A method and device for thermoacoustic imaging of magnetic nanoparticles based on single pulse magnetic field excitation

Abstract

A method and device for thermoacoustic imaging of magnetic nanoparticles based on single-pulse magnetic field excitation, wherein the mechanism for generating the thermoacoustic effect of magnetic nanoparticles under single-pulse magnetic field excitation is derived based on Langevin theory and the first law of thermodynamics. Based on this new mechanism, it is proposed to use a vortex coil to apply a single-pulse magnetic field to the object to be tested injected with magnetic nanoparticles, so that magnetization energy is generated in the magnetic nanoparticles, which leads to a magnetocaloric effect, and then a thermoacoustic signal is stimulated by thermal expansion. A 32-channel circular scanning device is designed to receive thermoacoustic signals, collect and process the received thermoacoustic signals, and then an image reconstruction method is used to obtain a concentration image of the magnetic nanoparticles in the object to be tested. Through the present invention, the concentration distribution of MNPs in the object to be tested can be reconstructed, and the 32-channel circular scanning device is used to receive signals, which can shorten the imaging time.

Description

Technical Field

[0001] The invention relates to the field of thermoacoustic imaging, and in particular to a magnetic nanoparticle thermoacoustic imaging method and device based on single-pulse magnetic field excitation. Background Art

[0002] A small amount of magnetic nanoparticles (MNP s ) has superparamagnetism and good biocompatibility. It can be targeted under the action of an external magnetic field and can produce thermoacoustic effects. Therefore, it is widely used in the biomedical field, such as cell labeling and separation, thermal therapy and imaging contrast agents. Using alternating magnetic fields to stimulate MNP s This phenomenon of thermoacoustic effect has been studied and applied in the field of medical imaging. s The imaging method that couples magnetic, thermal, and acoustic multiple physical fields can achieve high resolution of ultrasound imaging and high contrast generated by magnetic mediation.

[0003] Based on the Rosensweig model, Piao Daqing et al. proposed a magneto-thermoacoustic imaging method for MNPs excited by a short-time alternating magnetic field in 2013. Through simulation, it was proved that the short-time alternating magnetic field excitation can induce MHz-level thermoelastic ultrasound. The mechanism of the thermoacoustic effect is that MNPs s The magnetic relaxation loss generated under the alternating magnetic field and the heat loss power (unit: W / kg) are given as: Where μ0 is the vacuum magnetic permeability, χ0 is the magnetic susceptibility, ρ is the density, τ is the relaxation time, ω0 is the angular frequency, and H0 is the magnetic field intensity amplitude of the alternating magnetic field. In 2015, Nanyang Technological University in Singapore proposed a modulated short-time alternating magnetic field to excite MNPs for magnetothermoacoustic imaging. First, a bias magnetic field is applied to make the magnetic susceptibility of MNPs zero, forcing the magnetothermoacoustic process to fail to obtain the background field. A short-time alternating magnetic field with a frequency of 20 MHz is then used as the excitation signal. The thermoacoustic signal is then collected and the image is reconstructed. Finally, the reconstructed image is subtracted from the background field, further improving the contrast of the short-time alternating magnetic field to excite MNPs for magnetothermoacoustic imaging. It is worth noting that all of the above thermoacoustic imaging methods for MNPs use short-time alternating magnetic field excitation. The mechanism for producing the thermoacoustic effect is magnetic relaxation loss. According to the heat loss power (SLP CW ) formula, it can be seen that increasing the amplitude of the alternating magnetic field can enhance the thermoacoustic signal. However, since the excitation frequency of the short-term alternating magnetic field is at the MHz and GHz levels, resonance matching is difficult, and the current excitation signal amplitude is not high, resulting in a low signal-to-noise ratio of the generated thermoacoustic signal, and the imaging quality needs to be improved. At the same time, the detuning problem of resonance matching needs to be considered in complex electromagnetic environments. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the existing short-time alternating magnetic field excitation MNPs thermoacoustic imaging method, propose a mechanism for single-pulse magnetic field excitation MNPs to produce thermoacoustic effect, and based on this new mechanism, propose a magnetic nanoparticle thermoacoustic imaging method and device based on single-pulse magnetic field excitation, which is used to reconstruct the MNPs concentration in the test body.

[0005] Based on the Langevin theory and the first law of thermodynamics, the present invention proposes a mechanism for the generation of the thermoacoustic effect of MNPs under the excitation of a single pulse magnetic field. According to this new mechanism, a thermoacoustic imaging principle based on the excitation of MNPs by a single pulse magnetic field is proposed: a single pulse magnetic field (pulse width of 500ns, repetition frequency of 20Hz) is applied to the object to be tested injected with magnetic nanoparticles by a vortex coil. MNPs will generate magnetization energy under the excitation of a single pulse magnetic field, and then generate a magnetocaloric effect, and then stimulate a thermoacoustic signal by thermal expansion. A 32-channel circular scanning device is used to receive the thermoacoustic signal, and the received thermoacoustic signal is collected and processed, and then an image reconstruction method is used to obtain a concentration image of the magnetic nanoparticles in the object to be tested.

[0006] The present invention proposes the following technical solutions:

[0007] A magnetic nanoparticle thermoacoustic imaging method based on single-pulse magnetic field excitation, the method mainly includes three steps: the first step is to obtain the thermoacoustic signal of MNPs by a signal acquisition device; the second step is to reconstruct the thermoacoustic source distribution of MNPs using the obtained thermoacoustic signal; the third step is to reconstruct the concentration distribution of MNPs using the spatial distribution of the thermoacoustic source. The specific process is as follows:

[0008] Step 1: Acquire the thermoacoustic signal of magnetic nanoparticles by the signal acquisition device

[0009] The pulse excitation source excites the object to be tested injected with MNPs through the vortex coil. The MNPs are magnetized under the action of the single pulse magnetic field, thereby forming a certain magnetization energy, and its magnetization intensity is Under adiabatic conditions, this part of magnetization energy will be converted into the internal energy of MNPs, causing magnetocaloric effect, and then stimulated by thermal expansion to generate thermoacoustic signals, which are received by a 32-channel circular scanning device. The thermoacoustic signals received by each ultrasonic transducer are processed by the preamplifier circuit, filter circuit and postamplifier circuit, and then sampled and integrated and stored in the data acquisition card as the raw data for image reconstruction.

[0010] Step 2: Reconstruct the thermoacoustic source distribution of magnetic nanoparticles using the acquired thermoacoustic signals

[0011] It is known that the acoustic pressure wave equation under adiabatic conditions is:

[0012]

[0013] Where p(r, t) is the sound pressure, c s represents the speed of ultrasound, is the Laplace operator, C p represents the specific heat at constant pressure, β is the thermal expansion coefficient, Q(r) represents the thermoacoustic source, and δ′(t) is the derivative of the impulse function.

[0014] Under the stimulation of a single pulse magnetic field, the change in the internal energy of MNPs mainly comes from the accumulation of magnetization energy generated during the dynamic magnetization process. When MNPs are introduced into the magnetic field, the magnetic field energy increment in the entire space is:

[0015]

[0016] Due to the magnetization intensity outside the MNPs region is always zero, so the integral is limited to the space where the MNPs are located. In addition, for superparamagnetic MNPs, and The direction is consistent, so from formula (2), we can know that the magnetization energy density is B0 represents the magnetic induction intensity in vacuum The magnitude of M represents the magnetization intensity The amplitude of .

[0017] According to the first law of thermodynamics, under adiabatic conditions, the internal energy change per unit volume of magnetized MNPs is dU = W m , therefore, the thermal sound source Q(r) can be obtained as:

[0018]

[0019] Solving the sound pressure wave equation according to Green's function yields:

[0020]

[0021] Where r d is the position of the ultrasonic transducer, r is the position of the thermal sound source, R=|r d -r| represents the distance from the thermal acoustic source to the ultrasonic transducer. According to the time reversal method, the reconstruction expression of the thermal acoustic source is obtained:

[0022]

[0023] Where S d is the detection surface, n is the number of d The normal vector of the position. Set the solution domain of the thermal acoustic source and divide it into n×n square grid units. The heat source matrix Q can be solved by formula (5): n×n .

[0024] Step 3: Reconstructing the concentration distribution of magnetic nanoparticles

[0025] According to the Langevin theory, the magnetization intensity of MNPs can be obtained as:

[0026] M=Nm0L(a) (6)

[0027] Where N represents the concentration of MNPs solution, m0 represents the magnetic moment of MNPs, is the Langevin function, Where kB is the Boltzmann constant, H represents the magnitude of the magnetic field, T0 is the absolute temperature, and μ0 is the relative magnetic permeability;

[0028] Since the Langevin function L(a) can be expanded in series as When a<1, the Langevin function can be simplified to Then θ(r) can be expressed as:

[0029]

[0030] Here is defined Discretize C according to the grid cells of the solution domain, and its expression is:

[0031]

[0032] And the matrix C is calculated by nonlinear finite element method. n×n According to formula (7), the MNPs concentration distribution matrix N n×n The expression is:

[0033]

[0034] Then the heat source matrix Q n×n Substituting into formula (9), the concentration distribution of MNPs can be reconstructed.

[0035] Furthermore, the first step specifically includes the following steps:

[0036] Step 1:

[0037] Initialization parameters: Set the number of signals to be collected to m, where m is a positive integer multiple of 32, and set the trigger signal to rising edge trigger;

[0038] Step 2:

[0039] Start the pulse excitation source: apply a pulse current to the vortex coil, thereby generating a single pulse magnetic field in the imaging space, and the magnetic induction intensity of the magnetic field is

[0040] Step 3:

[0041] The signal acquisition device receives the thermoacoustic signal: the 32-channel circular scanning device is adjusted to acquire the signal through the ultrasonic transducer on the selected fault plane;

[0042] Step 4:

[0043] The collected thermoacoustic signals are processed by a preamplifier circuit, a filter circuit and a postamplifier circuit and then stored in a data acquisition card;

[0044] Step 5:

[0045] Determine whether the 32-channel circular scanning device needs to be rotated and perform signal acquisition again:

[0046] The 32-channel circular scanning device is rotated by a connected stepper motor. When n is equal to 1, the stepper motor does not need to move; when n is greater than 1, the host computer needs to control the stepper motor to drive the 32-channel circular scanning device to rotate, and rotate n-1 times in total. Each time, return to step 3 to obtain 32 thermoacoustic signals again, and finally sort all thermoacoustic signals from 0° to 360° and transmit them to the host computer; wherein,

[0047] The present invention also proposes a magnetic nanoparticle thermoacoustic imaging device based on single-pulse magnetic field excitation, wherein the imaging device mainly comprises four parts: a single-pulse magnetic field excitation device, a signal acquisition device, a signal processing module and an image reconstruction module; wherein the excitation source device comprises a single-pulse excitation source and a vortex coil;

[0048] The signal acquisition device includes a stepper motor, a 32-channel circular scanning device whose rotation is controlled by the stepper motor, and an ultrasonic transducer. 32 ultrasonic transducers are fixedly placed in the 32-channel circular scanning device, so that the 32-channel circular scanning device can simultaneously acquire 32 thermoacoustic signals, thereby improving sampling efficiency;

[0049] The signal processing module includes a preamplifier circuit, a filter circuit and a postamplifier circuit;

[0050] The reconstruction module includes a data processing module and an image display module;

[0051] The imaging device performs the imaging method described above.

[0052] After the imaging device collects data, it performs normalization processing on the collected data, and then reconstructs the concentration image of MNPs through an image reconstruction method, and displays it on a host computer.

[0053] The reconstruction module reconstructs the concentration image of MNPs and displays the concentration image on the host computer.

[0054] Furthermore, the 32-channel circular scanning device is controlled at different fault planes by adjusting the height connecting rod, so as to achieve the purpose of performing circular scanning on different fault planes.

[0055] Furthermore, the 32-channel circular scanning device has 32 equally spaced holes, which are used to place 32 ultrasonic transducers respectively.

[0056] The present invention has the following beneficial effects:

[0057] The present invention proposes a new mechanism of thermoacoustic effect produced by MNPs stimulated by a single pulse magnetic field, which can reconstruct the concentration distribution of MNPs in the object to be tested, and designs a 32-channel circular scanning device for receiving signals, which can shorten the imaging time. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the technical solution of the present invention, the drawings required for the embodiments are briefly introduced below.

[0059] Figure 1 It is a schematic diagram of a magnetic nanoparticle thermoacoustic imaging device based on single pulse magnetic field excitation of the present invention;

[0060] Figure 2 It is a schematic diagram of a signal acquisition device and a 32-channel circular scanning device according to a specific embodiment of the present invention;

[0061] Figure 3 The figure is a working flow chart of an imaging device according to a specific embodiment of the present invention.

[0062] In the figure: A1 pulse excitation source, A2 vortex coil, A3 object to be tested, A4 host computer, A5 stepper motor, A632 channel circular scanning device, A7 ultrasonic transducer, A8 preamplifier circuit, A9 filter circuit, A10 post-amplifier circuit, A11 data processing module, A12 image display module, B1 height connecting rod, B2 height connecting rod, B3 height connecting rod. DETAILED DESCRIPTION

[0063] The present invention will be described clearly, in detail and completely below in conjunction with the accompanying drawings and specific implementation methods of the embodiments of the present invention.

[0064] like Figure 1As shown, the imaging device involved in the reconstruction method of the present invention mainly includes four parts: a single-pulse magnetic field excitation device, a signal acquisition device, a signal processing module and an image reconstruction module. The single-pulse magnetic field excitation device includes a pulse excitation source A1 and a vortex coil A2. The diameter of the vortex coil A2 is 5 cm and the number of turns is 4. The vortex coil A2 is placed at the bottom of the box, and then the object to be tested A3 is placed inside the box and located directly above the vortex coil A2, so that the object to be tested A3 and the vortex coil A2 are isolated from each other by air. The pulse excitation source A1 is set to output a narrow pulse signal through the host computer A4, and the pulse excitation source A1 is connected to the vortex coil A2. A single pulse current is generated in the vortex coil A2 through power amplification, and it is used as an excitation signal to generate a single pulse magnetic field. The signal acquisition device is as follows Figure 2 As shown, it includes a stepper motor A5, a 32-channel circular scanning device A6 and an ultrasonic transducer A7, wherein the 32-channel circular scanning device A6 has 32 equally spaced holes for placing ultrasonic transducers, and there is a M3 top screw hole just above each hole for fixing the ultrasonic transducer. The 32-channel circular scanning device A6 can be controlled to rotate by the stepper motor A5. The 32-channel circular scanning device A6 is controlled to be at different fault planes by adjusting the height connecting rod (B1, B2, B3), so as to achieve the purpose of circular scanning of different fault planes. Among them, 32 ultrasonic transducers A7 need to be placed in the 32-channel circular scanning device A6, and each ultrasonic transducer A7 is fixed with an M3 top screw. The signal processing module includes a preamplifier circuit A8, a filter circuit A9 and a postamplifier circuit A10. The ultrasonic transducer A7 is coupled to the object to be measured through oil or deionized water, and the ultrasonic transducer A7 is used to receive the thermal acoustic signal. The received thermoacoustic signal is amplified by the preamplifier circuit A8, where the preamplifier circuit A8 uses an amplifier with a small amplification factor and low noise. The filter circuit A9 uses a fourth-order Butterworth bandpass filter circuit to filter the thermoacoustic signal after the first-stage amplification, and then the filtered thermoacoustic signal passes through the post-amplifier A10, where the amplifier mainly increases the amplification factor. In this way, a thermoacoustic signal with a high signal-to-noise ratio is obtained. The image reconstruction module includes a data processing module A11 and an image display module A12. The thermoacoustic signal passing through the signal detection module is stored in a data acquisition card, normalized, and then transmitted to the host computer, and the concentration image of MNPs is obtained by an image reconstruction method, and finally displayed on the host computer.

[0065] Reconstructing the concentration image of MNPs mainly includes three steps: the first step is to obtain the thermoacoustic signal of MNPs by the signal acquisition device; the second step is to reconstruct the thermoacoustic source distribution of MNPs using the acquired thermoacoustic signal; the third step is to reconstruct the concentration distribution of MNPs using the spatial distribution of the thermoacoustic source. The specific process is as follows:

[0066] Step 1: Acquire the thermoacoustic signal of magnetic nanoparticles by the signal acquisition device

[0067] The pulse excitation source A1 excites the test body A3 injected with MNPs through the vortex coil. The MNPs are magnetized under the action of the single pulse magnetic field, thereby forming a certain magnetization energy, and its magnetization intensity is Under adiabatic conditions, this part of magnetization energy will be converted into the internal energy of MNPs, causing magnetocaloric effect, and then stimulated by thermal expansion to generate thermoacoustic signals, which are received by the 32-channel circular scanning device A6. The thermoacoustic signals received by each ultrasonic transducer A7 are processed by the preamplifier circuit A8, the filter circuit A9 and the post-amplifier circuit A10, and then stored in the data acquisition card after sampling and integration as the original data for image reconstruction.

[0068] Step 2: Reconstruct the thermoacoustic source distribution of magnetic nanoparticles using the acquired thermoacoustic signals

[0069] It is known that the acoustic pressure wave equation under adiabatic conditions is:

[0070]

[0071] Where p(r, t) is the sound pressure, c s represents the speed of ultrasound, is the Laplace operator, C p represents the specific heat at constant pressure, β is the thermal expansion coefficient, Q(r) represents the thermoacoustic source, and δ′(t) is the derivative of the impulse function.

[0072] Under the stimulation of a single pulse magnetic field, the change in the internal energy of MNPs mainly comes from the accumulation of magnetization energy generated during the dynamic magnetization process. When MNPs are introduced into the magnetic field, the magnetic field energy increment in the entire space is:

[0073]

[0074] Due to the magnetization intensity outside the MNPs region is always zero, so the integral is limited to the space where the MNPs are located. In addition, for superparamagnetic MNPs, and The direction is consistent, so from formula (2), we can know that the magnetization energy density is B0 represents the amplitude of magnetic induction intensity in vacuum, and M represents the amplitude of magnetization intensity. According to the first law of thermodynamics, under adiabatic conditions, the internal energy change of magnetized MNPs per unit volume is dU=W m , therefore, the thermal sound source Q(r) can be obtained as:

[0075]

[0076] Solving the sound pressure wave equation according to Green's function yields:

[0077]

[0078] Where r d is the position of the ultrasonic transducer, r is the position of the thermal sound source, R=|r d -r| represents the distance from the thermal acoustic source to the ultrasonic transducer. According to the time reversal method, the reconstruction expression of the thermal acoustic source is:

[0079]

[0080] Where S d is the detection surface, n is the number of d The normal vector of the position. Set the solution domain of the thermal acoustic source and divide it into n×n square grid units. The heat source matrix Q can be solved by formula (5): n×n .

[0081] Step 3: Reconstructing the concentration distribution of magnetic nanoparticles

[0082] According to the Langevin theory, the magnetization intensity of MNPs can be obtained as:

[0083] M=Nm0L(a) (6)

[0084] Where N represents the concentration of MNPs solution, m0 represents the magnetic moment of MNPs, is the Langevin function, where k B is the Boltzmann constant, H represents the magnitude of the magnetic field, T0 is the absolute temperature, and μ0 is the relative magnetic permeability. Since the Langevin function L(a) can be expanded in series as When a<1, the Langevin function can be simplified to Then Q(r) can be expressed as:

[0085]

[0086] Here is defined Discretize C according to the grid cells of the solution domain, and its expression is:

[0087]

[0088] And the matrix C is calculated by nonlinear finite element method. n×n According to formula (7), the MNPs concentration distribution matrix N n×n The expression is:

[0089]

[0090] Then the heat source matrix Qn×n Substituting into formula (9), the concentration distribution of MNPs can be reconstructed.

[0091] like Figure 3 It is the specific workflow of the imaging device, and its working steps include:

[0092] Step 1: Initialize parameters: set the number of signals to be collected to m (m is a positive integer multiple of 32), the sampling frequency to 100MHz, the trigger type to rising edge trigger, and the sampling integration times of signal collection to 512;

[0093] Step 2: Start the excitation device to generate a single pulse magnetic field: Start the pulse excitation source A1, apply a pulse current to the vortex coil A2, and then generate a single pulse magnetic field in space, the magnetic induction intensity is

[0094] Step 3: The signal acquisition device collects the thermoacoustic signal: the height connecting rods (B1, B2, B3) are adjusted to control the 32-channel circular scanning device A6 and the ultrasonic transducer A7 to collect signals on the selected fault plane;

[0095] Step 4: The collected thermoacoustic signal is processed by the preamplifier circuit A8, the filter circuit A9 and the postamplifier circuit A10, and then stored in the data acquisition card;

[0096] Step 5: Determine whether the 32-channel circular scanning device A6 needs to be rotated, and collect signals again. The 32-channel circular scanning device A6 can be rotated by the connected stepper motor A5. When n=1, the stepper motor A5 does not need to move; when n is greater than 1, the host computer A4 needs to control the stepper motor A5 to drive the 32-channel circular scanning device A6 to rotate, and rotate n-1 times in total. Each time, return to step 3 to obtain 32 thermoacoustic signals again, and finally sort all thermoacoustic signals from 0° to 360°, and then transmit them to the host computer A4; wherein,

[0097] Step 6: Normalize the data;

[0098] Step 7: Reconstruct the concentration image of MNPs through the image reconstruction method and display it on the host computer A4.

[0099] The above specific description of the exemplary embodiment is some relevant principles of the present invention explained with reference to the accompanying drawings, and the protection scope of the present invention is not limited to this exemplary embodiment. All embodiments that make various possible substitutions and changes based on the above description are considered to belong to the protection scope of the claims of the present invention.

Claims

1. A magnetic nanoparticle thermoacoustic imaging method based on single pulse magnetic field excitation, characterized in that: The method comprises the following steps: In the first step, the thermoacoustic signal of MNPs is obtained by a signal acquisition device, including: The pulse excitation source excites the object to be tested injected with MNPs through the vortex coil. The MNPs are magnetized under the action of the single pulse magnetic field, thereby forming a certain magnetization energy, and its magnetization intensity is Under adiabatic conditions, this part of magnetization energy is converted into the internal energy of MNPs, causing magnetocaloric effect, and then the thermal expansion excites the thermoacoustic signal. The thermoacoustic signal is received by a 32-channel circular scanning device. The thermoacoustic signal received by each ultrasonic transducer is processed by a preamplifier circuit, a filter circuit and a post-amplifier circuit, and then stored in a data acquisition card after sampling and integration as the raw data for image reconstruction; In the second step, the acquired thermoacoustic signals were used to reconstruct the spatial distribution of the thermoacoustic source of MNPs; The third step is to reconstruct the concentration distribution of MNPs using the spatial distribution of the thermoacoustic source; Wherein, the MNPs are magnetic nanoparticles; The reconstruction of the spatial distribution of the thermal sound source in the second step is achieved by the following method: It is known that the acoustic pressure wave equation under adiabatic conditions is: （1） In the above formula, is the sound pressure, represents the speed of ultrasound, is the Laplace operator, represents the specific heat capacity at constant pressure, is the coefficient of thermal expansion, represents the thermal sound source, is the derivative of the impulse function; Under the stimulation of a single pulse magnetic field, the change in the internal energy of MNPs mainly comes from the accumulation of magnetization energy generated during the dynamic magnetization process. When MNPs are introduced into the magnetic field, the magnetic field energy increment in the entire space is: （2） The magnetization energy density is , Represents the magnetic induction intensity The amplitude of Represents the magnetization intensity The amplitude of According to the first law of thermodynamics, under adiabatic conditions, the internal energy change per unit volume of magnetized MNPs is , so the thermal sound source is for: （3） Solving the sound pressure wave equation according to the Green's function yields: （4） In the above formula, is the position of the ultrasonic transducer, is the location of the thermal sound source, Represents the distance from the thermoacoustic source to the ultrasonic transducer; Further solving formula (4) according to the time reversal method, we get the following reconstructed expression of the thermal sound source: （5） In the above formula, is the detection surface, On the detection surface The normal vector of the position is set to the solution domain of the thermal sound source and divided into The heat source matrix can be solved by using formula (5) for square grid cells: , thus reconstructing the spatial distribution of thermal sound sources; The reconstruction of the concentration distribution of MNPs in the third step is achieved by the following method: According to the Langevin theory, the magnetization intensity of MNPs is: （6） In the above formula, Characterize the concentration of MNPs solution, represents the magnetic moment of MNPs, is the Langevin function, ,in is the Boltzmann constant, represents the magnitude of the magnetic field strength, is the absolute temperature, is the relative magnetic permeability; Since the Langevin function It can be expanded into series as ,when When , the Langevin function can be simplified to ,and then It can be expressed as: （7） Here is defined ,Will Discretize according to the grid units of the solution domain, and its expression is: （8） The matrix is ​​transformed by nonlinear finite element method To solve; According to formula (7), the MNPs concentration distribution matrix The expression is: （9） Then the heat source matrix Substituting into formula (9), the concentration distribution of MNPs can be reconstructed.

2. The method for magnetic nanoparticle thermoacoustic imaging based on single pulse magnetic field excitation according to claim 1, characterized in that: The first step specifically comprises the following steps: Step 1: Initialization parameters: Set the number of signals to be collected to m, where m is a positive integer multiple of 32, and set the trigger signal to rising edge trigger; Step 2: Start the pulse excitation source: Apply a pulse current to the vortex coil, thereby generating a single pulse magnetic field in the imaging space. The magnetic induction intensity of this magnetic field is ; Step 3: The signal acquisition device receives the thermoacoustic signal: the height of the 32-channel circular scanning device in the signal acquisition device is adjusted to acquire the signal through the ultrasonic transducer on the selected fault plane; Step 4: The collected thermoacoustic signals are processed by a preamplifier circuit, a filter circuit and a postamplifier circuit and then stored in a data acquisition card; Step 5: Determine whether the 32-channel circular scanning device needs to be rotated and perform signal acquisition again: The 32-channel circular scanning device is rotated by a connected stepper motor. When n is equal to 1, the stepper motor does not need to move; when n is greater than 1, the host computer needs to control the stepper motor to drive the 32-channel circular scanning device to rotate, and rotate n-1 times in total. Each time, return to step 3 to obtain 32 thermoacoustic signals again, and finally sort all thermoacoustic signals from 0° to 360° and transmit them to the host computer; wherein, .

3. A magnetic nanoparticle thermoacoustic imaging device based on single pulse magnetic field excitation, characterized in that: The imaging device is composed of four parts: a single pulse magnetic field excitation device, a signal acquisition device, a signal detection module and an image reconstruction module; The single pulse magnetic field excitation device comprises a single pulse excitation source and a vortex coil; The signal acquisition device includes a stepper motor, a 32-channel circular scanning device whose rotation is controlled by the stepper motor, and an ultrasonic transducer. The ultrasonic transducer is fixedly placed in the 32-channel circular scanning device, so that the 32-channel circular scanning device can simultaneously acquire 32 thermoacoustic signals, thereby improving sampling efficiency; The signal detection module includes a preamplifier circuit, a filter circuit and a post-amplifier circuit; The reconstruction module includes a data processing module and an image display module; The imaging device performs the imaging method according to any one of claims 1 to 2.

4. The magnetic nanoparticle thermoacoustic imaging device based on single pulse magnetic field excitation according to claim 3, characterized in that: The reconstruction module reconstructs the concentration image of MNPs and displays the concentration image on the host computer.

5. The magnetic nanoparticle thermoacoustic imaging device based on single pulse magnetic field excitation according to claim 4, characterized in that: The 32-channel circular scanning device is controlled at different fault planes by adjusting the height connecting rod, so as to achieve the purpose of performing circular scanning on different fault planes.

6. The magnetic nanoparticle thermoacoustic imaging device based on single pulse magnetic field excitation according to claim 5, characterized in that: The 32-channel circular scanning device has 32 holes with equal spacing, which are used to place 32 ultrasonic transducers respectively.

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