WO2021109663A1 - 具有高时域信号稳定性的磁共振成像射频线圈组件 - Google Patents

具有高时域信号稳定性的磁共振成像射频线圈组件 Download PDF

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WO2021109663A1
WO2021109663A1 PCT/CN2020/113869 CN2020113869W WO2021109663A1 WO 2021109663 A1 WO2021109663 A1 WO 2021109663A1 CN 2020113869 W CN2020113869 W CN 2020113869W WO 2021109663 A1 WO2021109663 A1 WO 2021109663A1
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radio frequency
coil unit
receiving coil
magnetic resonance
frequency receiving
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PCT/CN2020/113869
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English (en)
French (fr)
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张孝通
高阳
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浙江大学
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Publication of WO2021109663A1 publication Critical patent/WO2021109663A1/zh
Priority to US17/395,406 priority Critical patent/US20210364585A1/en

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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
    • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
    • G01R33/341—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils
    • G01R33/3415—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils comprising arrays of sub-coils, i.e. phased-array coils with flexible receiver channels
    • 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
    • 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
    • G01R33/3621—NMR receivers or demodulators, e.g. preamplifiers, means for frequency modulation of the MR signal using a digital down converter, means for analog to digital conversion [ADC] or for filtering or processing of the MR signal such as bandpass filtering, resampling, decimation or interpolation
    • 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
    • G01R33/3642—Mutual coupling or decoupling of multiple coils, e.g. decoupling of a receive coil from a transmission coil, or intentional coupling of RF coils, e.g. for RF magnetic field amplification
    • 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
    • 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
    • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
    • G01R33/34007—Manufacture of RF coils, e.g. using printed circuit board technology; additional hardware for providing mechanical support to the RF coil assembly or to part thereof, e.g. a support for moving the coil assembly relative to the remainder of the MR system
    • 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
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/4806—Functional imaging of brain activation

Definitions

  • the invention relates to the field of magnetic resonance imaging systems, and specifically designs a magnetic resonance imaging radio frequency coil assembly with high time domain signal stability.
  • Magnetic resonance imaging technology uses this principle by adding a pulse gradient magnetic field to the main magnetic field to selectively excite the nuclei in the human body at the required position, and then receive the magnetic resonance signal generated by the nucleus, and finally perform Fourier in the computer. Transform, frequency-encode and phase-encode these signals to create a complete magnetic resonance image.
  • the magnetic resonance imaging device includes a radio frequency transmitting coil and a radio frequency receiving coil.
  • the radio frequency transmitting coil is used to generate a radio frequency pulse for exciting protons
  • the radio frequency receiving coil is used to receive a magnetic resonance signal generated by an atomic nucleus.
  • the uniformity of the magnetic field generated by the radio frequency transmitting coil, the high transmission efficiency, and the high signal-to-noise ratio of the signal received by the radio frequency receiving coil are the key factors for obtaining high-quality images.
  • the design of the birdcage radio frequency transmitting coil working in orthogonal excitation mode can meet the requirements of the uniformity of the transmitting magnetic field within the range of the human body.
  • the body radio frequency transmitting coil adopting this design is integrated into a conventional field-strength magnetic resonance system as a conventional configuration, which can meet the imaging requirements of any part.
  • a large-aperture ultra-high field magnetic resonance system with a main magnetic field stronger than 3 Tesla that can be used for human imaging is generally not equipped with a volume radio frequency transmitting coil.
  • the large-aperture ultra-high field magnetic resonance system that can be used for human imaging it is different from the conventional field strength medical magnetic resonance system. It is necessary to consider adding the design of the RF transmitting coil while designing and fabricating the RF receiving coil, and additional circuits need to be added to avoid RF The signal coupling problem between the transmitting coil and the radio frequency receiving coil.
  • the design with both the radio frequency transmitting coil and the radio frequency receiving coil can also be compatible with the medical magnetic resonance system under the normal field strength, and vice versa.
  • the multi-channel phased array radio frequency receiving coil design is widely used at present, which can ensure the realization of high signal-to-noise ratio requirements in a large imaging range.
  • the multi-channel phased array radio frequency receiving coil can be used with parallel imaging technology to accelerate image acquisition and improve image quality.
  • the functional magnetic resonance imaging method used to capture brain nerve activity requires high-performance special magnetic resonance radio frequency coil components.
  • the conventional magnetic resonance radio frequency coil assembly is very sensitive to the movement of the object during the imaging process, and then introduces the time-domain noise signal, which becomes a key bottleneck factor restricting the quality of the functional magnetic resonance imaging signal.
  • time-domain noise when the image signal-to-noise ratio is high, the main component of the time-domain noise is the fluctuation of the radio frequency receiving coil signal caused by the displacement of the load object; when the image signal-to-noise ratio is low, the main component of the time-domain noise is The main component begins to become the fluctuation of the thermal noise level of the RF receiving coil caused by the displacement of the load object.
  • the thermal noise source of the radio frequency coil includes two parts, one part is the thermal noise caused by the conduction current inside the electronic device of the radio frequency coil, and the other part is the thermal noise caused by the displacement current inside the load object. Since the second part of the displacement current level is determined by the relative position of the RF coil and the load object, it will also be affected by the displacement of the load object during the imaging process. Therefore, a radio frequency coil with high time-domain signal stability should have two characteristics at the same time: first, the signal level of the radio frequency coil, that is, the sensitivity is not easily affected by the displacement of the load object; second, the thermal noise level of the radio frequency coil is not easily affected by the load object Displacement influence.
  • the purpose of the embodiments of the present invention is to provide a magnetic resonance imaging radio frequency coil assembly with high time domain stability, which solves the problem that the signal and noise characteristics of the radio frequency coil in the process of magnetic resonance brain imaging, especially brain functional imaging, are easily affected by imaging.
  • the problem of the influence of object displacement is to provide a magnetic resonance imaging radio frequency coil assembly with high time domain stability, which solves the problem that the signal and noise characteristics of the radio frequency coil in the process of magnetic resonance brain imaging, especially brain functional imaging, are easily affected by imaging.
  • the problem of the influence of object displacement is to provide a magnetic resonance imaging radio frequency coil assembly with high time domain stability, which solves the problem that the signal and noise characteristics of the radio frequency coil in the process of magnetic resonance brain imaging, especially brain functional imaging, are easily affected by imaging.
  • the embodiment of the present invention provides a magnetic resonance imaging radio frequency coil assembly with high time-domain signal stability, including a radio frequency transmitting coil unit and several radio frequency receiving coil units.
  • the radio frequency transmitting coil unit needs to have a plane area larger than all the radio frequency receiving coil units.
  • the total area of the layout plane, several radio frequency receiving coil units are arranged inside the radio frequency transmitting coil unit, the array of radio frequency receiving coil units must be larger than the imaging area, and the circumference of each radio frequency receiving coil unit must be less than tenth of the wavelength of the vacuum electromagnetic wave.
  • the thermal noise from the load has a small proportion in the radio frequency receiving coil unit, that is, the quality factor of the radio frequency receiving coil unit in the no-load state is more than twice the quality factor of the radio frequency receiving coil unit in the load state.
  • radio frequency receiving coil unit and the radio frequency receiving unit are directly placed inside the housing, and their positions relative to each other are fixed.
  • a fixing device interface is provided on the outer side of the housing to prevent relative displacement between the housing and the imaging object.
  • both the radio frequency transmitting coil unit and the radio frequency receiving coil unit are metal conductors.
  • the radio frequency transmitting coil unit and the radio frequency receiving coil unit adopt copper wires with insulating coatings.
  • the radio frequency transmitting coil unit and the receiving coil unit include a diode circuit to ensure that the radio frequency transmitting coil unit and the radio frequency receiving coil unit will not be in the working state at the same time.
  • a parallel LC resonant circuit with a diode is connected in series to the radio frequency receiving coil unit, and the working frequency of the resonant circuit is the same as that of the radio frequency receiving coil unit; when the diode is forward biased, the radio frequency receiving coil unit is The series parallel LC circuit is in a resonance state, and the radio frequency transmitting coil unit is therefore in an out-of-resonance state and does not work, and vice versa.
  • a diode circuit is connected in series to the radio frequency transmitting coil unit, and when the diode is forward biased, the radio frequency transmitting coil unit is in a resonance state and works, otherwise it does not work.
  • signal isolation measures are adopted between the radio frequency receiving units, that is, the signal isolation between the radio frequency receiving coil units is realized by geometric overlap; the radio frequency receiving coil unit is directly connected to the preamplifier to reduce the loss of the coaxial line, and it is packaged in Inside the shell structure.
  • the working frequency of the radio frequency transmitting coil unit and the radio frequency receiving coil unit is 297.2 MHz
  • the radio frequency transmitting coil unit has a ring structure
  • the diameter of the radio frequency transmitting coil unit is 7 cm
  • the diameter of the radio frequency receiving coil unit is 1.5 cm.
  • the embodiment of the present invention has the following beneficial effects: by combining three technical features at the same time, the overall size of the array formed by the radio frequency transmitting coil and the radio frequency receiving coil unit is larger than the size of the imaging area; each radio frequency receiving unit has a larger size Less than the vacuum electromagnetic wave wavelength (that is, the circumference of each radio frequency receiving coil unit needs to be less than one-tenth of the vacuum electromagnetic wave wavelength), the thermal noise from the load accounts for a small proportion of the radio frequency receiving coil unit, and the radio frequency receiving coil unit under no load
  • the quality factor is more than 2 times the quality factor of the RF receiving coil unit under load; with the help of the shell design that can interface with external fixtures, there is no relative displacement between the RF transmitting coil, the RF receiving coil and the imaging object, which can achieve the imaging object
  • the effect of reducing the interference of movement on the time-domain stability of the imaging signal to a minimum has a potentially huge application prospect in the functional magnetic resonance imaging application field that has high requirements for the time-domain stability of the signal
  • FIG. 1 is a schematic diagram of the layout of the radio frequency receiving coil array in the magnetic resonance imaging radio frequency coil assembly with high time domain signal stability according to an embodiment of the present invention.
  • 1 is the radio frequency receiving coil
  • FIG. 2 is a schematic diagram of the connection between the radio frequency receiving coil array and the radio frequency preamplifier.
  • 1 is the radio frequency receiving coil and 2 is the radio frequency preamplifier;
  • FIG. 3 is a schematic diagram of the layout of the radio frequency receiving coil array and the radio frequency transmitting coil.
  • 1 is the radio frequency receiving coil
  • 2 is the radio frequency preamplifier
  • 3 is the radio frequency transmitting coil unit
  • Figure 4 is a structural diagram of the housing of the radio frequency coil assembly, 4 is the device interface, and 5 is the housing;
  • FIG. 5 is a schematic diagram of the overall structure of an embodiment of the present invention.
  • 1 is a radio frequency receiving coil
  • 2 is a radio frequency preamplifier
  • 3 is a radio frequency transmitting coil unit
  • 4 is a fixing device interface
  • 5 is a housing;
  • FIG. 6 is a schematic diagram of a magnetic resonance imaging experimental design used to study the effects of the RF receiving coil signal and thermal noise on the displacement of the load object.
  • 6 is the RF coil assembly including the RF transmitting coil unit and the RF receiving coil unit, and 7 It is a Teflon backing plate, and 8 is a cylindrical liquid model (conductivity and dielectric constant are similar to human tissue);
  • Figure 7 is a comparison result of the thermal noise level of different types of RF receiving coils affected by the displacement of the load object
  • Figure 8 shows the comparison results of the time-domain fluctuations of the thermal noise level of the radio frequency receiving coils with different no-load/load-quality factor ratios under the condition of displacement of the loaded object
  • Fig. 9 is a graph showing the comparison result of the amplitude of signal fluctuations in the time domain when the load object has a displacement of the radio frequency receiving coils with different diameters.
  • the quality factor ratio of no load/load it specifically refers to the ratio of the quality factor of the RF receiving coil unit under no load to the quality factor of the RF receiving coil unit under load.
  • the embodiment of the present invention provides a magnetic resonance imaging radio frequency coil assembly with high time-domain signal stability, which mainly includes a single-channel radio frequency transmitting coil unit 3 and several radio frequency receiving coil units 1.
  • the receiving coil 1 array is arranged on a circular plane (that is, several radio frequency receiving coil units 1 are arranged in the form of a circular array), and the receiving coil units overlap each other, and the mutual inductance is generated to enhance the radio frequency receiving coils.
  • the central axis of the radio frequency transmitting coil 3 coincides with the central axis of the circular plane where the array of the radio frequency receiving coil 1 is located, and the coverage area is greater than the sum of the coverage areas of all the radio frequency receiving coils 1;
  • the radio frequency receiving coil array is compatible with the parallel imaging function of the magnetic resonance imaging system, It is helpful to shorten the scanning time and improve the image quality;
  • each RF receiving coil is connected in series with a capacitor and then respectively connected to the respective preamplifier 2.
  • the capacitor is used for impedance matching and enhancing the decoupling performance between RF coil channels.
  • the single-channel radio frequency transmitting coil 3 is used to match the electromagnetic load of the imaging object to achieve high transmission efficiency and uniform excitation of the brain.
  • a further technical solution is to obtain the ratio of the quality factor of the RF receiving coil unit in the unloaded state to the quality factor of the RF receiving coil unit in the loaded state through the radio frequency network analyzer test.
  • the quality factor of the RF receiving coil unit in the unloaded state is
  • the quality factor of the RF receiving coil unit under load is more than 2 times.
  • each radio frequency receiving coil 1 has a ring structure with an effective diameter of 1.5 cm, and the multiple radio frequency receiving coils 1 are arranged in an array with an effective coverage diameter of 7 cm.
  • the resonance frequency of the radio frequency coil shown in the embodiment is equal to 297.2 MHz, which can be used in a magnetic resonance system with a main magnetic field strength of 7 Tesla or more and a magnetic resonance system with a bodyless transmitting coil.
  • the integrated design reduces the space occupied by the coil to facilitate the fixation with the imaging object, that is, to maintain No relative displacement.
  • the overlapping range of the geometrical overlap of the radio frequency receiving coil 1 is determined by the network analyzer measuring the overlapping range when the forward transmission coefficient S21 between the channels is less than -15dB.
  • the radio frequency transmitting coil unit 3 and the radio frequency receiving coil unit 1 may use copper wires with insulating coatings.
  • the three fixing device interfaces 4 are installed in the radio frequency coil housing 5, which can be conveniently fixed with external fixing devices, so as to be used for mechanical fixing during the magnetic resonance imaging process to ensure that there is no opposition between the radio frequency transmitting coil, the radio frequency receiving coil and the imaging object Displacement.
  • the working principle of the magnetic resonance imaging radio frequency coil assembly with high time-domain signal stability of the present invention is as follows:
  • Fig. 6 is a schematic diagram of a magnetic resonance imaging experimental design used to study the influence of the radio frequency receiving coil signal and thermal noise on the displacement of the load object.
  • the experimental design uses a cylindrical liquid model 9 with conductivity and dielectric constant similar to that of human tissue as the load object.
  • a Teflon backing plate 8 with different thicknesses (3mm, 6mm, 8mm) was placed between the RF coil and the load object to simulate the influence of the displacement of the load object on the signal and thermal noise characteristics of the RF coil.
  • the dielectric constant of the Teflon backing plate 8 is close to the vacuum dielectric constant.
  • the radio frequency coil described in the embodiment of the present invention used in the experiment and the other three toroidal radio frequency coils with both receiving and transmitting functions are equipped with different types of preamplifiers. All experiments are performed on a 7T ultra-high field large aperture human magnetic resonance system.
  • FIG. 7 is the comparison result of the thermal noise fluctuation amplitude of the RF receiving coil.
  • the data comes from the 7T ultra-high field magnetic resonance water model imaging experiment.
  • the thermal noise is obtained by turning off the RF excitation energy collection.
  • the figure shows that the radio frequency coil array according to the embodiment of the present invention has the smallest thermal noise fluctuation, which is embodied in the smallest variance between groups, followed by a 2 cm diameter toroidal radio frequency coil with both receiving and transmitting functions, and finally with both receiving And a 5 cm diameter toroidal radio frequency coil with transmitting function.
  • the radio frequency receiving unit of the radio frequency coil array has the smallest size, followed by a 2 cm diameter toroidal radio frequency coil with both receiving and transmitting functions, and finally a 5 cm diameter toroidal radio frequency with both receiving and transmitting functions. Coil. From the consensus in the field of magnetic resonance radio frequency coils, generally, the smaller the physical size of the radio frequency receiving coil unit, the smaller the thermal noise contribution from the load, that is, the smaller the quality factor ratio of the radio frequency receiving unit under no load/load. Therefore, it can be inferred that the thermal noise contribution from the load is smaller, that is, the thermal noise level of the radio frequency receiving coil unit is not sensitive to load displacement under no load/load with a smaller quality factor ratio.
  • Figure 8 shows the ratio of the variance of the time-domain thermal noise of various RF receiving coils to the quality factor of the RF receiving unit under no load/load. It can be seen that the time-domain noise variance of the RF receiving coil is correlated with the ratio of the quality factor of the RF receiving unit under no load/load. Therefore, it can be inferred that the smaller the thermal noise contribution from the load, that is, the lower the quality factor ratio of the radio frequency receiving coil under no load/load, the less sensitive the thermal noise level is to the load displacement.
  • the radio frequency preamplifiers used by various radio frequency receiving coils that is, the radio frequency preamplifiers used between the radio frequency coils and other radio frequency coils in this embodiment may potentially cause data to be correlated. Sexual deviation.
  • Figure 9 shows the comparison results of the quality factor of the RF receiving unit under no load/load under the time-domain fluctuations of various RF receiving coil signals.
  • the radio frequency receiving coil array according to the embodiment of the present invention has the largest coverage, followed by a 5 cm diameter toroidal radio frequency coil with both receiving and transmitting functions, a 3.5 cm diameter toroidal radio frequency coil with both receiving and transmitting functions, and finally A 2 cm diameter toroidal radio frequency coil with both receiving and transmitting functions. It can be seen from the graphical results that the radio frequency coil with the larger radio frequency receiving coil coverage has the smaller signal time-domain fluctuation.
  • the radio frequency receiving coil array described in the embodiment of the present invention is composed of the smallest size radio frequency receiving coil unit
  • the overall radio frequency receiving coil composed of several radio frequency receiving coil unit arrays has the largest effective coverage and still exhibits the smallest signal time-domain fluctuations. . Therefore, it can be inferred that the smaller the coverage of the RF receiving coil is, the less sensitive the signal level of the RF receiving coil is to the load displacement.

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Abstract

一种具有高时域信号稳定性的磁共振成像射频线圈组件（6），主要由射频发射线圈单元（3）、若干射频接收线圈单元（1）、外壳（5）结构组成，射频发射线圈单元（3）需满足平面面积大于所有射频接收线圈单元（1）布局平面面积总和，若干射频接收线圈单元（1）排布在射频发射线圈单元（3）内侧，射频接收线圈单元（1）所构成阵列需满足总体尺寸大于成像区域尺寸，每个射频接收线圈单元（1）周长需小于真空电磁波波长的十分之一，来自负载的热噪声在射频接收线圈单元（1）中的占比小；射频发射线圈单元（3）和射频接收线圈单元（1）直接放置在外壳（5）结构内部，彼此相对位置固定；外壳（5）具有固定装置接口（4），用于使射频线圈组件（6）与成像物体之间无相对位移。具有高时域信号稳定性的磁共振成像射频线圈组件（6）可以更好的满足磁共振成像任务要求。

Description

具有高时域信号稳定性的磁共振成像射频线圈组件 技术领域
本发明涉及磁共振成像系统领域，具体设计一种具有高时域信号稳定性的磁共振成像射频线圈组件。
背景技术
磁共振成像的基本原理来自于1946年美国学者Bloch和Purcell的发现，在外磁场的作用下，某些绕主磁场(外磁场)进动的自旋的质子(包括人体中的氢质子)在短暂的射频电波作用下，进动角增大；当射频电波停止后，那些质子又会逐渐恢复到原来的状态，并同时释放与激励波频率相同的射频信号，这一物理现象被称为核磁共振。磁共振成像技术便是利用这一原理，通过在主磁场中附加一个脉冲梯度磁场，选择性地激发所需要位置的人体内的原子核，然后接收原子核产生的磁共振信号，最后在计算机中进行傅立叶变换，对这些信号进行频率编码和相位编码，从而建立一幅完整的磁共振图像。
磁共振成像装置包括射频发射线圈以及射频接收线圈，射频发射线圈用于产生激励质子的射频脉冲，射频接收线圈用于接收原子核产生的磁共振信号。在磁共振成像系统中，射频发射线圈所产生的磁场均匀性好、发射效率高以及射频接收线圈所接收的信号信噪比高是获得高质量图像的关键因素。对于主磁场强度较低的磁共振系统(不高于3特斯拉)，正交激励方式工作的鸟笼射频发射线圈设计可以满足在人体范围内发射磁场均匀性的要求。采用该设计的体射频发射线圈作为常规配置集成于常规场强磁共振系统中，可以满足任何部位的成像需求。但是对于主磁场场强大于3特斯拉的可用于人体成像的大孔径超高场磁共振系统，一般不配备体射频发射线圈。针对可用于人体成像的大孔径超高场磁共振系统，区别于常规场强的医用磁共振系统，需要在设计制作射频接收线圈的同时考虑增加射频发射线圈的设计，需要增加额外电路以避免射频发射线圈和射频接收线圈之间的信号耦合问题。同时，同时具有射频发射线圈和射频接收线圈的设计也可以兼容常规场强下的医用磁共振系统，反之则不兼容。对于射频接收线圈，目前广泛采用多通道相控阵射频接收线圈设计，可以保证在大的成像范围内实现高信噪比的要求。同时多通道相控阵射频接收线圈可以配合并行成像技术用于加速图像采集，改善图像质量。
用于捕捉大脑神经活动的磁共振功能成像方法，需要高性能的特殊磁共振射频线圈组件。常规的磁共振射频线圈组件对于物体在成像过程中的运动非常敏感，进而会引入时域噪声信号，成为制约功能磁共振成像信号质量的关键瓶颈因素。对于时域噪声，当图像信噪比较高 的时候，构成时域噪声的主要成分是射频接收线圈信号受负载物体位移引起的波动；当图像信噪比较低的时候，构成时域噪声的主要成分开始变为射频接收线圈热噪声水平受负载物体位移引起的波动。其中射频线圈的热噪声来源包括两个部分，一部分是射频线圈电子器件内部的传导电流引起的热噪声，另一部分是负载物体内部的位移电流引起的热噪声。由于第二部分由于位移电流的水平受到射频线圈和负载物体相对位置所决定，因而也会受到负载物体在成像过程中位移的影响。因而具有高时域信号稳定性的射频线圈应该同时具备两个特征：第一，射频线圈的信号水平即敏感度不易受负载物体位移影响；第二，射频线圈的热噪声水平也不易受负载物体位移影响。
发明内容
鉴于此，本发明实施例的目的是提供一种具有高时域稳定性的磁共振成像射频线圈组件，解决了磁共振脑成像尤其是脑功能成像过程中射频线圈的信号和噪声特性容易受成像物体位移影响的问题。
本发明实施例所采用的技术方案如下：
本发明实施例提供一种具有高时域信号稳定性的磁共振成像射频线圈组件，包括射频发射线圈单元和若干射频接收线圈单元，所述射频发射线圈单元需满足平面面积大于所有射频接收线圈单元布局平面面积总和，若干射频接收线圈单元排布在射频发射线圈单元内侧，射频接收线圈单元所构成阵列需满足总体尺寸大于成像区域尺寸，每个射频接收线圈单元的周长需小于真空电磁波波长的十分之一，来自负载的热噪声在射频接收线圈单元中的占比小，即无负载状态下射频接收线圈单元的品质因数是负载状态下射频接收线圈单元品质因数的2倍以上。
进一步的，所述射频接收线圈单元和射频接收单元直接放置在外壳内部，彼此相对位置固定。
进一步的，所述外壳外侧具有固定装置接口，用于使外壳与成像物体之间无相对位移。
进一步的，射频发射线圈单元和射频接收线圈单元均为金属导体。
进一步的，射频发射线圈单元和射频接收线圈单元采用带绝缘涂层的铜导线。
进一步的，射频发射线圈单元和接收线圈单元中包含二极管电路，用于保证射频发射线圈单元和射频接收线圈单元不会同时处于工作状态。
进一步的，射频接收线圈单元上串联了带有二极管的并联LC谐振电路，且该谐振电路的工作频率与射频接收线圈单元的工作频率相同；二极管正向偏置的情况下，射频接收线圈单元中串联的并联LC电路处于谐振状态，射频发射线圈单元因而处于失谐振状态，不工作，反 之则工作。
进一步的，射频发射线圈单元上串联了二极管电路，在二极管正向偏置的情况下，射频发射线圈单元处于谐振状态，工作，反之则不工作。
进一步的，射频接收单元之间采用信号隔离措施，即射频接收线圈单元之间通过几何重叠实现信号隔离；射频接收线圈单元与前置放大器直接连接，以减小同轴线损耗，一并封装于外壳结构内侧。
进一步的，射频发射线圈单元和射频接收线圈单元的工作频率为297.2兆赫兹，射频发射线圈单元为环形结构，射频发射线圈单元的直径是7厘米，射频接收线圈单元的直径都是1.5厘米。
与现有技术对比，本发明实施例具有的有益效果如下：通过同时结合三种技术特征，包括射频发射线圈和射频接收线圈单元所构成阵列总体尺寸大于成像区域尺寸；每个射频接收单元尺寸远小于真空电磁波波长(即每个射频接收线圈单元的周长需小于真空电磁波波长的十分之一)，来自负载的热噪声在射频接收线圈单元中的占比小，无负载状态下射频接收线圈单元的品质因数是负载状态下射频接收线圈单元品质因数的2倍以上；借助可与外部固定装置接口的外壳设计，射频发射线圈，射频接收线圈以及与成像物体之间无相对位移，可以达到将成像物体移动对成像信号时域稳定性的干扰降低到最小的效果，在对于信号时域稳定性有高要求的功能磁共振成像应用领域有着潜在巨大的应用前景。
附图说明
为了更清楚地说明本方案实施例的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本方案的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动性的前提下，还可以根据这些附图获得其它的附图。
图1是本发明实施例的具有高时域信号稳定性的磁共振成像射频线圈组件中的射频接收线圈阵列布局方式示意图，图中，1为射频接收线圈；
图2是射频接收线圈阵列与射频前置放大器连接方式的示意图，图中，1为射频接收线圈，2为射频前置放大器；
图3是射频接收线圈阵列与射频发射线圈布局方式示意图，图中，1为射频接收线圈，2为射频前置放大器，3为射频发射线圈单元；
图4是射频线圈组件外壳的结构图，4为装置接口，5为外壳；
图5是本发明实施例的整体构架示意图，图中，1为射频接收线圈，2为射频前置放大器，3为射频发射线圈单元，4为固定装置接口，5为外壳；
图6是用于研究射频接收线圈信号与热噪声受负载物体位移影响的磁共振成像实验设计示意图，图中，6为包括射频发射线圈单元和射频接收线圈单元等在内的射频线圈组件，7为特氟龙垫板，8为圆柱形液体模型(电导率和介电常数与人体组织相近)；
图7是不同类型的射频接收线圈热噪声水平受负载物体位移影响的比较结果；
图8是具有不同的无负载/负载下品质因数比值的射频接收线圈，在负载物体存在位移情况下，热噪声水平的时域波动情况比较结果；
图9是具有不同直径的射频接收线圈，在负载物体存在位移情况下，信号时域波动幅度比较结果图。
具体实施方式
为使本申请的目的、技术方案和优点更加清楚，下面将结合本申请具体实施例及相应的附图对本申请技术方案进行清楚、完整地描述。显然，所描述的实施例仅是本申请一部分实施例，而不是全部的实施例。基于本申请中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本申请保护的范围。
当说明中提及无负载/负载下品质因数比值，具体指代无负载状态下射频接收线圈单元的品质因数与负载状态下射频接收线圈单元品质因数的比值。
如图1-5所示，本发明实施例提供一种具有高时域信号稳定性的磁共振成像射频线圈组件，主要包括单通道的射频发射线圈单元3和若干射频接收线圈单元1，所有射频接收线圈1阵列排布在一个圆形平面上(即采用圆形阵列的形式对若干射频接收线圈单元1进行排布)，接收线圈单元之间相互重叠，通过产生的互感来增强射频接收线圈间信号隔离；射频发射线圈3中轴线与射频接收线圈1阵列所在圆形平面中轴线重合，覆盖范围大于所有射频接收线圈1覆盖范围的总和；射频接收线圈阵列兼容磁共振成像系统的并行成像功能，有助于缩短扫描时间和改善图像质量；各射频接收线圈均串联一电容后分别与各自的前置放大器2相连，电容用于阻抗匹配和增强射频线圈通道间去耦合性能。
采用单通道射频发射线圈3，匹配成像物体的电磁负载，实现高的发射效率和大脑范围均匀激发。
进一步的技术方案是，通过射频网络分析仪测试，获得无负载状态下射频接收线圈单元的品质因数与负载状态下射频接收线圈单元品质因数的比值，无负载状态下射频接收线圈单元的品质因数是负载状态下射频接收线圈单元品质因数的2倍以上。对于实施例，每个射频接收线圈1为有效直径都是1.5厘米的环形结构，多个射频接收线圈1阵列排布的有效覆盖直径7厘米。
进一步的技术方案是，实施例所示射频线圈的共振频率等于297.2MHz，可用于磁共振系统主磁场场强大于和等于7特斯拉的磁共振系统和无体发射线圈的磁共振系统。
将前置放大器2直接与各射频接收线圈1连接，在避免同轴线损耗、提升成像信噪比的同时，通过整合式设计缩小线圈所占空间，方便与成像物体之间进行固定，即保持无相对位移。
射频接收线圈1几何上重叠的重叠范围通过网络分析仪测定通道间正向传输系数S21小于-15dB时的重叠范围决定。射频发射线圈单元3和射频接收线圈单元1可以采用带有绝缘涂层的铜导线。
三个固定装置接口4安装在射频线圈外壳5，可方便与外部固定装置进行固定，从而用于磁共振成像过程中的机械固定，保证射频发射线圈、射频接收线圈以及与成像物体之间无相对位移。
本发明所述具有高时域信号稳定性的磁共振成像射频线圈组件的工作原理如下：
基于实验测量数据，有效覆盖范围越大的射频线圈，信号受物体-线圈位移的影响越小；单个射频接收单元尺寸越小，无负载下品质因数与负载下品质因数比值越小，即来自负载的热噪声占热噪声总体水平比值越小，热噪声水平受物体-线圈位移影响越小。我们提出了由小尺寸射频接收线圈单元构成的大规模高密集度射频接收线圈阵列，具备大的信号覆盖范围。同时可以通过可以与外部固定装置接口的外壳设计，进一步从根源最小化磁共振成像过程中物体-线圈位移。从而总体效果可以实现高时域信号稳定性。
图6是用于研究射频接收线圈信号与热噪声受负载物体位移影响的磁共振成像实验设计示意图。实验设计采用了具有与人体组织电导率和介电常数相近的圆柱形液体模型9作为负载物体。实验中，在射频线圈和负载物体之间放置了具有不同厚度(3mm，6mm，8mm)的特氟龙垫板8，来模拟负载物体位移对于射频线圈信号和热噪声特性的影响。其中特氟龙垫板8的介电常数接近于真空介电常数。在使用每一个厚度的特氟龙垫板8的情况下，针对射频线圈6信号水平和热噪声水平，在2分钟的时间里，各进行了60次成像采样，以获取负载物体无位移情况下射频接收线圈信号和热噪声时域波动数据和方差。通过将使用不同厚度特氟龙垫板7时，获取的信号和热噪声时域波动数据进行随机混合，获得模拟的负载物体位移情况下的数据。实验采用了四种具有不同尺寸和无负载/负载下品质因数比值的射频线圈6，包括：本发明实施例所述的射频线圈，同时具备接收和发射功能的2厘米直径的环形射频线圈，同时具备接收和发射功能的3.5厘米直径的环形射频线圈，同时具备接收和发射功能的5厘米直径的环形射频线圈。其中实验中所采用的本发明实施例所述的射频线圈与其他三种同时具有接收和发射功能的环形射频线圈，装备有不同种类的前置放大器。所有实验是在7T超高场大孔径人用磁共振系统上进行的。
图7是射频接收线圈热噪声波动幅度比较结果图，数据来自7T超高场磁共振水模型成像实验，通过关闭射频激励能量采集得到热噪声。图示表明本发明实施例所述的射频线圈阵列具有最小的热噪声波动，具体体现为组间方差最小，其次是同时具备接收和发射功能的2厘米直径的环形射频线圈，最后是同时具备接收和发射功能的5厘米直径的环形射频线圈。同时本发明实施例所述的射频线圈阵列的射频接收单元尺寸最小，其次是同时具备接收和发射功能的2厘米直径的环形射频线圈，最后是同时具备接收和发射功能的5厘米直径的环形射频线圈。从磁共振射频线圈专业领域的共识来看，通常射频接收线圈单元物理尺寸越小，来自负载的热噪声贡献就越小，即无负载/负载下射频接收单元品质因数比值越小。因而可以推断来自负载的热噪声贡献就越小，即无负载/负载下品质因数比值越小的射频接收线圈单元的热噪声水平对于负载位移不敏感这一结论。
图8给出了各种射频接收线圈时域热噪声方差与无负载/负载下射频接收单元品质因数的比值结果。可以看出射频接收线圈的时域噪声方差与无负载/负载下射频接收单元品质因数的比值具有相关性。因而可以推断来自负载的热噪声贡献就越小，即无负载/负载下品质因数比值越小的射频接收线圈，其热噪声水平对于负载位移越不敏感这一结论。最后，需要讨论的是各种射频接收线圈所采用的射频前置放大器的类型差异，即本实施例所述射频线圈和其他射频线圈之间采用了不同射频前置放大器，会潜在造成数据在相关性上的偏差。
图9给出了各种射频接收线圈信号时域波动性无负载/负载下射频接收单元品质因数的比较结果。本发明实施例所述射频接收线圈阵列具有最大的覆盖范围，其次是同时具备接收和发射功能的5厘米直径的环形射频线圈，同时具备接收和发射功能的3.5厘米直径的环形射频线圈，最后是同时具备接收和发射功能的2厘米直径的环形射频线圈。从图示结果可以看出，具有越大射频接收线圈覆盖范围的射频线圈具有越小的信号时域波动性。虽然本发明实施例所述的射频接收线圈阵列由最小尺寸的射频接收线圈单元构成，但由若干射频接收线圈单元阵列构成的总体射频接收线圈有效覆盖范围最大，依然展现出最小的信号时域波动。因而可以推断射频接收线圈覆盖范围越小的射频接收线圈，射频接收线圈的信号水平对于负载位移越不敏感这一结论。
以上所述仅为本发明的优选实施例，并非因此限制本发明的专利范围，凡是利用本发明说明书及附图内容所作的等效结构变换，或直接或间接运用在其他相关的技术领域，均同理包括在本发明的专利保护范围内。

Claims (10)

  1. 一种具有高时域信号稳定性的磁共振成像射频线圈组件，其特征在于，包括射频发射线圈单元和若干射频接收线圈单元，所述射频发射线圈单元需满足平面面积大于所有射频接收线圈单元布局平面面积总和，若干射频接收线圈单元排布在射频发射线圈单元内侧，射频接收线圈单元所构成阵列需满足总体尺寸大于成像区域尺寸，每个射频接收线圈单元的周长需小于真空电磁波波长的十分之一，无负载状态下射频接收线圈单元的品质因数是负载状态下射频接收线圈单元品质因数的2倍以上。
  2. 根据权利要求1所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，所述射频发射线圈单元和射频接收单元直接放置在外壳内部，彼此相对位置固定。
  3. 根据权利要求2所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，所述外壳外侧具有固定装置接口，用于使外壳与成像物体之间无相对位移。
  4. 根据权利要求1所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频发射线圈单元和射频接收线圈单元均为金属导体。
  5. 根据权利要求4所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频发射线圈单元和射频接收线圈单元采用带绝缘涂层的铜导线。
  6. 根据权利要求1所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频发射线圈单元和射频接收线圈单元中包含二极管电路，用于保证射频发射线圈单元和射频接收线圈单元不会同时处于工作状态。
  7. 根据权利要求6所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频接收线圈单元上串联了带有二极管的并联LC谐振电路，且该谐振电路的工作频率与射频接收线圈单元的工作频率相同；二极管正向偏置的情况下，射频接收线圈单元中串联的并联LC电路处于谐振状态，射频发射线圈单元因而处于失谐振状态，不工作，反之则工作。
  8. 根据权利要求6所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频发射线圈单元上串联了二极管电路，在二极管正向偏置的情况下，射频发射线圈单元处于谐振状态，工作，反之则不工作。
  9. 根据权利要求1所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频接收线圈单元之间通过几何重叠实现信号隔离；射频接收线圈单元与前置放大器直接连接，以减小同轴线损耗，一并封装于外壳结构内侧。
  10. 根据权利要求1所述的用于磁共振成像系统进行脑功能成像的射频线圈组件，其特征在于，射频发射线圈单元和射频接收线圈单元的工作频率为297.2兆赫兹，射频发射线圈 单元为环形结构，射频发射线圈单元的直径是7厘米，射频接收线圈单元的直径都是1.5厘米。
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