Abstract:
A temperature sensor is included within a lead in proximity to distal electrodes. The temperature sensor measures temperature change at the electrode to tissue interface. Actions can be taken when the temperature exceeds a threshold due to heating from current induced by radio frequency energy from an MRI scan. The actions may include sending a signal via telemetry from the implanted device to an external device to produce an alarm to alert an MRI technician or to instruct the MRI scanner to alter the MRI scan. The actions may include activating a switch in the conduction path of an implanted lead to block some of the RF energy and/or to activate a shunt in the conduction path to divert some of the RF energy. The temperature sensor may be of various forms and may be mounted in various locations within the lead.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments relate to implantable medical leads and systems. More particularly, embodiments relate to implantable medical leads and systems where a temperature change in proximity to an electrode is being monitored so that the temperature change at the electrode may be limited when necessary. 
       BACKGROUND 
       [0002]    Implantable medical leads include electrodes at a distal end in order to provide electrical stimulation to tissue at a target site within the body and/or to provide sensing of physiological signals at the target site. The implantable medical leads have a proximal end that is coupled to an implantable medical device (IMD) that performs the electrical stimulation and/or physiological sensing. Electrical conductors extend through the implantable medical leads from the IMD, which is positioned at a convenient implantation site, to the electrodes located at the target site. 
         [0003]    During procedures such as a magnetic resonance imaging (MRI) scan where a level of radio frequency electromagnetic energy much greater than in normal ambient conditions is present, an electrical current is induced onto the electrical conductors of the lead. This electrical current passes through the electrode to generate heating of the electrode to tissue interface, and this heating can be potentially dangerous to the patient having the implantable medical system that includes the IMD and one or more leads. Various techniques may be used to reduce the degree of heating. One example is to include a shield within the lead that surrounds the electrical conductors. Other examples include increasing the impedance of the electrical conductors via chokes and the like. 
         [0004]    These techniques have been proven effective for implantable medical systems where the lead is routed well below the surface of the body of the patient. However, in some cases, the lead may be routed near the surface, such as for peripheral nerve stimulation therapy. In such a case, the shallow depth of the lead within the body results in exposure to higher levels of the RF energy. These higher levels may exceed the capabilities of a lead to prevent excessive heating at the electrode to tissue interface for a lead designed originally to lessen the effects of RF energy for a deeper type of implantation. 
       SUMMARY 
       [0005]    Embodiments address issues such as these and others by providing for monitoring of temperature changes within an implantable medical lead nearby an electrode in order to limit the degree of heating. A temperature sensor may be positioned within the lead and in proximity to the electrode and may have a signal path back to a temperature probe processor. When the monitored temperature as determined by the temperature probe processor exceeds a threshold, an action may be taken to limit the degree of heating. For instance, the implantable medical device or separate probe processing device may trigger an alarm via telemetry that alerts an MRI technician to stop the MRI scan or may submit a machine instruction via telemetry that stops the MRI scan. The implantable medical device may also trigger a series switch in the conduction path to open to attempt to block conduction of the RF energy and/or may trigger a shunt in parallel to the conduction path to become active to divert some of the RF energy away from the electrode being heated. 
         [0006]    Embodiments provide a method of reducing heating at an electrode of an implantable medical system during an MRI scan. The method involves monitoring a temperature at the electrode during an MRI scan and upon detecting that the temperature being monitored exceeds a threshold, then reducing the current reaching the electrode. 
         [0007]    Embodiments provide an implantable medical lead that includes a lead body and an electrical conductor within the lead body. The lead further includes an electrode on a distal end of the lead body and electrically coupled to the electrical conductor and includes a temperature sensor within the lead body adjacent to the electrode, the temperature sensor having a signal path within the lead body. 
         [0008]    Embodiments provide an implantable medical system that includes an implantable medical device. The implantable medical device includes a stimulation engine and an electrical connector that is electrically coupled to an output of the stimulation engine. The implantable medical system further includes an implantable medical lead. The implantable medical lead includes a lead body, a proximal contact on a proximal end of the lead body, the proximal contact being electrically coupled to the electrical connector, and a distal electrode on a distal end of the lead body. The implantable medical lead further includes an electrical conductor that electrically interconnects the proximal contact to the distal electrode, with the proximal contact electrically coupled to the electrical connector. Additionally, the implantable medical lead includes a temperature sensor within the lead body and adjacent to the distal electrode and a signal path extending proximally from the temperature sensor through the lead body. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows an operating environment for various embodiments of an implantable medical system. 
           [0010]      FIG. 2  shows an embodiment of an implantable medical device and an implantable medical lead coupled together and providing a fiber optic cable as a signal path for a temperature sensor. 
           [0011]      FIG. 3  shows an embodiment of an implantable medical device and an implantable medical lead coupled together and providing an electrical conductor and header connection as a signal path for a temperature sensor. 
           [0012]      FIG. 4  shows an embodiment of an implantable medical device and an implantable medical lead coupled together and providing a signal path for a temperature sensor that terminates to a separate probe processing unit attached to the implantable medical device. 
           [0013]      FIG. 5  shows an embodiment of an implantable medical device and an implantable medical lead coupled together and providing a signal path for a temperature sensor that terminates to a separate probe processing unit attached to the implantable medical lead. 
           [0014]      FIG. 6  shows a lateral cross-sectional view of the separate probe processing unit and lead of the embodiment of  FIG. 5 . 
           [0015]      FIG. 7  shows an embodiment of a distal end of the lead where a temperature sensor is positioned within a distal electrode. 
           [0016]      FIG. 8  shows an embodiment of a distal end of the lead where a temperature sensor is positioned longitudinally adjacent to a distal electrode. 
           [0017]      FIG. 9  shows an embodiment where the temperature sensor is affixed to an interior surface of an electrode. 
           [0018]      FIG. 10  shows an embodiment where the temperature sensor is encapsulated in a conductive filler material that may be positioned within an electrode or elsewhere within the lead. 
           [0019]      FIG. 11  shows an embodiment of a distal end of the lead where a temperature sensor is positioned longitudinally adjacent to a distal electrode and within a separate conductive ring. 
           [0020]      FIG. 12  shows a lateral cross-sectional view of the temperature sensor within the separate ring. 
           [0021]      FIG. 13  shows a perspective view of the embodiment of  FIG. 12  with the temperature sensor within the separate ring. 
           [0022]      FIG. 14  shows an embodiment of a distal end of the lead where a temperature sensor is positioned longitudinally adjacent to a distal electrode and within a conductive filler material at the tip of the lead. 
           [0023]      FIG. 15  shows an embodiment of a distal end of the lead where a temperature sensor is positioned longitudinally adjacent to a distal electrode and within a metal tip of the lead. 
           [0024]      FIG. 16  shows a perspective view of the embodiment of  FIG. 15  with the temperature sensor positioned within the metal tip. 
           [0025]      FIG. 17  shows an embodiment of an implantable medical system where the signal path for the temperature sensor is an electrical conductor that is also coupled to the electrode. 
           [0026]      FIG. 18  shows a set of operations that an implantable medical device or separate probe processing device may perform to monitor the temperature change at the electrode and limit the temperature increase when using a fiber optic cable or electrical conductor not shared with an electrode. 
           [0027]      FIG. 19  shows a set of operations that an implantable medical device may perform to monitor the temperature change at the electrode and limit the temperature increase when the temperature probe shares an electrical conductor with the electrode. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Embodiments provide for temperature monitoring at an electrode of an implantable medical lead to allow detection of excessive heating and to further allow for action to be taken to limit further heating. The temperature sensor is positioned within the lead either within or adjacent to an electrode. A signal path for the temperature sensor may be of various forms such as an electrical conductor or a fiber optic cable. The signal path may lead to a connection within a header of an IMD or may lead to a separate probe processing unit, and the probe processing unit may be attached to either the IMD or to the lead. 
         [0029]      FIG. 1  shows an example of an implantable medical system  100  that includes an IMD  102  with a header  104  and a lead  106  coupled to the header  104  of the IMD  102 . Electrodes  108  are positioned at a distal end of the lead  106  and are located at a target site within the body of a patient  110 . In this example, the target site is within the arm of the patient  110  to provide peripheral nerve stimulation, but it will be appreciated that the target site may be located in other areas of the body of the patient  110 . 
         [0030]      FIG. 2  shows an embodiment of an implantable medical system  200  where an IMD  202  includes a header portion  204  that includes a first bore  205  where a proximal end of a lead  206  is inserted. The header portion  204  also includes a second bore  224  where a proximal portion  222  of a fiber optic cable is inserted. The proximal portion  222  exits from a body  207  of the lead  206  at an exit point  223 . The remaining portion  220  extends distally through the body  207  of the lead  206  to the temperature sensor (not shown in this view). 
         [0031]    The proximal portion  222  engages a port of a probe processing unit  226  that is positioned within the header portion  204 . The probe processing unit  226  may be a conventional temperature sensing unit with a fiber optic port suitable for receiving the fiber optic cable portion  222 . The probe processing unit  226  may have an output that is electrically coupled to a stimulation engine  208  and/or to a controller  210  of the IMD  202 . The probe processing unit  226  may then output a signal representative of the measured temperature to a controller  210 , and the controller  210  may then compare the temperature to a threshold and take further action. For instance, the controller  210  may cause the stimulation engine  208  or other circuit to operate a switch to shunt energy to the device casing or other heat sink or to operate a switch near the electrodes to open the circuit or shunt the energy elsewhere. The controller may additionally or alternatively telemeter an alarm or machine instruction to an external device to cause the MRI scan to be altered. The controller  210  has telemetry circuitry either integral or coupled thereto which allows communication with external devices. Alternatively, the probe processing unit may perform additional logic such as comparing the temperature to a threshold and then sending an interrupt signal to the stimulation engine  208  or other circuit to cause further action to be taken such as shunting away the energy to the casing or a heat sink. 
         [0032]    The lead  206  also includes one or more electrical conductors  218  that lead to one or more corresponding electrodes on the distal end (not shown in this view) and to one or more corresponding electrical contacts  216  on the proximal end. The header portion  204  includes conventional electrical connectors  212  that are electrically connected via feed through conductors  214  passing from the header portion  204  through a conventional feed through hermetic seal to the stimulation engine  208 . 
         [0033]      FIG. 3  shows an embodiment of an implantable medical system  300  where an IMD  302  includes a header portion  304  that includes a first bore  305  where a proximal end of a lead  306  is inserted. The proximal end includes an electrical contact  322  that is dedicated to temperature sensor signals and couples to a dedicated electrical connector  324  of the header portion  304 . An electrical conductor  320  that is dedicated to the temperature sensor located at the distal end of the lead (not shown in this view) passes through the lead body  307  and is electrically coupled to the dedicated electrical contact  322 . Likewise, an electrical feedthrough conductor  326  passes the electrical signal of the temperature sensor from the electrical connector  324  through a conventional feedthrough hermetic seal to a probe processing unit  328  within the IMD  302 . 
         [0034]    As with  FIG. 2 , the probe processing unit  328  may be a conventional temperature sensing unit but in this case has an electrical connection suitable for receiving the electrical signals produced by the temperature sensor connected to the electrical conductor  320 . The probe processing unit  328  may have an output that is electrically coupled to a stimulation engine  308  and/or to a controller  310  of the IMD  302 . The probe processing unit  328  may then output a signal representative of the measured temperature to the controller  310 , and the controller  310  may then compare the temperature to a threshold and take further action such as causing the stimulation engine  308  or other circuit to operate a switch to shunt energy to the device casing or other heat sink or to operate a switch near the electrodes to open the circuit or shunt the energy elsewhere or to telemeter an alarm or machine instruction to an external device to cause the Mill scan to be altered. Alternatively, the probe processing unit  328  may perform additional logic such as comparing the temperature to a threshold and then sending an interrupt signal to the stimulation engine  308  or other circuit to cause further action to be taken such as shunting the energy away. 
         [0035]    The lead  306  also includes one or more electrical conductors  318  that lead to one or more corresponding electrodes on the distal end (not shown in this view) and to one or more corresponding electrical contacts  316  on the proximal end. The header portion  304  includes conventional electrical connectors  312  that are electrically connected via feed through conductors  314  passing from the header portion  304  through a conventional feed through hermetic seal to the stimulation engine  308 . 
         [0036]      FIG. 4  shows an embodiment of an implantable medical system  400  where a separate probe processing enclosure  430  is attached to the IMD  402 . The probe processing enclosure  430  may include various mounting structures  432  such as a flange that compresses against the IMD  402  to fix the enclosure  430  in place. The enclosure  430  includes a bore  431  where a proximal end of an implantable medical lead  406  passes entirely through the enclosure  430  in order to enter a bore  405  of a header portion  404  of the IMD  402 . 
         [0037]    In this example, the lead  406  includes an electrical contact  422  that is electrically coupled to a dedicated electrical conductor  420  within a lead body  407  of the lead  406 . The electrical conductor  420  extends distally to a temperature sensor at a distal end of the lead  406  (not shown in this view). The electrical contact  422  electrically couples to an electrical connector  424  within the bore  431  of the enclosure  430 . 
         [0038]    A probe processing unit  428  is within the enclosure  430  and therefore separate from the IMD  402 . In this example, an electrical conductor  426  electrically couples the probe processing unit  428  to the electrical connector  424 . The conductor  426  may pass the electrical signal of the temperature sensor from the electrical connector  424  through a conventional feedthrough hermetic seal of the enclosure  430  to the probe processing unit  428 . 
         [0039]    While this example shows an electrical conductor  420  and an electrical signal path to the probe processing unit  428 , it will be appreciated that a fiber cable could be used with the enclosure  430  and separate probe processing unit. For example, a fiber optic cable within the lead body  407  may exit the lead body distally of the enclosure  430  and then be received within a bore of the enclosure  430 , in a similar fashion to the manner in which the header portion  204  receives the fiber cable portion  222  in a bore  205  in  FIG. 2 . 
         [0040]    As with the prior examples, the probe processing unit  428  may be a conventional temperature sensing unit. The probe processing unit  428  may either be configured to receive an electrical signal via the conductor  426  or receive a fiber cable in the alternative. The probe processing unit  428  may have a telemetry circuit to allow the probe processing unit  428  to provide the temperature information to the controller  210 . Alternatively, the probe processing unit  428  may perform additional logic such as comparing the temperature to a threshold and then triggering a switch to trigger to shunt the energy away. 
         [0041]    The lead  406  also includes one or more electrical conductors  418  that lead to one or more corresponding electrodes on the distal end (not shown in this view) and to one or more corresponding electrical contacts  416  on the proximal end. The header portion  404  includes conventional electrical connectors  412  that are electrically connected via feed through conductors  414  passing from the header portion  404  through a conventional feed through hermetic seal to the stimulation engine  408  which is being controlled by a controller  410 . 
         [0042]      FIG. 5  shows an embodiment of an implantable medical system  500  where a separate probe processing enclosure  530  is attached to a lead  506 . The probe processing enclosure  530  may include mounting structures such as an interference fit as shown in the lateral cross-sectional view of  FIG. 5  that compresses against the lead  506  to fix the enclosure  530  in place. The enclosure  530  includes a bore  531  where a proximal end of the implantable medical lead  506  passes entirely through the enclosure  530  in order to enter a bore  505  of a header portion  504  of the IMD  502 . 
         [0043]    In this example, the lead  506  includes an electrical contact  522  that is electrically coupled to a dedicated electrical conductor  520  within a lead body  507  of the lead  506 . The electrical conductor  520  extends distally to a temperature sensor at a distal end of the lead  506  (not shown in this view). The electrical contact  522  electrically couples to an electrical connector  524  within the bore  531  of the enclosure  530 . 
         [0044]    A probe processing unit  528  is within the enclosure  530  and therefore separate from the IMD  502 . In this example, an electrical conductor  526  electrically couples the probe processing unit  528  to the electrical connector  524 . The conductor  526  may pass the electrical signal of the temperature sensor from the electrical connector  524  through a conventional feedthrough hermetic seal of the enclosure  530  to the probe processing unit  528 . 
         [0045]    While this example shows an electrical conductor  520  and an electrical signal path to the probe processing unit  528 , it will be appreciated that a fiber cable could be used with the enclosure  530  and separate probe processing unit. For example, a fiber optic cable within the lead body  507  may exit the lead body distally of the enclosure  530  and then be received within a bore of the enclosure  530 , in a similar fashion to the manner in which the header portion  204  receives the fiber cable portion  222  in a bore  205  in  FIG. 2 . 
         [0046]    As with the prior examples, the probe processing unit  528  may be a conventional temperature sensing unit. The probe processing unit  528  may either be configured to receive an electrical signal via the conductor  526  or receive a fiber cable in the alternative. The probe processing unit  528  may have telemetry to communicate with the controller as described above for the probe processing unit of  FIG. 4 . Alternatively, the probe processing unit  528  may perform additional logic such as comparing the temperature to a threshold and then trigger a switch to shunt the energy away. 
         [0047]    The lead  506  also includes one or more electrical conductors  518  that lead to one or more corresponding electrodes on the distal end (not shown in this view) and to one or more corresponding electrical contacts  516  on the proximal end. The header portion  504  includes conventional electrical connectors  512  that are electrically connected via feed through conductors  514  passing from the header portion  504  through a conventional feed through hermetic seal to the stimulation engine  508  which is being controlled by a controller  510 . 
         [0048]    For each of the examples discussed above where an electrical signal is being generated by the temperature sensor, if two conduction paths back to the probe processing unit are required, then the conductor in the lead body may be two separately insulated conductors and that terminate in two separate electrical contacts on the lead. The probe processing unit may then be coupled to two separate electrical connectors that electrically couple to the two separate electrical contacts. As an alternative, one of the conduction paths may be through the body tissue of the patient where the probe processing unit is coupled to a conductive encasement that contacts the body tissue to complete the conduction path to the probe processing unit.  FIG. 7  shows an example of a distal end of a lead  700  which may be used with the various embodiments discussed above in relation to  FIGS. 1-6 . In this example, the lead  700  includes a lead body  706  having an electrode  702  separated by an insulator region  708  from another electrode  704 . An electrical conductor  712  is coupled to the electrode  702  and extends proximally back to a proximal end contact as discussed above in relation to the proximal end of the lead examples. Likewise, an electrical conductor  714  is coupled to the electrode  704  and extends proximally back to a proximal end contact. An insulative tip section  710  may be included distally of the electrode  704 . 
         [0049]    A temperature sensor  718  may be included within one of the electrodes as shown. The temperature sensor  718  may be of various conventional forms such as a thermocouple with an electrical or optical output. A temperature sensor  718  having an electrical output may require a two conductor path back to the probe processing unit and in such a case, multiple insulated conductors may be present within the signal path  716  and multiple electrical contacts may be present on the lead and in the header or separate enclosure at the proximal end to receive the two conduction paths. As another example, the conduction of the body tissue may be utilized as one of the conduction paths. 
         [0050]      FIG. 8  shows another example of a distal end of a lead  800  which may be used with the various embodiments discussed above in relation to  FIGS. 1-6 . In this example, the lead  800  includes a lead body  806  having an electrode  802  separated by an insulator region  808  from another electrode  804 . An electrical conductor  812  is coupled to the electrode  802  and extends proximally back to a proximal end contact as discussed above in relation to the proximal end of the lead examples. Likewise, an electrical conductor  814  is coupled to the electrode  804  and extends proximally back to a proximal end contact. An insulative tip section  810  may be included distally of the electrode  804 . 
         [0051]    A temperature sensor  818  may be included within the insulative region  808  adjacent to the electrodes  802 ,  804  as shown. The temperature sensor  818  may be of various conventional forms as discussed above in relation to  FIG. 7 . As discussed above, the temperature sensor  818  having an electrical output may require a two conductor path back to the probe processing unit and in such a case, multiple insulated conductors may be present within the signal path  816  and multiple electrical contacts may be present on the lead and in the header or separate enclosure at the proximal end to receive the two conduction paths. As another example, the conduction of the body tissue may be utilized as one of the conduction paths. 
         [0052]      FIG. 9  shows an example  900  of mounting a temperature sensor  904  within an electrode  902  which has the form of a ring. The temperature sensor  904  may be attached to an inside surface of the electrode  902 . A medical adhesive  906  may be applied to fix the temperature sensor  904  in position. Furthermore, a conductive epoxy  908  may backfilled within the electrode around the temperature sensor  904 . 
         [0053]      FIG. 10  shows an example  1000  where a temperature sensor  1004  is suspended within a conductive epoxy filler  1008 . The filler  1008  forms a cylinder  1002  that may fit within the insulative region of the lead body between electrodes or within one of the electrodes. 
         [0054]      FIG. 11  shows an alternative configuration for including a temperature sensor  1112  adjacent to an electrode  1104 . In this example the temperature sensor  1112  is located between an electrode  1102  and the electrode  1104  and is positioned within an aperture  1116  of a conductive ring  1114 . This configuration of the temperature sensor  1112  and ring  1114  can be further seen in the lateral cross-sectional view of  FIG. 12  and the perspective view of  FIG. 13 . The conductive ring  1114  is present between insulative portions  1106  and  1108  of the lead  1100  and holds the temperature sensor  1112  in a fixed position. The conductive ring  1114  may be encapsulated within the lead body or may be exposed to the tissue of the patient. The temperature sensor  1112  provides a signal over the signal path  1110  that extends proximally to the proximal end of the lead. 
         [0055]      FIG. 14  shows another example of a distal end of a lead  1400  that has electrodes  1402  and  1404  attached to a lead body  1406  and electrical conductors  1412  and  1414  attached to the respective electrodes  1402 ,  1404 . An insulative region  1408  is present between the electrodes  1402 ,  1404 . In this example, the temperature sensor  1418  is located within a tip  1410  of the lead  1400 . The tip  1410  may be constructed of an insulative material and then backfilled with a conductive filler material  1420  to fix the temperature sensor  1418  within the tip  1410 . The temperature sensor  1418  provides a signal over the signal path  1416  that extends proximally to the proximal end of the lead  1400 . 
         [0056]      FIG. 15  shows another example of a distal end of a lead  1500  that has electrodes  1502  and  1504  attached to a lead body  1506  and electrical conductors  1512  and  1514  attached to the respective electrodes  1502 ,  1504 . An insulative region  1508  is present between the electrodes  1502 ,  1504 . In this example, the temperature sensor  1518  is located within a tip  1511  of the lead  1500 . The tip  1511  is constructed of metal and includes a bore  1509  that the temperature sensor  1518  is tightly positioned within to fix the position of the temperature sensor  1518  within the tip  1511 . An additional insulative portion  1510  may be present to separate the metal tip  1511  from the electrode  1504 . The configuration of the temperature sensor  1518  and insulative region  1510  can be further seen in the perspective view of  FIG. 16 . The temperature sensor  1518  provides a signal over the signal path  1516  that extends proximally to the proximal end of the lead  1500 . 
         [0057]      FIG. 17  an embodiment of an implantable medical system  1700  where an IMD  1702  includes a header portion  1704  that includes a first bore  1705  where a proximal end of a lead  1706  is inserted. The proximal end includes an electrical contact  1716  that provides the multiple functions of carrying stimulation or physiological sensing signals and also carrying temperature sensor signals. The electrical contact  1716  couples to an electrical connector  1712  of the header portion  1704 . An electrical conductor  1718  that passes through the lead body  1707  to a temperature sensor  1738  and to an electrode  1730  and is electrically coupled to the electrical contact  1716 . Likewise, an electrical feedthrough conductor  1714  passes the stimulation signals and electrical signals of the temperature sensor  1738  from the electrical connector  1712  through a conventional feedthrough hermetic seal. Within the IMD  1702 , the signal path branches to both a stimulation engine  1708  though portion  1721  and to a probe processing unit  1722  through portion  1720 . 
         [0058]    The distal end of the lead  1706  includes a tip  1734  and one or more electrodes  1730 . While this example shows the temperature sensor  1738  within the electrode  1730 , it will be appreciated that any of the temperature sensor mounting methods discussed above may be utilized such as including the temperature sensor in the insulative region  1732  adjacent the electrode  1730  or within the tip  1734 . 
         [0059]    In order to isolate the temperature sensor  1738  from stimulation or sensed physiological signals during normal operation outside of an MRI scan, a magnetically sensitive switch  1740  configured to close when in the presence of a strong magnetic field from an MRI is in series with the temperature sensor  1738  to isolate the temperature sensor. Likewise, to isolate the electrode  1730  from temperature sensor signals, a second magnetically sensitive switch  1736  configured to open when in the presence of a strong magnetic field from an MRI may be in series with the electrode  1730  to isolate the electrode  1730 . An example of the magnetically sensitive switch may be found in U.S. Application No. 61/981,768 which is incorporated herein by reference. 
         [0060]    Returning to the IMD  1702 , the probe processing unit  1722  may be a conventional temperature sensing unit but in this case has an electrical connection suitable for receiving the electrical signals produced by the temperature sensor  1738 . The probe processing unit  1722  may have an output that is electrically coupled to the stimulation engine  1708  and/or to a controller  1710  of the IMD  1702 . The probe processing unit  1722  may then output a signal representative of the measured temperature to the controller  1710 , and the controller  1710  may then compare the temperature to a threshold and take further action such as causing the stimulation engine  1708  or other circuit to shunt any energy to the device case or other heat sink, or to telemeter an alarm or machine instruction to an external device to cause the MRI scan to be altered. Alternatively, the probe processing unit  1722  may perform additional logic such as comparing the temperature to a threshold and then sending an interrupt signal to the stimulation engine  1722  or other circuit to cause further action to be taken such as operating a switch to shunt the energy away. 
         [0061]      FIG. 18  shows a set of operations that may be performed by a controller of an IMD as introduced for the various embodiments above where a temperature sensor with a fiber optic signal path or a dedicated electrical path is being used. In this example, the controller receives an MRI trigger at an operation  1802 , such as receiving a telemetry signal that instructs the controller to enter an MRI mode of operation and/or by having a sensor like that of  FIG. 17  that detects the magnetic field of the MRI. Upon receiving the MRI trigger, the controller then pings the temperature sensor by communicating with the probe processing unit of the temperature sensor to cause the probe processing unit to begin providing a temperature reading at an operation  1804 . The controller continues to monitor the sensor at an operation  1806 . 
         [0062]    While monitoring the temperature readings, the controller detects whether the temperature being read exceeds a temperature threshold at a query operation  1810 . This process continues until another MRI trigger is received at an operation  1808 , such as receiving a telemetry signal indicating the controller should exit the MRI mode of operation or by a sensor no longer detecting the magnetic field of an MRI. If the controller detects that the temperature does exceed a threshold at the query operation  1810 , then the controller initiates an action to limit any further increase in the temperature at an operation  1812 . 
         [0063]    The actions taken may be of various forms. For example, the controller may telemeter an alarm that an external device may produce to cause an MRI technician to reduce the RF power or to terminate the MRI scan. As another example, the controller may telemeter a machine instruction to the MRI scanner that causes the MRI scanner to reduce the RF power or to terminate the MRI scan. As another example, the controller may trigger a switch in series with the electrodes to transition to an open state to block some RF energy from reaching the electrodes, and/or the controller may trigger a shunt in parallel with the electrodes to become active to divert some RF energy away from the electrodes. 
         [0064]      FIG. 19  shows a set of operations that may be performed by a controller of an IMD as introduced for the various embodiments above where a temperature sensor shares an electrical path with an electrode. In this example, the controller receives an MRI trigger at an operation  1902 , such as receiving a telemetry signal that instructs the controller to enter an MRI mode of operation and/or by having a sensor like that of  FIG. 17  that detects the magnetic field of the MRI. Upon receiving the MRI trigger, the controller then enters an MRI mode of operation where the stimulation path is blocked while the temperature signal path is activated for the shared signal path at an operation  1904 . The controller then pings the temperature sensor by communicating with the probe processing unit of the temperature sensor to cause the probe processing unit to begin providing a temperature reading at an operation  1906  by communicating with the temperature sensor over the shared electrical signal path. The controller continues to monitor the sensor at an operation  1908 . 
         [0065]    While monitoring the temperature readings, the controller detects whether the temperature being read exceeds a temperature threshold at a query operation  1914 . This process continues until another MRI trigger is received at an operation  1910 , such as receiving a telemetry signal indicating the controller should exit the MRI mode of operation or by a sensor no longer detecting the magnetic field of an MRI. The MRI mode is then exited at operation  1912  where the temperature signal path is then blocked while the stimulation signal path is activated for the shared signal path. If the controller detects that the temperature does exceed a threshold at the query operation  1914 , then the controller initiates an action to limit any further increase in the temperature at an operation  1916 . The actions taken may include those discussed above in  FIG. 18 . 
         [0066]    Thus, by monitoring the temperature at the electrodes, it can be determined whether operation of the MRI scan may continue as currently configured or whether some action to limit the temperature increase at the electrodes is necessary. Accordingly, the temperature monitoring aids in avoiding risks associated with heating at the electrode to tissue interface due to induced RF energy. 
         [0067]    While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.