Design Method of Magnetic Resonance Double-Loop Resonant Self-Decoupling Coil Based on Multi-Objective Optimization

Abstract

This invention provides a design method for a magnetic resonance double-loop resonant self-decoupling coil based on multi-objective optimization, comprising the following steps: Step 1, constructing a physical model of the magnetic resonance double-loop resonant self-decoupling coil; Step 2, establishing an electromagnetic model based on coupled-mode theory to solve for the system's characteristic modes; Step 3, determining the solution method for complex current distribution; Step 4, determining the spatial magnetic field distribution index; Step 5, constructing a multi-objective optimization function, including: a field strength gain term, aiming to maximize the central field strength; and a uniformity penalty term, calculating the difference ratio between the central field and the edge field; Step 6, setting physical constraints; and Step 7, performing nonlinear global optimization.

Description

Technical Field

[0001] This invention relates to the field of magnetic resonance imaging, and in particular to a design method for a magnetic resonance dual-ring resonant self-decoupling coil based on multi-objective optimization. Background Technology

[0002] Magnetic resonance imaging (MRI) is a commonly used non-invasive diagnostic technique in clinical practice. Its principle involves using a radio frequency transmitting coil to emit radio frequency pulses at Larmor frequencies to disrupt the precession equilibrium of atomic nuclei within an external reinforced static magnetic field. After the radio frequency pulses cease, a radio frequency receiving coil, based on Faraday's law of electromagnetic induction, receives the magnetic resonance signal as the atomic nuclei return to their precession equilibrium, and then uses a computer to reconstruct the image.

[0003] Improving the signal-to-noise ratio (SNR) is of great significance in magnetic resonance imaging (MRI). Array design and parallel imaging techniques can enhance the SNR and imaging speed of high-field MRI systems while accommodating multi-site and multi-modal imaging needs. However, the destructive electromagnetic coupling between multi-channel coils is a key challenge in its design and application. Excessive coupling strength can compromise channel independence, reduce SNR and energy transfer efficiency, and introduce image artifacts and specific absorption rate (SAR) risks. These problems become more pronounced with higher channel density and higher field strength. Therefore, reducing the coupling between multi-channel coil units is crucial for MRI.

[0004] Traditional multi-channel coil decoupling schemes include geometric decoupling, capacitive / inductive decoupling, and preamplifier decoupling. Their core drawbacks lie in limited decoupling effectiveness, narrow bandwidth, susceptibility to introduced losses / noise, high design complexity, and difficulty in adapting to high-density arrays. Essentially, they are passive compensations or physical isolations of "mutual inductance-capacitance coupling," failing to fundamentally solve the complex coupling problems under multi-channel, high-field-strength conditions. Patent application CN202610089111.7 provides a magnetic resonance self-decoupling coil based on a double-open-loop reverse coupling mechanism. However, in the complex double-loop coupling structure, how to simultaneously and accurately tune the capacitances of both loops while suppressing external interference and optimizing the strength and uniformity of the internal field is a complex problem involving electromagnetics, geometry, and nonlinear optimization. Traditional empirical tuning methods or single-variable search methods struggle to find the globally optimal solution among "target operating frequency," "external field suppression," and "internal field strength and high uniformity," limiting the design and application of high-performance self-decoupling coils. Therefore, finding a collaborative optimization method that can obtain suitable parameters and ensure extremely low external electromagnetic interference and extremely high internal field uniformity in the target frequency operating mode is a problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a design method for a magnetic resonance dual-loop resonant self-decoupling coil based on multi-objective optimization. The aim is to easily determine the relevant parameters of the self-decoupling coil, enabling its operating mode to resonate at the Larmor frequency corresponding to the magnetic resonance imaging system. Furthermore, the method ensures that the external magnetic field strength generated by the self-decoupling coil is sufficiently weak without excessively weakening the internal magnetic field strength, thus achieving excellent imaging performance. To achieve the above objectives, this invention employs the following technical solution: A design method for a magnetic resonance dual-ring resonant self-decoupling coil based on multi-objective optimization is proposed. The magnetic resonance dual-ring resonant self-decoupling coil includes a dielectric substrate and concentric outer and inner rings fixed on the dielectric substrate and nested together. The outer ring is provided with an outer ring tuning capacitor. The inner ring is equipped with an inner ring tuning capacitor. It includes the following steps: Step 1: Construct a physical model of a magnetic resonance dual-loop resonant self-decoupling coil: Set the inner loop side length according to the imaging range. Define the outer ring side length as a fixed value. Outer ring tuning capacitor and inner ring tuning capacitor The vector of variables to be optimized Set target Lamoir frequency Set decoupling control points , Step 2: Establish an electromagnetic model based on coupled-mode theory to solve for the characteristic modes of the system and the high-frequency modes of the system. With low frequency mode Calculate the self-inductance of the inner and outer loops. and mutual induction Solving the high-frequency modes of the system using coupled-mode theory With low frequency mode ; Step 3: Determine the solution method for complex current distribution; Step 4: Determine the spatial magnetic field distribution indicators: Determine the three types of magnetic fields, namely the central field strength. Edge field strength and external field strength i.e., decoupling control point The residual magnetic field at the location The calculation method; Step 5: Construct a multi-objective optimization function; the function includes: Field strength gain term, to maximize the center field strength For the goal; The uniformity penalty term calculates the proportion of the difference between the central field and the peripheral field. This is introduced as a penalty factor into the objective function to optimize the spatial distribution of magnetic induction intensity; Step 6: Set physical constraints; Step 7: Perform nonlinear global optimization to search for the parameter combination that optimizes the objective function and satisfies all physical constraints. .

[0006] Furthermore, step one also includes setting boundary constraints for each variable to be optimized.

[0007] Furthermore, step two includes determining the impedance at a specific angular frequency by establishing a circuit impedance matrix based on Kirchhoff's laws. Complex current distribution in the inner and outer rings and ; Furthermore, step six specifically includes: Frequency constraints require the system to operate in high-frequency modes. Equal to the target Larmor frequency of the magnetic resonance system ; Self-decoupling constraints, calculate the decoupling control points of the inner and outer loops. The residual magnetic field at the location It is required that it be lower than a preset inhibition threshold. .

[0008] Furthermore, step seven involves employing a sequential quadratic programming (SQP) algorithm. By collaboratively adjusting the outer ring size and the ratio of the two-ring capacitors, the algorithm searches for the parameter combination that optimizes the objective function and satisfies all physical constraints. .

[0009] Furthermore, step seven also includes: based on the parameter combination obtained in step five. A model is established and simulated. Based on the simulation results, specific parameters are fine-tuned to achieve tuning and impedance matching. At the same time, this working mode has decoupling performance.

[0010] The beneficial effects of this invention are as follows: The reverse current mode, i.e., the high-frequency characteristic mode, coupled from the double-ring resonator achieves magnetic field cancellation, significantly reducing electromagnetic radiation from the coil without adding additional decoupling components or circuits, and effectively suppressing mutual inductance coupling between channels. Based on coupled-mode theory and Kirchhoff's laws, the capacitance calculated by this method to make the high-frequency characteristic mode resonate at the corresponding Larmor frequency of the magnetic resonance system is relatively accurate, and it can effectively cancel the magnetic fields generated by the currents on the inner and outer rings, achieving good decoupling performance. Simultaneously, this method balances the magnetic fields of the inner and outer rings through a specific uniformity penalty mechanism, avoiding the central field collapse problem that may be caused by the magnetic field cancellation mechanism. Furthermore, this method achieves cross-dimensional joint optimization of geometric and circuit parameters, greatly shortening the design cycle of the self-decoupling coil. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the topology of a magnetic resonance self-decoupling coil unit structure based on a double-open-loop reverse coupling mechanism provided in an embodiment of the present invention; Figure 2 The equivalent circuit diagram of the double-open-loop reverse coupling mechanism provided in this embodiment of the invention is shown on the left, which is the equivalent circuit of the outer loop, and on the right, which is the equivalent circuit of the inner loop. Figure 3 A flowchart of multi-objective parameter optimization for a parameter determination method for self-decoupling receiving coils in multi-field intensity MRI provided in an embodiment of the present invention; Figure 4 A schematic diagram of the coordinate system at the start of the multi-objective parameter optimization process for the parameter determination method of self-decoupling receiving coil for multi-field intensity MRI provided in this embodiment of the invention (z=0). Figure 5 The high-frequency mode provided in this embodiment of the invention, based on parameter values ​​obtained from a multi-objective parameter optimization algorithm. A comparison chart of the S-parameters and the fine-tuned S-parameters; Figure 6 The high-frequency mode of a pair of self-decoupling coils provided in the embodiments of the present invention after parameter determination. S-parameter plot; Figure 7 The magnetic field strength distribution diagram provided for an embodiment of the present invention, in which only one of a pair of self-decoupling coils is excited after the parameters are determined. Detailed Implementation

[0012] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are all within the scope of protection of this invention.

[0013] This embodiment uses a 1.5T magnetic resonance imaging system (operating frequency) Taking a self-decoupling RF coil as an example. Refer to the appendix. Figure 1 The hardware topology constructed in this embodiment is a concentric double square ring structure, including: an inner ring: serving as the imaging working ring, with a side length of Fixed as Series tuning capacitor Outer ring: serving as a decoupling ring, with a side length of... For the geometric variables to be optimized, series tuning capacitors See attached document. Figure 2 The equivalent circuit of this structure can be modeled as consisting of mutual inductance. Two coupled together Resonator, in which These are the equivalent resistance, equivalent capacitance, and equivalent inductance of the outer ring metal, respectively. These represent the equivalent resistance, equivalent capacitance, and equivalent inductance of the inner ring metal, respectively. The wire width... Vacuum permeability .

[0014] See attached document Figure 3 The specific optimization process in this embodiment is as follows: Step 1: Input initial parameters and design goals. Set the target frequency. See attached document. Figure 4 A decoupling control point is set at a distance of 0.05 m from the center of the coil, i.e., the origin of the coordinate system. ,Right now The residual magnetic field at that point is required. No more than Define the vector of variables to be optimized. Construct the parameter vector. To ensure physical feasibility, boundary constraints are set: , .

[0015] Step 2: Establish an electromagnetic model based on coupled-mode theory to solve for the system's characteristic modes. Calculate the equivalent self-inductance of each loop using Grover's approximation formula: ,in The permeability of free space, and Let the side length and line width of the inner or outer ring be respectively; then calculate the mutual inductance between the inner and outer rings based on the Neumann equation: ,in and Let be the side lengths of the inner and outer rings, respectively. This represents the distance between corresponding edges of the inner and outer rings; simultaneously, the mutual inductance coefficient can be obtained. ,in For mutual intuition, and These are the equivalent self-inductances of the outer ring and the inner ring, respectively; and as shown in the appendix... Figure 1 With appendix Figure 2 As shown, for the proposed structure, the equivalent capacitance of the metal wires in each ring is connected in parallel with the adjustable capacitor and can be ignored. Therefore, the equivalent capacitance in the equivalent circuit can be considered as the adjustable capacitor. The resonant frequency With capacitor ,inductance The relationship is: Then, based on the coupled-mode theory formula: The characteristic frequencies of the system can be solved, where and These are the resonant frequencies of the outer and inner rings, respectively. The coupling coefficient between the inner and outer rings. For the calculated high-frequency mode ( ) and low-frequency mode ( These represent the reverse current mode and the same current mode, respectively. For self-decoupling applications of magnetic resonance imaging coils, high-frequency modes should be used for excitation; therefore, extraction... As a frequency locking target.

[0016] Step 3: Solve for the complex current distribution. Construct the impedance matrix of the system. Apply outer ring The equivalent excitation voltage is obtained by solving the equation. : ,in , is the self-impedance of the outer ring. , is the self-impedance of the inner loop. Given the mutual impedance between the inner and outer loops, we can solve for the induced current generated in the inner and outer loops at the target frequency, i.e., the outer loop current. With the inner ring .

[0017] Step 4: Calculate the spatial magnetic field distribution index. Calculate the magnetic field at each point using the Biot-Savart law: ,in The permeability of free space, The current intensity passing through the coil conductor, This represents the distance vector from the current source point to the observation point. Magnetic field calculation function. Consider the superposition of four current elements in a square. This method involves three types of magnetic fields: the central field strength... That is, coordinate points Total field strength at the edge, field strength at the edge That is, selecting the edge points of the region of interest (ROI). Field strength at the location, external field strength i.e., decoupling control point The field strength at that location.

[0018] Step 5: Construct and evaluate the multi-objective optimization function. Define the evaluation scoring function. ,in, , where represents the ratio of the difference between the central field and the edge field. This evaluation scoring function aims to maximize the central field strength while using a penalty term to force the algorithm to reduce the difference in field strength between the center and the edge, thereby improving uniformity.

[0019] Step Six: Set constraints and determine whether the requirements and limitations are met. Establish equality constraints. To ensure the system operates close to the Larmor frequency, inequality constraints are established. This ensures the self-decoupling effect.

[0020] Step 7: Perform optimization and output the results. A sequential quadratic programming (SQP) algorithm is used for nonlinear search.

[0021] Step 8: After iterating through the above steps, a set of optimal implementation parameters is obtained. Based on this set of parameters, a model is built and simulated in the full-wave simulation software. The results are observed, and then fine-tuned according to the actual situation.

[0022] Based on the above steps, the results are calculated as follows: when the inner ring side length is set... When the length is 0.05m, calculate the length of the outer ring side. The outer ring capacitance is 0.054m. The inner ring capacitor is 326.36pF. The value is 68.92 pF. Based on this parameter, a model was built and simulated in full-wave simulation software, and the results are shown in the attached figure. Figure 5 As shown, under no-load conditions, the system's high-frequency mode The resonance frequency is 58.6MHz. Based on the system's geometry and resonant frequency at this point, further fine-tuning of the capacitor and impedance matching are needed for its application. In this example, the outer loop capacitor... Adjusting the impedance to 97pF and connecting an external matching network for impedance matching can achieve high-frequency mode switching of the system. It resonates at 63.8MHz and exhibits good performance, as shown in the attached figure. Figure 5 As shown.

[0023] To verify the self-decoupling performance of the coils, a simulation was performed with two coil units placed side by side. The S-parameter simulation results for a 5mm spacing between the two coils are shown in the attached figure. Figure 6 As shown, both coils resonate at 63.8MHz, at which point the S0 of the two coils... 11 The parameters are all -25.12dB, indicating excellent coil matching and high efficiency; while the S between the two coils 21 The parameter is -34.96dB, meaning that only about 0.03% of the power output from port 1 of coil 2 is transmitted to port 2. This indicates that the isolation between the two coils is very high, with almost no interference between them. To further confirm this, the magnetic field strength distribution of the two coils is shown in the attached figure. Figure 7 As shown, when only one coil is excited, it can be seen that the magnetic field is mainly concentrated in the range of the excited coil, and there is almost no magnetic field induced by coupling in the range of the unexcited coil. This indicates that the two coils are almost uncoupled, and the coil unit has excellent self-decoupling performance.

[0024] In summary, the parameter determination method for multi-field MRI self-decoupling receiving coils proposed in this invention successfully achieves efficient active self-decoupling of the designed coil, and also achieves extremely high magnetic field spatial uniformity in the internal region of interest. This invention provides a method for the engineering design and implementation of multi-channel magnetic resonance coils, demonstrating strong feasibility and effectiveness.

[0025] Those skilled in the art will understand that the above refers to square coils and The system description is only a preferred embodiment. The method of this invention is also applicable to irregularly shaped coils such as circular and polygonal coils, to coils of different geometric dimensions based on imaging requirements, and to ultra-low fields such as 50mT, 70mT, and 100mT. 5.0T This invention relates to the design of radio frequency coils for ultra-high field magnetic resonance systems. In the design process based on this method, some parameters can be modified according to actual needs and limitations. Furthermore, based on actual conditions, the outer loop capacitor can be adjusted individually during manual adjustment, the inner loop capacitor can be adjusted individually, or both the inner and outer loop capacitors can be adjusted simultaneously. Additionally, different matching networks can be selected as needed; this does not limit the scope of protection of this invention.

[0026] Matters not covered in this invention are common knowledge.

[0027] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A design method for a magnetic resonance dual-ring resonant self-decoupling coil based on multi-objective optimization, wherein the magnetic resonance dual-ring resonant self-decoupling coil includes a dielectric substrate and concentric outer and inner rings fixed together on the dielectric substrate, the outer ring being provided with an outer ring tuning capacitor. The inner ring is equipped with an inner ring tuning capacitor. It includes the following steps: Step 1: Construct a physical model of a magnetic resonance dual-loop resonant self-decoupling coil: Set the inner loop side length according to the imaging range. Define the outer ring side length as a fixed value. Outer ring tuning capacitor and inner ring tuning capacitor The vector of variables to be optimized ; Set target Lamoer frequency Set decoupling control points ; Step 2: Establish an electromagnetic model based on coupled-mode theory to solve for the characteristic modes of the system and the high-frequency modes of the system. With low frequency mode Calculate the self-inductance of the inner and outer loops. and mutual induction Solving the high-frequency modes of the system using coupled-mode theory With low frequency mode ; Step 3: Determine the solution method for complex current distribution; Step 4: Determine the spatial magnetic field distribution indicators: Determine the three types of magnetic fields, namely the central field strength. Edge field strength and external field strength i.e., decoupling control point The residual magnetic field at the location The calculation method; Step 5: Construct a multi-objective optimization function, including: Field strength gain term, to maximize the center field strength For the goal; The uniformity penalty term calculates the proportion of the difference between the central field and the peripheral field. This is introduced as a penalty factor into the objective function to optimize the spatial distribution of magnetic induction intensity; Step 6: Set physical constraints; Step 7: Perform nonlinear global optimization to search for the parameter combination that optimizes the objective function and satisfies all physical constraints. .

2. The design method for a magnetic resonance double-ring resonant self-decoupling coil based on multi-objective optimization according to claim 1, characterized in that, Step one also includes setting boundary constraints for each variable to be optimized.

3. The design method for a magnetic resonance double-ring resonant self-decoupling coil based on multi-objective optimization according to claim 1, characterized in that, Step two involves establishing the circuit impedance matrix and determining the impedance at a specific angular frequency based on Kirchhoff's laws. Complex current distribution in the inner and outer rings and .

4. The design method for a magnetic resonance double-ring resonant self-decoupling coil based on multi-objective optimization according to claim 1, characterized in that, Step six specifically includes: Frequency constraints require the system to operate in high-frequency modes. Equal to the target Larmor frequency of the magnetic resonance system ; Self-decoupling constraints, calculate the decoupling control points of the inner and outer loops. The residual magnetic field at the location It is required that it be lower than a preset inhibition threshold. .

5. The design method for a magnetic resonance double-ring resonant self-decoupling coil based on multi-objective optimization according to claim 1, characterized in that, Step seven involves employing a Sequential Quadratic Programming (SQP) algorithm. By collaboratively adjusting the outer loop size and the ratio of the two loop capacitors, the algorithm searches for the parameter combination that optimizes the objective function and satisfies all physical constraints. .

6. The design method for a magnetic resonance double-ring resonant self-decoupling coil based on multi-objective optimization according to claim 1, characterized in that, Step seven is followed by: based on the parameter combination obtained in step five. A model is established and simulated. Based on the simulation results, specific parameters are fine-tuned to achieve tuning and impedance matching. At the same time, this working mode has decoupling performance.

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