the voltage differential across a small resistor. In another usable embodiment, control circuitry  162  is configured to receive a signal, such as a telemetry signal, that is initiated outside of the patient&#39;s body, and to control switch S i  in response to a received signal. For example, a person such as a physician, medical technician, nurse, or patient can initiate a first signal prior to beginning an MRI. The first signal can be initiated, for example, by activating a switch on a device. In addition, a person such as a physician, medical technician, nurse, or patient can also initiate a second signal after an MRI is completed. The second signal can also be initiated by activating a switch on a device. 
         [0035]    In operation, switch S i  is preferably placed in an open position when a patient is undergoing an MRI procedure and is preferably placed in a closed position when a patient is not undergoing an MRI procedure. In one embodiment, control circuitry  162  is configured to open switch S i  in response to the detection by sensor of a magnetic field associated with MRI. In another embodiment, control circuitry  162  is configured to open switch S i  in response to the detection by sensor  166  of low frequency induced current. In another embodiment, control circuitry  162  is configured to open switch S i  in response to a received first signal  68  that was initiated outside of the patient&#39;s body. Control circuitry  162  is further configured to close switch S i  when the sensor does not detect a magnetic field associated with MRI. In another embodiment, control circuitry  162  is configured to close switch S i  when sensor does not detect low frequency induced current. In another embodiment, control circuitry  162  is configured to close switch S i  in response to a received second signal that was initiated outside of the patient&#39;s body. 
         [0036]    Other embodiments of the invention are usable. An alternative embodiment of a pacemaker  272  constructed according to the principles of the present invention is depicted in  FIG. 6 . Pacemaker  272  of  FIG. 6  is constructed similarly to pacemaker  160  of  FIG. 5 . However, pacemaker  272  does not include switch S i , but instead has a resistor  274  in series between pacemaker housing  244  and capacitor C p . Resistor  274  can be in series with diode D 1 . Although resistor  274  is shown in  FIG. 6  as being between diode D 1  and pulse generator  238 , resistor  274  can also be positioned between diode D 1  and housing  244 . Resistor  274  thus forms part of a conductive loop  276  through capacitor C p  and switch S p  (when switch S p  is closed), or through switch S r  (when switch S r  is closed), and through capacitor C r , conductor  240 , electrode  236 , tissue path  250 , diode D 1 , and resistor  274 . Resistor  274  preferably has a sufficient resistance to substantially limit any induced current from flowing within conductive loop  276  during an MRI procedure, yet also preferably has a small enough resistance to prevent the potential of the ground side of capacitor C p  from drifting significantly from the potential of housing  244 . In one embodiment, resistor  274  has a resistance between 1,000 and 300,000 ohms, and in another embodiment, resistor  274  has a resistance between 10,000 and 30,000 ohms. In operation, although a conductive loop  276  is formed and current is capable of being inducted into loop  276  during an MRI procedure, the presence of resistor  274  limits the amount of current flowing in loop  276  to an insignificant amount. For example, in one embodiment resistor  274  limits the LFIC in loop  276  to no more than 0.1 milliamps. In other embodiments, resistor  274  limits the LFIC in loop  276  to no more than 0.5 milliamps, and in further embodiments, resistor  274  limits the LFIC in loop  276  to no more than 1.0 milliamp. 
         [0037]    Yet another embodiment of the invention is depicted in  FIG. 7 . Pacemaker  378  of  FIG. 7  is constructed similarly to pacemaker  272  of  FIG. 6  and pacemaker  160  of  FIG. 5 . However, pacemaker  378  does not include an electrically conductive connection between housing  344  and capacitor C p . When switch S m  is configured to select a bipolar operation mode (that is, where switch S m  connects conductor  342  to terminal  1  of switch S m ), there is no electrically conductive path from housing  344  to conductor  340  or pulse generator  338 . Accordingly, there is no loop in which LFIC can be generated. 
         [0038]    The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices. 
         [0039]    The above specification provides a complete description of the structure and use of the invention. Since many of the embodiments of the invention can be made without parting from the spirit and scope of the invention, the invention resides in the claims.