Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
  • G01R33/3852—Gradient amplifiers; means for controlling the application of a gradient magnetic field to the sample, e.g. a gradient signal synthesizer

Definitions

• the following relates to the diagnostic imaging arts. It particularly relates to controlled power supplies for driving magnetic field gradient coils of magnetic resonance imaging scanners, and will be described with particular reference thereto. However, the following relates more generally to precision controlled power supplies for various applications .
• spatial encoding is typically performed by generating magnetic field gradients in the main magnet bore. These gradients are produced by a set of magnetic field gradient coils. Gradient coils are typically provided for producing independent magnetic field gradients in each of the x- , y- , and z-directions .
• Magnetic resonance imaging techniques such as echo planar imaging employ high field strength, high frequency magnetic field gradient waveforms.
• the gradient slew rate, waveform shape, amplitude, and other parameters of the magnetic field gradients vary widely for different types of imaging sequences.
• magnetic field gradient coil power supplies should deliver arbitrary power waveforms at high voltages (e.g. peak voltages of 100 volts or higher), high currents (e.g. hundreds of amperes) and high frequencies (e.g., hundreds of cycles per second) and slew rates.
• the power supply should be able to source or sink current at bipolar voltages (4-quadrant operation) .
• Pulse-width modulated switching amplifiers are commonly used in magnetic field gradient power supplies.
• a pulse-width modulated control signal triggered by a carrier signal oscillating at about 40 kHz switches the switching power supply at the carrier signal frequency to deliver pulse-width modulated power to the gradient coil .
• Power oscillations at the carrier frequency are removed by low-pass filtering inherent in
• These amplifiers include component transistors which meet stringent voltage, current, and speed (frequency) specifications.
• the transistors should have both a maximum voltage rating exceeding the maximum voltage applied to the gradient coil, and a maximum frequency rating exceeding the carrier frequency.
• the present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.
• a controlled power supply for driving a magnetic field gradient coil of a magnetic resonance imaging apparatus.
• a plurality of switching power regulators are electrically connected in series to deliver power to the gradient coil.
• a control circuit delivers phase-staggered pulse-width modulated control signals to the switching power regulators.
• a method for applying controlled power to a magnetic field gradient coil of a magnetic resonance imaging apparatus.
• Phase-staggered pulse-width modulated control signals are generated.
• a plurality of switched power outputs are produced.
• Each switch power output is switched by one of the phase-staggered pulse-width modulated control signals.
• the switched power outputs are combined in series.
• the series-combined switched power outputs are applied to the gradient coil.
• an apparatus for applying controlled power to a magnetic field gradient coil of a magnetic resonance imaging apparatus .
• a means is provided for generating phase-staggered pulse-width modulated control signals.
• a means is provided for producing a plurality of switched power outputs each of which is switched by one of the phase-staggered pulse-width modulated control signals.
• a means is provided for combining the switched power outputs in series.
• a means is provided for applying the series-combined switched power outputs to the gradient coil.
• One advantage resides in reduced voltage loads on high-speed transistors of the power amplifier. Another advantage resides obtaining a higher frequency power output without a corresponding increase in switching frequency.
• the invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations.
• the drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention.
• FIGURE 1 shows a magnetic resonance imaging apparatus that employs multi-stage magnetic field gradient amplifiers.
• FIGURE 2 shows an electrical circuit of one of the magnetic field gradient amplifiers of the imaging apparatus of FIGURE 1.
• FIGURE 3 shows an electrical circuit of one of the magnetic field gradient controllers of the imaging apparatus of FIGURE 1.
• FIGURE 4 diagrammatically shows low output voltage switching operation of the magnetic field gradient power supply of FIGURES 2 and 3.
• FIGURE 5 diagrammatically shows intermediate output voltage switching operation of the magnetic field gradient power supply of FIGURES 2 and 3.
• FIGURE 6 diagrammatically shows intermediate output voltage switching operation of the magnetic field gradient power supply of FIGURES 2 and 3, in which the output voltage is higher than the output voltage in FIGURE 5.
• FIGURE 7 diagrammatically shows high output voltage switching operation of the magnetic field gradient power supply of FIGURES 2 and 3.
• a magnetic resonance imaging scanner 10 includes a cylindrical main magnet 12, which is preferably superconducting and cryoshrouded.
• the main magnet 12 defines a magnet bore 14 inside of which a patient or other imaging subject is placed for imaging.
• the main magnet 12 produces a spatially and temporally constant and uniform main magnetic field oriented along a longitudinal axis of the bore
• Magnetic field gradient coils 16 produce magnetic field gradients in the bore 14 for spatially encoding magnetic resonance signals, for producing magnetization-spoiling field gradients, or the like.
• the magnetic field gradient , coils 16 include coils configured to produce magnetic field gradients in three orthogonal directions including the longitudinal axial direction parallel to the main magnetic field.
• a whole body radio frequency coil assembly 18 generates radio frequency pulses for exciting magnetic resonances.
• the radio frequency coil assembly 18 also serves to detect magnetic resonance signals.
• additional local radio frequency coils or phased radio frequency coil arrays are included for exciting and/or detecting magnetic resonances at localized areas in the bore 14.
• Gradient pulse amplifiers 20 deliver controlled electrical currents to the magnetic field gradient coils 16 to produce selected magnetic field gradients.
• Magnetic field gradient controllers 22 control the gradient pulse amplifiers
• Each of the gradient coils of the three orthogonal directions preferably has a corresponding gradient pulse amplifier 20 and magnetic field gradient controller 22 so that independent magnetic field gradients can be produced in the x- , y- , and z-directions .
• a radio frequency receiver 26 also coupled to the radio frequency coil assembly 18 receives magnetic resonance signals. If more than one radio frequency coil is provided (such as a local coil or phased coil array) , then different coils are optionally used for the magnetic resonance excitation and detection operations.
• a sequence controller 30 communicates with the gradient controllers 22 and the radio frequency transmitter 24 to produce selected transient or steady state magnetic resonance configurations in the subject, to spatially encode such magnetic resonances, to selectively spoil magnetic resonances, or otherwise generate selected magnetic resonance signals characteristic of the subject.
• the generated magnetic resonance signals are detected by the radio frequency receiver 26, and stored in a k-space memory 34.
• the imaging data is reconstructed by a reconstruction processor 36 to produce an image representation that is stored in an image memory 38.
• the reconstruction processor 36 performs an inverse Fourier transform reconstruction.
• the resultant image representation is processed by a video processor 40 and displayed on a user interface 42, which is preferably a personal computer, workstation, or other type of computer. Rather than producing a video image, the image representation can be processed by a printer driver and printed, transmitted over a computer network or the Internet, or the like.
• the user interface 42 also allows a radiologist or other operator to communicate with the magnetic resonance sequence controller 30 to select magnetic resonance imaging sequences, modify imaging sequences, execute imaging sequences, and so forth.
• an individual gradient pulse amplifier 20 x controls a gradient coil 16 x .
• the gradient coil 16 ! is one of the gradient coils 16 of FIGURE 1, while the gradient pulse amplifier 20.. is one of the gradient pulse amplifiers 20 of FIGURE 1.
• the gradient coil l ⁇ 1 is typically one of a pair of gradient coils that produce selected magnetic field gradients in the x-direction, the y-direction, or the longitudinal z-direction.
• the gradient pulse amplifier 20 x includes four switching power regulators 50, 52, 54, 56, which receive phase-staggered pulse-width modulated control signals A, B, C,
• each switching power regulator 50, 52, 54, 56 is a half-bridge amplifier that regulates a voltage V in .
• Each regulator 50, 52, 54, 56 includes two high-speed field effect transistors 60, 62. The gate of the transistor 60 receives the pulse-width modulated control signal
• the transistors 60, 62 act as a two-state switch.
• the transistor 60 In a high voltage state, the transistor 60 is conducting while the transistor 62 is non-conductive . This places the voltage V ln over non-conducting transistor 62. In a low voltage state, the transistor 60 is non-conductive while the transistor 62 is conducting. The output is taken over the transistor 62.
• the outputs of the four switching power regulators 50, 52, 54, 56 are connected in series (note that the lower output terminal of power regulator 52 connects with the upper output terminal of power regulator 54, as indicated by the connection point S) .
• the series-connected power regulators 50, 52, 54, 56 collectively produce one of five voltage levels: 0V, V ln , 2V in , 3V ln , or 4V ln , depending upon the state of the pulse-width modulated control signals A, B, C, D.
• the series-connected power regulators 50, 52, 54, 56 collectively provide a voltage resolution of V in , which is one-fourth of the voltage maximum output voltage 4V in .
• a bipolar circuit 70 receives the output of the series-connected power regulators 50, 52, 54, 56.
• the bipolar circuit 70 is a full-bridge amplifier that includes two pairs of insulated gate bipolar transistors 72, 74.
• the bipolar circuit 70 applies the output of the series-connected power regulators 50, 52, 54, 56 to the magnetic field gradient coil 16- ⁇ at a selected polarity.
• inputs P, N applied to the transistors 72, 74, respectively, select the polarity.
• a first polarity is provided if the input P places the transistors 72 in a conducting state while the input N places the transistors 74 in a non-conducting state.
• a second polarity opposite the first polarity is provided if the input P places the transistors 72 in a non-conducting state while the input N places the transistors 74 in a conducting state.
• an ammeter 76 measures electrical current flowing through the magnetic field gradient coil 16.,_.
• the bipolar circuit 70 which is separate from the power regulators 50, 52, 54, 56. As discussed later, this arrangement has certain advantages. However, it is also contemplated to integrate polarity selection with the power regulators 50, 52,
• the pulse-width modulated control signals A, B, C, D are generated by a magnetic field gradient controller 22 1#
• the magnetic field gradient controller 22 1# The magnetic field gradient controller
• 22 x is one of the gradient controllers 22 of FIGURE 1, and is associated with the gradient pulse amplifier 20 x and gradient coil 16. . of FIGURE 2.
• An a.c. carrier signal 80 defines a frequency of the pulse-width modulated control signals A, B, C, D.
• the a.c. carrier signal 80 oscillates at 40 kHz.
• Phase shifting circuits 82, 84, 86 shift the phase of the a.c. carrier signal 80 by 90°, 180°, and 270°, respectively.
• the carrier signal 80 is input into a pulse width modulator 90, while the outputs of the phase shifting circuits 82, 84, 86 are input into pulse-width modulators 92, 94, 96.
• Each pulse-width modulator 90, 92, 94, 96 produces a pulse train with a frequency and phase defined by the input carrier signal .
• the pulse trains are the pulse-width modulated control signals A,
• control signals A, B, C, D are phase-staggered at 90° intervals.
• a feedback controller 100 compares a set point 102 with an output of the ammeter 76 that measures current in the gradient coil 16 .
• the set point 102 is provided by the sequence controller 30 (see FIGURE 1) based on the desired magnetic field gradient called for in the imaging sequence. It will be appreciated that the set point 102 can dynamically vary, for example as the magnetic field gradient slews. Moreover, other feedback signals besides gradient coil current can be used for control, such as a gradient coil voltage or a measured characteristic of the magnetic field gradient produced by the gradient coil 16-_.
• FIGURE 4 shows operation in which the feedback controller 100 is generating a control signal corresponding to a short pulse width. Specifically, in FIGURE 4 the duty cycle of pulse-width modulated control signals A, B, C, D is less than 25%. Since the phase staggering is 90° (i.e. 25% of the carrier period T carrier ) the pulses of the four control signals A,
• an output voltage 110 varies discretely between zero volts and V ln .
• the output voltage 110 has a period which is one-fourth of the carrier period T oarrler . Hence, the output voltage 110 has a frequency that is four times the carrier frequency. This is a consequence of the phase staggering.
• the high frequency components of the output voltage 110 are filtered out by intrinsic low-pass filtering of the magnetic field gradient coil 16...
• FIGURE 5 shows operation in which the feedback controller 100 is generating a control signal corresponding to a pulse width of duty cycle between 25% and 50%.
• the feedback controller 100 is generating a control signal corresponding to a pulse width of duty cycle between 25% and 50%.
• an output voltage 120 varies discretely between V in and 2V ln .
• the frequency is four times the carrier frequency.
• FIGURE 6 shows operation in which the feedback controller 100 is generating a control signal corresponding to a pulse width of duty cycle between 50% and 75%.
• the control signals A, B, C, D are simultaneously on at any given time. That is, either two or three pulses are overlapping at any given time.
• an output voltage 130 varies discretely between 2 ln and 3V ia .
• the frequency is four times the carrier frequency.
• FIGURE 7 shows operation in which the feedback controller 100 is generating a control signal corresponding to a pulse width of duty cycle between 75% and 100%.
• the control signals A, B, C, D are simultaneously on at any given time. That is, either three or four pulses are overlapping at any given time.
• an output voltage 140 varies discretely between 3V la and 4V ln .
• the frequency is four times the carrier frequency.
• one advantage of the gradient controller 22 x and gradient amplifier 20 ! is that for higher desired output voltages (i.e. voltages that exceed one-fourth of the maximum voltage 4V in ) , the output voltage does not drop to zero volts. Similarly, as demonstrated in FIGURES 4-6, for lower voltages (i.e. voltages below three-fourths of the maximum voltage 4V in ) , the output voltage does not rise to the maximum voltage 4V in . Indeed, for any given voltage the instantaneous variation is only one-fourth of the maximum voltage, which provides improved instantaneous voltage resolution.
• Another advantage of the gradient controller 22- ⁇ and gradient amplifier 20 x is that for a given switching frequency, that is, for a given carrier frequency, the output power switching frequency is four times the carrier frequency. This enables more precise tailoring of gradient slews without using higher-speed electronics.
• the high-speed field effect transistors 60, 62 never see more than a maximum voltage of V ln , which is one-fourth of a rated output voltage amplitude
• the switching voltage regulators 50, 52, 54, 56 can be constructed using high speed field effect transistors 60, 62 specified to have a reduced maximum operating voltage that exceeds V in but is well below
• the insulated gate bipolar transistors 72, 74 of the bipolar circuit 70 do see the full voltage amplitude V rated applied to the gradient coil 16 x . However, the transistors 72, 74 do not need to switch at high frequency.
• the transistors 72, 74 are preferably selected for high power operation with reduced speed specification.
• the bipolar circuit 70 can be constructed using transistors 72, 74 specified to have a frequency rating which is substantially below the carrier frequency of the pulse-width modulated control signals A, B, C, D.
• the illustrated half-bridge power regulators 50, 52, 54, 56 include fewer components than a similar circuit that incorporates polarity selection, such as a full -bridge regulator.
• the phase staggering is preferably selected as 360°/N.
• the output power frequency is N times the carrier frequency.
• the voltage resolution is the rated voltage V rated divided by the number of voltage regulator stages N, that is voltage resolution is V rated /N.
• each additional voltage regulator increases the maximum voltage that can be applied over the gradient coil 16 x .
• six voltage regulators each regulating an input voltage V lr . can output a maximum voltage of 6V ln with a voltage resolution of V in . Since the high speed field effect transistors 60, 62 each regulate a maximum voltage of
• V in adding more voltage regulators does not affect the choice of high speed transistors 60, 62. Indeed, only the four transistors 70, 72 of the amplifier 20 x , which do not operate at high speeds, may need to be modified to accommodate a higher maximum voltage produced when more voltage regulator stages are added .
• the rated voltage can be retained as regulator stages are added by lowering the voltage regulated by each voltage regulator.
• voltage resolution increases as regulator stages are added, and the high speed field effect transistors 60, 62 process lower maximum voltages.

Landscapes

• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=32850884

Family Applications (1)

Country Status (4)

Cited By (1)

Families Citing this family (19)

Family Cites Families (4)

• 2004
  • 2004-01-22 WO PCT/IB2004/000180 patent/WO2004070411A1/en active Application Filing
  • 2004-01-22 JP JP2006502365A patent/JP2006516439A/ja not_active Withdrawn
  • 2004-01-22 CN CN 200480003381 patent/CN1745315A/zh active Pending
  • 2004-01-22 EP EP04704295A patent/EP1595159A1/de not_active Withdrawn

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events