A magnetic resonance imaging contactless balun and a method for determining a matching capacitance thereof and applications

Abstract

This invention discloses a contactless balun for magnetic resonance imaging and its matching capacitance determination method and application. First, a contactless balun model is established, and the value of the matching capacitance is calculated to make the balun resonant frequency consistent with the magnetic resonance signal frequency. Second, considering the skin effect, the matching capacitance value calculated by the model is corrected by electromagnetic simulation. This invention also proposes a method for manufacturing and using the contactless balun, which can further improve the common-mode suppression effect.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging technology, specifically to a non-contact balun (common-mode suppressor) for magnetic resonance imaging and a method for determining its matching capacitance value, as well as its application. Background Technology

[0002] Magnetic resonance imaging (MRI) is an advanced imaging technique that encompasses structural imaging, functional imaging, and local detection of chemical substances, and is widely used in the diagnosis of diseases inside the human body. Radio frequency (RF) coils are a crucial component of an MRI system, installed inside the magnet. The transmitting coil generates a uniform radio frequency field, while the receiving coil acquires the magnetic resonance signal from the sample. The performance of the RF coil directly determines the quality of the MRI image.

[0003] Coaxial transmission lines are typically used to transmit signals between the radio frequency (RF) coil and the spectrometer. During the RF coil's transmission in a magnetic resonance imaging (MRI) system, the high input power generates a strong alternating magnetic field inside the system's magnet. Since the transmission line is located inside the magnet, this alternating magnetic field induces a common-mode current in the coaxial transmission line's shielding layer, which is in the same direction as the current in the inner conductor. This affects coil tuning, causing inhomogeneity distortion and a decrease in the signal-to-noise ratio in MRI imaging. More seriously, it can lead to severe burns to the patient.

[0004] To address the common-mode current suppression issue in coaxial transmission line shielding, contact baluns can be used. Baluns are typically made by winding coaxial cable into a solenoid shape. By welding a capacitor between the cable and the coaxial shielding, the resonant frequency can be adjusted to the magnetic resonance signal frequency. At the resonant frequency, an impedance of several thousand ohms can be generated, effectively suppressing common-mode current. However, the disadvantages are that contact baluns are usually complex to manufacture, have a fixed and immovable position, and limited applicability.

[0005] A team led by DASEEBER published a paper in the journal "Floating Shield Current Suppression Trap" using a contactless balun (floating trap) to suppress common-mode current. The contactless balun consists of two coaxial cylindrical conductors with a matching capacitor welded between them, utilizing the mutual inductive coupling impedance between the shielding layer and the suppressor to suppress common-mode current. Chinese patent CN213069147U proposes a novel balun assembly where the outer conductive tube is detachable and includes a connected heat dissipation structure. Based on the principles of nuclear magnetic resonance, in addition to traditional... 1 Outside the H nucleus, a large number of spin nuclei, such as 39 K, 23 Na、 19 F, and 31 P and other particles can also be used for magnetic resonance imaging. The main magnetic field strength of different magnetic resonance imaging (MRI) devices varies.1 The resonance frequencies of H nuclei are not uniform, and the resonance frequencies of different spin nuclei are also different. Therefore, it is often necessary to fabricate baluns with different operating frequencies. However, in practical applications, there is no fast and accurate method for determining the matching capacitance value for frequency tuning. Current methods rely on trial and error to obtain a suitable balun matching capacitance value, which is time-consuming and difficult to achieve optimal suppression. Therefore, there is an urgent need to design a method for determining the non-contact balun matching capacitance value for magnetic resonance imaging. Summary of the Invention

[0006] This invention proposes a design method for determining the matching capacitance value of a contactless balun for magnetic resonance imaging (MRI). First, a model of the contactless balun (common-mode suppressor) is established, and the value of the matching capacitance is calculated to ensure that the balun resonant frequency matches the frequency of the MRI signal. Second, considering the skin effect, the matching capacitance value calculated by the model is corrected. This invention also proposes a method for fabricating and using the balun, which can further improve the common-mode suppression effect.

[0007] The technical solution of the present invention:

[0008] A method for determining the matching capacitance of a contactless balun in magnetic resonance imaging includes the following steps:

[0009] 1) The matching capacitance value of the contactless balun is obtained through modeling and solving; specifically:

[0010] 11) The contactless balun is modeled as a closed-loop circuit consisting of a matching capacitor, inner and outer coaxial cylindrical conductors, and a bottom conductor; the structure of the contactless balun includes two coaxial cylindrical conductors, a bottom conductor connecting the two coaxial cylindrical conductors, and a matching capacitor; and the matching capacitor is used as a series capacitor-resistor element; the transmission line is coupled to the balun through electromagnetic induction.

[0011] 12) Calculate the mutual inductance between the transmission line and the balun; calculate the self-inductance and self-capacitance between the coaxial cylindrical conductors; calculate the resistance of the inner and outer coaxial cylindrical conductors; calculate the resistance R of the bottom conductor. 底 ;

[0012] The main structure of a contactless balun consists of two concentric (coaxial) cylindrical conductors. Therefore, it can be considered a rectangular closed loop along the cross-section of the transmission line. When current flows through the transmission line, a magnetic field is generated around it. The integral of the magnetic field within the rectangular area is the magnetic flux through the balun. From this, the mutual inductance between the transmission line and the balun can be calculated. The self-inductance and self-capacitance between the coaxial cylindrical conductors can be calculated using formulas. Considering the skin effect at high frequencies, the resistances of the inner and outer coaxial cylindrical conductors can also be obtained using resistance formulas. In addition to the two coaxial cylindrical copper conductors, the balun also includes a bottom conductor connecting the two cylindrical conductors and a matching capacitor. For the bottom conductor, the inductance and capacitance have relatively small effects, so only its resistance is considered. Similarly, considering the skin effect, the resistance of the bottom conductor is calculated using integration. The matching capacitor has parasitic resistance, and resistance has a significant impact on the balun's effect; therefore, the matching capacitor should be treated as a series capacitor-resistor element when building the model. The balun model is a closed-loop circuit composed of the matching capacitor, the inner and outer concentric cylindrical conductors, and the bottom conductor. The team led by Gregory H. Griffin published a paper in the journal "Miniaturizing Floating Traps to Increase RF Safety of Magnetic-Resonance-Guided Percutaneous Procedures" which analyzed the modeling of contactless baluns. In order to simplify the model, the influence of the bottom conductor was ignored. However, the model proposed in this invention takes into account the non-negligible role of the bottom conductor in the coupling impedance.

[0013] In this invention, the self-resistance R of the conductor portion t The sum of the resistances, which can be divided into three parts—inner and outer conductors and bottom conductor—is expressed as:

[0014] R t =R 内 +R 外 +R 底

[0015] Where R 内 R 外 and R 底 Let represent the resistances of the inner and outer conductors of the concentric cylinder and the annular conductor at one end (the bottom conductor), respectively. According to the definition of resistance, the formula for calculating the resistance value is: Where l is the conductor length, σ is the conductor conductivity, and A is the conductor cross-sectional area. At high frequencies, the skin effect causes non-uniform current density across the conductor cross-section, with the current distribution concentrated on the conductor surface. The formula for calculating the skin depth is: Where σ is the conductor conductivity. The current direction is along the length of the balun. When the skin depth δ is much smaller than the conductor thickness, the current flows through the conductor's cross-sectional area A≈2π·a·δ. The resistance of the conductor inside the cylinder is expressed as:

[0016]

[0017] Where ρ is resistivity;

[0018] The resistance of a cylindrical outer conductor is expressed as:

[0019]

[0020] The resistance of the conductor at the bottom of the balun can be calculated using calculus:

[0021]

[0022] dR represents the differential resistance of the bottom conductor, R 底 Indicates the total resistance of the bottom conductor;

[0023] Wherein, the current path is between the inner and outer circles, the current I is in the radial direction, and dr represents the differentiation of the circular conductor in the radial direction;

[0024] Integrating over the radius r, we can obtain the total resistance of the circular conductor as:

[0025]

[0026] R 底 In actual calculations, the self-resistance R of the conductor portion accounts for approximately t Five percent of this is not negligible, and its value will affect the coupling impedance Z. coupled This can affect the assessment of the inhibitory effect of balen.

[0027]

[0028] The matching capacitance value c is obtained by solving the matching capacitance formula, and is expressed as: in Matching resistors affect coupling impedance, which in turn affects balun suppression performance.

[0029] 2) Use the transmission line model to analyze the transmission line with balun and calculate the suppression effect of balun;

[0030] The transmission line and balun are coupled via electromagnetic induction. Using the analysis method of transformer primary and secondary impedance changes, the transmission line is considered the primary side and the balun the secondary side, allowing calculation of the equivalent impedance from the secondary side to the primary side. After coupling the balun's impedance to the transmission line, the transmission line impedance changes within the balun. This invention uses a transmission line model to analyze transmission lines with baluns. This model divides the transmission line into three different parts, each corresponding to a different transmission line impedance. The transmission matrix of each part is analyzed, and then the suppression effect of the balun is calculated based on the relationship between the transmission matrix and scattering parameters.

[0031] The transmission coefficients calculated from the transmission matrix of the transmission line are expressed as:

[0032]

[0033] S 21 =S 21_1 ×S 21_2 ×S 21_3

[0034] Among them, S 21_1 S is the transmission coefficient of the first transmission line segment. 21_2 S is the transmission coefficient of the second transmission line segment. 21_3 S is the transmission coefficient of the third transmission line. 21 This represents the total transmission coefficient of the transmission line;

[0035] From this, the transmission characteristic S of the contactless balun transmission line can be calculated. 21 Variation with frequency.

[0036] 3) Determine the suppression effect of the balun based on the transmission characteristics of the contactless balun transmission line;

[0037] 4) Correct the balun matching capacitance value obtained in step 1); determine the balun matching capacitance value through electromagnetic simulation.

[0038] This invention provides a method for correcting the matching capacitance value calculated from a model based on electromagnetic simulation software (HFSS). The specific steps of this method are as follows:

[0039] This invention uses the electromagnetic simulation software HFSS to perform a 1:1 modeling and simulation of the designed contactless balun, and calculates the transmission coefficient S of the transmission line with the balun. 21 Meanwhile, the balun matching capacitance value obtained by constructing the model and calculating it in step 1) is used as the initial value to perform parameterized calculation on the balun matching capacitance, thereby obtaining the corrected balun matching capacitance value.

[0040] Based on the matching capacitance value obtained from the model analysis, a balun transmission line model was established using HFSS simulation software according to the actual structural parameters of the suppressor. Considering the skin effect, the finite element method was used for analysis to obtain the corrected matching capacitance value. Specific implementation steps include:

[0041] 41) Run HFSS and create a new project. Set the solution type to mode-driven solution type and set the default length unit used when creating the model for the current design.

[0042] 42) Define design variables and add initial values, including: balun inner radius, balun outer radius, balun length, transmission line length, and matching capacitance.

[0043] 43) Create a transmission line model with a balun, set the port excitation to wave port excitation, apply the excitation to the transmission line shield during the simulation, and observe the balun's suppression effect on common-mode current; create a cylinder with a radiating boundary and set its boundary conditions to radiating boundary conditions.

[0044] 44) Select the lumped parameter to set the matching capacitor. The matching capacitor is placed along the circumference at one end of the balun (the other end is the bottom conductor). The initial value is the matching capacitor value calculated in step 1).

[0045] 45) Solver settings;

[0046] In practice, to analyze the case with a magnetic field strength of 1.5T, the solution frequency was set to 64MHz, the maximum number of iterations for adaptive mesh generation was 20, the convergence error was 0.02, and a frequency sweep setting of 50MHz to 100MHz was added. The frequency sweep type was selected as Fast to analyze the transmission coefficient of the transmission line at 64MHz.

[0047] 46) Set the parameterized scan for the matching capacitor c and tune it to the solution frequency (64MHz).

[0048] 47) Design checks and run simulation calculations to obtain the corrected matching capacitor value;

[0049] 48) Post-processing of HFSS balun data to derive transmission coefficient diagrams of transmission lines, electromagnetic field distribution on the balun surface, etc.

[0050] This invention also provides a method for determining the matching capacitance of a contactless balun in magnetic resonance imaging to design and fabricate a physical balun. The specific steps of the design method are as follows:

[0051] After determining the structural parameters (inner radius, outer radius, and length) of the contactless balun, Teflon was selected as the filling medium, and the shape was a hollow cylinder. Copper foil was attached to the inner and outer layers of the hollow cylinder as concentric cylindrical conductors. One end of the hollow cylindrical dielectric used copper foil as the connection between the inner and outer conductors, and a matching capacitor was soldered between the inner and outer conductors at the other end. The specific capacitance value was the matching capacitance value obtained using the method described above, forming a closed loop. The transmission line impedance and transmission coefficient S with the balun are then determined. 21 The frequency of the balun can be obtained by measuring with a network analyzer and based on the transmission coefficient.

[0052] This invention also proposes a method for using baluns, in which two baluns are placed at the radio frequency (RF) coil and at a distance of one-quarter wavelength from the RF coil in a magnetic resonance imaging system, respectively. The wavelength is determined according to the operating frequency of the magnetic resonance imaging. Compared with placing the baluns at the coil alone, the method of this invention can effectively improve the signal-to-noise ratio of magnetic resonance imaging.

[0053] The technical solution of this invention has the following advantages:

[0054] 1. The contactless balun modeling method and transmission line model-based analysis method provided by this invention first analyze the electromagnetic induction relationship between the balun and the transmission line, establish a corresponding physical model, calculate the balun impedance value, and quickly obtain the preliminary matching capacitance value. Then, the balun circuit model is coupled to the transmission line through impedance transformation, and the transmission line model is used to analyze the transmission line with the balun to verify whether the balun with the current structural parameters can meet the suppression requirements.

[0055] 2. This invention provides a method for correcting the matching capacitance value obtained from model analysis and calculation. The method uses the electromagnetic simulation software HFSS to simulate a transmission line with a balun. For a balun with determined structural parameters, the model-calculated matching capacitance value is used as the initial value. The transmission line impedance change, the resonant frequency of the balun, and the transmission coefficient S are then calculated. 21 The matching capacitance value calculated by the model is corrected, and the corrected matching capacitance value is more accurate, making the production of the balun faster and more efficient.

[0056] 3. This invention provides a physical design scheme for a balun and a method for using the balun. Teflon is used as the supporting medium between the inner and outer conductors. Copper foil is used as the concentric cylindrical conductor of the balun and the annular conductor for connecting the inner and outer conductors. Capacitors are welded to both ends of the balun. The capacitance value is a correction capacitance value. The manufacturing steps are simple, and a suitable matching capacitor can be quickly and accurately selected to manufacture a balun that meets the requirements.

[0057] 4. The balun usage method proposed in this invention involves placing two baluns at the coil and at a distance of one-quarter wavelength from the coil, which, compared with the traditional method of placing the balun at the coil, can effectively improve the signal-to-noise ratio of imaging. Attached Figure Description

[0058] Figure 1 Flowchart for determining the balun matching capacitance value;

[0059] Figure 2 A model analysis diagram of baluns and transmission lines;

[0060] Where a is the balun circuit model diagram; b is the coupling circuit model diagram between the balun and the transmission line.

[0061] Figure 3 A schematic diagram for calculating balun resistance;

[0062] Where a is the calculation diagram of the inner conductor resistance; b is the calculation diagram of the bottom conductor resistance of the balun.

[0063] Figure 4 A schematic diagram of the Baron model calculation;

[0064] Where a is the curve of coupling impedance versus frequency; b is the transmission coefficient S of the transmission line with balun. 21 Graph showing the variation with frequency.

[0065] Figure 5 The image shows a balun simulation based on HFSS software.

[0066] Where, a is a diagram of a balun transmission line model built using HFSS simulation software; b is a diagram of the simulation results of the transmission coefficient;

[0067] Figure 6 This is a schematic diagram of Barron's experiment;

[0068] Where a is the Baron experiment test diagram;

[0069] In the diagram: 1-coaxial cable; 2-outer conductor; 3-inner conductor; 4-dielectric layer; 5-matching capacitor; 6-bottom conductor used to connect the inner and outer conductors;

[0070] b is a graph showing the transmission coefficient test results; Figure 6 c shows the results of the coupling impedance test. Detailed Implementation

[0071] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments herein are only used to illustrate the implementation steps of the present invention, and are not intended to limit the specific scope of the present invention.

[0072] Figure 1 The flowchart for determining the matching capacitance value of the balun proposed in this scheme includes the following steps: 1- Input balun structure parameters: inner radius a, outer radius b, length l; 2- Establish balun model and solve for the matching capacitance value; 3- Calculate the balun suppression effect using transmission line model; 4- Determine whether the suppression effect meets the suppression requirements; 5- Correct the matching capacitance value based on HFSS software; 6- Fabricate a physical prototype based on the corrected matching capacitance value and balun structure parameters.

[0073] The first step in implementation is the selection of parameters for the contactless balun:

[0074] The structural parameters of the contactless balun designed in this invention are as follows: inner conductor radius a is 6 mm, outer conductor radius b is 15 mm, and length l is 74 mm.

[0075] Step two involves the process of modeling the contactless balun and solving for the matching capacitance value:

[0076] When current flows through the conductor of a transmission line, an electromagnetic induction occurs between the contactless balun and the transmission line, which can be equivalent to adding a series impedance to the transmission line. Figure 2'a' represents the balun circuit model, where the coaxial line is equivalent to a straight wire, and the suppressor is equivalent to a series circuit of an inductor, capacitor, and resistor (the sum of the resistances of the inner and outer conductors and the bottom conductor). L t For balun self-inductance, R in R out and R t C represents the resistance of the inner conductor, outer conductor, and bottom conductor of the balun, respectively. t Let c be the balun self-capacitance and c be the matching capacitor. The effect of the balun on the transmission line current can be calculated based on the circuit model diagram.

[0077] Figure 2 The magnitude of the current flowing through transmission line a is I, and the magnetic field strength distribution around the transmission line is as follows:

[0078]

[0079] Where B is the magnetic field strength; μ0 is the permeability; I is the current; and r is the radius.

[0080] The radii of the inner and outer conductors of the balun are a and b, respectively. The magnetic flux generated by the transmission line magnetic field within the conductor loop is... for:

[0081]

[0082] Where S is the magnetic flux area, φ is the magnetic flux, μ0 is the permeability, and l is the length.

[0083] At the same time, the balun also has its own inductance. Due to its special concentric cylindrical structure, self-inductance and mutual inductance have the same calculation formula. Self-inductance is expressed as:

[0084]

[0085] Where M represents mutual inductance; L t For balun self-inductance.

[0086] Mutual inductance is used for transmission and transformation in circuits. The coupling circuit model between a balun and a transmission line is as follows: Figure 2 As shown in Figure b, M is the mutual inductance between the balun and the transmission line, Z and Y are the distributed impedance and distributed admittance of the transmission line, ΔZ and ΔY are the changes in the distributed impedance and distributed admittance of the transmission line with the balun, and dl represents the unit length of the transmission line. Using an analysis method similar to the impedance transformation between the primary and secondary sides of a transformer, the transmission line is considered as the primary side and the balun as the secondary side. The impedance transformation relationship between the transmission line and the balun is derived, and the impedance of the secondary side balun circuit can be equivalently represented on the primary side transmission line.

[0087] Viewed from the primary side, the transmission line is a passive one-port network. According to the definition of equivalent impedance, the coupling impedance Z generated by the secondary-side balun to the primary-side transmission line is...coupled Represented as:

[0088]

[0089] Where ω is the frequency, Z t The total impedance of the balun is given by j, where j is the imaginary unit; the self-capacitance of the concentric cylindrical conductor is C. t The calculation formula is:

[0090]

[0091] Where ε is the dielectric constant.

[0092] Self-resistance R of the conductor section t It can be divided into the sum of the resistance of the inner and outer conductors and the total resistance of the bottom conductor:

[0093] R t =R 内 +R 外 +R 底

[0094] Where R 内 R 外 and R 底 Let represent the resistances of the inner and outer conductors of the concentric cylinder and the conductor at one end of the circular ring (bottom conductor), respectively. According to the definition of resistance, the formula for calculating the resistance value is: Where l is the conductor length, σ is the conductor conductivity, and A is the conductor cross-sectional area. At high frequencies, the skin effect causes non-uniform current density across the conductor cross-section, with the current distribution concentrated on the conductor surface. The formula for calculating the skin depth is: Where σ is the conductor conductivity. Figure 3 'a' represents the diagram for calculating the resistance of the inner conductor. The direction of the current I is along the length of the balun, and 'dl' represents the resistance calculated along the length. When the skin depth δ is much smaller than the conductor thickness, the current flowing through the cross-sectional area of ​​the conductor is approximately A≈2π·a·δ. The resistance of the inner conductor of the cylinder can be calculated as follows:

[0095]

[0096] Where ρ is resistivity and μ is permeability;

[0097] Similarly, the resistance of the outer conductor of a cylinder can be calculated as follows:

[0098]

[0099] Figure 3 b is the diagram for calculating the resistance of the conductor at the bottom of the balun. Using calculus, we analyze it. The current path is between the inner and outer circles, and the current I is in the radial direction. dr represents the differentiation of the circular conductor in the radial direction, yielding:

[0100]

[0101] Where dR represents the differential resistance of the conductor at the bottom of the range corresponding to dr;

[0102] Integrating over the radius r, we can obtain the total resistance of the circular conductor as:

[0103]

[0104] The structural parameters of the contactless balun designed in this invention are as follows: inner conductor radius a is 6 mm, outer conductor radius b is 15 mm, length l is 74 mm, and the magnetic permeability of copper material is μ0 = 4π × 10⁻⁶. -7 H / m, dielectric material is Teflon, vacuum dielectric constant ε0=8.85×10 -12 With a dielectric constant of 2.2 and an F / m, taking 1.5T magnetic resonance imaging as an example (other magnetic field strengths can be extrapolated accordingly), and an imaging operating frequency of 64MHz, the self-inductance L at 64MHz is calculated. t =13.56nH, self-capacitance C t = 9.88pF, resistance R t =0.006Ω. The balun's self-capacitance is too small to make the suppressor resonate. Based on the calculated self-inductance value, the required matching capacitor value should be 455.6pF. Figure 4 The graph shows the coupling impedance as a function of frequency. The maximum impedance value is 4923.4Ω at 64MHz.

[0105] The third step involves calculating the transmission characteristics of the transmission line with a balun using the transmission line model, and verifying the balun's suppression effect on common-mode current with the above structural parameters:

[0106] Figure 2 b represents an improved transmission line circuit model. Due to the electromagnetic induction between the transmission line and the balun, the impedance of the middle part of the transmission line changes. Therefore, the transmission line is divided into three segments (corresponding to three different transmission coefficients) for analysis. The lengths of the three segments are l1 = 50 mm, l2 = 74 mm, and l3 = 50 mm, respectively.

[0107] For the commonly used RG-58 coaxial transmission line, the inner conductor radius is 0.45 mm, the intermediate dielectric radius is 1.475 mm, the shielding layer radius is 1.75 mm, the inner conductor material is copper, and the conductivity is 5.98 × 10⁻⁶. 7 S / m, dielectric material is PE, complex permittivity ε r=2.25-j*0.01, from which the distributed parameters of the coaxial line at 64MHz can be calculated as follows: distributed capacitance C = 105.4pF / m, distributed conductance G = 188.4μS / m, distributed inductance L = 239.8nH / m, distributed resistance R = 0.9634Ω / m, skin depth δ = 8.3μm. The characteristic impedance and complex propagation constant of the lossy transmission line can be calculated through the distributed parameters, and their magnitudes are respectively: characteristic impedance Z0 = 47.7046-j0.1323Ω / m, complex propagation constant γ = 0.0146+j2.0217.

[0108] Assume the characteristic impedances of the first and third transmission lines are both Z0, and the characteristic impedance of the second transmission line after coupling with a contactless balun is:

[0109]

[0110] Where C is the distributed capacitance, G is the distributed conductance, L is the distributed inductance, R is the distributed resistance, Z is the coupling impedance, j is the imaginary unit, ω is the frequency, and Ω is the unit of resistance, in ohms. coupled l1 represents the coupling impedance generated by the secondary balun to the primary transmission line; l2 represents the length of the second transmission line segment; the bottom resistance affects the characteristic impedance Z1 of the second transmission line after coupling with the contactless balun, and thus also affects the transmission coefficient S of the second segment. 21_2 and total transmission coefficient S 21 This affects the inhibitory effect.

[0111]

[0112] S 21 =S 21_1 ×S 21_2 ×S 21_3

[0113] If the third transmission line is subsequently connected to a transmission line with a characteristic impedance of Z0, then there will be no reflection from its terminal.

[0114] Based on the transmission characteristic equation of the second transmission line, the voltage and current wave equations can be obtained as follows:

[0115]

[0116]

[0117] Among them, Γ 23 The reflection coefficient of the second transmission line is:

[0118]

[0119] The formula for calculating the input impedance of a port is:

[0120]

[0121] The relationship between the input and output voltages and the input and output currents of the transmission line ports can be established through the transmission matrix of a two-port network.

[0122]

[0123] Substituting the above equation into the formulas for voltage and current waves, we can obtain a transmission matrix that meets the requirements:

[0124]

[0125] Where γ1 is the complex propagation constant of the second transmission line.

[0126] Therefore, when both ends of the second transmission line are connected to transmission lines with characteristic impedance Z0, the formula for calculating the transmission coefficient of the second transmission line can be obtained as follows:

[0127]

[0128] S 21_2 Let Z1 represent the transmission coefficient of the second transmission line, A, B, C, and D be the matrix elements of the matrix obtained above, and Z1 represent the characteristic impedance of the second transmission line.

[0129] Simultaneously, the transmission matrices of the first and third transmission lines can be obtained, and the transmission coefficients calculated from the transmission matrices are:

[0130]

[0131] S 21 =S 21_1 ×S 21_2 ×S 21_3

[0132] S 21_1 S represents the transmission coefficient of the first transmission line segment. 21_3 Let γ0 represent the transmission coefficient of the third transmission line, γ0 be the complex propagation constant of the first and third transmission lines, and l1 be the length of the first and third transmission lines (the lengths are equal).

[0133] From this, the transmission coefficient S of the contactless balun transmission line can be calculated. 21 With frequency variation, Figure 4 b is S 21 Graph showing variation with frequency. Figure 4 b, The frequency at which the transmission coefficient is at its minimum is 64.02MHz, and the minimum transmission coefficient S 21 It is -43.6643dB.

[0134] The fourth step is to determine whether the inhibition effect meets the inhibition requirements:

[0135] The calculation results in step three show that the balun with the above structural parameters meets the suppression requirements for common-mode current. 21 If the value is less than -15dB, the suppression requirement is met, and the smaller the value, the better the suppression effect. However, in practice, the balun suppression effect obtained from the previous step may not meet the suppression requirement. In this case, it is necessary to reselect a suitable balun structure, perform modeling and solving, and use the transmission line model to calculate the common-mode suppression effect.

[0136] If step five satisfies the suppression requirement, then the matching capacitance value calculated by the model is corrected:

[0137] The above analysis is based on the assumption that the inner and outer concentric cylindrical conductors are ideal conductors with uniform current distribution. However, in reality, eddy current shielding occurs on the surfaces of the inner and outer conductors due to the skin effect, affecting the actual parameters in the balun. At high frequencies, the skin effect results in a smaller internal magnetic flux linkage that lags behind the surface magnetic flux linkage. Consequently, the voltage across the inner and outer cylindrical conductors can be divided into real and imaginary parts. The real part depends on the portion where the internal and surface magnetic flux linkages are 90° out of phase, representing an equivalent resistive voltage drop and characterizing eddy current power loss. The imaginary part depends on the portion where the internal and surface magnetic flux linkages are in phase, representing a passive voltage drop and characterizing self-inductance. Therefore, the voltage generated by the self-inductance of the inner and outer cylindrical conductors is only determined by the portion of the internal magnetic flux linkage that is in phase with the surface magnetic flux. The eddy current shielding effect causes the actual impedance to be smaller than the theoretical value, resulting in a further decrease in the actual inductance as the frequency increases.

[0138] To make the analysis of the balun more consistent with reality, this invention, based on the matching capacitance value obtained from model analysis, establishes a transmission line model with a balun using HFSS simulation software, according to the actual structural parameters of the suppressor. Considering the skin effect, the finite element method is used for analysis to obtain the corrected matching capacitance value. The specific steps of the method for correcting the balun matching capacitance value through simulation are as follows:

[0139] 1) Run HFSS and create a new project. Set the solution type to mode-driven solution type and set the default length unit used by the current design when creating the model to millimeters.

[0140] 2) Define design variables and add initial values: inner radius of balun a = 6mm, outer radius of balun b = 15mm, length of balun l = 74mm, transmission line length l1 = 150mm, matching capacitor c = 455.6pF.

[0141] 3) Create a transmission line model with baluns. Figure 5Model a is a transmission line model with a balun built using HFSS simulation software. The dimensions of the balun are the same as those described above. The port excitation is set to wave port excitation. Since the balun is designed to eliminate common-mode current on the shielding layer, the excitation is applied to the transmission line shielding layer during the simulation to observe the balun's suppression effect on common-mode current. A cylinder with a radiating boundary is created and its boundary conditions are set to radiating boundary conditions.

[0142] 4) Use lumped parameters to set the matching capacitors. Place four matching capacitors along the circumference at one end of the balun. The initial value is the matching capacitor value calculated in step one.

[0143] 5) Solution settings: To analyze the case with a magnetic field strength of 1.5T, the solution frequency is set to 64MHz, the maximum number of iterations for adaptive mesh generation is 20, the convergence error is 0.02, and a frequency sweep setting of 50MHz to 100MHz is added. The frequency sweep type is selected as Fast to analyze the transmission coefficient of the transmission line at 64MHz.

[0144] 6) Set the parameterized scan for the matching capacitor c and tune it to 64MHz.

[0145] 7) Design and run the simulation calculation to obtain the corrected balun matching capacitance value. During the simulation calculation, the progress bar window in the lower right corner of the working interface will display the solution progress, and the information management window will also provide corresponding information. A completion prompt message will be given after the simulation calculation is completed.

[0146] 8) Post-processing of HFSS balun problem data, exporting transmission coefficient diagrams of transmission lines, electromagnetic field distribution on the balun surface, etc., and simulation results of transmission coefficients are shown below. Figure 5 As shown in Figure b, when the value of a single matching capacitor is 124pF and the total capacitance is 496pF, the resonant frequency of the balun is 64MHz. At this time, the transmission coefficient S of the transmission line is... 21 With a value of -19.7dB, the balun's suppression effect on the common-mode current on the transmission line shielding layer meets the suppression requirements.

[0147] 9) Save the design for future reference.

[0148] Experimental step six involves the fabrication of the balun prototype and its application in a magnetic resonance imaging system:

[0149] To analyze real-world balun scenarios, this invention, based on theoretical derivation, model solving, and simulation, uses a vector network analyzer to measure the balun's transmission coefficient S. 21 This was used to verify the suppression effect of common-mode current.

[0150] Figure 6Figure a shows the experimental test diagram of the balun. The transmission line (1) passes through the central hole of the balun without physical contact between them. The balun consists of three parts: a dielectric, a conductor, and a matching capacitor. The dielectric part is a hollow cylinder (4) made of Teflon. The conductor part consists of inner and outer concentric cylindrical conductors (2) and (3) and a bottom conductor (6). The matching capacitor is soldered to one end of the suppressor, serving to connect the inner and outer conductors to form a closed loop and to adjust the resonant frequency of the balun. The structural parameters of the balun are consistent with those described above, and the edges of the balun are connected by solder. Using the corrected capacitance value as the actual matching capacitance value, the test results are as follows. Figure 6 b、 Figure 6 As shown in Figure c, at 64MHz, the common-mode current suppressor achieves a suppression effect of -30dB on the common-mode current of the coaxial cable shield, with a coupling impedance of 2172+j3405Ω.

[0151] This invention proposes a method for using baluns, in which two baluns are placed at the coil and at a quarter wavelength away from the coil, respectively. Both the placement of the baluns at the coil and the new method are applied to an imaging experiment of a 1.5T magnetic resonance imaging system. The signal-to-noise ratio of the images obtained by the two methods is calculated to verify that the baluns can reduce system noise and improve the signal-to-noise ratio of the images.

[0152] In this embodiment, the function of the contactless balun is verified by measuring spin echo images using a head coil of magnetic resonance imaging (MRI). The test coil is a 250mm diameter head coil, and the test model is a cylindrical water phantom made of copper sulfate and sodium chloride with a diameter of 200mm. The sequence selected for MRI is a spin echo sequence with the following parameters: repetition time TR = 350ms, echo time TE = 16ms, flip angle FA = 90°, field of view FOV = 250x250mm, number of acquisitions NEX = 2, and slice thickness THK = 5mm. In this embodiment, MRI was performed on the cylindrical water phantom, and the signal-to-noise ratio (SNR) of the images was measured. The results are shown in Table 1. The SNR of MRI without the balun is 22.56, the SNR of MRI with the balun at the coil is 39.97, and the SNR of MRI with both baluns at the coil and at a quarter wavelength away from the coil are 49.45. Therefore, the method proposed in this invention has a better suppression effect.

[0153] Table 1. Signal-to-noise ratio of magnetic resonance imaging

[0154]

[0155] It should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the scope of the claims.

Claims

1. A method for determining the matching capacitance of a contactless balun in magnetic resonance imaging, comprising the following steps: 1) Modeling and solving for the matching capacitance value of the contactless balun; including: 11) The contactless balun is modeled as a closed-loop circuit consisting of a matching capacitor, inner and outer coaxial cylindrical conductors, and a bottom conductor; the structure of the contactless balun includes two coaxial cylindrical conductors, a bottom conductor connecting the two coaxial cylindrical conductors, and a matching capacitor; and the matching capacitor is used as a series capacitor-resistor element; the transmission line is coupled to the balun through electromagnetic induction. 12) Calculate the mutual inductance between the transmission line and the balun; Calculate the self-inductance and self-capacitance between coaxial cylindrical conductors; calculate the resistance of the inner and outer coaxial cylindrical conductors and the resistance of the bottom conductor; Self-resistance R of the conductor section t The sum of the resistances of the inner and outer conductors and the bottom conductor is expressed as: in, and These represent the resistances of the inner and outer conductors and the bottom conductor of the concentric cylinder, respectively. The current direction is along the length of the balun. When the skin depth is much smaller than the conductor thickness, the resistance of the conductor inside the cylinder is expressed as: in; Resistivity; The conductor length; For deep skin care, ; The conductivity of a conductor; the cross-sectional area through which current flows. S is the magnetic flux area; a is the radius of the inner conductor; ω is the frequency. Permeability; The resistance of a cylindrical outer conductor is expressed as: Where b is the radius of the outer conductor; The total resistance of the circular conductor at the bottom of the Baron's end was calculated using calculus as follows: Where r is the radius of the circular conductor; 13) The matching capacitance value c is obtained by using the matching capacitance calculation formula, and is expressed as: ,in , M represents mutual inductance; For balun self-inductance; φ is the current, µ0 is the permeability, φ is the magnetic flux, and ε is the permittivity. 2) Use the transmission line model to analyze the transmission line with a contactless balun and calculate the suppression effect of the balun; The transmission line model is used to analyze the transmission line with balun. The transmission line is divided into three different parts, each with a different transmission line impedance. The transmission matrix of each part of the transmission line is analyzed. Then, based on the relationship between the transmission matrix and the scattering parameters, the transmission coefficient of the transmission line with contactless balun is calculated as a function of frequency, thus obtaining the balun suppression effect. 3) Determine the suppression effect of the balun based on the transmission coefficient of the contactless balun transmission line; 4) Correct the balun matching capacitance value obtained in step 1) using electromagnetic simulation to determine the correct balun matching capacitance value. This includes: performing a 1:1 modeling simulation of the designed contactless balun physical object to calculate the transmission coefficient of the transmission line with the balun; using the balun matching capacitance value obtained in step 1) through model building and calculation as the initial value, performing parameterized calculations on the balun matching capacitance to obtain the corrected balun matching capacitance value. The specific steps are as follows: 41) Run HFSS and create a new project, set the solution type to mode-driven solution type, and set the default length unit used when creating the model for the current design; 42) Define design variables and add initial values, including: balun inner radius, balun outer radius, balun length, transmission line length, and matching capacitance; 43) Create a transmission line model with a balun, set the port excitation to wave port excitation, apply the excitation to the transmission line shield during the simulation, and calculate the balun's suppression effect on common-mode current; create a cylinder with a radiating boundary and set its boundary conditions to radiating boundary conditions; 44) Select the lumped parameter setting for the matching capacitor. The matching capacitor is placed along the circumference at one end of the balun, with the initial value being the matching capacitor value calculated in step 1). 45) Solver settings; including: solver frequency, maximum number of iterations for adaptive mesh generation, convergence error, sweep frequency, sweep frequency type, and transmission coefficient of transmission lines; 46) Set the parametric scan for the matching capacitor and tune it to the solution frequency; 47) Design check and run simulation calculation to obtain the corrected matching capacitor value; The above steps enable the determination of the matching capacitance for a contactless balun in magnetic resonance imaging.

2. The method for determining the matching capacitance of a contactless balun in magnetic resonance imaging as described in claim 1, characterized in that, In step 3), the transmission coefficient of the contactless balun transmission line is calculated, including: Assuming the characteristic impedances of the first and third transmission lines are both Z0, the characteristic impedance of the second transmission line after coupling with a contactless balun is: Where C is the distributed capacitance; G is the distributed conductance; and L is the distributed inductance. Distributed resistance; Where j is the coupling impedance; j is the imaginary unit. For frequency; Second segment transmission coefficient and total transmission coefficient They are respectively: If the third transmission line is subsequently connected to a transmission line with a characteristic impedance of Z0, then there will be no reflection from its terminal. Based on the transmission characteristic equation of the second transmission line, the voltage and current wave equations can be obtained as follows: in, The reflection coefficient of the second transmission line is: The formula for calculating the input impedance of a port is: The relationship between the input and output voltages and the input and output currents at the ports of a two-port network is established using the transmission matrix, and is expressed as follows: The transfer matrix is ​​obtained as follows: in, The complex propagation constant of the second transmission line; The transmission coefficient S with the contactless balun transmission line can be calculated from this. 21 Variation with frequency.

3. The method for determining the matching capacitance of a contactless balun in magnetic resonance imaging as described in claim 2, characterized in that, A threshold is set. If the calculated transmission coefficient of the contactless balun transmission line is less than the threshold, the suppression requirement is met. The smaller the transmission coefficient value, the better the suppression effect.

4. The method for determining the matching capacitance of a contactless balun for magnetic resonance imaging as described in claim 3, characterized in that, The set threshold is -15dB.

5. The method for determining the matching capacitance of a contactless balun for magnetic resonance imaging as described in claim 1, characterized in that, In step 45), the solution frequency is set to 64MHz, the maximum number of iterations for adaptive mesh generation is 20, the convergence error is 0.02, and a frequency sweep setting of 50MHz to 100MHz is added. The frequency sweep type is fast frequency sweep. The transmission coefficient of the transmission line at 64MHz is analyzed.

6. A method for producing a contactless balun, characterized in that: First, determine the structural parameters of the non-contact balun, including its inner radius, outer radius, and length; Teflon was chosen as the filling medium, and the shape was a hollow cylinder. Copper foil is attached to the inner and outer layers of the hollow cylinder to form concentric cylindrical conductors. The hollow cylindrical conductor dielectric uses copper foil as the connection between the inner and outer conductors at one end, and a matching capacitor is welded between the inner and outer conductors at the other end. The matching capacitance value used in the contactless balun physical object is the matching capacitance value obtained by the matching capacitance determination method of the magnetic resonance imaging contactless balun as described in claim 1. This forms a closed loop, resulting in a contactless balun.

7. A method of using a non-contact balun in a magnetic resonance imaging system, characterized in that, Two contactless baluns are fabricated using the method described in claim 6; the two contactless baluns are placed at the radio frequency coil of the magnetic resonance imaging system and at a distance of one-quarter wavelength from the radio frequency coil, respectively; The wavelength is determined based on the operating frequency of magnetic resonance imaging, thereby improving the signal-to-noise ratio of magnetic resonance imaging.

Images