Abstract:
A circuit for isolating line voltage from a patient in an MRI environment, uses a filter network tuned to line voltage and incorporating blocking capacitors to eliminate the need for bulky air core transformers, complex energy transfer systems, and ferromagnetic components, while providing reduced heating such as might produce patient discomfort.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     --  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     --  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates generally to magnetic resonance imaging (MRI) equipment and in particular to a method of providing electrically isolated power to patient monitoring equipment used in conjunction with MRI.  
         [0004]     Magnetic resonance imaging can provide sophisticated images of the human body by detecting faint nuclear magnetic resonance (NMR) signals primarily from concentrations of hydrogen protons. In MRI, the patient is located in a strong polarizing magnetic field and hydrogen protons of the patient&#39;s tissues are excited into precession with a strong radio frequency (RF) pulse. A series of applied gradient magnetic fields are switched on and off to spatially encode the protons by phase and frequency after which sensitive antennas are used to detect the NMR signals which can then be reconstructed into images.  
         [0005]     The MRI machine is a difficult environment for other electrical instrumentation. The switched magnetic gradients and the RF pulse create electromagnetic interference and the MRI receiving antenna is sensitive to interference from other devices. Magnetic materials such as those used in transformer and inductor cores must be eliminated from the region of the MRI machine because of the extremely strong polarizing magnetic field.  
         [0006]     Often it is necessary to monitor physiological data of the patient in the MRI room before or during scanning. Such monitoring equipment may, for example, include blood pressure meters, anesthetic gas monitors, oximeters and ECG amplifiers requiring a source of electrical power.  
         [0007]     If line power is used to power such monitoring equipment, care must be taken to isolate patient contacting portions of the equipment from the source of the line power, typically, 120 to 240 volts and 20 amperes or more. Conventional iron-core transformers can provide such isolation, but are impractical in the MRI environment because of their ferromagnetic cores. Ferrites, in contrast, can become saturated in the magnetic field of the MRI magnet, often 0.5 Tesla or more. Air core transformers, such as provide adjacent windings of copper conductors without a ferromagnetic core, may be used; however, air core transformers are bulky and inefficient, and this latter drawback generates heat that can be a problem in a patient contacting device.  
         [0008]     U.S. Pat. No. 4,737,712 describes a method of providing isolated power to a patient contacting instrument by converting electrical power into another form (e.g. optical, mechanical, or acoustic power) and then reconverting those alternative forms of the electrical energy back into electrical energy for use by the device. The conversion process provides inherent limitations in power transfer. This approach requires complex mechanisms and is practically limited to extremely low power transfer.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present inventors have recognized that power can be effectively transferred and isolated in an MRI environment by the dielectric of high voltage but small capacitance capacitors forming part of a tuned circuit blocking the normal frequency (60 Hertz) of line current. This simple approach does not require complex conversions from one type of power to another and uses a low cost circuit design. The transferred power is shifted in frequency to avoid the blocking effect of the tuned circuit.  
         [0010]     Unlike an air core transformer approach, the present invention produces very little radiated electrical power such as may interfere with the MM machine and the circuit may be compact. Isolation is provided in two directions limiting power to the patient and limiting power to the equipment.  
         [0011]     Specifically, the present invention provides an MRI compatible patient monitor having a patient sensor circuit communicating with the patient and including an electrical load. The invention provides an alternating current power source attached to a source of line power and having a frequency different from a line frequency and an electrical filter connecting the alternating current power source to the electrical load. The electrical filter has a rejection band encompassing the line frequency.  
         [0012]     It is thus is one object of at least one embodiment of the invention to provide a simple isolating circuit compatible with the environment of an MRI machine, and that prevents line voltage faults from being communicated to patient contacting portions of a patient sensor.  
         [0013]     The electrical filter may provide isolating capacitors between the alternating current power source and the patient sensor circuit.  
         [0014]     It is thus another object of at least one embodiment of the invention to block all flow of direct current.  
         [0015]     The total capacitance between the alternating power source and the load may be less than 1000 picofarads.  
         [0016]     It is thus another object of at least one embodiment of the invention to provide a high impedance limiting all low frequency current flow.  
         [0017]     The electrical filter may be a series resonant inductance and capacitance.  
         [0018]     It is thus one object of at least one embodiment of the invention to provide a passive filter blocking line current frequencies without significant heating.  
         [0019]     The alternating current power source may provide a power and return conductor and the electrical filter may provide a capacitor in series with each of the power and return.  
         [0020]     It is thus another object to the invention to prevent dangerous current flows on a return conductor.  
         [0021]     Each capacitor may have a breakdown voltage of no less than one thousand volts.  
         [0022]     It is thus another object of at least one embodiment of the invention to prevent the flow of current at extremely high voltages, for example, that may be associated with other medical equipment.  
         [0023]     The alternating current power source may provide a first phase of alternating current on a power conductor and an opposite phase of alternating current on a return conductor.  
         [0024]     It is thus another object of at least one embodiment of the invention to provide extremely low radio emissions from the power and return conductor by providing balanced current flows whose fields cancel each other out.  
         [0025]     Each power conductor and return conductor may provide a series resonant, series connected, inductor and capacitor or may use a simple resistance-capacitor circuit.  
         [0026]     It is thus another object of at least one embodiment of the invention to provide balance in the filter circuitry among the power and return conductors to reduce electromagnetic interference and to ensure that neither the power nor return conductor can conduct fault currents.  
         [0027]     The circuit may include a resistance in series with the series connected inductor and capacitor.  
         [0028]     It is thus one object of at least one embodiment of the invention to provide a range of blocked frequencies to accommodate variations about normal line frequency.  
         [0029]     The circuit may further include a parallel-connected inductor and capacitor bridging the power and return lines.  
         [0030]     It is thus another object of at least one embodiment of the invention to reduce high frequency harmonics of the line frequency such as those created by the non-linear loads of rectifier diodes and thereby reduce radio emissions.  
         [0031]     The alternating current power source may have a frequency at least 100 times the line frequency.  
         [0032]     Thus it an object of at least one embodiment of the invention to shift the frequency of the source of power away from the line frequency to prevent attenuation of the transmitted power by the electronic filter.  
         [0033]     The alternating current power supply may have a frequency of at least 10 kilohertz.  
         [0034]     It is another object of at least one embodiment of the invention to provide a high frequency alternating current power source allowing adequate power transfer across small isolating capacitors.  
         [0035]     The patient sensor may measure any physiological parameter, including for example, a blood pressure, temperature, respiration, specific blood oxygen, or ECG.  
         [0036]     Thus it is another object of at least one embodiment of the invention to provide an isolating system generally useful for a wide variety of patient monitoring apparatus.  
         [0037]     The electronic filter may have an impedance of greater than five mega-ohms at the line frequency.  
         [0038]     It is thus another object of at least one embodiment of the invention to provide predetermined limits on the amount of current flow that may occur in a direct fault situation.  
         [0039]     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  
       [0040]      FIG. 1  is a simplified perspective view of an MRI machine including an MRI coil assembly for transmitting and receiving radio frequency electronic signals and showing positioning of a patient prior to a scan, the patient having an on-patient sensor unit provided with power from a monitoring unit connected to line power;  
         [0041]      FIG. 2  is a schematic of the isolation circuit of the present invention as positioned in the monitoring unit or the on-patient sensor unit of  FIG. 1  showing the conversion of line power to balanced high frequency alternating current (AC) waveforms passing through a filter to be used by patient contacting sensors; and  
         [0042]      FIG. 3  is a plot of the impedance of the filter of  FIG. 2  showing the frequency of the AC power source and line frequency.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     Referring now to  FIG. 1 , an MRI machine  10  may include a polarizing magnet  12 , for example, that creates a strong polarizing magnetic field. The polarizing magnetic field is such as to saturate ferrites and prevent the use of ferromagnetic materials in the region of the MRI machine  10 .  
         [0044]     The polarizing magnet  12  may include a bore  14  into which the patient  25  may be placed for scanning. Within the bore  14 , the patient  25  is surrounded by a coil set  16  providing for an RF excitation pulse, one or more gradient magnetic fields, and an antenna for detecting a faint NMR signal, as will be understood to those of ordinary skill in the art. The coil set  16  communicates with remote analysis signal processing electronics and computers (not shown) via cables  18 .  
         [0045]     The present invention provides a patient monitoring system  20  having an on-patient sensor unit  22  requiring a source of electrical power. The on-patient sensor unit  22  may, for example, include transducers for detecting blood pressure, amplifiers for ECG signals; light emitting diode/photodetector pairs for specific blood oxygen measurement and the like. The general construction of such sensor units are well know in the art.  
         [0046]     The on-patient sensor unit  22  communicates via lead  26  to a patient monitoring system  20 . The lead  26  is sized to allow the patient  25  to be moved into the bore  14  for scanning and ideally for the patient monitoring system  20  to operate during that scanning process.  
         [0047]     The patient monitoring system  20  in turn may receive a source of line power  24  through a conventional power cord  30  plugging into a floor mounted outlet or the like. The on-patient sensor unit  22  must be electrically isolated from line power  24 , in the event of an unexpected fault condition that results in the breakdown of blocking elements or insulation normally between the patient  25  and line power  24 .  
         [0048]     Referring to  FIG. 2 , the present invention provides an isolating circuit  32  positioned between the patient  25  and line power  24  preventing the flow of line power  24  to the patient  25  in the event of a fault condition. It will be understood, that the isolating circuit  32  may be placed anywhere between line power  24  and on-patient sensor unit  22 , and thus can be positioned in the on-patient sensor unit  22  or in the monitoring unit  28  or prior to monitoring unit  28  in the power cord  30 . For the purposes of clarity and discussion, it will be assumed that the isolating circuit  32  will be placed in part in on-patient sensor unit  22  and in part in monitoring unit  28  as indicated by a dividing line  34 .  
         [0049]     To the left of dividing line  34 , the isolating circuit  32  provides an alternating current power source  36  receiving a source of line power  24  typically being 120 volts at 60 hertz referenced with respect to a ground lead  37 . The alternating current power source  36  converts the line power  24  into two out-of-phase, equal amplitude, waveforms, one at a V+ output  38  and the second at a V− output  40 . In the preferred embodiment, the waveform at V+ output  38  is a sine wave of frequency of no less than ten kilohertz and preferably thirty megahertz with approximately 20 volts amplitude and the V− output  40  is exactly 180 degrees out of phase with the V+ output  38 , but identical in frequency and amplitude.  
         [0050]     The alternating current power source  36  may employ a rectifier, such as a full wave rectifier, followed by a filter to convert the line power  24  into a DC voltage. The necessary waveforms may then be synthesized from the DC voltage according to methods well known in the art, for example, using a switching amplifier driven by a stable reference oscillator source.  
         [0051]     Referring still to  FIG. 2 , each of V+ output  38  and V− output  40  connect to one end of an inductor  42  and  44 , respectively. Inductor  42  is connected in series with resistor  46  and capacitor  48 , whereas inductor  44  is connected in series with resistor  50  and capacitor  52 .  
         [0052]     The capacitors  48  and  52  block all direct current flow past a dielectric barrier  49  produced by the insulating separators between the capacitor plates. In the preferred embodiment, the capacitors  48  and  52  are approximately 50 picofarads and have a breakdown voltage of at least 1000 volts and preferably 2.5 kilovolts to produce a leakage current of less than fifty microamperes at 60 hertz for a worst case line voltage fault. Generally the total capacitance between the alternating power source and the load is limited to be less than 1000 picofarads and preferably less than 100 picofarads and the total impedance between the alternating power source and the load is greater than five mega-ohms and preferably greater than 50 mega-ohms.  
         [0053]     The remaining terminal of capacitor  48  connects to a first input  54  of an AC to DC converter  56 , whereas the remaining terminal of capacitor  52  connects to a second input  58  of the AC to DC converter  56 . The AC to DC converter  56  may, for example, be a half wave or full wave rectifier or other rectification system known in the art followed by filter elements such as capacitors or inductors to provide an output of plus and minus DC voltage  68  referenced to a ground, for example, through the use of a divider. Referring also to  FIG. 3 , the value of each of inductors  42  and  44  and capacitors  48  and  52  are selected to provide a series resonance at a frequency  60  of the alternating current power source  36  being in the preferred embodiment thirty megahertz. The series resonance creates a pass band  64  around thirty megahertz where power transmitted through the isolating circuit  32  is maximized. The resistors  46  and  50  control the width of a rejection band  64  ensuring that slight deviations in frequency  60  of the alternating current power source  36  are passed by the resulting series resonant circuit.  
         [0054]     Because the frequency of resonance (and thus the center of the pass band  64 ) is proportional to a ratio of the values of the inductors  42  and  44  and the capacitors  48  and  52 , each of the series resonance circuits of inductor  42 , and capacitor  48  and of inductor  44  and capacitor  52  are together and individually tuned to the same frequency.  
         [0055]     The frequency  66  of the line power  24  is substantially lower than the frequency of the alternating current power source  36 , in the preferred embodiment, more than 100 times lower, thus ensuring that there is relatively little attenuation of the power generated by the alternating current power source  36  as received by the AC to DC converter  56 .  
         [0056]     The nonlinear characteristics of the diodes of the AC to DC converter  56  may create a varying load such as may induce asymmetric currents and high frequency harmonics in the leads  26  generally passing between capacitors  48  and  52  and inputs  54  and  58 . Accordingly, a parallel resonance circuit formed of capacitor  70  and inductor  72  may be placed across inputs  54  and  58  and tuned to create a low impedance at possible frequencies of radio frequency harmonics. The exact tuning of these devices can be determined empirically by observing, for example, on a spectrum analyzer, the frequencies of electromagnetic interference. Alternative loads to the AC to DC converter  56  include resistive loads and electrical lamps such as LEDs.  
         [0057]     The capacitors provide extremely lightweight isolation with low heating determined by the values of resistors  46  and  50 . The circuit employs no ferromagnetic components.  
         [0058]     As will be apparent from the above description, the present invention could alternatively employ an unbalanced approach in which the V− output  40  is simply a ground reference. Alternatively the V+ output  38  and V− output  40  may be out of phase square waves or other waveforms with the possible disadvantage of increased electromagnetic interference. Further, it will be recognized that a simple series resistor-capacitor circuit, providing, for example, a high pass filter allowing passage of the high frequency of the alternating current power source  36  but blocking the lower frequency of the line power  24  can be used. Other well-known passive filter designs can also be used.  
         [0059]     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.