A radio frequency coil design method for transmit array spatial encoding imaging

Abstract

The application relates to a radio frequency coil design method for transmit array spatial encoding imaging, and belongs to the technical field of magnetic resonance imaging. In view of the problems of low single-dimension coding efficiency, strong coupling interference between coils and nonlinear phase gradient of a traditional TRASE technology, axial and radial coding coils are designed based on a target field method and a magnetic dipole method, two-dimensional radio frequency phase coding is realized through reverse current solenoid decoupling combination, and excitation and signal receiving are realized in combination with a saddle coil. Through optimization of the coil structure and decoupling design, the technical scheme achieves multi-dimensional efficient coding, high-precision phase gradient, strong anti-interference and system light weight, and is suitable for the portability and dynamic monitoring scene of low-field magnetic resonance.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology and relates to a radio frequency coil design method for spatial coding imaging of a transmitter array. Background Technology

[0002] Magnetic Resonance Imaging (MRI), as a non-invasive medical imaging technique, plays an irreplaceable role in the clinical diagnosis of joint and brain diseases. Traditional MRI systems rely on a high-intensity main magnetic field (typically 1.5T or 3.0T) combined with a gradient coil system to achieve spatial encoding. However, its high manufacturing cost, large size, and high operating energy consumption limit its application in mobile healthcare and long-term monitoring scenarios. To meet the clinical needs for low cost, portability, and continuous monitoring, low-field (0.2T–1.0T) and ultra-low-field (<0.1T) MRI technologies have recently become a research hotspot. Low-field MRI systems have a higher tolerance for patient positioning errors, making them particularly suitable for children, critically ill patients, and other groups who cannot remain still. Furthermore, their soft tissue contrast is excellent in joint imaging. However, the image resolution and signal-to-noise ratio (SNR) of low-field MRI are significantly lower than those of high-field systems, making it difficult to meet the imaging requirements of fine structures (such as micro-trauma of cartilage and small brain lesions). This has become a major bottleneck for its clinical promotion.

[0003] Traditional MRI spatial coding relies on the linear gradient field generated by the main magnetic field (B0) gradient coil, and achieves k-space traversal through frequency coding and phase coding. However, the size, weight, and high power consumption of the gradient coil are particularly prominent in low-field systems, and its performance drops sharply in non-uniform B0 fields. To address this, Transmit Array Spatial Encoding (TRASE) technology was proposed, which directly performs spatial coding using the phase gradient field generated by the radio frequency (RF) coil, without relying on the B0 gradient coil. The core principle of TRASE is that multiple coils with different RF phase gradient directions alternately emit 180° refocusing pulses, causing the phase accumulation of the spin echo sequence to form a k-space trajectory, thereby achieving imaging. Compared with B0 gradient coding, TRASE technology has the following advantages: (1) the RF coil is small and lightweight, and can be customized for specific anatomical sites; (2) it can still work stably in highly non-uniform B0 fields; (3) the system power consumption and manufacturing cost are significantly reduced.

[0004] However, the practical application of existing TRASE technology still faces the following challenges:

[0005] Most TRASE systems use a single radio frequency phase gradient direction (such as axial or radial), which can only achieve one-dimensional spatial encoding. They rely on mechanical rotation or complex coil switching to achieve multi-dimensional imaging, resulting in low scanning efficiency and increased risk of motion artifacts.

[0006] Existing RF phase gradient coils are mostly based on empirical winding designs and lack rigorous electromagnetic field optimization. For example, the phase linearity of axial gradient coils is prone to distortion (error >10%) at the edge of the target region, while the field uniformity of radial gradient coils is limited by insufficient winding symmetry, resulting in image artifacts.

[0007] When multiple RF coils are used in combination, the mutual inductance between the coils introduces additional phase errors. Traditional decoupling methods (such as geometric orthogonality and capacitance compensation) have limited effectiveness under wide bandwidth conditions, especially in low-field systems where sensitivity decreases, further reducing the imaging signal-to-noise ratio.

[0008] To address the aforementioned issues, existing technologies attempt to improve performance by optimizing coil layout or introducing active shielding structures, but these often lead to increased coil size or a sharp increase in design complexity. Therefore, there is an urgent need for an RF coil design method that can balance multidimensional coding efficiency, phase gradient accuracy, and coil decoupling performance to promote the clinical application of low-field TRACE technology. Summary of the Invention

[0009] In view of this, the purpose of the present invention is to provide a radio frequency coil design method for spatial coding imaging of a transmitter array.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] A method for designing radio frequency coils for spatially coded imaging of a transmit array includes the following steps:

[0012] S1: Axial Encoding Coil Design: Based on the target field method and magnetic dipole method, within a radius of R... z Cylindrical surface design RF coil, R z Let be the radius of the cylindrical surface of the axial encoding coil, so that it generates a longitudinal radio frequency phase gradient field in the target region, wherein the Cartesian coordinate components of the target field satisfy:

[0013]

[0014] Among them, B X B Y B Z These represent the radio frequency magnetic field components along the X, Y, and Z axes in the Cartesian coordinate system, respectively; L represents half the length of the target region ROI along the Z axis; z and y are the coordinates of the target point along the Z and Y axes, respectively.

[0015] S2: Radial Encoding Coil Design: Based on the target field method and magnetic dipole method, in a radius of R... r Cylindrical surface design RF coil, R r Let the radius of the cylindrical surface of the radial encoding coil be such that it generates a transverse radio frequency phase gradient field in the target region, wherein the cylindrical coordinate components of the target field satisfy:

[0016]

[0017] Among them, B ρ B θ B z These represent the radial, circumferential, and axial radio frequency magnetic field components in cylindrical coordinates, respectively; μ0 represents the free permeability; I represents the coil input current; A represents the winding modulation amplitude of the radial encoding coil; h represents the coil turn spacing; R represents the radius of the coil cylinder; ρ represents the radial distance; θ represents the azimuth angle; and ψ represents the winding rotation direction angle.

[0018] S3: Coil Combination and Decoupling: The axial encoding coil and the radial encoding coil are combined and decoupled by a solenoid coil with reverse current in series, forming a two-dimensional radio frequency phase encoding system.

[0019] Furthermore, in S1, the radius R of the axial encoding coil z satisfy:

[0020] The design of the axial encoder coil includes: 0.8L≤R z ≤1.2L

[0021] The cylindrical surface is divided into Q mesh elements, with a flow function S q Describe the current distribution;

[0022] Solving the stream function S using the least squares method q The optimal distribution of B within the target region. Z The phase gradient linearity error of the component is less than 5%;

[0023] The minimum radius of curvature of the stream function contour lines is ≥2mm.

[0024] Furthermore, in S2, the radius R of the radial encoding coil r Satisfies: 6.0cm≤R r ≤7.0cm;

[0025] The optimized range of the winding modulation amplitude A is 4≤A≤7, and the turn spacing h≥3mm.

[0026] Furthermore, in S3, the decoupling design adopts any of the following methods:

[0027] Method 1: Connect a first solenoid coil in series at one end of the axial encoding coil and a second solenoid coil in series at one end of the radial encoding coil, with the current directions of the two solenoids being opposite.

[0028] Method 2: Connect a third solenoid coil with current in the same direction in series at both ends of the axial encoding coil, and connect a fourth solenoid coil with current in the same direction in series at both ends of the radial encoding coil, with the current directions of the third and fourth solenoids being opposite.

[0029] Furthermore, in S1 and S2, the optimization of the target field is based on a mathematical model:

[0030]

[0031] The constraints are:

[0032]

[0033] Among them, Q c S is the number of circumferential meshes on the cylindrical surface. q This is the unit stream function.

[0034] Furthermore, the target area has dimensions of 8cm × 4cm × 4cm, and the axial encoding coil radius R z = 8.5cm, radial encoding coil radius R r =6.6cm.

[0035] Furthermore, the method further includes:

[0036] The combined radio frequency coil is integrated with a saddle coil, which is used to transmit 90° excitation pulses and receive magnetic resonance signals;

[0037] The axial and radial encoding coils alternately emit a 180° refocusing pulse sequence to form a spin echo code.

[0038] Furthermore, the refocusing pulse sequence is: (CB) 2 -(CTS1) 3 -C, where:

[0039] C represents the saddle-shaped coil, TS1 represents the axial encoding coil, and B represents the radial encoding coil; the encoding direction is K. c -K TS1 With K c -K B The orthogonal locus.

[0040] Furthermore, the operating field strength of the radio frequency coil is 0.2T to 1.0T, which is suitable for ultra-low field magnetic resonance imaging of wrist joints or brain tissues.

[0041] A radio frequency coil system designed to implement the method described above includes:

[0042] Saddle-shaped coil module C: Arranged along the Z-axis, used to transmit 90° excitation pulses and receive magnetic resonance echo signals;

[0043] Axial phase gradient module CTS1: Composed of the axial encoding coil, it is coaxially sleeved along the axial direction of the saddle coil, i.e. the Z-axis, and is used to generate a Z-axis linear phase gradient.

[0044] Radial phase gradient module B: Composed of the radial encoding coil, orthogonally nested along the radial direction of the saddle-shaped coil, i.e. the Y-axis, to generate a linear phase gradient along the Y-axis;

[0045] Decoupling module: includes a first anti-solenoid group connected in series across both ends of the axial encoder coil, and a second anti-solenoid group connected in series across both ends of the radial encoder coil, wherein:

[0046] The current direction of the first reverse solenoid group is opposite to the driving current of the axial encoder coil;

[0047] The current direction of the second reverse solenoid assembly is opposite to the driving current of the radial encoder coil;

[0048] The RF output terminal of the saddle-shaped coil module is connected to the axial / radial phase gradient module through the first switching circuit;

[0049] The axial encoding coil and the radial encoding coil are alternately turned on by a second switching circuit to switch the transmission in a 180° refocusing pulse sequence;

[0050] The signal receiving end of the saddle coil module is independently connected to the data acquisition unit of the magnetic resonance imaging system.

[0051] The beneficial effects of this invention are as follows:

[0052] (1) By combining axial and radial radio frequency coils, this invention achieves two-dimensional spatial coding without mechanical rotation or complex switching, which significantly improves imaging efficiency and reduces image distortion caused by motion artifacts, enabling low-field magnetic resonance imaging to adapt to dynamic monitoring scenarios.

[0053] (2) Based on the target field method and magnetic dipole method, the coil optimization design ensures that the phase gradient of the radio frequency magnetic field in the target area is strictly linear and uniformly distributed, effectively overcoming the edge distortion problem of traditional coils and providing a precise phase coding basis for high-resolution imaging.

[0054] (3) The unique reverse current solenoid decoupling design greatly reduces the mutual inductance coupling effect between multiple coils, enabling the axial and radial encoding coils to work together without interfering with each other, ensuring the stability and reliability of signal acquisition under complex sequences.

[0055] (4) Abandoning the traditional gradient coil system, the system adopts a compact radio frequency coil array to achieve spatial coding, which significantly reduces the system size, weight and power consumption, making it particularly suitable for portable devices and mobile medical scenarios, while reducing hardware manufacturing costs.

[0056] (5) Radio frequency phase gradient coding has low dependence on the uniformity of the main magnetic field and can work stably in highly non-uniform magnetic field environments, expanding the application range of low field magnetic resonance imaging in complex clinical scenarios, such as bedside monitoring or intraoperative real-time imaging.

[0057] (6) The modular design of axial, radial, and saddle-shaped coils supports rapid adaptation to different anatomical sites. By adjusting coil parameters or combinations, it can be flexibly extended to three-dimensional coding or high-contrast imaging of specific tissues, improving the versatility of the technology.

[0058] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0059] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0060] Figure 1 This is a schematic diagram of the vector map of each component of the encoding coil along the axial (Z) axis in Case 1, along with the coil winding diagram. Figure 1 (a)~ Figure 1 (d) shows the vector distribution diagrams of Bx, By, Bz, and Btarget, respectively. Figure 1 (e) and Figure 1 (f) are the axially encoded stream function contour plots and 3D winding plots designed by the target field method and the magnetic dipole method, respectively;

[0061] Figure 2 This is a schematic diagram of the axially encoded transmit array spatial coding coil group described in Case 1. Figure 2 (a) is an axially encoded radio frequency phase gradient coil. Figure 2 (b) is a diagram showing the distribution of the Bz component in the plane where the coil is located when y = 3cm in the ZX plane. Figure 2 (c) shows the distribution of the Bz component, with the arrows indicating the directions of the magnetic field components;

[0062] Figure 3 The image shows the simulation results of the radial radio frequency coil (TS1) described in Case 1 at y = 4cm, 0cm, and -4cm in the yz plane. Figure 3 (a)~ Figure 3 (c) shows the phase diagram, phase gradient diagram, and B1 magnitude diagram of the coil simulation, respectively. Figure 3 (d) represents the actual phase, ideal phase, and linearity at y = 0 cm;

[0063] Figure 4 This is a schematic diagram of the vector map of each component of the encoding coil along the radial (Y-axis) axis in Case 2, along with the coil winding diagram. Figure 4 (a)~ Figure 4 (d) shows the vector distribution diagrams of Bx, By, Bz, and Btarget, respectively. Figure 4 (e) and Figure 4 (f) shows the radially encoded stream function contour plots and 3D winding plots designed by the target field method and the magnetic dipole method, respectively;

[0064] Figure 5 This is a schematic diagram of the radially encoded transmit array spatial coding coil group described in Case 2. Figure 5 (a) Radio frequency phase gradient coil encoded radially; Figure 5 (b) Distribution of Bx components in the plane where X = 3cm in the ZY plane; Figure 5 (c) is the distribution diagram of Bx components, with arrows indicating the directions of magnetic field components;

[0065] Figure 6 The decoupling model diagram shows a decoupling model in which regular solenoid coils are added to one and both ends of the axial and radial RF coils, respectively. Figure 6 (a)~ Figure 6 (d) are model diagrams of TS1+RS1, TS2+RS2, RS1′+TS1+RS1 and RS2′+TS2+RS2 respectively, with arrows indicating the direction of current;

[0066] Figure 7 The parameter diagram for decoupling coil S11 is shown below. Figure 7 (a)~ Figure 7 (d) represents the S11 parameters in the CST simulation for TS1+RS1, TS2+RS2, RS1′+TS1+RS1 and RS2′+TS2+RS2, respectively.

[0067] Figure 8 A schematic diagram of the K-space trajectory for two-dimensional ultra-low field magnetic resonance imaging using a spatial coding coil group of the transmitter array is shown. After 2D K-space trajectory excitation using a saddle coil C, a refocusing sequence (CB)2-(CA)3-C is executed using orthogonal coding directions (Kc-Ka) and (Kc-Kb). Detailed Implementation

[0068] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0069] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0070] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0071] Please see Figures 1-3 This is a schematic diagram of an axially encoded radio frequency phase coil according to Example 1 of the present invention, as shown below. Figure 1 As shown, the method for designing this coil includes: using the target field method and the magnetic dipole method to design the general structure of the coil, and continuously optimizing each parameter to obtain the optimal structure of the coil. The design consists of the following four steps:

[0072] (1) The method for the radio frequency phase encoding coil along the Z-axis in Case 1 is as follows: The current-carrying elliptical cylindrical surface is divided into several current-carrying rings. Each arc-shaped current ring can be regarded as a magnetic dipole. The magnetic field in the target region ROI (Region of Interest) is formed by the superposition of the magnetic fields generated by these unit magnetic dipoles. If the surface of any coil is divided into Q grid units with side length a, when a is sufficiently small, the flow function in element q can be approximated by the constant Sq, and the surface of the element can be approximated by a. 2 Let q be the source point. The element q at the source point will generate a magnetic field at a point in the target region, which is equivalent to the magnetic field of a magnetic dipole.

[0073] (2) In Case 1, the Bx, By, and Bz components generated by the desired magnetic field are superimposed to obtain Btarget. The Btarget vector diagram produces a uniform axial phase change along the Z-axis. The coil is constrained to a cylindrical surface using the target field method and the magnetic dipole method. The following longitudinal phase gradient target field is set in the target region:

[0074]

[0075] In the formula, B X B Y and B z These represent the magnetic field components along the X, Y, and Z axes, respectively. Btarget is the matrix form of the desired magnetic field. The vector diagrams for each component are shown below. Figure 1 (a)~ Figure 1 As shown in (d), the value of L can be modified to determine the size of the region of interest (ROI) based on the size of the human wrist joint.

[0076] (3) The radius of the coil parameters is set to 8cm to 10m, with a step size of 0.5cm. By evaluating the winding distribution of each generated coil, sharp turns and coils that are too small are avoided. A coil with a radius of 8.5cm is selected as the optimal radius. The target area is set as a cuboid of 8cm×4cm×4cm based on the size of an adult's wrist joint. Figure 1 (e) and Figure 1 (f) shows the stream function contour plot and three-dimensional winding diagram of an 8.5cm coil, respectively;

[0077] (4) The data calculated in MATLAB, with one column representing the Z (m) coordinate and the other column representing Φ (rad), is processed using the formulas x = ρcosθ and y = ρsinθ to obtain the x, y, and z coordinates. This data is then imported into COMSOL Multiphysics to design a coil for electromagnetic field simulation, such as... Figure 2 (a)~ Figure 2 (c) In Case 1, the coil encoding along the axial direction changes the magnetic field component Bz along the z-axis from negative to positive, while the magnetic field component Bx along the x-axis remains essentially unchanged. The phase of B1 can be determined by... Calculations show that φ is the radio frequency phase and B1 is the amplitude. |B1| represents the radio frequency field, and the coil encoding along the z-axis achieves the desired encoding. The gradient linearity formula is as follows:

[0078]

[0079] In the formula, Δφ is the phase difference (the simulated or measured phase versus the ideal phase φ). target The maximum difference between ) where the ideal phase is determined by the following formula:

[0080]

[0081] In the formula, L is half the length of the ROI in the z direction. To accurately evaluate the phase gradient, a second-order polynomial and formula are fitted to the original Bx and Bz measurements, and the formula is used as the B1 phase gradient:

[0082]

[0083] In the formula, the phase gradient of B1 is evaluated at some equidistant points on the x-axis, and Δx is used to evaluate the phase gradient of B1. i Defined as the selected x-interval. For simulation parameters including phase, phase gradient, magnitude, and phase linearity, please refer to [link to relevant documentation]. Figure 3 (a)~ Figure 3 (d)

[0084] Figures 4-5 This is a schematic diagram of the radially encoded radio frequency phase coil of Example 2 of the present invention, as shown below. Figure 4 As shown, the method for designing this coil includes: using the target field method and the magnetic dipole method to design the general structure of the coil, and continuously optimizing the parameters such as the number of turns, radius, modulation amplitude, and turn spacing to obtain the optimal structure of the coil. The design consists of the following four steps:

[0085] (1) Case 2 uses the target field method and magnetic dipole method to design a coil constrained to a cylindrical surface, and sets the following transverse phase gradient target field in the target region:

[0086]

[0087] B ρ B is the radial component. θ B is the circumferential component. z For the axial components, define Cartesian coordinates; (x,y,z)=(ρcosθ,ρsinθ,z), and convert these field components to Cartesian coordinates:

[0088] B X =-B θ sinθ+B ρ cosθ

[0089] B y =+B θ cosθ+B ρ sinθ

[0090]

[0091] For the chosen coil rotation direction (ψ = 0), the B1 phase gradient occurs only in the direction perpendicular to the x-axis (i.e., the y-axis), because the Bz component is uniform, and By varies only as a function of x. It can also be expressed as follows:

[0092]

[0093] Using the trigonometric identities x = ρ·cosθ and ψ = 0, we can simplify to:

[0094]

[0095] Written in consistent matrix form:

[0096]

[0097] The Bx, By, and Bz components generated by the desired magnetic field are superimposed to obtain Btarget. The Btarget vector diagram exhibits a uniform lateral phase change along the Y-axis, as shown below. Figure 4 (a)~ Figure 4 As shown in (d);

[0098] (2) The target region is set as a cuboid of 8cm×4cm×4cm based on the size of an adult's wrist joint. Using the target field method and the equivalent magnetic dipole method, with the goal of minimizing energy loss, the energy consumption expression for all magnetic dipole units is derived. Within the target region, the magnetic fields generated by the coils are required to be orthogonal to each other on the X and Y axes and as close as possible to the desired value. Based on this, a mathematical model is constructed:

[0099]

[0100] (3) The optimal distribution of the stream function on the cylindrical surface is solved by the least squares method. After obtaining the optimal value of the stream function Sq, the contour lines of the stream function are drawn to obtain the coil structure. The optimal values ​​of the parameters of the radio frequency coil in Case 2 are obtained by parametric scanning: A = 5.5, I = 3A, h = 0.3cm, θ = 45°, where A is the coil modulation amplitude. After optimization, it was found that the coil shape is better when A = 5.5; I is the input current. The range of I in this coil is suitable from 1 to 3A; h is the distance between two turns of the coil. This distance cannot be too large, which will cause the coil to be not compact enough and affect its uniformity. It cannot be too small either, which will cause the coils to be too close to each other and affect the impedance matching. h ≥ 3mm is most suitable; the value of θ is a cylindrical coordinate that varies between -Nπ and Nπ, where N represents the number of coil turns; Φ represents the winding displacement. The coil parameters were set with a radius range of 6.0cm-6.8cm and a step size of 0.2cm. The optimal radius obtained from the scan was 6.6cm. The target area was set as a cuboid of 8cm×4cm×4cm based on the size of an adult's wrist joint. Figure 4 (e) and Figure 4 (f) shows the stream function contour plot and 3D winding diagram of a 6.6cm coil.

[0101] (4) The data from the MATLAB run, with one column representing the Z (m) coordinate and the other column representing Φ (rad), is processed using the formulas x = ρcosθ and y = ρsinθ to obtain the x, y, and z coordinates. This data is then imported into COMSOL Multiphysics to design a coil for electromagnetic field simulation. The simulated coil is shown below. Figure 5 (a)~ Figure 5 As shown in (c), in Case 2, the radially encoded coil with a radius of 6.6 cm has a magnetic field component along the y-axis that changes from negative to positive, and the magnetic field distribution of the Bx component achieves the expected obvious symmetry on both the left and right sides.

[0102] Please see Figures 6-7 The decoupling model diagram and S11 parameter diagram are provided for adding regular solenoid coils to one end and both ends of the axial RF coil in Case 1 and the radial RF coil in Case 2, respectively. When combining them, a decoupling design is considered, that is, adding a conventional solenoid coil segment in series on each of the two solenoid coil segments, with the current directions of the two series-connected solenoid coil segments being opposite. The specific decoupling method is as follows:

[0103] Method 1: Connect a conventional solenoid coil (RS1) in series at one end of the axial RF coil (TS1), and connect a conventional solenoid coil (RS2) in series at one end of the radial RF coil (TS2). The current flows in opposite directions in RS1 and RS2.

[0104] Method 2: Connect a conventional solenoid coil (RS1 and RS1') in series at both ends of the axial RF coil (TS1), and connect a conventional solenoid coil (RS2 and RS2') in series at both ends of the radial RF coil (TS2). The current flows in the same direction in RS1 and RS1', and the current flows in the same direction in RS2 and RS2', but the current flows in opposite directions in RS1 and RS2.

[0105] The modeling formula for a conventional solenoid is x = rcos(2π*s), y = rsin(2π*s), z = pitch*s, s∈[0,N] trunsIn COMSOL Multiphysics, after parametrically scanning the number of turns (N_turns) and pitch of the added conventional solenoid coil, it was determined that N_turns = 4 and pitch = 3 cm resulted in the best simulation effect. The created decoupling model was imported into CST to calculate the S11 parameters. The simulation results showed that both methods one and two achieved decoupling design for the coil. TRASE, as an independent encoding mechanism, can be used in highly non-uniform B0 fields without requiring the B1+ phase gradient to dominate the change in B0. Unlike B0 gradient coils, RF coils are physically lightweight, easy to handle, and can be customized into arrays for each anatomical situation because smaller coils are more efficient. A basic TRASE imaging pulse sequence consists of an initial 90° excitation pulse used for moving magnetization. It initially reaches equilibrium along the z-axis, then enters the transverse X, Y plane, generating its own radio frequency field, followed by a series of equally spaced 180° refocusing pulses (spin echo sequence). The refocusing pulses in the echo sequence are applied alternately by coils A and B, and a magnetic resonance signal can be observed after each refocusing pulse. Two-dimensional radio frequency phase-coded imaging is achieved using a combination of two radio frequency coils encoded along different orthogonal directions. This consists of axial and radial encoding radio frequency coils and a saddle-shaped coil that generates uniform phase. A 90° pulse excitation is emitted from the saddle-shaped coil, followed by alternating 180° refocusing pulse sequences emitted from the axial and radial radio frequency coils. The magnetic resonance echo signal is received at the saddle-shaped coil, thus achieving two-dimensional imaging encoding in k-space. After 2D k-space trajectory excitation using the saddle-shaped coil C, orthogonal encoding directions K... c -K TS1 and K c -K B Execute the refocusing sequence (CB). 2 -(CTS1) 3 -C, where coil A represents the axial RF coil TS1 of Case 1, coil B represents the axial RF coil TS2 of Case 2, and coil C represents the saddle-shaped coil that generates a uniform phase. Figure 8 A schematic diagram of the K-space trajectory for two-dimensional ultra-low field magnetic resonance imaging using a spatial coding coil group for the transmitting array.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for designing radio frequency coils for spatial coding imaging of a transmit array, characterized in that: Includes the following steps: S1: Axial Encoding Coil Design: Based on the target field method and magnetic dipole method, with a radius of... R z Cylindrical surface design for RF coils, R z Let be the radius of the cylindrical surface of the axial encoding coil, so that it generates a longitudinal radio frequency phase gradient field in the target region, wherein the Cartesian coordinate components of the target field satisfy: in, , , These represent the radio frequency magnetic field components along the X, Y, and Z axes in the Cartesian coordinate system, respectively. L This represents half the length of the target region (ROI) along the Z-axis. z , y These are the coordinates of the target point along the Z-axis and Y-axis, respectively; S2: Radial Encoding Coil Design: Based on the target field method and magnetic dipole method, with a radius of... R r Cylindrical surface design for RF coils, R r Let the radius of the cylindrical surface of the radial encoding coil be such that it generates a transverse radio frequency phase gradient field in the target region, wherein the cylindrical coordinate components of the target field satisfy: in, , , These represent the radial, circumferential, and axial radio frequency magnetic field components in cylindrical coordinates, respectively. Indicates the permeability of free space; Indicates the coil input current; This indicates the winding modulation amplitude of the radial encoding coil; Indicates the coil turn spacing; Indicates the radius of the cylindrical surface of the coil; Indicates radial distance; Indicates azimuth; Indicates the direction angle of rotation around the wire; In S1 and S2, the optimization of the target field is based on a mathematical model: The constraints are: in, Q c The number of grid cells in the circumferential direction of the cylindrical surface. S q For cell flow functions; S3: Coil Combination and Decoupling: The axial encoding coil and the radial encoding coil are combined and decoupled by a solenoid coil with reverse current in series, forming a two-dimensional radio frequency phase encoding system.

2. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: In S1, the radius of the axial encoding coil R z satisfy: The design of the axial encoder coil includes: 0.8 L ≤ R z ≤1.2 L The cylindrical surface is divided into Q Each grid cell, with a flow function S q Describe the current distribution; Solving the flow function using the least squares method S q The optimal distribution within the target area B Z The phase gradient linearity error of the component is less than 5%; The minimum radius of curvature of the stream function contour lines is ≥2 mm.

3. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: In S2, the radius of the radial encoding coil R r Satisfies: 6.0cm≤ R r ≤7.0cm; Wire-wound modulation amplitude A The optimization range is 4≤ A ≤7, turn spacing h ≥3mm.

4. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: In S3, the decoupling design adopts any of the following methods: Method 1: Connect a first solenoid coil in series at one end of the axial encoding coil and a second solenoid coil in series at one end of the radial encoding coil, with the currents in the two solenoids in opposite directions; Method 2: Connect a third solenoid coil with current in the same direction in series at both ends of the axial encoding coil, and connect a fourth solenoid coil with current in the same direction in series at both ends of the radial encoding coil, with the current directions of the third and fourth solenoids being opposite.

5. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: The target area has dimensions of 8cm × 4cm × 4cm, and the axial encoding coil radius is... R z =8.5cm, radial encoding coil radius R r =6.6cm.

6. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: The method further includes: The combined radio frequency coil is integrated with a saddle coil, which is used to transmit 90° excitation pulses and receive magnetic resonance signals; The axial and radial encoding coils alternately emit a 180° refocusing pulse sequence to form a spin echo code.

7. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 6, characterized in that: The refocusing pulse sequence is as follows: ,in: C Indicates a saddle-shaped coil. B represents the axial encoding coil, and B represents the radial encoding coil; the encoding direction is... and The orthogonal locus.

8. The radio frequency coil design method for spatial coding imaging of a transmit array according to claim 1, characterized in that: The operating field strength of the radio frequency coil is 0.2T~1.0T, which is suitable for ultra-low field magnetic resonance imaging of wrist joints or brain tissue.

9. A radio frequency coil system designed according to any one of claims 1 to 8, characterized in that: include: Saddle-shaped coil module C: Arranged along the Z-axis, used to transmit 90° excitation pulses and receive magnetic resonance echo signals; Axial phase gradient module CTS1: Composed of the axial encoding coil, it is coaxially sleeved along the axial direction of the saddle coil, i.e. the Z-axis, and is used to generate a Z-axis linear phase gradient. Radial phase gradient module B: Composed of the radial encoding coil, orthogonally nested along the radial direction of the saddle-shaped coil, i.e. the Y-axis, to generate a linear phase gradient along the Y-axis; Decoupling module: includes a first anti-solenoid group connected in series across both ends of the axial encoder coil, and a second anti-solenoid group connected in series across both ends of the radial encoder coil, wherein: The current direction of the first reverse solenoid group is opposite to the driving current of the axial encoder coil; The current direction of the second reverse solenoid assembly is opposite to the driving current of the radial encoder coil; The RF output terminal of the saddle-shaped coil module is connected to the axial / radial phase gradient module through the first switching circuit; The axial encoding coil and the radial encoding coil are alternately turned on by a second switching circuit to switch the transmission in a 180° refocusing pulse sequence; The signal receiving end of the saddle coil module is independently connected to the data acquisition unit of the magnetic resonance imaging system.

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