Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Application ______ filed ______. 
     
    
     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 machine.  
         [0003]     Magnetic resonance imaging (MRI) allows images to be created of soft tissue from faint electrical resonance signals (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 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 forces of magnetic attraction 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” sensors on the patient connected by electrical or optical cables to a monitoring unit outside of the bore.  
         [0006]     Long runs of 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.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a wireless patient monitor that may be placed in the bore of the MRI machine with the patient during scanning. Resistance to the extreme electrical environment within the bore is provided by a shielding system that works with the wireless transmitter in the patient monitor. The shielding system is also designed to minimize eddy current induced vibration allowing the patient monitor to be attached to the patient. In this latter case, the patient monitor can be attached to the patient&#39;s shoulder to provide good access to data sent wirelessly from the patient monitor to a remote receiver. Wireless communication eliminates the cabling which must pass from the bore to remote monitoring equipment and the ability to place the monitor in the bore itself reduces the length of leads communicating with sensor elements on the patient, for example, electrodes or SPO 2  optics.  
         [0008]     Specifically then, the present invention provides an electronic patient monitor providing at least one sensor for receiving a patient signal from the patient and having a transmitter system for transmitting data communicating the patient signal. A shield housing surrounds the electronic patient monitor to block free space radio frequency signals therethrough allowing operation of the electronic patient monitor within a bore of the MRI machine during scanning and to suppress eddy currents from the MRI gradients, reducing vibration of the monitor. An antenna attaches to the outside of the shield housing and communicates with the wireless transmitter through an aperture in the shield housing.  
         [0009]     Thus it is one object of at least one embodiment of the invention to provide a monitor unit that may be near to or on the patient during scanning without excessive vibration.  
         [0010]     The sensor system may include a shell surrounding the shield housing.  
         [0011]     It is thus another object of at least one embodiment of the invention to provide a housing that may be safely placed on or near the patient and that is resistant to damage.  
         [0012]     The antenna may be covered by the shell and may be, for example, a micro strip antenna.  
         [0013]     It is thus another object of at least one embodiment of the invention to prevent the antenna from interfering with placement of the monitor.  
         [0014]     The shield housing may comprise separate sections joined by eddy current blocking capacitors.  
         [0015]     It is thus another object of at least one embodiment of the invention to provide a patient monitor that may be comfortably placed on the patient without eddy current induced vibration as might be disturbing or uncomfortable to a patient touching the monitor.  
         [0016]     The shield housing may be a substantially rectangular parallelepiped having each face electrically joined to an adjacent face by DC blocking capacitors.  
         [0017]     It is thus another object of at least one embodiment of the invention to provide a manufacturable shield housing that provides for ample contained volume.  
         [0018]     The shield housing may be mesh.  
         [0019]     It is thus another object of at least one embodiment of the invention to provide a lightweight shield material that accommodates the viewing of a display that may be associated with the monitor.  
         [0020]     The monitor may include a display, for example, an LCD panel.  
         [0021]     It is thus another object of at least one embodiment of the invention to provide an in-bore patient monitor that can also serve as a primary patient monitor outside of the MRI room.  
         [0022]     The electronic patient monitor may include an LED visible outside the shield housing through at least one aperture in the shield housing.  
         [0023]     It is thus another object of at least one embodiment of the invention to provide a display that allows verifying the operation of the patient monitor from outside the bore of the magnet simply by inspection.  
         [0024]     The system may include a mount adapted to hold the electronic patient monitor to the patient.  
         [0025]     It is thus another object of at least one embodiment of the invention to reduce possible stress on the leads attached to the patient by attaching the patient monitor to the patient.  
         [0026]     The mount is adapted to hold the electronic patient monitor with the antenna removed from the patient, and when an LED is used, to allow the LED to be visible by a person observing the patient outside the bore of the magnet. Preferably this mount is to the patient&#39;s shoulder.  
         [0027]     It is thus another object of at least one embodiment of the invention to allow a location of the patient monitor to improve communication between the patient monitor and remote sensing systems.  
         [0028]     The patient mount may include a harness fitting around the patient&#39;s shoulder.  
         [0029]     It is thus another object of at least one embodiment of the invention to provide a convenient means of attaching the patient monitor to the patient.  
         [0030]     The mount may include a harness supporting leads attaching the sensors to the patient.  
         [0031]     It is thus another object of at least one embodiment of the invention to provide a method of managing the leads between the sensor and the patient to prevent them from being tangled or obstructing access to the patient.  
         [0032]     The sensor may include batteries held within the shield housing to power the electronic patient monitor.  
         [0033]     It is thus another object of at least one embodiment of the invention to provide a source of portable power that is compatible with operation in the MRI machine during scanning and that eliminates the need for remote power sources.  
         [0034]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a simplified, perspective view of an MRI system showing the MRI magnet and the location of an in-bore patient unit and an out-of-bore receiving unit;  
         [0036]      FIG. 2  is a block diagram of the patient unit of  FIG. 1  configured for ECG collection and showing blocks of a microprocessor-controlled diversity transmitter employing a contained strip antenna and an on-board display;  
         [0037]      FIG. 3  is a block diagram of the receiving unit of  FIG. 1  showing multiple diversity receivers with switched antennas communicating with a programmable controller to select accurate data for outputting to a display screen;  
         [0038]      FIG. 4  is a timing diagram of digital data packet transmitted using the diversity system of the present invention with one packet enlarged showing time diversity transmission of ECG data with a trailing error-correction code;  
         [0039]      FIG. 5  is a figure similar to that of  FIG. 4  showing a digital data packet that may be transmitted from the processing unit to the in-bore patient unit for providing commands to that transmitting unit;  
         [0040]      FIG. 6  is a plan view of an alternative embodiment of the patient unit of  FIG. 2  having a graphic display;  
         [0041]      FIG. 7  is a schematic cross-sectional representation of the graphic display employing an LED backlighting system with an LCD panel;  
         [0042]      FIG. 8  is a perspective view of a shield container for the in-bore patient unit of  FIG. 6  providing eddy-current reduction; and  
         [0043]      FIG. 9  is a partial plan view of a patient showing a harness system for holding the patient unit of  FIG. 2  to the patient in the bore for minimizing motion transmitting obstructions and lead entanglement. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0044]     Referring now to  FIG. 1 , an MRI magnet room  10  containing an MRI magnet  14  may have shielded walls  12  blocking and reflecting radio waves. The MRI magnet  14  may have a central bore  16  for receiving a patient (not shown) supported on a patient table  18 . As used henceforth, bore shall refer generally to the imaging volume of an MRI machine and should be considered to include the patient area between pole faces of open frame MRI systems.  
         [0045]     During the MRI scan, the patient is held within the bore  16  and may be monitored via wireless patient unit  20  attached to the patient or patient table  18  and within the bore  16  during the scan. The patient unit  20  transmits via radio waves  22  physiological patient data and status data (as will be described) to processing unit  24  outside the bore  16  useable by personnel within the magnet room  10 . The processing unit  24  typically will include controls  26  and a display  28  providing an interface for the operator, and may be usefully attached to an IV pole  30 . The IV pole  30  may have hooks  32  for holding IV bags (not shown) and a rolling, weighted base  34  that may be freely positioned as appropriate without the concern for wires between the patient unit  20  and processing unit  24 .  
         [0046]     Referring now to  FIG. 2 , the patient unit  20  holds an interface circuit  35  for receiving physiological patient signals including, but not limited to, signals indicating: respiration, blood oxygen, blood pressure, pulse, and temperature, each from an appropriate sensor  37 . Only ECG signals will be described henceforth for clarity.  
         [0047]     When used to sense ECG signals, the interface circuit  35  may receive two or more ECG leads  36 , being connected to, for example, the right arm, the right leg, the left arm and the left leg. The signals from these ECG leads  36  are connected to electrode amplifier and lead selector  39  which provides signals I, II and V, in a normal lead mode to be described below, or signals X, Y and Z in a vector lead mode (not shown), each attached to a corresponding electrode providing the sensor  37 . The leads  36  may be high impedance leads so as to reduce the induction of eddy currents within those leads during the MRI process. The electrode amplifier and lead selector  39  provides the signals to an interface circuit  35  which controls signal offset and amplification, provides a gradient filter having variable filter settings to reduce interference from the MRI gradient fields, and converts the signals to digital words that may be transmitted to a contained processor  38 . In a preferred embodiment, the ECG signals are sampled and digitized at a rate of 1,000 samples per second or faster so that they may be used for gating purposes. Other signals, such as those of blood oxygen may be sampled at a slower rate, for example, 250 samples per second.  
         [0048]     The processor  38  communicates with flash memory  41  which may be used to buffer and store data from ECG leads  36  and which may have a stored program controlling the operation of the patient unit  20  as will be described below.  
         [0049]     The processor  38  may communicate with an operator indicator  40 , in this case a bi-colored LED, which may display operating information according to the following states:  
                                                   LED color   Meaning                           Blinking Green   Good ECG Signals           Solid Green   No ECG Signal           Blinking Red   ECG, Poor               Communication           Solid Red   No ECG, Poor               Communication                      
 
         [0050]     The operator indicator  40  has a lens which protrudes from a housing of the patient unit  20  so that it can be viewed by an operator sighting along the bore from a variety of attitudes. Importantly, the operator indicator  40  may be used during preparation of the patient outside of the bore, even in the absence of the processing unit  24  in the patient&#39;s hospital room.  
         [0051]     The processor  38  of the patient unit  20  may also communicate with a transceiver  42 . A suitable transceiver  42  provides multi-band Gaussian frequency shift keying (GFSK) in the 2.4 GHz ISM band and is capable of operating on battery power levels to produce powers of 0 dBm such as a type commercially available from Nordic Semiconductors of Norway under the trade name nRF24E1.  
         [0052]     The transceiver  42  provides for transmission and reception of digital data packets holding samples of the ECG data with calculated error-correction codes over radio channels that may be selected by processor  38 . Preferably the radio channels are selected to provide a substantial frequency difference between the channels to reduce the possibility of any interfering source of radio frequency from blocking both channels at the same time. The selection of channels  1  and  9  provide for an 8 MHz separation between channels.  
         [0053]     The transceiver  42  connects to a microstrip antenna  44  which may be wholly contained within a housing  46  of the patient unit  20  outside of Faraday shield  83  to be described in more detail below. The housing  46 , may for example be an insulating plastic material or other material. A battery  48  having no ferromagnetic terminal or other components, such as a polymer battery, is used to provide power to each of the interface circuit  35 , processor  38 , transceiver  42  and operator indicator  40 , all held within the Faraday shield  83 .  
         [0054]     Referring now to  FIG. 3 , the processing unit  24  contains two transceivers  50   a  and  50   b  compatible with transceiver  42 , and each switching between one of at least two channels depending on the frequency of transmission by the transceiver  42 . Each of the transceivers  50  and  50   b  are connected to two antennas: antennas  52   a  and  52   b  for transceiver  50   a,  and antennas  54   a  and  54   b  for transceiver  50   b,  via a solid-state antenna switches  56   a  and  56   b,  respectively. A controller  58  receives data from and provides data to each of transceivers  50   a  and  50   b  for communication with the patient unit  20 . The controller  58  also provides signals to the switches  56   a  and  56   b  to control which antennas are connected to transceiver  50   a  and  50   b.    
         [0055]     Antennas  52  and  54  are both spatially diverse and have different polarizations. Ideally, antennas  52   a  and  54   a  are vertically polarized and antennas  52   b  and  54   b  are horizontally polarized. Further, the antennas  52  and  54  are spaced from each other by approximately an odd multiple of a quarter wavelength of the frequencies of transmission by the patient unit  20  representing an expected separation of nodal points. This spacing will be an odd multiple of approximately 3 cm in the 2.4 GHz ISM frequency band.  
         [0056]     With these diverse antennas  52   a,    52   b,    54   a,  and  54   b,  drop-off or adverse polarization of the waves at the processing unit  24 , may be accommodated by switching of the antennas  52  and  54 . Generally, this switching may be triggered when the signal from a given transceiver  50   a  or  50   b  is indicated to be corrupted by the error-correction code attached to data packets received by the given transceiver  50   a  or  50   b  as detected by program executed by the controller  58 . Alternatively, the signal quality, for example, the signal strength or the length of time that the signal has been above a predetermined threshold, may be used to trigger the switching to the better of the two antennas  52  and  54 .  
         [0057]     The controller  58  communicates with a memory  60  such as may be used to store data and a program controlling operation of the processing unit  24 . The controller  58  may also communicates with the display  28  that may display the physiological data collected by the patient unit  20  and user controls  26  that allow programming of that processing unit  24  and control of the display  28  according to methods well-known in the art.  
         [0058]     Referring now to  FIGS. 2 and 4 , during operation, the processor  38  of the patient unit  20  executes a stored program in memory  60  to collect data from ECG leads  36  and to transmit it in time-diverse forward data packets  65  over multiple time frames  66 . During a first time frame  66   a,  the processor  38  may switch the frequency of transmission of the transceiver  42  and provide a settling period of approximately 220 microseconds. As will be described, the frequency need not be changed at this time, but allowance is made for that change.  
         [0059]     At time frame  66   b,  forward data packet  65 , being physiological data from the patient, is transmitted from patient unit  20  to processing unit  24 . This forward data packet will include a header  68   a  which generally provides data needed to synchronize communication between transceivers  42  and  50   a  and  50   b,  and which identifies the particular data packet as a forward data packet  65  and identifies the type of physiological data, e.g.: ECG, SPO 2 , etc.  
         [0060]     Following the header  68   a,  data  68   b  may be transmitted providing current samples in 16-bit digital form for the ECG signals at the current sampling time (e.g., LI 0 , LII 0 , LV 0 ). This is followed by data  68   c  providing corresponding samples in 16 bit digital form for the ECG signals at the next earlier sampling time (e.g., LI −1 , LII −1 , LV −1 ) as buffered in the patient unit  20 . This in turn is followed by data  68   d  providing corresponding samples in 16 bit digital form for the ECG signals at the next earlier sampling time before data  68   d  (e.g., LI −2 , LII −2 , LV −2 ) again as buffered in the patient unit  20 . In the vector mode, the samples may be X n , Y n , and Z n .  
         [0061]     Thus, a rolling window of three successive sample periods (one new sample and the two previous samples for each lead) is provided for each forward data packet  65 . This time diversity allows data to be transmitted even if two successive forward data packets  65  are corrupted by interference.  
         [0062]     Status data  68   e  follows data  68   c  and provides non-physiological data from the patient unit  20  indicating generally the status of the patient unit  20  including, for the example of ECG data, measurements of lead impedance, device temperature, operating time, battery status, test information, information about the lead types selected, the gradient filter settings selected, and the next or last radio channel to be used to coordinate the transceivers  42  and  50   a  and  50   b.  The status data  68   e  may also include a sequence number allowing the detection of lost forward data packet  65 . Different status data  68   e  is sent in each forward data packet  65  as indexed by all or a portion of the bits of the sequence number. This minimized the length of each forward data packets  65 .  
         [0063]     Finally status data  68   e includes an error detection code  68   f,  for example, a cyclic redundancy code of a type well known in the art, computed over the total forward data packet  65  of header  68   a,  data  68   b,  data  68   c,  data  68   d,  and status data  68   e  that allows detection of corruption of the data during its transmission process by the controller  58 . Detection of a corrupted forward data packet  65  using this error detection code  68   f  causes the controller to first see if an uncorrupted packet is available form the other transceiver  50   a  or  50   b,  and second to see if an uncorrupted packet is available from the following two forward packets. The antenna of the transceiver  50   a  or  50   b  is in any event switched to see if reception can be improved. Alternatively, signal quality, as described above, may be used to select among packets.  
         [0064]     Referring still to  FIG. 4 , the forward data packet  65  of time frame  66   b  is followed by another channel changing time frame  66   c  which allows changing of the channel, if necessary, which is followed by a backward data packet  67  of time frame  66   d  providing data from the processing unit  24  to the patient unit  20 .  
         [0065]     Referring now to  FIG. 5 , the backward data packet  67  may include a header frame  70   a  followed by command frame  70   b  and an error detection code  70   c.  The commands of the command frame  70   b  in this case may be instructions to the patient unit  20 , for example, pulse the LED of the operator indicator  40  for testing or initiate a test of the hardware of the patient unit  20  according to diagnosis software contained therein, or to select the lead type of vector or normal described above, or to change the gradient filter parameters as implemented by the interface circuit  35 , or to provide a calibration pulse, or to control the filling of flash memory on the patient unit  20  as may be desired.  
         [0066]     Referring again to  FIG. 4 , an uncommitted time frame  66   e  may be provided for future use followed again by a channel change time frame  66   f  which typically will ensure that the radio channel used during the following forward data packet  65  of time frame  66   g  is different from the radio channel used in the previous forward data packet  65  of time frame  66   b.  This ensures frequency diversity in successive forward data packet  65  further reducing the possibility of loss of a given sample.  
         [0067]     Referring now to  FIG. 6 , the present invention contemplates that the patient unit  20  may be used for setup of the patient without the need for processing unit  24 , for example, in the patient&#39;s room before the patient is transported to the magnet room  10  or as a portable patient monitor that may be used for short periods of time in the patient room or during transportation of the patient and providing some of the features of the processing unit  24 . For this purpose the patient unit  20  may include not only light for operator indicator  40 , but graphic display  72  being similar to display  28  providing, for example, an output of physiological signal wave forms  74  and alphanumeric data  76 .  
         [0068]     Referring to  FIG. 7 , the display  72  to be suitable for use in the MRI environment, may comprise a liquid crystal panel  77  driven by processor  38  according to well known techniques but backlit by a series of solid state lamps, preferably white light-emitting diodes (LEDs)  80  communicating to the rear surface of the LCD panel  78  by a light pipe  82  instead of a common cold cathode fluorescent lamp. The LEDs  80  may be driven by a DC source to be unmodulated so as to reduce the possibility of creating radio frequency interference in the magnet bore caused by switching of the LEDs  80 . The use of LEDs  80  also eliminates the high voltage interference that can occur from operation of cold cathode fluorescent tubes and the magnet components inherent in such tubes.  
         [0069]     Referring now to  FIG. 8 , the circuitry of the patient unit  20  shown in  FIG. 2 , with the exception of the microstrip antenna  44 , may be contained within a Faraday shield  83  held within the housing  46  and comprised of a box of conductive elements  84  formed of a mesh material, such as a screen or wire cloth. The microstrip antenna  44  may connect with the circuitry of the patient unit  20  with a conductor threaded through the mesh, through a waveguide, or a small aperture in the mesh, which blocks only free space radio frequency electromagnetic signals. The screen elements  84  may provide a mesh size smaller than the wavelength of the MRI gradient fields but ample to allow the display  72  to be viewed therethrough. Alternatively, the display  72  may be positioned outside of the Faraday shield  83 . The light (preferably an LED) for the operator indicator  40  may protrude through the Faraday shield  83  to provide greater visibility to an operator outside the magnet bore.  
         [0070]     The screen elements  84  providing radio frequency shielding for each face of the box forming the Faraday shield  83  may be insulated from each other with respect to direct currents, but yet joined by capacitors  86  at the corner edges of the box to allow the passage of a radio frequency current. The effect of these capacitors is to block the flow of lower frequency eddy currents induced by the magnetic gradients such as can vibrate the patient unit  20  when it is positioned on the patient. Alternatively, the capacitors  86  may be replaced with resistors (not shown) to dissipate the eddy currents through resistive heating.  
         [0071]     Referring now to  FIG. 9 , the patient unit  20  may desirably be held by a harness  90  to the body, for example the shoulder of the patient  92 , so as to be free from interference with the patient while maintaining a position conducive to transmission of wireless operator indicator  40 . As positioned on the shoulder of the patient  92 , the microstrip antenna  44  is removed from the patient  92  for line of sight transmission out of the bore and the LED operator indicator  40  is exposed for viewing outside the magnet bore. The harness may provide a guide for the ECG leads  36  reducing their entanglement and simplifying installation of the unit on the patient  92 .  
         [0072]     Referring now to  FIG. 1 , the present invention further contemplates that a gating unit  100  may be positioned in the magnet room  10  to receive signals both from the processing unit  24  and patient unit  20 , and thereby to generate gating signals that may be used for gating the MRI machine. This gating unit may eavesdrop on the transmissions between the patient unit  20  and the processing unit  24  reducing the transmission overhead required of using these signals for gating.  
         [0073]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but 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. For example, the diversity techniques as described herein may be applicable to optical and other wireless transmission methods. In the case of optical transmission, for example, different frequencies of light, modulation types, modulation frequencies, polarizations, orientations may be used to provide diversity.

Technology Category: 3