Patent Application
20060206024
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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.
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.
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.
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.
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.
Desirably, a wireless method of monitoring a patient in the MRI magnet bore would be developed, however, conventional radio transmission faces severe obstacles in the MRI environment. First, the bore of the magnet itself is shielded, restricting the free transmission of radio signals. Second, the frequency and strength of wireless transmissions must be limited to prevent interference with the faint magnetic resonance signals detected by the MRI machine and to accommodate practical battery-powered operation of the transmitter. Third, the radio frequency excitation pulse, that is part of the MRI process, can interfere with wireless transmissions. Finally, the room in which the MRI machine is held may be shielded electrically and magnetically creating problems of reflection of wireless signals such as can produce “dead spots” in the room.
These problems are compounded by the requirement that patient signals, unlike voice signals, for example, must be robust and reliable in real time, even in the face of interference. Particularly, when monitoring signals are used to gate the MRI machine, even short periods of signal dropout or delay are unacceptable. Accordingly, conventional wireless transmission techniques may prove impractical.
The present invention provides a wireless, in-bore patient monitoring system that provides the necessary high-data transmission rate and robustness against interference in an MRI environment. The invention addressed the difficult environment of MRI by using multiple diversity techniques including frequency diversity, antenna location diversity, antenna polarization diversity and time diversity in the transmitted signals. In the preferred embodiment of the invention, error detection codes attached to the signals or the signal quality of the signals are monitored to select among diverse pathways, dynamically, allowing low error rates and high bandwidth at practical transmission power.
Specifically, the present invention provides a wireless patient sensor system for MRI imaging having a patient unit positionable adjacent to the patient within a bore of an MRI magnet, the patient unit providing at least one sensor receiving a patient signal from the patient and having a wireless transmitter system for transmitting digital data packets communicating the patient signal via wireless signals. A receiving unit having a wireless receiver system receiving the digital data packets from outside the bore of the MRI magnet outputting information of the digital data packets to an operator or as a relay to another device. The wireless transmitter system and wireless receiver system communicate using diverse multiple channels between the patient unit and receiving unit.
It is thus one object of at least one embodiment of the invention to provide a practical wireless communication of patient data from inside a magnet bore in an MRI system.
The receiving unit may compute an error checking code for the digital data packets transmitted on at least two diverse multiple channels to select one diverse multiple channel from which to obtain a digital data packet for outputting. Alternatively the receiving unit may compute a signal quality (e.g., signal strength, time between drop outs, etc.) on the diverse multiple channels to select one diverse multiple channel from which to obtain a digital data packet for outputting.
It is another object of at least one embodiment of the invention to provide a simple and robust method of selecting among the diverse multiple channels to identify accurate data.
The diverse multiple channels may be at least two different frequencies of radio waves between the radio transmitter system and radio receiver system.
It is thus another object of at least one embodiment of the invention to use frequency diversity to eliminate potential sources of interference while nevertheless ensuring that the wireless frequencies do not interfere with the MRI machine's detection of NMR signals.
The different frequencies of radio waves are transmitted alternately in time, for example, using at least one radio receiver switching between the different frequencies for reception and at least one radio transmitter switching between the different frequencies for transmission. For the radio transmitter, predetermined settle time may occur after switching and before transmitting a digital data packet.
It is thus another object of at least one embodiment of the invention to provide maximum diversity with each communicating transmitter and receiver.
The diverse multiple channels may be provided by different antennas having different polarization and or having different spatial locations.
It is thus another object of at least one embodiment of the invention to address problems unique to the shielded bore and magnet room such as create modal hot spots and drop-out zones.
The different spatial locations may be an odd multiple of one-quarter wavelength of a frequency of radio signals used to transmit the digital data packets or another distance.
It is thus another object of at least one embodiment of the invention to provide a system avoiding dead zones caused by interfering reflections off the shielded magnet room wall.
The radio receiver system may include multiple radio receivers each with switchable different antennas and wherein the receiving unit computes an error checking code or signal quality for digital data packets received on a radio receiver to selectively switch an antenna on the radio receiver when the error checking code indicates an error in. or that the signal quality comparatively low for. the digital data packet.
It is thus another object of at least one embodiment of the invention to dynamically adapt to changing conditions of the MRI room.
The diverse multiple channels may be data samples of the patient signal repeated at diverse times. For example, multiple sequential data samples of the patient signal may be collected in each digital data packet according to a rolling time window applied to the patient signal that provides for redundant data samples to be transmitted in successive digital data packets. The receiving unit may compute an error checking code or signal quality for at least two corresponding digital data packets received at different times to select one data sample of the corresponding digital data packets for outputting.
It is thus another object of at least one embodiment of the invention to create a system robust against short-duration data losses without complex and time-consuming handshaking routines.
The receiving unit may further include a radio transmitter for transmitting control instructions to the patient unit and wherein the patient unit further includes a radio receiver for receiving the control instructions from the receiving unit, for example, instructions controlling recording of data or selecting from among the patient signals or outputting an operator output display on the patient monitor.
It is yet another object of at least one embodiment of the invention to provide both data from the patient and control of the patient monitor from a remote location to the patient outside of the bore.
The radio transmitter system of the patient unit may further transmit digital data packets communicating non-patient signals via radio signals to the receiving unit.
It is thus another object of at least one embodiment of the invention to allow the patient monitor to communicate status information related to monitor hardware and operation out of the bore during monitoring when the device is not easily accessible.
The patient unit may include a battery for powering the radio transmitter system.
It is thus another object of at least one embodiment of the invention to provide diversity in a compact unit that can be contained within the bore and powered by a relatively modest battery power supply.
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.
Referring now to
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 19. 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.
Referring now to
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.
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.
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:
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's hospital room.
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.
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.
The transceiver 42 connects to a microstrip antenna 44 which may be wholly contained within an insulating plastic housing 46 of the patient unit 20 outside of Faraday shield 83 to be described in more detail below. A polymer battery 48 having no ferromagnetic terminal or other components 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.
Referring now to
Antennas 52 and 54 are both spatially diverse and have different polarizations. Ideally, antennas 52a and 54a are vertically polarized and antennas 52b and 54b 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.
With these diverse antennas 52a, 52b, 54a, and 54b, 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 50a or 50b is indicated to be corrupted by the error-correction code attached to data packets received by the given transceiver 50a or 50b 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.
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.
Referring now to
At time frame 66b, 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 50a and 50b, and which identifies the particular data packet as a forward data packet 65 and identifies the type of physiological data, e.g.: ECG, SPO2, etc.
Following the header 68a, data 68b may be transmitted providing current samples in 16 bit digital form for the ECG signals at the current sampling time (e.g., LI0, LII0, LV0). This is followed by data 68c providing corresponding samples in 16 bit digital form for the ECG signals at the next earlier sampling time (e.g., LL-1, LII-1, LV-1) as buffered in the patient unit 20. This in turn is followed by data 68d providing corresponding samples in 16 bit digital form for the ECG signals at the next earlier sampling time before data 68d (e.g., LI-2, LII-2, LV-2) again as buffered in the patient unit 20. In the vector mode, the samples may be Xn, Yn, and Zn.
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.
Status data 68e follows data 68c 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 50a and 50b. The status data 68e may also include a sequence number allowing the detection of lost forward data packet 65. Different status data 68e 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.
Finally status data 68e includes an error detection code 68f, for example, a cyclic redundancy code of a type well known in the art, computed over the total forward data packet 65 of header 68a, data 68b, data 68c, data 68d, and status data 68e 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 68f causes the controller to first see if an uncorrupted packet is available form the other transceiver 50a or 50b, and second to see if an uncorrupted packet is available from the following two forward packets. The antenna of the transceiver 50a or 50b 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.
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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.
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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.
