Circularly polarized coil and probe for low-field magnetic resonance spectrometer

Abstract

The invention provides a circularly polarized coil and a probe for a low-field magnetic resonance spectrometer, and relates to the field of magnetic resonance spectrums, and the circularly polarized coil comprises a group of orthogonal saddle-shaped coils and Helmholtz coils. The supporting structure and size of a single-turn coil are determined according to structural parameters of a detection target area, the optimal number of turns of the circularly polarized coil is determined by using Ansys Maxwell magnetic field simulation software, a CuSO4 solution is tested by using a self-developed circularly polarized probe in combination with a peripheral radio frequency circuit, and the performance of the circularly polarized coil is evaluated according to the test result. Experimental results show that compared with a single-channel saddle-shaped coil and a Helmholtz coil, the SNR of the combined circularly polarized coil is respectively improved by 28.78% and 29.21%, the transmitting power is saved by about 37.5%, and it is proved that the circularly polarized coil can effectively improve the performance of the desktop magnetic resonance spectrometer probe.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear magnetic resonance spectroscopy, and particularly relates to a circularly polarized coil and a probe for a low-field nuclear magnetic resonance spectrometer. Background Art

[0002] Nuclear Magnetic Resonance (NMR) spectroscopy, as a non-destructive analysis technique, has become an important tool for analyzing the multi-dimensional structure, dynamic process, and reaction mechanism of biochemical systems due to its characteristics such as non-ionizing radiation and non-destructive sample detection. It has important application prospects in fields such as food quality monitoring, organic synthesis characterization, drug research and development, and polymer material research. In recent years, desktop NMR spectrometers based on permanent magnet technology have developed rapidly in the scientific research and industrial fields. Compared with traditional superconducting nuclear magnetic systems, although the spectral resolution and signal-to-noise ratio are relatively low due to the limitation of the magnetic field strength (usually 0.5 - 2T), the breakthrough advantages of desktop NMR spectrometers are reflected in two aspects: one is the compact volume; the other is the non-liquid helium magnet, which greatly reduces the installation and maintenance costs. Therefore, desktop nuclear magnetic resonance spectrometers show unique application potential in scenarios such as university teaching laboratories, pharmaceutical process monitoring, and industrial on-site detection, providing an important supplement to traditional high-field nuclear magnetic technologies (>10T).

[0003] The probe is the core component of the magnetic resonance system. In low-field nuclear magnetic resonance spectrometers, a single-channel radio frequency coil structure design is usually used, such as a saddle coil, a Helmholtz coil structure, etc. However, the signal-to-noise ratio of conventional single-channel coils is limited. Seeking a more efficient radio frequency coil is crucial for improving the performance of the desktop nuclear magnetic resonance spectrometer system. The circularly polarized coil design is a commonly used method to improve the signal-to-noise ratio of the probe. This method realizes the circularly polarized reception effect of the probe by adding a coil perpendicular to the single channel in space, and realizes The improvement of the signal-to-noise ratio (SNR) by several times. Most of the domestic and foreign research on magnetic resonance circularly polarized probes focuses on the imaging field. In 2024, the team of Xu Zheng from Chongqing University studied a knee joint orthogonal receiving coil for an ultra-low field magnetic resonance imaging system. The support structure of this system consists of orthogonal coils with a radius of 80 mm and a length of 200 mm, achieving an effect of increasing the image SNR by about 30%. Hoult et al. proposed a theory for orthogonal detection of circularly polarized coils in imaging experiments, predicting that using circularly polarized coils can increase the image signal-to-noise ratio by 41% compared to single coils and save half of the transmit power. The measured image quality was improved by 40%, and the transmitter power was reduced by 1 / 2. The above work focuses on the imaging system. To meet the large field of view required for imaging, the coil volume is relatively large and not suitable for desktop magnetic resonance systems. No researcher has been found to use circularly polarized probes in desktop magnetic resonance spectrometers. The desktop magnetic resonance spectrometer based on permanent magnets has characteristics such as a compact structure and a small probe size, resulting in limited space for the design of the radio frequency coil, which is the main challenge in the design of circularly polarized coils in compact magnetic resonance spectrometers. Summary of the Invention

[0004] To achieve the above objects and other advantages of the present invention, the first object of the present invention is to provide a circularly polarized coil for a low-field magnetic resonance spectrometer, including a set of orthogonal saddle coils and Helmholtz coils. The radio frequency fields generated by the saddle coils and the Helmholtz coils are both perpendicular to the direction of the main magnetic field, and the radio frequency fields generated by the saddle coils and the Helmholtz coils are also perpendicular to each other.

[0005] Furthermore, the saddle coils and the Helmholtz coils are designed by means of flexible circuit board printing.

[0006] Furthermore, the ratio of the height to the diameter of the saddle coil is 2:1, and the coil angle is 120°.

[0007] Furthermore, the height of the Helmholtz coil is half of its diameter.

[0008] Furthermore, the diameter of the saddle coil is 8 mm, and the height of the saddle coil is 16 mm.

[0009] Furthermore, the diameter of the Helmholtz coil is 8 mm, and the height of the Helmholtz coil is 4 mm.

[0010] Furthermore, the number of turns of the saddle coils and the Helmholtz coils is obtained by calculating the comprehensive performance. The comprehensive performance calculation formula is:

[0011]

[0012] Among them, FoM is the comprehensive performance, SNRc is the relative signal-to-noise ratio of the coil, δ is the radiofrequency field inhomogeneity, B max and B min are the maximum and minimum values of the magnetic induction intensity of the radiofrequency field, B mean is the average value of the magnetic induction intensity of the radiofrequency field within the region of interest, I is the excitation current applied to the coil, and R is the AC resistance of the coil.

[0013] Furthermore, the number of turns of the saddle coil is 2, and the number of turns of the Helmholtz coil is 5.

[0014] The second object of the present invention is to provide a circularly polarized probe for a low-field nuclear magnetic resonance spectrometer, including the above-mentioned circularly polarized coil, coil bracket, tuning and matching circuit, shielding case, feeding structure, T / R switch. The circularly polarized coil, the coil bracket, and the tuning and matching circuit are installed inside the shielding case; among them,

[0015] The coil bracket is used to fix the circularly polarized coil so that signals can be excited and received within a specific area;

[0016] The tuning and matching circuit is connected to the circularly polarized coil. The tuning and matching circuit is used to tune so that the circuit resonance frequency is consistent with the NMR frequency, and to receive the NMR signal generated by the sample, and is also used to match so that the circuit impedance matches the transmission line impedance;

[0017] The feeding structure is connected to the circularly polarized coil. The feeding structure is used to achieve power distribution during radiofrequency pulse transmission and signal acquisition during reception of magnetic resonance signals;

[0018] The T / R switch is connected to the feeding structure. The T / R switch is used to achieve the conversion of the working mode.

[0019] Furthermore, the feeding structure adopts a Hybrid bridge. The first port of the Hybrid bridge is connected to the radiofrequency power amplifier, the second port and the third port are connected to the saddle coil and the Helmholtz coil of the circularly polarized coil, and the fourth port is connected to the Rx receiving end.

[0020] Furthermore, a lumped-parameter Hybrid bridge is designed based on a λ / 8 transmission line.

[0021] Furthermore, an amplifier is also included. The amplifier adopts a cascade of a first-stage and a second-stage low-noise preamplifier, and is used to amplify and process the detected signal.

[0022] Furthermore, a passive T / R switch is fabricated based on a λ / 4 transmission line.

[0023] Furthermore, the tuning and matching circuit adopts an L-type tuning and matching network.

[0024] Further, it further includes an electromagnetic shielding module, and the electromagnetic shielding module is fixed on the coil bracket.

[0025] Further, the electromagnetic shielding module uses copper tape, and the copper tape is pasted on the upper and lower ends of the coil bracket.

[0026] Further, the shielding case uses a shielding aluminum case.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] The present invention provides a circularly polarized coil and a probe for a low-field nuclear magnetic resonance spectrometer. The circularly polarized coil includes a set of orthogonal saddle coils and Helmholtz coils. The present invention determines the support structure and size of a single-turn coil according to the structural parameters of the detection target area, uses Ansys Maxwell magnetic field simulation software to determine the optimal number of turns of the circularly polarized coil, and uses a self-developed circularly polarized probe combined with a peripheral radio frequency circuit to test a CuSO4 solution and evaluate the performance of the circularly polarized coil. The experimental results show that compared with a single-channel saddle coil and a Helmholtz coil, the SNR of the combined circularly polarized coil is increased by 28.78% and 29.21% respectively, and the transmission power is saved by about 37.5%, proving that the circularly polarized coil can effectively improve the performance of the probe of the desktop nuclear magnetic resonance spectrometer.

[0029] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly and implement it according to the content of the specification, the following takes the preferred embodiments of the present invention and combines the accompanying drawings to describe in detail as follows. The specific implementation manners of the present invention are given in detail by the following embodiments and their accompanying drawings. Description of the Drawings

[0030] The drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and the schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0031] Figure 1 It is a schematic diagram of the main magnetic field of the Halbach array magnet and the radio frequency field generated by the circularly polarized coil;

[0032] Figure 2 It is a schematic diagram of the saddle coil, the Helmholtz coil and the copper strip;

[0033] Figure 3 It is a simulation schematic diagram of the saddle coil and the Helmholtz coil;

[0034] Figure 4 It is a schematic diagram of the L-type matching network;

[0035] Figure 5Three-dimensional schematic diagram of a circularly polarized coil after optimizing parameters;

[0036] Figure 6 Schematic diagram of the measurement results of S parameters of saddle-shaped and Helmholtz coils;

[0037] Figure 7 Schematic diagram of the signal principle block diagram of a Hybrid bridge;

[0038] Figure 8 Schematic diagram of a Hybrid bridge of a lumped element circuit equivalent to λ / 8;

[0039] Figure 9 Schematic diagram of the insertion loss and phase shift of ports 2, 3 and port 1 in the S parameters of a Hybrid bridge;

[0040] Figure 10 Schematic diagram of the isolation between port 2 and port 3 in the S parameters of a Hybrid bridge;

[0041] Figure 11 Schematic diagram of the test system;

[0042] Figure 12 Measured by a single-channel coil and a circularly polarized coil 1 H NMR spectrum. Detailed implementation manners

[0043] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0044] In the accompanying drawings, for clarity, the shapes and dimensions may be enlarged, and the same reference numerals will be used throughout the drawings to indicate the same or similar components.

[0045] In the following description, terms such as center, thickness, height, length, front, back, rear, left, right, top, bottom, upper, lower, etc. are defined with respect to the structures shown in the respective drawings. In particular, "height" corresponds to the dimension from top to bottom, "width" corresponds to the dimension from left to right, and "depth" corresponds to the dimension from front to back. They are relative concepts and may therefore change accordingly depending on their different positions and different usage states. Therefore, these or other orientations should not be construed as restrictive terms.

[0046] Terms related to attachment, connection, etc. (e.g., "connect" and "attach") refer to the relationship in which these structures are directly or indirectly fixed or attached to each other through an intermediate structure, as well as a movable or rigid attachment or relationship, unless otherwise explicitly stated.

[0047] The signal-to-noise ratio (SNR) is a key parameter affecting the signal quality of a desktop nuclear magnetic resonance (NMR) spectrometer. Using orthogonal circularly polarized coils in the probe is an effective means to improve the SNR. Due to the compact magnet structure of the desktop NMR spectrometer, the space left for the probe is limited, which poses a challenge to the design of circularly polarized coils. No other researchers have used circularly polarized coils in desktop NMR spectrometers. Based on the self-developed desktop NMR spectrometer platform in the laboratory, the present invention designed and fabricated a transceiver-integrated circularly polarized coil based on a flexible board. The coil is composed of a set of orthogonal saddle coils and Helmholtz coils. First, the support structure and dimensions of a single-turn coil were determined according to the structural parameters of the detection target area. Then, the optimal number of turns of the circularly polarized coil was determined using the Ansys Maxwell magnetic field simulation software. Finally, the performance of the circularly polarized coil was evaluated by testing a CuSO4 solution using a self-developed circularly polarized probe combined with a peripheral radio frequency circuit. The experimental results show that compared with a single-channel saddle coil and a Helmholtz coil, the SNR of the combined circularly polarized coil is increased by 28.78% and 29.21% respectively, and the transmitted power is saved by about 37.5%, proving that the circularly polarized coil can effectively improve the performance of the probe of the desktop NMR spectrometer.

[0048] Example 1

[0049] A circularly polarized coil for a low-field nuclear magnetic resonance spectrometer includes a set of orthogonal saddle coils and Helmholtz coils. The radio frequency fields generated by the saddle coils and the Helmholtz coils are both perpendicular to the direction of the main magnetic field, and the radio frequency fields generated by the saddle coils and the Helmholtz coils are also perpendicular to each other. That is, the radio frequency fields generated by the circularly polarized coil should be orthogonal to each other and orthogonal to the main magnetic field of the magnet.

[0050] As Figure 1 shown, the direction of the main magnetic field B0 of the Halbach array magnet is radial, which is the x-axis direction in this embodiment. The radio frequency fields generated by the two sets of coils in the orthogonal coils should be perpendicular to the direction of the main magnetic field, and the radio frequency fields generated by the two sets of coils themselves should also be perpendicular. The three-dimensional configurations of the saddle coil and the Helmholtz coil are both axially cylindrical, which are suitable for the cylindrical space of the Halbach array magnet. The saddle coil can generate a radial radio frequency excitation field, and the Helmholtz coil can generate an axial radio frequency excitation field. Therefore, in this embodiment, a saddle coil is selected to generate a radio frequency field B s , which is the y-axis direction in this embodiment; a Helmholtz coil is used to generate a radio frequency field B h , which is the z-axis direction in this embodiment. Among them, Figure 1The arrow on the middle coil represents the direction of the current.

[0051] According to the structure of the magnetic resonance probe and the sample tube (5 mm standard sample tube), first determine the dimensions of the single-turn saddle and Helmholtz coils, then optimize the coil turns through simulation software, and obtain the optimal number of turns of the coil through the defined comprehensive performance. Finally, combine the tuning and matching circuit to match the RF coil, so that the coil can work correctly at the corresponding frequency of the laboratory low-field magnet.

[0052] In this embodiment, the circularly polarized coil is applied to the desktop nuclear magnetic resonance spectrometer, which helps to solve the problems of poor signal-to-noise ratio and low emission efficiency caused by low magnetic field strength.

[0053] According to the size of the sample tube, the region of interest is set as a cylinder with a diameter of 5 mm and a height of 4 mm. As Figure 2 shown, for the saddle coil, theoretically, when the ratio of the height H s to the diameter D s is 2:1 and the coil angle α is 120°, the most uniform magnetic field can be generated; for the Helmholtz coil, theoretically, when its height H h is half of the diameter D h , the most uniform magnetic field can be generated. At the same time, considering the size of the coil bracket, the diameter of the saddle coil is set to 8 mm and the height is 16 mm; the diameter of the Helmholtz coil is 8 mm and the height is 4 mm. Since the coil size is small and the manufacturing difficulty is large, this embodiment uses the flexible printed circuit board (FPC) method for coil design. The corresponding wire simulation model is a rectangular copper strip. Considering that the coil needs to be designed with multiple turns, the coil width is designed to be 0.5 mm. Figure 2 In (a), (b), and (c) are the schematic diagrams of the saddle coil size, the Helmholtz coil size, and the copper strip width, respectively.

[0054] The performance of the circularly polarized coil is determined by the RF field non-uniformity δ and the signal-to-noise ratio SNR c it generates. The calculation formula for the RF field non-uniformity δ is:

[0055]

[0056] where B max and B min are the maximum and minimum values of the magnetic induction intensity of the RF field, and B mean is the average value of the magnetic induction intensity of the RF field within the region of interest, with the unit of T.

[0057] The relative signal-to-noise ratio SNR c of the coil is calculated as:

[0058]

[0059] Among them, I is the excitation current applied to the coil, with the unit of A; R is the AC resistance of the coil, with the unit of Ω.

[0060] The Ansys Maxwell simulation software is used to model and analyze the saddle coil and the Helmholtz coil, with the excitation current intensity of 1 A and the frequency of 22.5 MHz. Figure 3 It shows the magnetic field distributions of the two coils in the region of interest. Figure 3 In (a) and (b), they are the simulations of the saddle coil and the Helmholtz coil respectively. The color inside the cylinder changes from blue to red, indicating that the magnetic induction intensity (μT) increases from weak to strong. The magnetic field direction of the saddle coil is along the y direction, and the magnetic field direction of the Helmholtz coil is along the z direction.

[0061] According to the Ansys Maxwell simulation software, the maximum value B of the magnetic induction intensity of the RF field in the region of interest, max and the minimum value B min can be obtained. The average value B of the magnetic induction intensity of the RF field in the region of interest mean and the AC resistance R of the coil can be obtained. Thus, the RF field non-uniformity δ and the relative signal-to-noise ratio SNR of the coil can be calculated. c .

[0062] The comprehensive performance FoM (Figure of Merics) of the coil is defined as the relative signal-to-noise ratio SNR of the coil c divided by the RF field non-uniformity δ. Among them, the relative signal-to-noise ratio is proportional to the coil performance, so it is placed in the numerator; the RF field non-uniformity is inversely proportional to the coil performance, so it is placed in the denominator. Due to the limitation of the spatial size, the maximum number of turns of the saddle coil is 4, and the maximum number of turns of the Helmholtz coil is 6. The number of turns of the two groups of coils is simulated and optimized respectively. The calculation formula of the comprehensive performance FoM is as follows:

[0063]

[0064] The simulation results are shown in Table 1 and Table 2. It can be learned from Table 1 that when the number of turns of the saddle coil is 2, its comprehensive performance FoM is the largest; it can be learned from Table 2 that when the number of turns of the Helmholtz coil is 5, its comprehensive performance FoM is the largest. This optimization result is used to manufacture the physical circularly polarized coil.

[0065] Table 1 Comprehensive performance of the saddle coil

[0066]

[0067]

[0068] Table 2 Comprehensive performance of the Helmholtz coil

[0069]

[0070] The actual circularly polarized coil adopts the FPC process and is designed and manufactured using the JLCPCB EDA software. In addition to the circularly polarized coil, the circularly polarized probe also includes a coil holder and a tuning and matching circuit. For a detailed description of the circularly polarized probe, reference can be made to the corresponding description in the following embodiments of the circularly polarized probe, which will not be elaborated here.

[0071] In this embodiment, a compact transceiver integrated circularly polarized radio frequency probe and a supporting Hybrid bridge circuit are designed based on the 0.53T Halbach magnet structure. Based on the saddle coil and Helmholtz coil configurations, the electromagnetic simulation software Ansys Maxwell is used to analyze the coil performance indicators in the region of interest of the sample, and the corresponding coil parameter models are optimized; a supporting Hybrid bridge circuit is designed and performance tested; a physical object is made based on the optimized results of the coil structure simulation, and it is installed and debugged in combination with the tuning and matching circuit; finally, based on the magnet platform, a radio frequency link test is carried out with the peripheral Hybrid bridge, transmit / receive switch, low-noise preamplifier, and radio frequency power amplifier to verify the performance of the circularly polarized probe.

[0072] Embodiment 2

[0073] A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer includes the circularly polarized coil for a low-field nuclear magnetic resonance spectrometer provided in Embodiment 1. For a detailed description of the circularly polarized coil, reference can be made to the corresponding description in the above embodiments of the circularly polarized coil, which will not be elaborated here.

[0074] Specifically, the circularly polarized probe for a low-field nuclear magnetic resonance spectrometer includes the above-mentioned circularly polarized coil, coil holder, tuning and matching circuit, shielding case, feeding structure, and T / R switch. The circularly polarized coil, the coil holder, and the tuning and matching circuit are installed inside the shielding case; among them,

[0075] The coil holder is used to fix the circularly polarized coil so that it can stably excite and receive signals in a specific area;

[0076] The tuning and matching circuit is connected to the circularly polarized coil. The tuning and matching circuit is the core component of the NMR probe and is mainly used for tuning and matching. Among them, tuning ensures that the circuit resonance frequency is consistent with the NMR frequency, ensures efficient transmission of radio frequency energy to the sample, and receives the NMR signal generated by the sample. Matching makes the circuit impedance match the transmission line impedance, thereby maximizing the transmission efficiency of radio frequency energy and reducing reflection loss.

[0077] The feeding structure is connected to the circularly polarized coil, and the feeding structure is used to achieve power distribution during radio frequency pulse transmission and signal acquisition during reception of magnetic resonance signals;

[0078] The T / R switch is connected to the feeding structure, and the T / R switch is used to realize the conversion of working modes.

[0079] In some embodiments, the tuning and matching circuit adopts an L-type tuning and matching network. As Figure 4 shown, the L-type network is the simplest matching network that can meet the system requirements at the lowest cost. Among them, Lc and Rc are the equivalent inductance and equivalent resistance of the coil, and the capacitors C1 to C3 are used for tuning, and the capacitors C4 to C9 are used for matching.

[0080] Referring to the parameters of the optimal saddle coil and Helmholtz coil in Embodiment 1, the FPC planar coil structure is designed in JLCPCB EDA. The top layer of the FPC is the saddle coil structure, and the bottom layer is the Helmholtz coil structure. They are evenly pasted and fixed on the 3D-printed coil bracket with high-temperature insulating tape. Since the region of interest is a 4-mm-high cylinder and the height of the saddle coil is 16 mm, in some embodiments, an electromagnetic shielding module is further included. The electromagnetic shielding module is fixed on the coil bracket. Further, the electromagnetic shielding module adopts copper tape, and the copper tape is pasted on the upper and lower ends of the coil bracket. For example, 7-mm-wide copper tape is used to paste on the upper and lower ends of the cylindrical bracket respectively. Enameled wire is used to connect the coil to the tuning and matching circuit board, and the coil and the tuning and matching circuit are installed in the designed shielding aluminum shell. The magnetic field directions of the two-channel coils are orthogonal to each other to achieve spatial decoupling. The circularly polarized coil is as Figure 5 shown. Since the actually manufactured FPC coil needs to consider the feeding port, there is a certain deviation between the configuration of the actual coil and the theoretical simulation model.

[0081] In this embodiment, a hydrogen nucleus is selected as the sample signal detection nucleus, and the capacitors on the tuning and matching circuit board are adjusted to make the coil resonate at the Larmor frequency (22.5 MHz) of the hydrogen nucleus at 0.53 T. The Keysight E5061B vector network analyzer is used to measure the echo loss parameter S11 and the isolation parameter S21 of the two coils at 22.5 MHz. S11 and S22 respectively reflect the impedance matching between the probe and the RF source and the sample. A low value indicates high energy transfer efficiency; S21 measures the isolation performance of the probe, and a low value indicates small signal crosstalk. The results are as Figure 6 shown, where the abscissa is the frequency, from 10 MHz to 30 MHz, and the ordinate is dB. The S11 and S22 of the saddle and Helmholtz coils at 22.5 MHz are -28.814 dB and -27.333 dB respectively, and the isolation S21 is -26.126 dB.

[0082] For the probe to generate a circularly polarized field, two feeding ports are required, and the two ports are excited by sources with equal amplitudes and a 90° phase difference. In some embodiments, the feeding structure employs a Hybrid bridge. As Figure 7 shown, the Hybrid bridge consists of 4 ports. Port 1 is connected to the RF power amplifier, ports 2 and 3 are connected to two independent coils x and y, and port 4 is connected to the Rx receiver. In the transmitting state, port 4 is isolated from port 1. The pulsed power is distributed to ports 2 and 3 in an orthogonal mode, thereby generating a rotating circularly polarized magnetic field. In the receiving state, the rotating magnetization induces signals with a 90° voltage phase difference in the x coil and the y coil. They are input to ports 2 and 3 of the mixer, and ports 2 and 3 are isolated from each other. The signals at ports 1 and 4 have a phase shift. The two orthogonal signals that appear in phase at port 4 are added, while they appear out of phase at port 1 and thus cancel each other out, ensuring that no signal leaks to the transmitting load.

[0083] The Hybrid bridge circuit is actually fabricated using lumped elements. Based on a λ / 8 transmission line, a circuit suitable for a frequency of 22.5 MHz is designed. Figure 8 It is the circuit diagram of the Hybrid lumped elements. Finally, a bridge with an insertion loss of -3.3 dB and an isolation of about -20 dB is designed.

[0084] Figures 9 - 10 They are the parameters of the Hybrid bridge for actual debugging. Among them Figure 9 the 1st label and the 2nd label respectively represent the insertion losses of ports 2, 3 and port 1. At 22.5 MHz, S21 is -3.31 dB and -3.28 dB; Figure 9 the 3rd label and the 4th label respectively represent the phase shift degrees of ports 2, 3 and port 1. At 22.5 MHz, they are -97.69° and -7.62° respectively, and their phase difference is -90.07°. Figure 10 In the curve S21, it is the isolation between ports 2 and 3. The 1st label indicates that S21 is -21.04 dB at 22.5 MHz, showing that the isolation between ports 2 and 3 is good. It is proved that in the transmitting state, this Hybrid bridge can achieve excitation at port 1, and ports 2 and 3 generate signals with equal amplitudes and a 90° difference; due to the symmetric structure of the circuit, in the receiving state, S12 is equal to S21, and their phase differences are also equal; S13 is equal to S31, and their phase differences are also equal. Therefore, this circuit can be used in a circularly polarized probe.

[0085] In the probe system, in order to achieve an integrated design of the transmitting and receiving functions, a transceiver integrated coil structure is adopted, and the working mode conversion is realized through a T / R switch. This switching device is used to accurately switch between the transmitting state and the receiving state to ensure accurate control of the system timing. In this embodiment, a passive and non-powered T / R switch is fabricated based on a λ / 4 transmission line, and circuit design and physical production are carried out. The final test results show that the insertion loss of the switch in the transmitting and receiving states is better than -0.2 dB, the isolation is better than -20 dB, and the switching time is about 600 ns.

[0086] Since the electromagnetic signal intensity collected by the magnetic resonance detection system is relatively low, usually in the μV order of magnitude, it is necessary to perform preprocessing before the signal enters the spectrometer. In this embodiment, low-noise amplification technology is used to perform primary amplification processing on the detection signal. A transistor integrated circuit is selected as the core device to construct a single-stage amplifier circuit, and the circuit topology is modeled and parameter optimized through the ADS simulation platform. The second-stage low-noise preamplifier uses a commercial MAX2611 chip with a fixed gain of 10 dB. The first-stage and second-stage low-noise preamplifiers are cascaded to form the final amplifier. Finally, the measured amplifier gain is about 36 dB, the noise figure is less than 0.7 dB, and the voltage reflection coefficient of each port is less than 2.

[0087] The performance of the circularly polarized probe is tested on the desktop magnetic resonance platform in the laboratory: the magnet is a 0.53 T Halbach permanent magnet with a uniformity of 3 ppm (1 ppm = 1×10 -6 ) in a 5 mm spherical region. The T / R switch, LNA, and Hybrid bridge are all RF components suitable for 22.5 MHz. The RF power amplifier uses the BT00500-AlphaSA model of the American company Tecmag, and the spectrometer uses the Tecmag Scout LF1 model of the Tecmag company. Each component is interconnected through coaxial cables. The block diagram of the test system is as Figure 11 shown.

[0088] Using a 90° hard pulse excitation, the sample in the channel is a 6.4 g / L CuSO4 solution. Hydrogen signal tests are performed on a single-channel saddle coil, a single-channel Helmholtz coil, and the optimal circularly polarized coil after combination. The sequence parameters of the 90° hard pulse for the single-channel Helmholtz and saddle coils are: pulse width of 8 μs, spectral width of 20 kHz, and number of data points of 4096. The sequence parameters of the 90° hard pulse for the quadrature coil are: pulse width of 5 μs, spectral width of 20 kHz, and number of data points of 4096. The signal-to-noise ratio is calculated by evaluating the ratio of the maximum amplitude of the signal to the noise level. The maximum amplitude of the signal is directly extracted from the signal, and the noise level is estimated through the statistical characteristics of the signal, based on the variance and product of differences of the signal, ensuring that it can accurately reflect the noise level. The calculation formula for defining the signal-to-noise ratio SNR is as follows:

[0089]

[0090] where maxval is the maximum value of the signal within the signal range, and the formula for noise is as follows:

[0091]

[0092] where N is the total number of sampling points within the signal range, n is (N - 1) / 2, and y(i) is the intensity value of the i-th point within the signal range.

[0093] Ten repeated experiments are carried out to collect the signal, and the optimal set of signals is selected for display. The optimal spectra of the single-channel coil and the circularly polarized coil are as shown in Figure 12 shown below. Figure 12 In it, the blue curve is the HNMR signal measured by the single-channel 90° pulse saddle coil 1 ; the red curve is the HNMR signal measured by the single-channel 90° pulse Helmholtz coil 1 ; the green and pink curves are the HNMR signals measured by the circularly polarized coil with 90° and 450° pulses respectively 1 . Figure 12 In it, the best signal-to-noise ratio SNR of the hydrogen spectrum signal measured by the single-channel saddle coil in the blue curve is 204.32, the best signal-to-noise ratio SNR of the hydrogen spectrum signal measured by the single-channel Helmholtz coil in the red curve is 205.01, and the best signal-to-noise ratio of the hydrogen spectrum signal measured by the optimal circularly polarized coil after combination in the green curve is 264.01. It can be seen from this that compared with the single-channel Helmholtz coil and the single-channel saddle coil, the signal-to-noise ratio of the circularly polarized coil designed in this embodiment is increased by 28.78% and 29.21% respectively. Figure 12The pink curve is the signal spectrogram at a flip angle of 450°, with an intensity of approximately 1286. The ratio of this intensity to the signal intensity of 1461 at a 90° flip angle is approximately 88%. Generally, if the ratio is required to be greater than 85%, the B1 field is considered to be relatively uniform, indicating that the radiofrequency field uniformity of the circularly polarized coil in this design is good. From the sequence information, the hard pulse length of the single-channel is 8 μs, and the hard pulse length of the circularly polarized probe is 5 μs. On the premise that the pulse power amplitude remains unchanged, the transmit power can be simply regarded as being proportional to the pulse time length. Therefore, compared with the single-channel coil, the transmit power of the circular polarization is reduced by approximately 37.5%.

[0094] In this embodiment, a set of circularly polarized radiofrequency probe applied to a desktop nuclear magnetic resonance spectrometer is designed. The coil is composed of a saddle-shaped coil and a Helmholtz coil. The finite element simulation software Ansys Maxwell is used to compare the simulation performance of the coil turns to obtain the optimal number of turns, and then a physical object is made. The signal test is completed in cooperation with the peripheral Hybrid bridge, T / R switch, LNA and spectrometer system. The results show that the crosstalk between the two coils is small. The signal-to-noise ratio of the spectrogram of the circularly polarized coil is increased by a maximum of 29.21% compared with the single-channel coil, and the transmit power is reduced by approximately 37.5%. The feasibility of the circularly polarized probe designed in this embodiment in the desktop nuclear magnetic resonance spectrometer is verified.

[0095] The number of devices and the scale of processing described here are used to simplify the description of the present invention. Applications, modifications and variations of the present invention will be apparent to those skilled in the art.

[0096] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily made. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to the specific details and the illustrated examples described here.

[0097] It should also be noted that the term "comprising", "including" or any other variant thereof is intended to cover a non-exclusive inclusion, such that a process, method, commodity or device including a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such process, method, commodity or device. Without further limitation, an element defined by the statement "including one..." does not exclude the presence of another identical element in the process, method, commodity or device including the said element.

[0098] Each embodiment in this specification is described in a progressive manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple. For the relevant parts, reference can be made to the partial description of the method embodiment.

[0099] The above is only for the embodiments of this specification and is not intended to limit one or more embodiments of this specification. For those skilled in the art, various changes and modifications can be made to one or more embodiments of this specification. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of one or more embodiments of this specification shall be included within the scope of the claims of one or more embodiments of this specification.

Claims

1. A circularly polarized coil for a low-field nuclear magnetic resonance spectrometer, characterized in that: It includes a set of orthogonal saddle coils and Helmholtz coils. The RF fields generated by the saddle coils and the Helmholtz coils are both perpendicular to the direction of the main magnetic field, and the RF fields generated by the saddle coils and the Helmholtz coils are also perpendicular to each other.

2. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 1, wherein: The saddle coils and the Helmholtz coils are designed by means of flexible circuit board printing.

3. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 1, characterized in that: The ratio of the height to the diameter of the saddle coil is 2:1, and the coil angle is 120°.

4. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 3, characterized in that: The height of the Helmholtz coil is half of its diameter.

5. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 4, wherein: The diameter of the saddle coil is 8 mm, and the height of the saddle coil is 16 mm.

6. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 5, wherein: The diameter of the Helmholtz coil is 8 mm, and the height of the Helmholtz coil is 4 mm.

7. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 1, wherein: The number of turns of the saddle coil and the Helmholtz coil is obtained by calculating the comprehensive performance. The calculation formula for the comprehensive performance is: Among them, FoM is the comprehensive performance, SNRc is the relative signal-to-noise ratio of the coil, δ is the inhomogeneity of the radiofrequency field, B max and B min are the maximum and minimum values of the magnetic induction intensity of the radiofrequency field, B mean is the average value of the magnetic induction intensity of the radiofrequency field within the region of interest, I is the excitation current applied to the coil, and R is the AC resistance of the coil.

8. The circularly polarized coil for a low-field nuclear magnetic resonance spectrometer according to claim 7, wherein: The number of turns of the saddle coil is 2, and the number of turns of the Helmholtz coil is 5.

9. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer, characterized in that: It includes the circularly polarized coil, coil bracket, tuning and matching circuit, shielding case, feeding structure, T / R switch as described in any one of claims 1 to 8. The circularly polarized coil, the coil bracket, and the tuning and matching circuit are installed in the shielding case; wherein, The coil bracket is used to fix the circularly polarized coil so that signals can be excited and received in a specific area; The tuning and matching circuit is connected to the circularly polarized coil. The tuning and matching circuit is used to tune so that the resonant frequency of the circuit is consistent with the NMR frequency, receive the NMR signal generated by the sample, and is used to match so that the impedance of the circuit matches the impedance of the transmission line; The feeding structure is connected to the circularly polarized coil. The feeding structure is used to achieve power distribution during RF pulse transmission and signal acquisition during reception of magnetic resonance signals; The T / R switch is connected to the feeding structure. The T / R switch is used to achieve the conversion of the working mode.

10. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 9, characterized in that: The feeding structure adopts a Hybrid bridge. The first port of the Hybrid bridge is connected to the RF power amplifier, the second port and the third port are connected to the saddle coil and the Helmholtz coil of the circularly polarized coil, and the fourth port is connected to the Rx receiving end.

11. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 10, characterized in that: The lumped parameter Hybrid bridge is designed based on the λ / 8 transmission line.

12. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 11, characterized in that: It also includes an amplifier. The amplifier adopts a cascade of a first-stage and a second-stage low-noise preamplifier, which is used to amplify the detected signal.

13. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 11, characterized in that: A passive T / R switch is fabricated based on the λ / 4 transmission line.

14. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 9, characterized in that: The tuning and matching circuit adopts an L-type tuning and matching network.

15. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 9, characterized in that: It also includes an electromagnetic shielding module, which is fixed on the coil bracket.

16. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 15, characterized in that: The electromagnetic shielding module adopts copper tape, and the copper tape is pasted on the upper and lower ends of the coil bracket.

17. A circularly polarized probe for a low-field nuclear magnetic resonance spectrometer according to claim 9, characterized in that: The shielding case adopts a shielding aluminum case.

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