Abstract:
A wireless patient monitor for MRI provides for on-board filtering of physiological signals from the patient to provide improved assessment and processing of MRI noise before the signal is affected by the transmission process. A system of powering of a wireless patient monitor using capacitors is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application 60/799,884 filed May 12, 2006 and hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    — 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates generally to electronic patient monitors and, in particular, to patient monitors suitable for use in the severe electromagnetic environment of a magnetic resonance imaging machine. 
         [0004]    Magnetic resonance imaging (MRI) allows images to be created of soft tissue from faint electrical resonance (NMR) signals emitted by nuclei of the tissue. The resonance signals are generated when the tissue is subjected to a strong magnetic field and excited by a radiofrequency pulse. 
         [0005]    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. 
         [0006]    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” sensors on the patient connected by electrical or optical cables to a monitoring unit outside of the bore. 
         [0007]    Connecting an in-bore sensor to a monitoring unit may be done with long runs of electrical or optical cables. Such 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. 
         [0008]    One solution to these problems of cabling is described in 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. This patent application describes a wireless patient monitor that may be positioned near the patient to provide real-time monitoring of patient physiological signals during the MRI examination. 
         [0009]    The electrical environment of the MRI system produces a considerable amount of electrical noise that may interfere with the collection of physiological signals by in-bore sensors. The noise may include that generated by radio frequency pulses used to stimulate the protons of the patient&#39;s tissue into precession and noise induced by rapidly switched gradient magnetic fields. The character of the electrical noise produced by the MRI machine may change depending on the machine and on the particular scan protocols. 
         [0010]    In order to reduce the effect of the electrical noise on the acquired physiological signals obtained by in-bore sensors, it is known to provide different types of signal filters, appropriate for different imaging situations, at the monitoring unit outside the bore. An operator observing the physiological signals may select among the different filters to choose a filter that provides a physiological signal with the lowest noise. 
         [0011]    When wireless in-bore sensors are used, the transmitted signal must generally be pre-filtered before transmission to limit the bandwidth of the transmitted signal. This pre-filtering is not intended to separate all noise from the physiological signal and generally will not provide the entire optimum filter for this purpose. Nevertheless, to the extent that the pre-filter necessarily modifies the transmitted signal, it limits the ability of the operator to fully assess the amount of noise on the physiological signal and/or to fully control the filtration of the physiological signal. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The present invention provides improved filtration of a wirelessly transmitted physiological signal by providing a set of different electrical and/or firmware pre-filters at the transmitting unit that provide filtering of the physiological signal before or instead of the pre-filtering of the transmitter. In this way, more sophisticated filtration may be performed uncompromised by the pre-filter of the transmitter. The particular signal filter can be selected automatically or by wireless command. 
         [0013]    Specifically then, the present invention provides an MRI compatible wireless system for monitoring patient physiological signals including a transmitting unit positionable proximate to the patient during the MRI examination. The transmitting unit includes at least one sensor input for receiving a physiological signal from a sensor communicating with the patient, and a set of electrical and/or firmware filters having different characteristics adapted to provide for signal filtering of MRI electrical interference from the physiological signal. A filter selector within the transmitting unit receives a selection signal during an MRI scan to select one or more of the electrical and/or firmware filters for filtering the physiological signal, and provides this signal to a wireless transmitter which transmits to an external monitoring unit in a manner compatible with the MRI examination. 
         [0014]    It is thus one feature of at least one embodiment of the invention that it allows wireless transmission of reduced bandwidth data compatible with filtration optimized for removing MRI-induced noise. 
         [0015]    The transmitting unit may include a receiver receiving commands from the external monitor and the selection signal may be a command received from the external monitor. 
         [0016]    It is thus another aspect of at least one embodiment of the invention to permit manual selection of a transmitter-based pre-filter. 
         [0017]    Alternatively, the transmitting unit may include a noise analyzer monitoring noise in the physiological signal to automatically provide the selection signal. 
         [0018]    It is thus another aspect of at least one embodiment of the invention to allow filter selection at the transmitter using an analysis of the unfiltered signal before modification by transmission. 
         [0019]    The noise analyzer may monitor the physiological signal as filtered by each of the pre-filters to select the pre-filter providing an output with best characteristics. 
         [0020]    It is thus an aspect of at least one embodiment of the invention to provide a simple automatic assessment of signal quality. 
         [0021]    The set of electrical and/or firmware pre-filters may be a field programmable gate array and/or a digital signal processor. 
         [0022]    It is thus another feature of at least one embodiment of the invention to provide for an extremely compact yet versatile filter implementation. 
         [0023]    The set of electrical and/or firmware filters may be tuned to different modes of interference caused by interference from the MRI examination selected from the sources of: magnetic gradient coil interference and radiofrequency coil interference. 
         [0024]    It is thus another feature of at least one embodiment of the invention to allow filtration using ex ante knowledge about the MRI process. 
         [0025]    The physiological signal is selected from the group consisting of: heart beat, respiration, body temperature, and blood oxygen. 
         [0026]    It is an aspect of the present invention that it may provide for different filter types based on the type of physiological signal being transmitted. 
         [0027]    The invention also provides a power source for wireless patient monitors using so called “super capacitors”, generally capacitors that are one Farad or more in size and thereby avoiding problems of using batteries in the confined and intense magnetic environment of the MRI machine. 
         [0028]    More specifically, this aspect of the invention provides a wireless patient sensor for monitoring a patient during an MRI examination having a housing positionable near the patient during the MRI examination and an input circuit for receiving a physiological signal from the patient. The sensor further provides a transmitter for transmitting the physiological signal wirelessly to an external station in a manner compatible with operation of the MRI machine. The sensor is powered by a power source employing capacitor storage without chemical batteries. 
         [0029]    It is thus one feature of at least one embodiment of the invention to eliminate the need for costly battery systems that may adversely affect the imaging process through the incorporation of magnetic elements. 
         [0030]    The capacitor storage may provide a capacitance of at least one Farad. 
         [0031]    It is thus another feature of at least one embodiment of the invention to employ so-called super capacitors as a convenient short-term energy storage system. 
         [0032]    The sensor may further include a display displaying a charge on the capacitor storage. 
         [0033]    It is thus another feature of at least one embodiment of the invention to provide a simple and accurate assessment of the readiness the sensor. 
         [0034]    The display may show one or both of a voltage of the capacitor storage and/or an operating time of the physiological monitor when using the capacitor storage at a given voltage. 
         [0035]    It is thus another feature of at least one embodiment of the invention to provide an accurate reading of the charge on the capacitors which may be derived directly from their output voltage. 
         [0036]    The sensor may further include a DC-to-DC converter for providing constant voltage output as the capacitor storage voltage drops. 
         [0037]    It is thus another aspect of at least one embodiment of the invention to provide a power system that accommodates the steady drop in voltage of a capacitor as power is used. 
         [0038]    The DC-to-DC converter may be a boost converter. 
         [0039]    It is thus another feature of at least one embodiment of the invention to provide a power system that maximizes the time at which the voltage is at a usable level. 
         [0040]    The sensor may be used with a charging station providing receptacles for removable capacitor power sources to receive and charge the removable capacitor power sources when they are held in the receptacles. 
         [0041]    It is thus a feature of at least one embodiment of the invention to provide for a simple rapid charging station at the site of use of the portable systems. 
         [0042]    The charging station may provide a constant current source for charging the capacitor power sources. 
         [0043]    It is another aspect of at least one embodiment of the invention to provide for high-speed charging of the capacitor storage without damage. 
         [0044]    The charging station may further include an indicator showing a degree of completion of the charging of each capacitor power source in a receptacle. 
         [0045]    It is thus another feature of at least one embodiment of the invention to allow quick determination of the state of charge of the capacitor storage systems prior to use. 
         [0046]    The capacitor power sources may be constructed of non-ferromagnetic materials. 
         [0047]    It is thus an aspect of at least one embodiment of the invention to provide a power source with improved compatibility with the MRI environment. 
         [0048]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of at least one embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0049]      FIG. 1  is a fragmentary perspective view of a magnet of an MRI machine showing a patient positioned before a scan with a patient sensor transmitting patient physiological signals wirelessly to a base unit; 
           [0050]      FIG. 2  is a block diagram of the patient sensor and base unit incorporating a field programmable gate array (“FPGA”) used to implement multiple digital filters for the present invention; 
           [0051]      FIG. 3  is a block diagram of a charging stand attached to the base unit and holding supercapacitors for recharging before powering the patient sensor; 
           [0052]      FIG. 4  is a graphical representation of a physiological signal received by the patient sensor of  FIG. 1  before and after filtering; 
           [0053]      FIG. 5  is a block diagram of a set of filters implemented by the FPGA of  FIG. 2  showing implementation of multiple filters in parallel and the selection of individual filters by a selector receiving a selection signal either remotely or locally through an analysis of variance or other statistical measure of the physiological signals; 
           [0054]      FIG. 6  is a perspective view of the patient unit of  FIG. 2  showing two displays of capacitor power reserve in terms of operation time and percent charge; 
           [0055]      FIG. 7  is a plot of voltage versus time at the capacitor storage and after a boost converter receives the capacitor voltage; and 
           [0056]      FIG. 8  is a figure similar to that of  FIG. 3  showing a charging station accepting the entire patient unit without removal of the capacitor power source. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0057]    Referring now to  FIG. 1 , an MRI suite  10  may include an MRI magnet  12  holding various radio frequency and gradient magnetic field coils such as produce substantial electrical interference as is well understood in the art. 
         [0058]    A patient  14 , supported on a movable table  17 , may be positioned outside the bore of the magnet  12  to receive a wireless patient monitor  16  receiving signals from the patient  14  by leads  18 . The patient  14  may then be moved into the bore of the magnet  12  with the wireless patient monitor  16  allowing for continuous monitoring of the patient  14 . The wireless patient monitor  16  may incorporate its own power supply to transmit the monitored signals from the patient  14  via radio transmitted signal  19  or the like to a base station  24  positioned near the magnet  12  but outside of the bore. 
         [0059]    Methods of supporting a wireless transmitter of this kind on a patient are described, for example, in co-pending U.S. patent application 2006/0247512 entitled “Patient Supported In-Bore Monitor For MRI”, assigned to the assignee of the present invention and hereby incorporated by reference. 
         [0060]    Referring now to  FIG. 2 , the leads  18  of the wireless patient monitor  16  may receive physiological signals  22 , for example heartbeat, respiration, body temperature, and blood oxygen, that may be processed by signal conditioning circuitry  25  and provided to a microcontroller  26  to be converted from analog signals to digital signals at a given sampling rate. The microcontroller  26  may communicate with a display  28  to be used in adjusting the patient monitor  16  (for example, at a bedside prior to scanning), verifying its proper operation during commissioning communicating with the base station  24 . In this regard, the display  28  may provide for a representation of the received physiological signal  22  or indicator lights indicating the status of the signal. A display system for such a monitor is described in U.S. patent application 2006/0241384 entitled: “Wireless In-Bore Patient Monitor For MRI With Integral Display” assigned to the assignee of the present invention and hereby incorporated by reference. 
         [0061]    The display  28  may further be used to associate the wireless patient monitor  16  with a given transmission channel. In this latter regard, the microcontroller  26  may communicate with a transmitter/receiver  30  connected to antenna  32  for transmitting and receiving data with the base station  24  on the channel selected by operating controls  29  to select a transmission channel that is then displayed on the display  28 . 
         [0062]    Wireless transmission of physiological data in the electrically noisy environment of the MRI suite  10 , the noise caused by switched radio frequency and magnetic gradient fields of the MRI machine, without interference by the noise to the transmission and without interference by the transmission to the sensitive receiver electronics of the MRI machine, requires specialized transmission techniques such as those taught in U.S. patent application 2006/0206024 entitled: “Wireless In Bore Patient Monitor For MRI”, assigned to the same assignee as the present invention and hereby incorporated by reference. 
         [0063]    The microcontroller  26  may also communicate with a field programmable gate array (FPGA)  34  that provides various features of a digital signal processor (DSP) to implement multiple gradient filters used in an automatic gradient filter selection algorithm as will be described further below. 
         [0064]    In a preferred embodiment, the circuitry of the signal conditioning circuitry  25 , the microcontroller  26 , the transmitter/receiver  30 , and the FPGA  34  are powered by a supercapacitor  36 , being one or multiple discrete capacitors wired together in series or parallel having a capacitance of at least one Farad, contained by the patient monitor  16 . In the application of the patient monitor  16 , the supercapacitor  36  offers sufficient power density for operating the patient monitor  16  during a normal MRI scan, and avoids ferromagnetic materials or hazardous materials often found in batteries. Ferromagnetic materials can be a problem in the vicinity of the magnet  12  because such materials can be affected by strong forces of attraction of the polarizing magnetic field and/or interfere with the homogeneity of the magnetic field within the magnet bore, homogeneity that is critical to accurate imaging. 
         [0065]    Referring now to  FIGS. 2 and 7 , during use, the supercapacitor  36  provides an asymptotically declining voltage  37  that may be received by a DC-to-DC converter  35 . The DC-to-DC converter may be a so-called “boost” converter that may provide a constant DC voltage  37 ′ even as the voltage  37  drops below that constant level by incorporating a small non-ferromagnetic boost inductor according to techniques well known in the art. In this way, the full energy capacity of the supercapacitor  36  may be utilized despite the decline in capacitor voltage. 
         [0066]    Referring now to  FIGS. 1 and 2 , these supercapacitors  36  may be removable from the wireless patient monitor  16  and placed in a charging stand  38  conveniently located on the remote base station  24 . Alternatively, and as shown in  FIG. 6 , the supercapacitors  36  may be wholly incorporated within the RF shielding  41  of the patient monitor  16  with only their terminals  49  exposed and the entire patient monitor  16  may be placed in the charging stand  38  as shown in  FIG. 8 . In this latter embodiment, provisions for user-access to a battery compartment or the like can be wholly eliminated permitting more robust radiofrequency shielding. 
         [0067]    In either case, the charging stand  38  may provide for a series of charging pockets  40  holding either the supercapacitors  36  or the entire patient monitor  16  so that terminals  49  connected to the supercapacitors  36  are exposed to connect to corresponding terminals  42  in the pockets  40 . Once placed in the pocket  40 , the ground terminal of each supercapacitor  36  may be connected to a ground of a power supply  44  of the charging stand  38 , and the positive terminal of each supercapacitor  36  may be connected to an independent current source  46  implemented in the power supply  44 . Each current source  46  provides a controlled (and typically constant) current preset to a percentage of the maximum permissible charging current of each supercapacitor  36 . The use of the current sources  46  ensure the maximum charging speed of the supercapacitor  36  by changing the charging voltage as necessary to provide a consistent current for charging. 
         [0068]    The power supply  44  may also provide voltage signals  50  indicating the voltage on the supercapacitors  36  to indicator gauges  52  each being, in one embodiment, an LED bar gauge showing percentage of total charge on the supercapacitors  36  being a simple function of the voltage on the individual supercapacitors  36 . Typically, the supercapacitors  36  will recharge in only a fraction of the time required for comparable batteries and will have a many times higher recharging number than batteries. 
         [0069]    Referring again to  FIGS. 6 and 7 , the wireless patient monitor  16  may also provide displays  39  and  39 ′ providing an indication of the charge on the supercapacitor  36  when installed in a particular wireless patient monitor  16  away from the charging stand  38 . Display  39 , like indicator gauges  52 , may show the percentage of total charge of the supercapacitor  36  such as may be deduced from the voltage signals  50  such as defines a lower area  43  under the curve of asymptotically declining voltage  37  as compared to the total area under the curve of asymptotically declining voltage  37 . In addition, the wireless patient monitor  16  may provide for display  39 ′ indicating a remaining running time of the wireless patient monitor  16  deduced simply from the total running time obtained with a fully charged supercapacitor  36  multiplied by the percentage deduced through the analysis of area  43 . 
         [0070]    Referring now to  FIG. 4 , in general, the physiological signals  22  received by the patient monitor  16  will include significant electrical interference  60  caused by induced currents and voltages produced by the coils associated with the MRI magnet  12 . The particular type of interference will often be related to the type of imaging being performed, and the degree of interference with the underlying physiological signal  22  may depend on the type of physiological signal  22 . The electrical interference  60  will increase the variance  62  of the signal  22  and other statistical measurements, for example, the power spectrum width or the like. 
         [0071]    A filter  64 , for example, a slew filter, conventional bandpass, lowpass or highpass filter, or other filters well known in the art of digital signal processing, may be applied to physiological signal  22  to produce a clean signal  22 ′ better reflecting the underlying physiological signals  22 , with reduced electrical interference  60 . Generally, the clean signal  22 ′ will have a smaller variance  62 ′ relative to variance  62  of signal  22 . 
         [0072]    Referring to  FIG. 5 , the FPGA  34  may accordingly implement multiple filters  64   a ,  64   b  and  64   c  (or more) having filter parameters selected to be appropriate for particular MRI machines or imaging sequences or physiological signals  22 . Each of the filters  64   a ,  64   b , and  64   c  may operate in parallel on the signal  22  and electrical interference  60 . The outputs of each filter  64   a ,  64   b , and  64   c  are provided to a variance analyzer  66  to determine the filter  64   a ,  64   b , and  64   c  that appears to best be reducing the electrical interference  60 , for example, by lowest statistical variance. Other techniques of estimating noise reduction may alternatively be used. 
         [0073]    The variance analyzer  66  automatically selects through a switch array  68 , implemented in software, one filter  64   a ,  64   b  and  64   c  providing the best filtration without the need for operation intervention. Alternatively, and referring to  FIGS. 2 and 5 , the switch array  68  may be manually controlled by a control signal communicated wirelessly from the base station  24  via a transmitted signal  21 . In this case, the particular filter is selected by the operator viewing the transmitted physiological signal  22  (as filtered by the different filters  64   a ,  64   b , or  64   c ) entering a desired filter or filter parameter on keyboard or entry panel  65 . No pre-filtering of the transmitted signal  19  is required other than the selection of one of the selectable filters  64   a ,  64   b , or  64   c . Because one filter will be selected by default, in one embodiment, no separate bandwidth reducing filter is needed to provide for bandwidth reduction prior to wireless transmission. Or, in the case where a separate bandwidth reducing filter is retained, the analysis of the variance, for example, can occur before this bandwidth reduction providing filter. In either case, only the filtered signal  22 ′ needs to be transmitted; thus, significantly reducing the burden and bandwidth of the wireless transmission. 
         [0074]    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.