WO2016011973A1 - 磁共振成像装置、射频放大系统及方法 - Google Patents

磁共振成像装置、射频放大系统及方法 Download PDF

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Publication number
WO2016011973A1
WO2016011973A1 PCT/CN2015/085022 CN2015085022W WO2016011973A1 WO 2016011973 A1 WO2016011973 A1 WO 2016011973A1 CN 2015085022 W CN2015085022 W CN 2015085022W WO 2016011973 A1 WO2016011973 A1 WO 2016011973A1
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Prior art keywords
signal
digital
phase
carrier signal
radio frequency
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English (en)
French (fr)
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蒋鑫
宋婷婷
陈基锋
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General Electric Co
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General Electric Co
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    • 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
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
    • G01R33/3614—RF power amplifiers
    • 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
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
    • H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits
    • H03F1/3247—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits using feedback acting on predistortion circuits
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/189—High-frequency amplifiers, e.g. radio frequency amplifiers
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/165—A filter circuit coupled to the input of an amplifier
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/336—A I/Q, i.e. phase quadrature, modulator or demodulator being used in an amplifying circuit
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/451—Indexing scheme relating to amplifiers the amplifier being a radio frequency amplifier
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/471—Indexing scheme relating to amplifiers the voltage being sensed

Definitions

  • the present invention relates to the field of radio frequency technology, and in particular, to a magnetic resonance imaging apparatus, a radio frequency amplification system, and a method.
  • the RF carrier signal can be modulated to carry information.
  • the modulated RF carrier signal can be amplified and transmitted. In order to prevent information distortion contained in the modulated radio frequency carrier signal, in general, it is desirable to maintain the relative linearity of the amplification process.
  • MRI devices typically have a radio frequency amplification system that utilizes a radio frequency amplifier to drive a radio frequency coil located within its primary magnetic structure.
  • the RF amplifier receives a series of pulses generated by an external RF source as its input signal, producing a series of pulses of power enhancement as its output signal that is transmitted and used to drive the RF coil.
  • nonlinearities in the gain and phase of the RF amplifier may be introduced into the system, resulting in distortion of the MRI image.
  • the RF amplification system utilizes a feedback device to generate a feedback signal to correct for nonlinear factors introduced during the RF amplification process to maintain linearity.
  • the present invention is summarized in one or more aspects of the present invention, which is not intended to be a . Instead, the main purpose of the summary is to present some concepts of the invention in a simplified form.
  • the RF amplification system includes a first digital modulator, a digital to analog converter, a radio frequency amplifier, an analog to digital converter, a digital demodulator, and a feedback control device.
  • a first digital modulator is operative to modulate the first digital carrier signal using the digital envelope signal to produce a digital radio frequency pulse signal having a first frequency.
  • a digital to analog converter is used to convert a digital radio frequency pulse signal into an analog radio frequency pulse signal.
  • RF amplifier for placing analog RF pulse signals Large to generate RF output signals.
  • An analog to digital converter is used to convert an analog feedback signal sampled from the RF output signal into a digital feedback signal.
  • a digital demodulator is operative to demodulate the digital feedback signal using the second digital carrier signal to produce an I signal and a Q signal.
  • the feedback control device is configured to generate an amplitude setting signal and a phase setting signal for adjusting an amplitude of the digital envelope signal according to the I signal and the Q signal, wherein the phase setting signal is used to adjust a phase of the first digital carrier signal.
  • the second digital carrier signal has a predetermined frequency offset relative to the first digital carrier signal, and the second digital carrier signal has a predetermined phase offset relative to the first digital carrier signal.
  • the radio frequency amplification method includes modulating a first digital carrier signal using a digital envelope signal to generate a digital radio frequency pulse signal having a first frequency, converting the digital radio frequency pulse signal into an analog radio frequency pulse signal, and amplifying the analog radio frequency pulse signal to Generating an RF output signal, converting an analog feedback signal sampled from the RF output signal into a digital feedback signal; demodulating the digital feedback signal using the second digital carrier signal to generate an I signal and a Q signal, and generating an amplitude setting signal according to the I signal and the Q signal And a phase setting signal, the amplitude setting signal is used to adjust the amplitude of the digital envelope signal, and the phase setting signal is used to adjust the phase of the first digital carrier signal.
  • the second digital carrier signal has a predetermined frequency offset relative to the first digital carrier signal, and the second digital carrier signal has a predetermined phase offset relative to the first digital carrier signal.
  • the magnetic resonance imaging apparatus includes a radio frequency coil assembly and a radio frequency amplification system.
  • the RF amplification system includes a first digital modulator, a digital to analog converter, a radio frequency amplifier, an analog to digital converter, a digital demodulator, and a feedback control device.
  • a first digital modulator is operative to modulate the first digital carrier signal using the digital envelope signal to produce a digital radio frequency pulse signal having a first frequency.
  • a digital to analog converter is used to convert a digital radio frequency pulse signal into an analog radio frequency pulse signal.
  • the RF amplifier is used to amplify the analog RF pulse signal to produce an RF output signal that is used to excite the RF coil assembly to transmit RF signals.
  • An analog to digital converter is used to convert an analog feedback signal sampled from the RF output signal into a digital feedback signal.
  • a digital demodulator is operative to demodulate the digital feedback signal using the second digital carrier signal to produce an I signal and a Q signal.
  • the feedback control device is configured to generate an amplitude setting signal and a phase setting signal for adjusting an amplitude of the digital envelope signal according to the I signal and the Q signal, wherein the phase setting signal is used to adjust a phase of the first digital carrier signal.
  • the second digital carrier signal has a predetermined frequency offset relative to the first digital carrier signal, and the second digital carrier signal has a predetermined phase offset relative to the first digital carrier signal.
  • the magnetic resonance imaging device and the radio frequency amplifying system and method provided by the invention have the beneficial technical effects of simple structure, less electronic components and simple circuit design.
  • FIG. 1 is a schematic view of a magnetic resonance imaging apparatus of an embodiment.
  • FIG. 2 is a functional block diagram of a radio frequency amplifying system of the first embodiment.
  • FIG. 3 is a functional block diagram of an embodiment of the digital demodulator and feedback control device of FIG. 2.
  • FIG. 4 is a functional block diagram of an embodiment of the first and second carrier signal generators shown in FIG. 2.
  • FIG. 5 is a functional block diagram of a radio frequency amplifying system of a second embodiment.
  • FIG. 1 is a schematic diagram of a magnetic resonance imaging (MRI) device 10 .
  • MRI device 10 The magnetic system 11, the cradle 12, the gradient magnetic field drive circuit 13, the radio frequency amplification system 14, the magnetic resonance signal receiving circuit 15, the control unit 16, and the operator console 17 are included.
  • the magnetic system 11 includes a substantially cylindrical inner space (hole) 111, as shown in Figure 1, and a bracket 12 on which an object 20 (e.g., a patient) is placed, supported on a cushion, through a transport mechanism ( Not shown) moved into the hole 111.
  • an object 20 e.g., a patient
  • a transport mechanism not shown
  • a static magnetic field generator 112 In the magnetic system 11, a static magnetic field generator 112, a gradient coil assembly 113, and a radio frequency coil assembly 114 are placed near the magnetic center (scanning center position) of the hole 111.
  • the static magnetic field generator 112 forms a static magnetic field in the aperture 111.
  • the direction of the static magnetic field is, for example, parallel to the axial direction of the object 20. However, the direction of the static magnetic field may also be perpendicular to the axial direction of the object 20.
  • the gradient coil assembly 113 generates a gradient magnetic field to slope the intensity of the static magnetic field formed by the static magnetic field generator 112.
  • the magnetic resonance signals received by the RF coil assembly 114 provide three dimensional positional information.
  • the gradient coil assembly 113 includes three gradient magnetic field coil systems.
  • the RF coil assembly 114 emits a radio frequency pulse to excite a proton spin within the object 20 in the space of the static magnetic field formed by the static magnetic field generator 112, thereby generating a magnetic resonance signal. At the same time, the RF coil assembly 114 receives the magnetic resonance signals generated by the subject 20.
  • the RF coil assembly 114 can be configured to have a transmitting RF coil unit and a receiving RF coil unit, respectively, or configured to perform radio frequency pulse transmission and magnetic resonance signal reception by the same RF coil unit.
  • the gradient magnetic field drive circuit 13 supplies the drive signal DR to the gradient coil assembly 113 to generate a gradient magnetic field.
  • the gradient magnetic field drive circuit 13 includes three drive circuit systems, which are not shown, corresponding to the three gradient magnetic field coil systems of the gradient coil assembly 113.
  • the RF amplification system 14 includes a radio frequency pulse frequency synthesizer, which will be described later, and uses an RF pulse frequency synthesizer to generate an analog RF pulse signal.
  • An analog RF pulse signal is applied to the RF coil assembly 114, and the RF coil assembly 114 transmits a RF signal.
  • the magnetic resonance signal receiving circuit 15 acquires the analog magnetic resonance signal received by the radio frequency coil assembly 114, converts the analog magnetic resonance signal into a baseband digital magnetic resonance signal, and outputs the baseband digital magnetic resonance signal to the data in the operator console 17. Processing unit 171.
  • the control unit 16 controls the gradient magnetic field drive circuit 13 and the radio frequency amplification system 14 in accordance with a predetermined pulse sequence to generate a drive signal DR and an analog radio frequency pulse signal. Further, the control unit 16 controls the magnetic resonance signal receiving circuit 15.
  • the operator console 17 includes a data processing unit 171, an image database 172, an operation unit 173, and a display unit 174.
  • the data processing unit 171 integrally controls the magnetic resonance imaging apparatus 10, image reproduction processing, and the like.
  • the control unit 16 is connected to a data processing unit 171 which controls the operation of the control unit 16.
  • the image database 172, the operation unit 173, and the display unit 174 are connected to the data processing unit 171.
  • the image database 172 includes, for example, a hard disk device or the like capable of recording/reproducible to record reproduced image data and the like.
  • the operation unit 173 includes a keyboard, a mouse, and the like, and the display unit 174 includes a graphic display or the like.
  • the radio frequency coil assembly 114 is an example of a radio frequency coil assembly included in the present invention
  • the radio frequency amplification system 14 is an example of a radio frequency amplification system included in the present invention.
  • Embodiments of the present invention provide a radio frequency amplification system including a radio frequency amplifier and a feedback control device, which can achieve high linearity and high stability under different loads, and obtain a constant gain (refers to an output signal and an input). The magnitude ratio between the signals).
  • the feedback control device includes independent amplitude control loops and phase control loops for controlling the amplitude and phase independently of each other, so that the amplitude and phase responses can be independently optimized to obtain high bandwidth, for example, to meet the requirements of the MRI system. bandwidth.
  • the RF amplifier can be compensated at a fast response speed under different load conditions to achieve stability.
  • feedback control is performed to obtain a stable power gain or a stable current source.
  • the output power or output current of the radio frequency amplifier can be used as a feedback signal for feedback control, and a stable power gain can be obtained when the output power is controlled as a feedback signal.
  • a stable current source can be obtained.
  • FIG. 2 is a functional block diagram of the radio frequency amplification system 800 of the first embodiment.
  • the radio frequency amplification system 800 includes a first carrier signal generator 30, an envelope signal generator 32, a first digital modulator 34, a digital to analog converter 42, a filter 44, a radio frequency amplifier 45, a sampling device 46, and an analog to digital converter 48.
  • radio frequency amplification system 800 may not include first carrier signal generator 30, second carrier signal generator 50, and sampling device 46. In a non-limiting example, radio frequency amplification system 800 may not include filter 44.
  • the first carrier signal generator 30 is operative to generate a first digital carrier signal 300.
  • the envelope signal generator 32 is operative to generate a digital envelope signal 320.
  • the first digital modulator 34 modulates the first digital carrier signal 300 using the digital envelope signal 320 to A digital radio frequency pulse signal having a first frequency is generated.
  • the first digital modulator 34 is a digital mixer.
  • the digital to analog converter 42 is configured to convert the digital radio frequency pulse signal of the first frequency into an analog radio frequency pulse signal.
  • Filter 44 is used to filter out higher harmonic components in the analog RF pulse signal.
  • filter 44 is a band pass filter.
  • the RF amplifier 45 is used to amplify the filtered analog RF pulse signal to produce a RF output signal.
  • the RF output signal is used to excite the RF coil assembly 114 in the magnetic resonance imaging apparatus to transmit a radio frequency signal; it will be appreciated that the RF output signal can be provided to other loads, such as not applied to magnetic A common coil in a resonance imaging device.
  • the first frequency of the digital RF pulse signal is determined by the intensity of the static magnetic field. For example, in a device having a static magnetic field strength of 1.5 T (Tesla), the first frequency of the digital radio frequency pulse signal is about 64 MHz.
  • the sampling frequency fs of the digital to analog converter 42 must be greater than or equal to the Nyquist frequency. In Fig. 2, for example, the sampling frequency is 200 MHz.
  • the first frequency of the digital radio frequency pulse signal is about 128.1 MHz.
  • the sampling frequency fs of the digital to analog converter 42 must be greater than the sampling frequency fs must be greater than or equal to the Nyquist frequency. In FIG. 2, for example, the sampling frequency fs is 480 MHz or 640 MHz.
  • a sampling device 46 is operative to sample the RF output signal to produce an analog feedback signal.
  • the sampling device 46 can be a coupler for obtaining a power feedback signal (for example, a directional coupler, the English name is directional coupler), or a sensor for obtaining a current feedback signal (for example, a current sensor).
  • Analog to digital converter 48 is used to convert the analog feedback signal to digital feedback signal 480.
  • the second carrier signal generator 50 is operative to generate a second digital carrier signal 500.
  • Digital demodulator 60 is operative to quadrature demodulate the digital feedback signal using the second digital carrier signal to produce an I signal and a Q signal.
  • the specific demodulation mode will be described later in FIG.
  • the feedback control device 70 is configured to generate an amplitude setting signal 72 and a phase setting signal 74 based on the I signal and the Q signal.
  • the amplitude setting signal 72 is provided to the envelope signal generator 32 for adjusting the amplitude of the digital envelope signal 320.
  • the phase setting signal 74 is provided to the first carrier signal generator 30 for adjusting the phase of the first digital carrier signal 300.
  • the first carrier signal generator 30, the envelope signal generator 32, the second carrier signal generator 50, the digital demodulator 60, and the feedback control device 70 can be integrated into the FPGA. internal.
  • the first carrier signal generator 30, the envelope signal generator 32, the second carrier signal generator 50, the digital demodulator 60, and the feedback control device 70 are internal logic functional blocks of the FPGA.
  • the second digital carrier signal 500 has a predetermined frequency offset with respect to the first digital carrier signal 300, and the second digital carrier signal 500 has a predetermined phase offset with respect to the first digital carrier signal 300.
  • the frequency of the second digital carrier signal 500 is f3 and the frequency of the first digital carrier signal 300 is f1.
  • f3 fs - f1
  • fs the frequency difference between the second digital carrier signal 500 and the first digital carrier signal 300
  • fs the sampling frequency of the analog to digital converter 48.
  • the phase difference between the second digital carrier signal 500 and the first digital carrier signal 300 is zero.
  • FIG. 3 is a functional block diagram of an embodiment of the digital demodulator 60 and the feedback control device 70 shown in FIG. 2.
  • Digital demodulator 60 utilizes second digital carrier signal 500, digital modulators 62, 64, and 90 degree phase shifter 65, or may further include two filters 66, 68 (eg, low pass filters) to digitize
  • the vector of feedback signal 480 is converted to an orthogonal scalar to produce an I signal and a Q signal with a carrier at zero frequency.
  • the I signal and the Q signal are in-phase and quadrature components of the digital feedback signal 480, respectively.
  • the feedback control device 70 includes an amplitude feedback loop and a phase feedback loop.
  • the amplitude feedback loop is used to achieve amplitude adjustment.
  • the amplitude feedback loop includes an amplitude function 702, a comparator 704, and a controller 706.
  • Amplitude function 702 is used to derive feedback amplitude from the I and Q signals
  • Comparator 704 is operative to compare the feedback amplitude to a reference amplitude to produce an amplitude error signal.
  • Controller 706 is operative to generate amplitude set signal 72 as described above based on the amplitude error signal.
  • the phase feedback loop is used to achieve phase adjustment.
  • the phase feedback loop includes a comparator 712, an integrator 714, and a phase shifter 716.
  • Comparator 712 is operative to compare the I signal to the Q signal to produce a phase error signal. If I is equal to Q, it is determined that there is no phase error; if I is not equal to Q, it is determined that there is a phase error.
  • Integrator 714 and phase shifter 716 are respectively used to accumulate phase errors and respond to phase errors to generate phase set signal 74 as described above.
  • the first carrier signal generator 30 includes a first phase memory 302, a first phase accumulator 304, and a first waveform table 306.
  • the first phase memory 302 is used to store a first phase increment.
  • the first phase accumulator 304 is configured to set an initial value of the first phase at its initialization and calculate the first accumulated phase by increasing the first phase increment for each clock cycle.
  • the first waveform table 306 is configured to store the first digital carrier signal. When the first accumulated phase is input to the first waveform table 306, the first waveform table 306 outputs a first digital carrier corresponding to the first accumulated phase. signal.
  • phase of the first digital carrier signal generated by the first carrier signal generator 30 is adjusted by setting an initial value of the first phase.
  • the second carrier signal generator 50 includes a second phase memory 502, a second phase accumulator 504, and a second waveform table 506.
  • the second phase memory 502 is for storing a second phase increment.
  • the second phase accumulator 504 is configured to set an initial value of the second phase at its initialization and calculate a second accumulated phase by increasing the second phase increment for each clock cycle.
  • the second waveform table 506 is configured to store the second digital carrier signal; when the second accumulated phase is input to the second waveform table 506, the second waveform table 506 outputs a second digital carrier corresponding to the second accumulated phase signal.
  • the predetermined frequency offset between the second digital carrier signal and the first digital carrier signal is set by adjusting a second phase increment, and the predetermined phase between the second digital carrier signal and the first digital carrier signal The offset is set by adjusting the initial value of the second phase.
  • FIG. 5 is a functional block diagram of the radio frequency amplifying system 900 of the second embodiment.
  • the difference between the radio frequency amplification system 900 of the second embodiment and the radio frequency amplification system 800 of the first embodiment is that the radio frequency amplification system 900 further includes an intermediate stage carrier signal generator 36 and a second digital modulator 38.
  • the intermediate stage carrier signal generator 36 is operative to generate a third digital carrier signal 360.
  • the second digital modulator 38 is configured to modulate the digital radio frequency pulse signal having the first frequency generated by the first digital modulator 34 using the third digital carrier signal 360 to generate a digital radio frequency pulse signal having a second frequency; the second frequency is greater than First frequency.
  • the digital to analog converter 42 thereby converts the digital RF pulse signal of the second frequency into an analog RF pulse signal, and the RF amplifier 45 is used to amplify the analog RF pulse signal filtered by the filter 44 to produce a RF output signal.
  • the frequency of the analog RF pulse signal generated by the digital to analog converter 42 in the RF amplification system 900 is greater than the frequency of the analog RF pulse signal generated by the digital to analog converter 42 in the RF amplification system 800.
  • the second frequency of the digital RF pulse signal is determined by the intensity of the static magnetic field.
  • the first frequency of the digital radio frequency pulse signal is about It is 128.1MHz.
  • the sampling frequency fs of the digital to analog converter 42 must be greater than the sampling frequency fs must be greater than or equal to the Nyquist frequency. In FIG. 2, for example, the sampling frequency fs is 480 MHz or 640 MHz.
  • the frequency of the second digital carrier signal 500 is f3, the frequency of the first digital carrier signal 300 is f1, and the frequency of the third digital carrier signal 360 is f2.
  • f3 f1 + f2, that is, the frequency difference between the second digital carrier signal 500 and the first digital carrier signal 300 is f2.
  • the phase difference between the second digital carrier signal 500 and the first digital carrier signal 300 is zero.
  • the intermediate stage carrier signal generator 36 and the second digital modulator 38 are integrated within the digital to analog converter 42. Therefore, the radio frequency amplifying system 900 of the present invention can achieve the beneficial technical effects of less electronic components and relatively simple circuit design.

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  • Engineering & Computer Science (AREA)
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Abstract

一种射频放大方法，其包括使用数字包络信号（320）对第一数字载波信号（300）进行调制以产生具有第一频率的数字射频脉冲信号，将数字射频脉冲信号转换成模拟射频脉冲信号，对模拟射频脉冲信号进行放大以产生射频输出信号，将取样自射频输出信号的模拟反馈信号转换成数字反馈信号（480）；使用第二数字载波信号（500）解调数字反馈信号（480）以产生I信号和Q信号，根据I信号和Q信号产生幅度设置信号（72）及相位设置信号（74），幅度设置信号（72）用于调节数字包络信号（320）的幅度，相位设置信号（74）用于调节第一数字载波信号（300）的相位。第二数字载波信号（500）相对于第一数字载波信号（300）具有预定的频率偏移，第二数字载波信号（500）相对于第一数字载波信号（300）具有预定的相位偏移。还提供一种射频放大系统及磁共振成像装置。

Description

磁共振成像装置、射频放大系统及方法 技术领域
本发明涉及射频技术领域，特别涉及一种磁共振成像装置、射频放大系统及方法。
背景技术
很多形式的无线通信采用射频发射。可对射频载波信号进行调制以传载信息。经调制的射频载波信号可以被放大和发射。为了防止所述经调制的射频载波信号所载信息失真，一般地，需要保持所述放大过程的相对线性。
比如，磁共振成像(MRI)装置一般具有射频放大系统，该射频放大系统利用射频放大器来驱动位于其主要磁结构内的射频线圈。该射频放大器接收由外部的射频源产生的一系列脉冲作为其输入信号，产生功率增强的一系列脉冲作为其输出信号，该输出信号被发射并用来驱动所述射频线圈。然而在射频放大过程中，可能会将射频放大器的增益和相位的非线性引入系统中，导致MRI图像失真。为了避免这样的情况，该射频放大系统利用反馈装置来产生反馈信号以校正在射频放大过程中引入的非线性因素来保持线性。
然而，在传统的射频放大系统中，存在电子元器件较多，电路设计复杂等技术问题。因此，需要提出一种新的射频放大系统及方法。
发明内容
现在归纳本发明的一个或多个方面以便于本发明的基本理解，其中该归纳并不是本发明的扩展性纵览，且并非旨在标识本发明的某些要素，也并非旨在划出其范围。相反，该归纳的主要目的是在下文呈现更详细的描述之前用简化形式呈现本发明的一些概念。
本发明的一个方面，在于提供一种射频放大系统。该射频放大系统包括第一数字调制器、数模转换器、射频放大器、模数转换器、数字解调器及反馈控制装置。第一数字调制器用于使用数字包络信号调制第一数字载波信号以产生具有第一频率的数字射频脉冲信号。数模转换器用于将数字射频脉冲信号转换成模拟射频脉冲信号。射频放大器用于对模拟射频脉冲信号进行放 大以产生射频输出信号。模数转换器用于将取样自射频输出信号的模拟反馈信号转换成数字反馈信号。数字解调器用于使用第二数字载波信号解调数字反馈信号以产生I信号及Q信号。反馈控制装置用于根据I信号及Q信号产生幅度设置信号及相位设置信号，该幅度设置信号用于调节数字包络信号的幅度，该相位设置信号用于调节第一数字载波信号的相位。第二数字载波信号相对于第一数字载波信号具有预定的频率偏移，第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
本发明的另一个方面，在于提供一种射频放大方法。该射频放大方法包括使用数字包络信号对第一数字载波信号进行调制以产生具有第一频率的数字射频脉冲信号，将数字射频脉冲信号转换成模拟射频脉冲信号，对模拟射频脉冲信号进行放大以产生射频输出信号，将取样自射频输出信号的模拟反馈信号转换成数字反馈信号；使用第二数字载波信号解调数字反馈信号以产生I信号和Q信号，根据I信号和Q信号产生幅度设置信号及相位设置信号，幅度设置信号用于调节数字包络信号的幅度，相位设置信号用于调节第一数字载波信号的相位。第二数字载波信号相对于第一数字载波信号具有预定的频率偏移，第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
本发明的另一个方面，在于提供一种磁共振成像装置。该磁共振成像装置包括射频线圈组件及射频放大系统。该射频放大系统包括第一数字调制器、数模转换器、射频放大器、模数转换器、数字解调器及反馈控制装置。第一数字调制器用于使用数字包络信号调制第一数字载波信号以产生具有第一频率的数字射频脉冲信号。数模转换器用于将数字射频脉冲信号转换成模拟射频脉冲信号。射频放大器用于对模拟射频脉冲信号进行放大以产生射频输出信号，该射频输出信号用于激励射频线圈组件发射射频信号。模数转换器用于将取样自射频输出信号的模拟反馈信号转换成数字反馈信号。数字解调器用于使用第二数字载波信号解调数字反馈信号以产生I信号及Q信号。反馈控制装置用于根据I信号及Q信号产生幅度设置信号及相位设置信号，该幅度设置信号用于调节数字包络信号的幅度，该相位设置信号用于调节第一数字载波信号的相位。第二数字载波信号相对于第一数字载波信号具有预定的频率偏移，第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
本发明提供的磁共振成像装置、射频放大系统及方法，具有结构简单、电子元器件较少、电路设计简单等有益技术效果。
附图说明
通过结合附图对于本发明的实施方式进行描述，可以更好地理解本发明，在附图中：
图1为一种实施方式的磁共振成像装置的示意图。
图2为第一种实施方式的射频放大系统的功能模块图。
图3为图2所示数字解调器及反馈控制装置之一种实施方式的功能模块图。
图4为图2所示第一及第二载波信号发生器之一种实施方式的功能模块图。
图5为第二种实施方式的射频放大系统的功能模块图。
具体实施方式
以下将描述本发明的具体实施方式，需要指出的是，在这些实施方式的具体描述过程中，为了进行简明扼要的描述，本说明书不可能对实际的实施方式的所有特征均作详尽的描述。应当可以理解的是，在任意一种实施方式的实际实施过程中，正如在任意一个工程项目或者设计项目的过程中，为了实现开发者的具体目标，为了满足系统相关的或者商业相关的限制，常常会做出各种各样的具体决策，而这也会从一种实施方式到另一种实施方式之间发生改变。此外，还可以理解的是，虽然这种开发过程中所作出的努力可能是复杂并且冗长的，然而对于与本发明公开的内容相关的本领域的普通技术人员而言，在本公开揭露的技术内容的基础上进行的一些设计，制造或者生产等变更只是常规的技术手段，不应当理解为本公开的内容不充分。
除非另作定义，权利要求书和说明书中使用的技术术语或者科学术语应当为本发明所属技术领域内具有一般技能的人士所理解的通常意义。本发明专利申请说明书以及权利要求书中使用的“第一”、“第二”以及类似的词语并不表示任何顺序、数量或者重要性，而只是用来区分不同的组成部分。“一个”或者“一”等类似词语并不表示数量限制，而是表示存在至少一个。“包括”或者“包含”等类似的词语意指出现在“包括”或者“包含”前面的元件或者物件涵盖出现在“包括”或者“包含”后面列举的元件或者物件及其等同元件，并不排除其他元件或者物件。“连接”或者“相连”等类似的词语并非限定于物理的或者机械的连接，而是可以包括电气的连接，不管是直接的还是间接的。
请参阅图1，其为磁共振成像(MRI)装置10的示意图。MRI装置10 包括磁系统11、支架(cradle)12、梯度磁场驱动电路13、射频放大系统14、磁共振信号接收电路15、控制单元16以及操作员控制台17。
磁系统11包括实际上为横向圆柱形内部空间(孔)111，如图1所示，以及其上放置有对象20(例如病人)的支架12，支撑在垫子(cushion)上，通过输送机构(未示出)移动至孔111中。
在磁系统11中，在孔111的磁中心(扫描中心位置)附近，放置有静磁场发生器112、梯度线圈组件113和射频线圈组件114。
静磁场发生器112在孔111中形成静磁场。静磁场的方向为，例如与对象20的轴方向平行。然而，静磁场的方向也可与对象20的轴方向垂直。
梯度线圈组件113产生梯度磁场以倾斜(slope)由静磁场发生器112形成的静磁场的强度。这样射频线圈组件114所接收的磁共振信号即可提供三维位置信息。梯度线圈组件113所产生的梯度磁场有三种类型：用于薄片(slice)选择的梯度磁场、用于频率编码的梯度磁场，用于相位编码的梯度磁场。为提供这三种类型的梯度磁场，梯度线圈组件113包括三个梯度磁场线圈系统。
射频线圈组件114发射射频脉冲以激励由静磁场发生器112所形成的静磁场的空间中的对象20体内的质子自旋(spin)，由此产生磁共振信号。与此同时，射频线圈组件114接收由对象20产生的磁共振信号。射频线圈组件114可构造成分别具有发射射频线圈单元和接收射频线圈单元，或者构造成由同一个射频线圈单元执行射频脉冲发射及磁共振信号接收。
基于控制单元16发出的命令，梯度磁场驱动电路13向梯度线圈组件113提供驱动信号DR以产生梯度磁场。梯度磁场驱动电路13包括三个驱动电路系统，其未示出，对应于梯度线圈组件113的三个梯度磁场线圈系统。
射频放大系统14包括射频脉冲频率合成器，其将在随后描述，并使用射频脉冲频率合成器产生模拟射频脉冲信号。向射频线圈组件114施加模拟射频脉冲信号，射频线圈组件114发射射频信号。
磁共振信号接收电路15，获取射频线圈组件114所接收的模拟磁共振信号，将模拟磁共振信号转换为基带数字磁共振信号，并将基带数字磁共振信号输出至操作员控制台17内的数据处理单元171。
控制单元16根据预定脉冲序列控制梯度磁场驱动电路13和射频放大系统14以产生驱动信号DR和模拟射频脉冲信号。此外，控制单元16控制磁共振信号接收电路15。
操作员控制台17包括数据处理单元171、图像数据库172、操作单元173和显示单元174，如图1所示，数据处理单元171整体控制磁共振成像装置10、图像再现处理等等。控制单元16连接到数据处理单元171，数据处理单元171控制着控制单元16的运行。此外，图像数据库172、操作单元173和显示单元174连接到数据处理单元171。图像数据库172包括，例如能够记录/能够重现(recordable/reproducible)的硬盘设备或类似设备以记录再现的图像数据等。操作单元173包括键盘、鼠标等，并且显示单元174包括图形显示器或类似设备。
射频线圈组件114是本发明中包括的射频线圈组件的实例，射频放大系统14是本发明中包括的射频放大系统的实例。
本发明实施例提供了一种射频放大系统，其包括射频放大器和反馈控制装置，该反馈控制装置可在不同的负载下实现高线性和高稳定性，获得恒定的增益(是指输出信号和输入信号之间的量级比例)。反馈控制装置包括相互独立的幅度控制回路和相位控制回路，用来相互独立地控制幅度和相位，使得幅度和相位的反应可独立地进行优化，从而获得高带宽，比如，满足MRI系统要求的高带宽。这样，射频放大器可在不同的负载情况下以很快的反应速度获得补偿从而达到稳定。通过使用不同反馈信号，比如，射频放大器的输出功率或电流，来进行反馈控制，可获得稳定的功率增益或稳定的电流源。更具体地，在本文所描述的系统或方法中，射频放大器的输出功率或输出电流都可以用作反馈信号来进行反馈控制，当以输出功率为反馈信号进行控制时，可获得稳定的功率增益；当以输出电流为反馈信号进行控制时，可获得稳定的电流源。
请参阅图2，其为第一种实施方式的射频放大系统800的功能模块图。射频放大系统800包括第一载波信号发生器30、包络信号发生器32、第一数字调制器34、数模转换器42、滤波器44、射频放大器45、取样装置46、模数转换器48、数字解调器60及反馈控制装置70。
在非限定的示例中，射频放大系统800可以不包括第一载波信号发生器30、第二载波信号发生器50及取样装置46。在非限定的示例中，射频放大系统800可以不包括滤波器44。
第一载波信号发生器30用于产生第一数字载波信号300。
包络信号发生器32用于产生数字包络信号320。
第一数字调制器34使用数字包络信号320调制第一数字载波信号300以 产生具有第一频率的数字射频脉冲信号。在非限定的示例中，第一数字调制器34为数字混频器。
数模转换器42用于将该第一频率的数字射频脉冲信号转换成模拟射频脉冲信号。
滤波器44用于滤除模拟射频脉冲信号中的高次谐波分量。在非限定的示例中，滤波器44为带通滤波器。
射频放大器45用于对经过滤波的模拟射频脉冲信号进行放大以产生射频输出信号。在非限定的示例中，该射频输出信号用于激励磁共振成像装置中的射频线圈组件114发射射频信号；可以理解的是，该射频输出信号可以被提供给其他的负载，例如不是应用于磁共振成像装置中的普通线圈。
数字射频脉冲信号的第一频率依赖于静磁场的强度来确定。例如，在具有1.5T(特斯拉)的静磁场强度的装置中，数字射频脉冲信号的第一频率约为64MHz。数模转换器42的采样频率fs必须大于或等于尼奎斯特(Nyquist)频率。在图2中，例如采样频率为200MHz。
再例如，在具有3.0T(特斯拉)的静磁场强度的装置中，数字射频脉冲信号的第一频率约为128.1MHz。数模转换器42的采样频率fs必须大于采样频率fs必须大于或等于尼奎斯特(Nyquist)频率。在图2中，例如采样频率fs为480MHz或640MHz。
取样装置46用于对该射频输出信号进行取样以产生模拟反馈信号。取样装置46可以是一种用来获得功率反馈信号的耦合器(例如定向耦合器，英文名为directional coupler)，或是一种用来获得电流反馈信号的传感器(例如电流传感器)。
模数转换器48用于将模拟反馈信号转换成数字反馈信号480。
第二载波信号发生器50用于产生第二数字载波信号500。
数字解调器60用于使用第二数字载波信号对数字反馈信号进行正交解调以产生I信号及Q信号。具体解调方式将在后续图3中描述。
反馈控制装置70用于根据该I信号及Q信号产生幅度设置信号72及相位设置信号74。幅度设置信号72被提供给包络信号发生器32，以用于调节数字包络信号320的幅度。相位设置信号74被提供给第一载波信号发生器30，以用于调节第一数字载波信号300的相位。
在非限定的示例中，第一载波信号发生器30、包络信号发生器32、第二载波信号发生器50、数字解调器60及反馈控制装置70可以集成于FPGA的 内部。换句话说，第一载波信号发生器30、包络信号发生器32、第二载波信号发生器50、数字解调器60及反馈控制装置70为FPGA的内部逻辑功能模块。
其中，第二数字载波信号500相对于第一数字载波信号300具有预定的频率偏移，第二数字载波信号500相对于第一数字载波信号300具有预定的相位偏移。
由此可知，作为第一种非限定的示例，第二数字载波信号500的频率为f3，第一数字载波信号300的频率为f1，其中f3＝f1，也即第二数字载波信号500与第一数字载波信号300之间的频率差异为0。
作为第二种非限定的示例，第二数字载波信号500的频率为f3，第一数字载波信号300的频率为f1。其中f3＝fs-f1，也即第二数字载波信号500与第一数字载波信号300之间的频率差异为fs-2f1，其中fs为模数转换器48的采样频率。
在非限定的示例中，第二数字载波信号500与第一数字载波信号300之间的相位差异为0。
请一并参阅图3，其为图2所示数字解调器60及反馈控制装置70之一种实施方式的功能模块图。数字解调器60利用第二数字载波信号500、数字调制器62、64以及90度移相器65，或者还可再包括两个滤波器66、68(例如低通滤波器)，来将数字反馈信号480的矢量转换成正交标量以产生载波在零频的I信号和Q信号。其中，I信号和Q信号分别为数字反馈信号480的同相和正交分量。
反馈控制装置70包括幅度反馈回路及相位反馈回路。
幅度反馈回路用于实现幅度调节。幅度反馈回路包括幅度函数702、比较器704及控制器706。幅度函数702用于从I信号和Q信号中得到反馈幅度
Figure PCTCN2015085022-appb-000001
比较器704用于将反馈幅度与参考幅度进行比较以产生幅度误差信号。控制器706用于根据幅度误差信号产生上述幅度设置信号72。
相位反馈回路用于实现相位调节。在非限定的示例中，相位反馈回路包括比较器712、积分器714及移相器716。比较器712用于将I信号与Q信号进行比较以产生相位误差信号。若I等于Q，则判定不存在相位误差；若I不等于Q，则判定存在相位误差。积分器714及移相器716分别用于累积相位误差以及响应相位误差来产生上述相位设置信号74。
请一并参阅图4，其为图2所示第一载波信号发生器30及第二载波信号 发生器50之一种实施方式的功能模块图。第一载波信号发生器30包括第一相存储器302、第一相累积器304以及第一波形表306。第一相存储器302用于存储第一相位增量。第一相累积器304用于在其初始化时设定第一相位的初始值并且通过对每个时钟周期增加所述第一相位增量来计算第一累积相位。第一波形表306用于存储该第一数字载波信号，当向第一波形表306输入上述第一累积相位时，该第一波形表306输出与所述第一累积相位对应的第一数字载波信号。
具体地，第一载波信号发生器30所产生的第一数字载波信号的相位通过设置第一相位的初始值来调节。
第二载波信号发生器50包括第二相存储器502、第二相累积器504及第二波形表506。第二相存储器502用于存储第二相位增量。第二相累积器504用于在其初始化时设定第二相位的初始值并且通过对每个时钟周期增加所述第二相位增量来计算第二累积相位。第二波形表506用于存储该第二数字载波信号；当向第二波形表506输入上述第二累积相位时，该第二波形表506输出与所述第二累积相位对应的第二数字载波信号。具体地，该第二数字载波信号与第一数字载波信号之间的预定频率偏移通过调节第二相位增量来设定，该第二数字载波信号与第一数字载波信号之间的预定相位偏移通过调节第二相位的初始值来设定。
请一并参阅图5，其为第二种实施方式的射频放大系统900的功能模块图。第二种实施方式的射频放大系统900与第一种实施方式的射频放大系统800的区别是：射频放大系统900进一步包括中间级载波信号发生器36及第二数字调制器38。
中间级载波信号发生器36用于产生第三数字载波信号360。
第二数字调制器38用于使用第三数字载波信号360调制第一数字调制器34产生的具有第一频率的数字射频脉冲信号以产生具有第二频率的数字射频脉冲信号；该第二频率大于第一频率。从而数模转换器42将该第二频率的数字射频脉冲信号转换成模拟射频脉冲信号，射频放大器45用于对经过滤波器44滤波的模拟射频脉冲信号进行放大以产生射频输出信号。最终使得射频放大系统900中的数模转换器42产生的模拟射频脉冲信号的频率大于射频放大系统800中的数模转换器42产生的模拟射频脉冲信号的频率。
数字射频脉冲信号的第二频率依赖于静磁场的强度来确定。例如，在具有3.0T(特斯拉)的静磁场强度的装置中，数字射频脉冲信号的第一频率约 为128.1MHz。数模转换器42的采样频率fs必须大于采样频率fs必须大于或等于尼奎斯特(Nyquist)频率。在图2中，例如采样频率fs为480MHz或640MHz。
在非限定的示例中，第二数字载波信号500的频率为f3，第一数字载波信号300的频率为f1，第三数字载波信号360的频率为f2。其中f3＝f1+f2，也即第二数字载波信号500与第一数字载波信号300之间的频率差异为f2。
在非限定的示例中，第二数字载波信号500与第一数字载波信号300之间的相位差异为零。
在一种实施方式中，中间级载波信号发生器36及第二数字调制器38集成于数模转换器42的内部。因此，本发明所示射频放大系统900可以达到电子元器件较少，电路设计较为简单的有益技术效果。
虽然结合特定的实施方式对本发明进行了说明，但本领域的技术人员可以理解，对本发明可以作出许多修改和变型。因此，要认识到，权利要求书的意图在于覆盖在本发明真正构思和范围内的所有这些修改和变型。

Claims (15)

  1. 一种射频放大系统，其包括：
    第一数字调制器，用于利用数字包络信号调制第一数字载波信号以产生具有第一频率的数字射频脉冲信号；
    数模转换器，用于将数字射频脉冲信号转换成模拟射频脉冲信号；
    射频放大器，用于对模拟射频脉冲信号进行放大以产生射频输出信号；
    模数转换器，用于将取样自射频输出信号的模拟反馈信号转换成数字反馈信号；
    数字解调器，用于使用第二数字载波信号对数字反馈信号进行正交解调以产生I信号及Q信号；及
    反馈控制装置，用于根据I信号及Q信号产生幅度设置信号及相位设置信号，该幅度设置信号用于调节数字包络信号的幅度，该相位设置信号用于调节第一数字载波信号的相位；
    其中，该第二数字载波信号相对于第一数字载波信号具有预定的频率偏移，该第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
  2. 如权利要求1所述的射频放大系统，其特征在于：该射频放大系统还包括：
    中间级载波信号发生器，用于产生第三数字载波信号；及
    第二数字调制器，用于使用第三数字载波信号调制该具有第一频率的数字射频脉冲信号以产生具有第二频率的数字射频脉冲信号；该第二频率大于第一频率。
  3. 如权利要求2所述的射频放大系统，其特征在于：该第三载波信号发生器及第二数字调制器集成于该数模转换器的内部。
  4. 如权利要求2所述的射频放大系统，其特征在于：该第二数字载波信号的频率等于f1+f2，其中，f1为第一数字载波信号的频率，f2为第三数字载波信号的频率。
  5. 如权利要求1所述的射频放大系统，其特征在于：该射频放大系统还包括用于产生该第一数字载波信号的第一载波信号发生器，该第一载波信号发生器包括：
    第一相存储器，用于存储第一相位增量；
    第一相累积器，用于在其初始化时设定第一相位的初始值并且通过对每 个时钟周期增加所述第一相位增量来计算第一累积相位；及
    第一波形表，用于存储该第一数字载波信号；当向第一波形表输入所述第一累积相位时，该第一波形表输出与所述第一累积相位对应的第一数字载波信号；
    该第一数字载波信号的相位通过设置第一相位的初始值来调节。
  6. 如权利要求5所述的射频放大系统，其特征在于：该射频放大系统还包括用于产生该第二数字载波信号的第二载波信号发生器，该第二载波信号发生器包括：
    第二相存储器，用于存储第二相位增量；
    第二相累积器，用于在其初始化时设定第二相位的初始值并且通过对每个时钟周期增加所述第二相位增量来计算第二累积相位；及
    第二波形表，用于存储该第二数字载波信号；当向第二波形表输入所述第二累积相位时，该第二波形表输出与所述第二累积相位对应的第二数字载波信号；
    其中，该预定的频率偏移通过调节第二相位增量来设定，该预定的相位偏移通过调节第二相位的初始值来设定。
  7. 如权利要求1所述的射频放大系统，其特征在于，该反馈控制装置包括：
    幅度反馈回路，用于根据I信号及Q信号来计算幅度值，并根据该幅度值与幅度参考值的比较来产生该幅度设置信号；及
    相位反馈回路，用于根据I信号与Q信号的比较来产生该相位设置信号。
  8. 一种射频放大方法，其包括：
    使用数字包络信号对第一数字载波信号进行调制以产生具有第一频率的数字射频脉冲信号；
    将该数字射频脉冲信号转换成模拟射频脉冲信号；
    对该模拟射频脉冲信号进行放大以产生射频输出信号；
    将取样自射频输出信号的模拟反馈信号转换成数字反馈信号；
    使用第二数字载波信号解调该数字反馈信号以产生I信号和Q信号；
    根据I信号和Q信号产生幅度设置信号及相位设置信号，该幅度设置信号用于调节数字包络信号的幅度，该相位设置信号用于调节第一数字载波信号的相位；
    其中，该第二数字载波信号相对于第一数字载波信号具有预定的频率偏 移，该第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
  9. 如权利要求8所述的射频放大方法，其特征在于，该射频放大方法还包括：
    利用中间级载波信号发生器产生第三数字载波信号；
    使用第三数字载波信号对该具有第一频率的数字射频脉冲信号进行调制以产生具有第二频率的数字射频脉冲信号；该第二频率大于第一频率。
  10. 如权利要求8所述的射频放大方法，其特征在于，该射频放大方法还包括：
    存储第一相位增量于第一相存储器；
    在第一相累积器初始化时设定第一相位的初始值并且通过对每个时钟周期增加该第一相位增量来计算第一累积相位；及
    存储该第一数字载波信号于第一波形表；当向第一波形表输入所述第一累积相位时，该第一波形表输出与该第一累积相位对应的第一数字载波信号；
    其中，该第一数字载波信号的相位通过设置第一相位的初始值来调节。
  11. 如权利要求10所述的射频放大方法，其特征在于，该射频放大方法还包括：
    存储第二相位增量于第二相存储器；
    在第二相累积器初始化时设定第二相位的初始值并且通过对每个时钟周期增加所述第二相位增量来计算第二累积相位；及
    存储该第二数字载波信号于第二波形表；当向第二波形表输入所述第二累积相位时，该第二波形表输出与所述第二累积相位对应的第二数字载波信号；
    其中，该预定的频率偏移通过调节第二相位增量来设定，该预定的相位偏移通过调节第二相位的初始值来设定。
  12. 一种磁共振成像装置，其包括：
    射频线圈组件；及
    射频放大系统，其包括：
    第一数字调制器，用于使用数字包络信号调制第一数字载波信号以产生具有第一频率的数字射频脉冲信号；
    数模转换器，用于将数字射频脉冲信号转换成模拟射频脉冲信号；
    射频放大器，用于对模拟射频脉冲信号进行放大以产生射频输出信号，该射频输出信号用于激励射频线圈组件发射射频信号；
    模数转换器，用于将取样自射频输出信号的模拟反馈信号转换成数字反馈信号；
    数字解调器，用于使用第二数字载波信号解调数字反馈信号以产生I信号及Q信号；及
    反馈控制装置，用于根据I信号及Q信号产生幅度设置信号及相位设置信号，该幅度设置信号用于调节数字包络信号的幅度，该相位设置信号用于调节第一数字载波信号的相位；
    其中，该第二数字载波信号相对于第一数字载波信号具有预定的频率偏移，该第二数字载波信号相对于第一数字载波信号具有预定的相位偏移。
  13. 如权利要求12所述的磁共振成像装置，其特征在于：该射频放大系统还包括：
    中间级载波信号发生器，用于产生第三数字载波信号；及
    第二数字调制器，用于使用第三数字载波信号调制该具有第一频率的数字射频脉冲信号以产生具有第二频率的数字射频脉冲信号；该第二频率大于第一频率。
  14. 如权利要求12所述的磁共振成像装置，其特征在于：该射频放大系统还包括用于产生该第一数字载波信号的第一载波信号发生器，该第一载波信号发生器包括：
    第一相存储器，用于存储第一相位增量；
    第一相累积器，用于在其初始化时设定第一相位的初始值并且通过对每个时钟周期增加所述第一相位增量来计算第一累积相位；及
    第一波形表，用于存储该第一数字载波信号；当向第一波形表输入所述第一累积相位时，该第一波形表输出与所述第一累积相位对应的第一数字载波信号；
    该第一数字载波信号的相位通过设置第一相位的初始值来调节。
  15. 如权利要求14所述的磁共振成像装置，其特征在于，该射频放大系统还包括用于产生该第二数字载波信号的第二载波信号发生器，该第二载波信号发生器包括：
    第二相存储器，用于存储第二相位增量；
    第二相累积器，用于在其初始化时设定第二相位的初始值并且通过对每个时钟周期增加所述第二相位增量来计算第二累积相位；及
    第二波形表，用于存储该第二数字载波信号；当向第二波形表输入所述 第二累积相位时，该第二波形表输出与所述第二累积相位对应的第二数字载波信号；
    其中，该预定的频率偏移通过调节第二相位增量来设定，该预定的相位偏移通过调节第二相位的初始值来设定。
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