Abstract:
A patient monitoring system detects physiological signals from a patient during an MRI examination. The patient monitoring system wirelessly transmits data associated with the physiological signals to a remote base unit. The wireless transmission of data is carried out in a manner to not be disruptive to the MRI examination. The patient monitoring system has a removable, MRI magnet compatible battery.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application 60/799,884, filed May 12, 2006, the disclosure of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to electronic patient monitors and, in particular, to a wireless patient monitor suitable for use in the severe electromagnetic environment of a magnetic resonance imaging (MRI) machine. 
         [0003]    Magnetic resonance imaging allows images to be created of soft tissue from faint electrical resonance signals (NMR signals) emitted by atomic nuclei of the tissue. The resonance signals are generated when the tissue is subjected to a strong magnetic field and excited by a radio frequency pulse. 
         [0004]    The quality of the MRI image is in part dependent on the quality of the magnetic field, which must be strong and extremely homogenous. Ferromagnetic materials are normally excluded from the MRI environment to prevent unwanted magnetic forces on these materials and distortion of the homogenous field by these materials. 
         [0005]    A patient undergoing an MRI “scan” may be received into a relatively narrow bore, or cavity in the MRI magnet. During this time, the patient may be remotely monitored to determine, for example, heartbeat, respiration, temperature, and blood oxygen. A typical remote monitoring system provides “in-bore” patient sensors on the patient connected by electrical or optical cables to a base unit outside of the bore. Long runs of these optical or electrical cables can be a problem because they are cumbersome and can interfere with access to the patient and free movement of personnel about the magnet itself. 
         [0006]    Co-pending U.S. patent application Ser. Nos. 11/080,958, filed Mar. 15, 2005 and 11/080,743 filed Mar. 15, 2005, assigned to the assignee of the present invention and hereby incorporated by reference, describe a wireless patient monitor that may be positioned near the patient to provide real-time monitoring of patient physiological signals. The inventions described in these applications overcome problems of the electrically noisy environment of MRI by using combined diversity techniques including: frequency diversity, antenna location diversity, antenna polarization diversity, and time diversity in the transmitted signals. The quality of the signals is monitored to select among diverse pathways, dynamically, allowing low error rates and high bandwidth at practical transmission power. 
         [0007]    While wireless patient sensors offer considerable advantages for use in monitoring patients in the MRI environment, the elimination of wires connecting the patient sensors to a base unit outside the MRI machine (the latter which is normally connected to a power line) raises the problem of providing power to the patient sensor. This is particularly a problem for patient sensors that employ electromechanical devices such as pumps and motors, which can require significant amounts of power. 
         [0008]    Placing batteries in the patient sensor is one solution, but many conventional batteries are unsuitable for use in a patient sensor in the MRI machine because of their weight and potential for leakage. Moreover, batteries are generally placed in relative proximity to the circuitry to which they supply power. Patient sensors used with an MRI machine must be shielded against radio frequency interference to operate properly. As such, to reduce the size and simplify the construction of a patient sensor, the battery and the operational circuitry are contained within a common and electrically shielded housing. However, providing a shielded housing for the patient sensor that can be readily opened for the replacement of the batteries and then sealed in a manner that protects the internal circuitry from radio frequency interference can be difficult. 
         [0009]    During the scanning procedure the patient sensor is inaccessible and therefore batteries that become exhausted during a scan may require termination of the scan, which can waste valuable time on the MRI machine. Scheduled regular replacement of the batteries can be used to address this problem, but requires continuous attention of staff and inevitably involves replacing or recharging some batteries that still have additional life. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a wireless patient sensor having a battery pocket that houses a battery in such a manner to isolate the battery from operational circuitry that is powered by the battery. The operational circuitry is contained within a shielded portion of the housing whereas the battery pocket is contained within a portion that is not shielded from radio frequency interference. This construction is believed to avoid the problems associated with constructing an electrically tight housing that is repeatedly opened and closed while retaining electrical shielding integrity. 
         [0011]    Therefore, in accordance with one aspect, the present disclosure includes a wireless patient monitoring system operative with an MRI machine during an MRI examination. The monitoring system has a housing supporting an antenna for wireless transmission of data associated with physiological signals acquired from a patient during the MRI examination. First and second interior portions are defined within the housing, wherein the first interior portion is electrically isolated from the second interior portion. Circuitry is disposed in the first interior portion and a battery substantially free from ferromagnetic components is disposed in the second interior portion. 
         [0012]    In accordance with another aspect of the present disclosure, a wireless patient sensor operative with an MRI machine during an MM examination is presented. The sensor includes a housing having an interior volume and a chamber disposed within a first portion of the interior volume and defined by electrically conductive walls. A shielded circuitry housing is disposed within a second portion of the interior volume and a battery pocket is disposed within a third portion of the interior volume and is electrically isolated from the chamber. The patient sensor further includes electrical connections between circuitry contained within the shielded circuitry housing and the battery pocket through the chamber. 
         [0013]    According to a further aspect of the present disclosure, a method is disclosed that includes determining a battery charge of a patient sensor that has been commissioned for use during a scheduled MRI examination. The battery charge of the patient sensor is compared to a minimal charge value required for patient monitoring during the prescribed MRI examination. If the charge of the battery is below the minimal charge value, a signal is wirelessly transmitted to an operator indicating that the commissioned patient sensor lacks sufficient battery charge to be used for the scheduled MRI examination. 
         [0014]    Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
           [0016]    In the drawings: 
           [0017]      FIG. 1  is an exploded perspective view of a wireless patient sensor of the present invention showing a smart battery positioned above a battery pocket; 
           [0018]      FIG. 2  is a cross-sectional view of  FIG. 1  along lines  2 - 2  showing the radio frequency shielding of power and data lines connecting the battery to the circuitry of the patient sensor; 
           [0019]      FIG. 3  is a schematic block-diagram of the circuitry of the patient sensor of  FIG. 1  in communication with a base unit; and 
           [0020]      FIG. 4  is a flow chart of a communication synchronization program used by the present invention to ensure reliable operation of the patient monitor incorporating the patient sensor of  FIGS. 1-3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The present invention will be described with respect to the wireless acquisition and transmission of physiological data to a remote base unit that is operative in the magnetic field generated by an MRI magnet. However, it is understood that the present invention may also be useful in other applications involving high-flux magnetic fields. 
         [0022]    Referring now to  FIG. 1 , the present invention provides a wireless patient sensor  10  having housing  12  providing radio frequency shielding to internal circuitry (not shown in  FIG. 1 ). The housing  12  supports an external antenna  15  and receives external monitoring leads  14  for collecting physiological signals. In some embodiments, the housing may further provide a connection to a hose  16 , for example, providing a source of controlled air pressure for inflating a cuff for non-invasive blood measurements or sampling respiration gases or the like. Additional details on the construction of the patient sensor  10  may be found in the co-pending applications, referenced herein, assigned to the assignee of the present invention, and hereby incorporated by reference. 
         [0023]    Referring to  FIGS. 1 and 2 , the outer walls of the housing  12  may form a pocket  18  to receive all or part of a lithium ion rechargeable smart battery  20 . Smart batteries  20  of this type are well known in the art and include integrated circuitry that can identify the type of battery and/or the capacity of the battery and that can monitor the batteries usage and likely reserved capacity. The lithium ion smart battery  20  is substantially free from ferromagnetic components to resist magnetic attraction by the MRI magnet. 
         [0024]    The pocket  18  may be electrically isolated from an interior  21  of the housing  12  by substantially continuous and electrically conductive walls  40  of the housing  12 . In embodiments in which the battery  20  may fit wholly within the pocket  18 , the battery  20  may be covered by a cover  22  (shown in  FIG. 2 ) or may be held by latch fingers  24  (shown in  FIG. 1 ). In this latter embodiment, the latch fingers  24  extending over the top of the pocket  18  when the battery  20  is in place in the pocket  18  and are releasable by spring loaded buttons  26  or the like. Shown also in  FIG. 2  is an outer enclosure  23  of insulating material, such as a polymer, that may provide an opening aligned with the pocket  18  or which may cover the pocket  18  allowing access to the battery  20  by disassembly of the enclosure  23 . 
         [0025]    In each of these embodiments, the battery pocket  18  need not be shielded from radio frequency interference eliminating the need for electrically shielded pocket covers that may be difficult to use or unreliable in daily use. Instead, the present invention provides for a connection with terminals  28  on the battery  20  that blocks not only radio frequency interference coming along the power leads from the terminals  28  but also radio frequency interference that can affect reading of the smart data obtainable from the smart battery  20 . 
         [0026]    Referring now to  FIGS. 1 ,  2  and  3 , the bottom of the pocket  18  may provide a connector  30  interfacing with terminals  28  of the battery  20  and provide leads  31  for conducting power from the battery  20  and lead  31 ′ (referenced to one of leads  31 ) for providing data from the battery  20  on battery type, capacity, remaining charge, and the like. The leads  31  and  31 ′ pass through a series of feed through capacitors  32  in one wall of a quiet box  34 , the latter which provides a volume that is wholly enclosed by conductive walls  36  which may include some of the walls  40  of the housing  12  but which is nevertheless electrically isolated from the interior  21  of the housing  12 . 
         [0027]    Within the quiet box  34 , the leads  31  and  31 ′ from connector  30  are received by other filter elements  38  (e.g., radio frequency chokes) after which they pass through a second set of feed through capacitors  41  through a shared wall  36  of housing  12  into the interior  21  of the housing  12 . The filter elements  38  are selected to provide low pass filters for the power leads  31  with a break point (e.g., less than ten Hertz), and a band pass filter for the data lead  31 ′ narrowly centered on the power spectrum for normal data communication rate for the data lead  31 ′. 
         [0028]    Referring now to  FIG. 3 , within the housing interior  21 , the power leads  31  provide power and ground signals to control circuitry  42 , transmitter circuitry  44 , and battery status circuitry  46 , which may be realized as separate circuits or integrated together, for example, using a field programmable gate array. Control circuitry  42  executes a stored program to control the operation of the transmitter circuitry  44  and battery status circuitry  46  and to receive information from these circuits and from the leads  14 , which may be transmitted to the base station  50 . Battery information from the data lead  31 ′ is provided to battery status circuitry  46  which then may provide a signal to be transmitted by transmitter circuitry  44  wirelessly to a base station  50 , as described in U.S. Pat. No. 7,091,879, the disclosure of which is incorporated herein by reference. 
         [0029]    Referring to  FIGS. 3 and 4 , stored programs executing in the control circuitry  42  and in the base station  50  operate to require a communication synchronization of the patient sensor  10  with the base station  50  indicated by process block  52  prior to use of the patient sensor  10 . This communication synchronization process provides a logical mapping of data from the patient sensor  10  to a display portion of the base station  50 . 
         [0030]    After this communication synchronization, as indicated by process block  54 , a check of the battery  20  can be made at the base station  50  that received battery data relayed from the patient sensor  10  to determine that there is sufficient electrical power remaining in the battery  20  to amply complete the scheduled MRI scan. In this regard, the base station  50  may have software to determine a minimal change required for the scheduled MRI scan based on the particulars of the scheduled MRI scan. If the battery of the commissioned patient sensor lacks the necessary charge for the scheduled MRI scan, the operator is signaled as indicated by process block  56  to replace the battery  20  with a freshly charged or new battery  20 . By having the base station determine if the commissioned patient sensor has a battery of sufficient charge, an operator is not required to determine the amount of charge that is needed to complete patient monitoring during the scheduling MRI scan. Any replacement of the battery is simplified by the elimination of a possibly cumbersome radio frequency shielding enclosure around the battery  20 . 
         [0031]    If the battery  20  has sufficient charge, the patient sensor  10  may be used to transmit physiological data. The base station  50  may store the battery usage data to track usage of the batteries  20  to establish their proper maintenance. 
         [0032]    Some of the features of the present invention can also be used for other energy storage systems, including, for example, high-capacity capacitors where the capacitor is inserted into similar pocket structure. 
         [0033]    Additionally, the base unit may also communicate with a display unit, similar to that described in U.S. Ser. No. ______, filed on ______, the disclosure of which is incorporated herein. The wireless patient sensor  10  may also include a magnet-friendly audio system, similar to that described in U.S. Ser. No. ______, filed on ______, the disclosure of which is incorporated herein. 
         [0034]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.