Design method of double-plane spiral gradient coil for magnetoacoustic magnetic particle concentration imaging

Abstract

The invention discloses a design method of a double-plane spiral gradient coil for magnetoacoustic magnetic particle concentration imaging. The method can greatly reduce the excitation current of the coil. The method comprises the following steps: firstly, presetting parameters of a double-plane spiral gradient coil according to requirements of an imaging area in MACT-MI; setting a magnetic field gradient value required by the imaging area according to a gradient field requirement of the imaging area; and finally, searching a minimum current value and an optimal coil parameter which meet the requirement that the gradient magnetic field deviation is within a limited range by adopting a Loutu-mouse optimization algorithm. By the adoption of the method, the biplane spiral gradient coil special for MACT-MI is designed, pulse current excitation is greatly reduced on the premise that it is guaranteed that the gradient magnetic field of an imaging area is uniform and stable, the coil structure is simplified, and experimental research of MACT-MI is promoted.

Description

Technical Field

[0001] The present invention belongs to the field of inductive magnetoacoustic magnetic particle concentration imaging, and more particularly relates to a design method of a dual-plane spiral gradient coil for magnetoacoustic magnetic particle concentration imaging. Background Art

[0002] MCAT-MI is a new method for MNPs imaging. This method utilizes the transmissivity of the magnetic field and uses a time-varying magnetic field as the excitation signal, enabling it to deeply detect the interior of biological tissues. At the same time, the ultrasound generated by the vibration of MNPs is used as the detection signal. Utilizing the characteristics of the ultrasound signal such as anti-interference, low scattering, and high signal-to-noise ratio, while improving the imaging contrast, it also improves the drawback of mutual interference between coils in the system structure, greatly improving the space utilization rate. Chinese invention patent CN201911067260.X2 discloses a magnetoacoustic magnetic particle concentration imaging device. The gradient coil described in this patent uses a Maxwell coil. At least a pulsed current of about 200 A needs to be passed through a Maxwell coil with a radius of 0.4 m to generate a magnetoacoustic signal that can be captured by an ultrasound probe. The excessive current excitation amplitude makes it impossible to be applied to experiments. Chinese invention patent CN202011019361.2 discloses a matrix coil design method for magnetoacoustic magnetic particle concentration imaging. The gradient coil described in this patent uses a matrix coil composed of 74 circular coils. The radius of the coil plane is at least 290 mm and the maximum pulsed current passed through the coil is 8.8889 A, generating a gradient magnetic field of 10 mT / m in the imaging area. The structure of this coil is too complex and the gradient magnetic field is small, unable to meet the gradient magnetic field requirements of the imaging area. Based on this, the present invention starts from reducing current excitation, simplifying the coil structure, and increasing the gradient magnetic field, designs a gradient coil suitable for MACT-MI, and promotes the experimental research of MACT-MI. Summary of the Invention

[0003] The purpose of the present invention is to solve the problems in the MACT-MI system that the gradient coil excitation is too large and the equipment is extremely easy to be damaged and not conducive to being applied to experiments, the coil structure is relatively complex, and the gradient magnetic field cannot meet the requirements of the imaging area. A design method of a gradient coil dedicated to magnetoacoustic magnetic particle concentration imaging is provided. The gradient coil designed by the present invention greatly reduces the pulsed current excitation on the premise of ensuring the uniform and stable gradient magnetic field in the imaging area, simplifies the coil structure, increases the gradient magnetic field in the target area, and promotes the experimental research of MACT-MI.

[0004] The experimental system of magnetoacoustic magnetic particle concentration imaging is as Figure 1As shown in the figure, it includes: an electromagnetic excitation system, which consists of a signal generator, a pulsed excitation power supply, and a dual-plane spiral gradient coil, and passes the generated pulsed current into the gradient coil to generate a gradient magnetic field; an ultrasonic transducer drive system, which consists of a motor driver and a drive circuit, and is used to drive the immersion ultrasonic transducer to perform circular scanning to receive ultrasonic signals; an intelligent control system, which consists of an intelligent controller FPGA, and the intelligent controller FPGA is used to control the current output of the signal generator and the pulsed excitation power supply, the rotation angle of the motor driver, and the timer of the data acquisition card; a signal processing and visualization system, which consists of an immersion ultrasonic transducer, a filter, an amplifier, and an oscilloscope, and is used to remove the noise of the magnetoacoustic signal and visualize the magnetoacoustic signal; a signal acquisition and storage system, which consists of a data acquisition card and a computer, and is mainly used to acquire and store data.

[0005] The dual-plane spiral gradient coil described in the present invention is specifically used for the MACT-MI system. The magnetic field generated by the dual-plane spiral gradient coil will magnetize the MNPs in the imaging region. The magnetized MNPs will vibrate under the action of the gradient magnetic field and then generate ultrasonic waves. The different concentrations of MNPs distributed in biological tissues result in different magnetic forces, and thus different positions or amplitudes of the generated ultrasonic signals. The ultrasonic transducer can detect the ultrasonic signals generated by the MNPs. By using an imaging algorithm to process the acquired ultrasonic signal data, the concentration distribution image of the MNPs can be reconstructed, realizing magnetoacoustic magnetic particle concentration imaging.

[0006] The dual-plane spiral gradient coil described in the present invention is a planar coil structure composed of an inner spiral coil and an outer circular coil. The dual-plane spiral gradient coil is wound with copper enameled wire with a diameter of 2 mm. The radius of the inner spiral coil is 100 mm, the number of turns is 15, the radius of the outer circular coil is 102 mm, the number of turns is 30, the distance between the two coil planes is 120.2 mm, and the total diameter of the planar coil is 204 mm.

[0007] The imaging region of the dual-plane spiral gradient coil described in the present invention is a cylindrical region with a bottom diameter of 60 mm and a height of 60 mm, and the dual-plane spiral gradient coil is only used to generate a uniform longitudinal gradient magnetic field.

[0008] The dual-plane spiral gradient coil described in the present invention is supplied with current by a pulsed excitation power supply controlled by a signal generator. A sinusoidal decay truncated wave signal current in the counterclockwise direction is passed into the inner spiral coil of the dual-plane spiral gradient coil, and a sinusoidal decay truncated wave signal current in the clockwise direction is passed into the outer circular coil in the same plane. The current directions of the corresponding coils on the upper and lower planes are opposite.

[0009] The parameter optimization process of the dual - plane spiral gradient coil described in the present invention is as follows: 1) Select the base pattern of the dual - plane spiral gradient coil and construct the winding trajectory of the spiral gradient coil in the Cartesian coordinate system; 2) Discretize the dual - plane spiral gradient coil to determine the relationship between the discrete current sources and the magnetic field intensity in the imaging region; 3) Superimpose the magnetic fields generated by all discrete current sources to obtain the gradient magnetic field in the imaging region; 4) Establish an optimization model for the dual - plane spiral gradient coil of magneto - acoustic magnetic particle concentration imaging. Take the shape parameters and current parameters of the coil as optimization parameters, and the goal is to minimize the root - mean - square error between the theoretical value and the actual value of the magnetic field at the set points in the imaging region; 5) Use the lemming optimization algorithm to optimize the coil current magnitude and coil parameters, and then obtain the optimal parameters and the best excitation current of the dual - plane spiral coil. Substitute the optimized parameters into the coil model to obtain the final coil winding path; 6) Use the non - uniformity to verify the effectiveness of the design scheme. Superimpose the magnetic fields generated by all discrete current sources to obtain the gradient magnetic field in the imaging region Establish an optimization model for the dual - plane spiral gradient coil of magneto - acoustic magnetic particle concentration imaging. Take the shape parameters and current parameters of the coil as optimization parameters, and the goal is to minimize the root - mean - square error between the theoretical value and the actual value of the magnetic field at the set points in the imaging region ; 5) Use the lemming optimization algorithm to optimize the coil current magnitude and coil parameters, and then obtain the optimal parameters and the best excitation current of the dual - plane spiral coil. Substitute the optimized parameters into the coil model to obtain the final coil winding path; 6) Use the non - uniformity to verify the effectiveness of the design scheme.

[0010] The beneficial effects of the present invention are as follows: A dual - plane spiral gradient coil dedicated to MACT - MI is designed. On the premise of ensuring the uniform and stable gradient magnetic field in the imaging region, the pulsed - current excitation is significantly reduced, the coil structure is simplified, and the experimental research of MACT - MI is promoted. The above description is only an overview of the technical solution of the present invention. In order to understand the technical means of the present invention more clearly, it can be implemented according to the content of the description. And in order to make the purpose, features, and advantages of the present invention more obvious and easy to understand, the present invention will be described in detail below with reference to the accompanying drawings. Brief Description of the Drawings

[0011] Figure 1 is a schematic diagram of the experimental system principle after the improvement of the magneto - acoustic magnetic particle concentration imaging device according to the present invention; Figure 2 is a structural block diagram after the improvement of the magneto - acoustic magnetic particle concentration imaging device according to the present invention; Figure 3 is a flowchart of the design method of the dual - plane spiral gradient coil after the improvement of the magneto - acoustic magnetic particle concentration imaging device according to the present invention; Figure 4 is a schematic diagram of the winding trajectory of the dual - plane spiral gradient coil according to the present invention; Figure 5 is a flowchart of the optimization algorithm adopted in the design of the dual - plane spiral gradient coil according to the present invention; Figure 6 is a plan view of the dual - plane spiral gradient coil after the improvement of the magneto - acoustic magnetic particle concentration imaging device according to the present invention; Figure 73D perspective view of the improved dual-plane spiral gradient coil of the magnetoacoustic magnetic particle concentration imaging device of the present invention; Figure 8 Flux density in the z-direction generated by the improved dual-plane spiral gradient coil of the magnetoacoustic magnetic particle concentration imaging device of the present invention within the imaging region; Figure 9 Non-uniformity of the gradient magnetic field generated by the improved dual-plane spiral gradient coil of the magnetoacoustic magnetic particle concentration imaging device of the present invention within the imaging region; 1 - Dual-plane spiral gradient coil; 2 - Magnetic nanoparticle cluster; 3 - Water tank; 4 - Ultrasonic transducer; 5 - Biological tissue.

[0012] Specific implementation manner: To make the purpose, technical solution and advantages of the improved gradient magnetic field excitation unit clearer, it will be described in detail below with reference to the accompanying drawings. At the same time, to make the drawings concise and easy to understand, only the relevant parts of the present invention are schematically shown in each figure. The schematic diagram of the experimental system principle of the improved magnetoacoustic magnetic particle concentration imaging device is as Figure 1 shown. The magnetic field generated by the dual-plane spiral gradient coil will magnetize the MNPs in the imaging region. The magnetized MNPs will vibrate under the action of the gradient magnetic field and then generate ultrasonic waves. The intelligent controller FPDA controls the rotation angle of the motor driver. The ultrasonic transducer connected to the motor driver makes a circular motion around the imaging region to receive the ultrasonic signals generated by the MNPs. Different concentrations of MNPs distributed in the biological tissue result in different magnetic forces received, and also result in different peak positions or amplitudes of the generated ultrasonic signals. The filter, amplifier and oscilloscope perform denoising, amplification and visualization processing on the signals received by the ultrasonic transducer. The data acquisition card and computer store the filtered and amplified signals. By applying imaging algorithms to process the ultrasonic signal data stored in the computer, the concentration distribution image of MNPs can be reconstructed to achieve magnetoacoustic magnetic particle concentration imaging.

[0013] The experimental system structure of magnetoacoustic magnetic particle concentration imaging is as Figure 2As shown in the figure, it includes: an electromagnetic excitation system, which consists of a signal generator, a pulsed excitation power supply, and a biplanar spiral gradient coil, and passes the generated pulsed current into the gradient coil to generate a gradient magnetic field; an ultrasonic transducer driving system, which consists of a motor driver and a driving circuit, and is used to drive the immersion ultrasonic transducer to perform circular scanning to receive ultrasonic signals; an intelligent control system, which consists of an intelligent controller FPGA, and the intelligent controller FPGA is used to control the current output of the signal generator and the pulsed excitation power supply, the rotation angle of the motor driver, and the timer of the data acquisition card; a signal processing and visualization system, which consists of an immersion ultrasonic transducer, a filter, an amplifier, and an oscilloscope, and is used to remove the noise of the magnetoacoustic signal and visualize the magnetoacoustic signal; a signal acquisition and storage system, which consists of a data acquisition card and a computer, and is mainly used to acquire and store data.

[0014] The flow chart of the design method of the biplanar spiral gradient coil for magnetoacoustic magnetic particle concentration imaging is as Figure 3 shown, including: 1) Select the base pattern of the biplanar spiral coil for magnetoacoustic magnetic particle concentration imaging. Based on the fact that a circular Maxwell coil can generate a stable gradient field, a parameter circle is used as the base of the spiral gradient coil. The winding trajectory of the spiral coil is constructed in the Cartesian coordinate system as Figure 4 shown, is the total number of turns of the coil. The shape of the spiral coil is determined by , and the whole coil is drawn in the Cartesian coordinate system, and its trajectory is expressed by formula (1): (1) In the formula, is the total number of turns of the coil; is the starting point of the coil; is the starting point of the next turn of the coil; is the rotation angle of a certain point of the coil. By controlling the coil parameters, the winding mode of the spiral coil is changed to generate the required target magnetic field. 2) Discretize the two-dimensional planar spiral coil, construct the relationship between the discrete current and the magnetic field intensity in the imaging region, and superimpose the magnetic fields generated by all discrete energized wires to obtain the magnetic field intensity in the imaging region; after discretizing the wire into N parts, according to the Biot-savart law, the magnetic field intensity generated by the wire at any point in space is: (2) In the formula, is the vacuum permeability; is the current intensity passed into the coil; is the target field point coordinate; is the discrete wire coordinate; is the number of discrete spiral wires; The sum of the magnetic fields generated by all discrete wires at the imaging point; 3) Superimpose the magnetic fields generated by all discrete current sources to obtain the magnetic field intensity in the target area of the spiral coil generated at the direction of: (3) The magnetic field intensity generated by the outer ring coil in the target area generated at the direction of is: (4) In the formula, ; is the radius of the ring coil; is the distance from the center of the coil to the target point in the imaging area; is the angle between the line connecting any point on the coil and the center of the ring and the axis; is the distance from the plane where the coil is located to the center of the imaging area. Combining the above two formulas, the magnetic flux density in the target area can be obtained: (5) Through the above formula, the magnetic field intensity in a certain direction at a point in the target area can be obtained, and then the gradient magnetic field value at this point can be obtained, as follows: (6) (6) According to the relationship between the magnetic nanoparticle concentration distribution and the magnetic field gradient in magnetic acoustic magnetic particle concentration imaging, determine the gradient magnetic field required in the imaging area. The relationship between the magnetic nanoparticle concentration distribution and the magnetic field gradient is: (7) In the formula is the particle magnetic moment, is the distribution of magnetic force sound sources, is the Boltzmann constant, is the temperature of the magnetic nanoparticle cluster area, is the excitation magnetic field axis component, is the excitation magnetic field direction gradient, is direction unit vector. 4) Establish a gradient coil optimization model, and define a fitness function according to the requirements of the gradient field in the target area. In this paper, the root mean square error between the theoretical value and the actual value of the magnetic field at the set points in the imaging area is used as the fitness function: (8) The constraint conditions for model optimization are: Wherein, is the magnetic flux density generated by the coil at the target point within the imaging region; is the target magnetic flux density at this point in the imaging region; is the minimum spacing of the coil; is the maximum current applied to the coil; is the maximum spacing between the coil planes. The optimization process of the ALA-optimized coil parameters is as shown in Figure 5 The specific steps are as follows: The first step: Set the initial values of the optimization algorithm, where the number of iterations D = 100, the population size S = 1000, I max = 40 A, D min = 65 mm, D max = 200 mm, and the initial values of the gradient coil are the maximum values of each parameter. The second step: Construct a fitness function based on the initial coil parameters and calculate the fitness function values of each lemming individual. Define the fitness values as the positions of the lemming population. The third step: Adjust the positions of the lemming population and calculate the energy coefficient E. The fourth step: Determine whether the lemmings enter the exploration stage or the exploitation stage according to the magnitude of the energy coefficient. After entering any stage, select four strategies (migration strategy, burrowing strategy, foraging strategy, predator avoidance strategy) according to the probability of E. The fifth step: Calculate the current fitness function value and update the optimal fitness value of the population. The sixth step: Determine whether the fitness function converges or reaches the maximum number of iterations. If either condition is met, output the optimal solution; otherwise, continue to execute the second step - the sixth step until the condition is satisfied. 5) Use the magnetic field inhomogeneity as the effectiveness of the coil design, where the inhomogeneity is: (9) When the inhomogeneity of the magnetic field within the imaging region is less than 5%, it proves the rationality of the coil design.

[0015] The coil winding result obtained by designing the biplanar spiral gradient coil according to the above process is as shown in Figure 6 The inner side is a spiral coil, and the outer side is a circular ring coil. The three-dimensional structure is as shown in Figure 7 The excitation current applied to the spiral coil is 18.11 A, and the excitation current applied to the circular ring coil is 20.61 A. The gradient magnetic field reaches 100 mT / m. Compared with the Maxwell coil, the current amplitude is reduced by about 90%, and the coil size is reduced by about 75%; compared with the matrix coil, the gradient magnetic field is increased by 10 times, and the coil size is reduced by about 30%. The magnetic field generated within the imaging region is as shown in Figure 8 The magnetic field in the imaging region is evenly distributed, and the inhomogeneity is as shown in Figure 9As shown, the imaging area in the middle black frame area is a square with a side length of 60 mm, and the outer black line is the demarcation line with a non-uniformity of 5%. It can be concluded from the image that the imaging area is within the 5% design range, meeting the coil design requirements.

[0016] The main innovation of the present invention lies in proposing a combination of a spiral coil and a circular coil to replace the original Maxwell coil and matrix coil as the gradient magnetic field excitation unit of the magnetoacoustic magnetic particle concentration imaging device. The dual-plane spiral gradient coil significantly reduces the pulsed current excitation on the premise of ensuring the uniform and stable gradient magnetic field in the imaging area, simplifies the coil structure, increases the gradient magnetic field in the target area, and promotes the experimental research of MACT-MI.

[0017] The above is the specific implementation manner of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. It should be pointed out that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and changes can still be made, and these improvements and changes are also regarded as the protection scope of the present invention.

Claims

1. A design method of a dual - plane spiral gradient coil for magneto - acoustic magnetic particle concentration imaging, characterized in that: A double - plane spiral gradient coil is used as the gradient magnetic field excitation unit for magneto - acoustic magnetic particle concentration imaging; for biological tissues labeled with MNPs, a time - varying gradient magnetic field generated by the double - plane spiral gradient coil is applied. After the MNPs are magnetized, they interact with the time - varying magnetic field and generate ultrasonic signals due to the magnetic force vibration. Different concentrations of MNPs in the tissue will generate different magnetic forces and different ultrasonic signals, and this signal can be used for inverse imaging.

2. The dual-plane spiral gradient coil design method for magnetoacoustic magnetic particle concentration imaging according to claim 1, wherein: The gradient magnetic field excitation part in the magneto - acoustic magnetic particle concentration imaging device is replaced by a double - plane spiral gradient coil from the combination of Maxwell coils, matrix coils, Maxwell - Helmholtz coils and a single - sided conical magnetic core. The double - plane spiral gradient coil is a coil structure composed of an inner spiral coil and an outer circular - ring coil.

3. The dual-plane spiral gradient coil design method for magnetoacoustic magnetic particle concentration imaging according to claim 1, characterized in that: The double - plane spiral gradient coil is wound with copper enameled wire with a diameter of 2 mm. The radius of the inner spiral coil is 100 mm, the number of turns is 15, the radius of the outer circular - ring coil is 102 mm, the number of turns is 30, and the distance between the two coil planes is 120.2 mm.

4. The method for designing a dual-plane spiral gradient coil for magnetoacoustic magnetic particle concentration imaging according to claim 1, characterized in that: The imaging region of magneto - acoustic magnetic particle concentration imaging is a cylindrical region with a bottom diameter of 60 mm and a height of 60 mm. The parameter range of the double - plane spiral gradient coil is set according to the imaging region.

5. The dual-plane spiral gradient coil design method for magnetoacoustic magnetic particle concentration imaging according to claim 1, characterized in that: A sinusoidal decay - truncated wave signal current in the counter - clockwise direction is passed through the inner spiral coil of the double - plane spiral gradient coil, while a sinusoidal decay - truncated wave signal current in the clockwise direction is passed through the outer circular - ring coil in the same plane. The magnitudes of the corresponding coil currents on the upper and lower planes are equal and the directions are opposite.

6. The method for designing a dual-plane spiral gradient coil for magnetoacoustic magnetic particle concentration imaging according to claim 1, characterized in that: The design process of the double - plane spiral gradient coil is as follows: 1) Select the base pattern of the double - plane spiral gradient coil and construct the winding trajectory of the spiral gradient coil in the Cartesian coordinate system; 2) Discretize the biplanar spiral gradient coil. The discretized coil can be equivalent to a current source, and establish the relationship between the discrete current source and the magnetic field strength of the imaging region. relation; 3) Superimpose the magnetic fields generated by all discrete current sources to obtain the gradient magnetic field in the imaging region ; 4) Establish a dual-plane spiral gradient coil optimization model for magnetoacoustic magnetic particle concentration imaging, taking the coil shape parameters and current parameters as optimization parameters, with the goal of minimizing the root mean square error between the theoretical and actual magnetic field values at set points within the imaging region; 5) Use the lemming optimization algorithm to optimize the coil current magnitude and coil parameters, and then obtain the optimal parameters and the best excitation current of the double - plane spiral coil. Substitute the optimized parameters into the coil model to obtain the final coil winding path; 6) Use the non-uniformity to verify the effectiveness of the coil design scheme.

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