A magnetic particle imaging system based on gradient field

Abstract

The present invention discloses a magnetic particle imaging system based on a gradient field, comprising: an excitation magnetic field module, comprising a uniform magnetic field excitation unit and a gradient magnetic field excitation coil pair in three directions; the gradient magnetic field excitation coil pairs in each direction are used to provide a gradient magnetic field with varying intensity after being loaded with a varying reverse alternating current; a control module is used to select a gradient magnetic field excitation coil pair in at least one specific direction to provide a varying reverse alternating current to change the magnitude of its magnetic field multiple times according to the target imaging dimension and the target imaging plane, so as to change the direction and magnitude of the total gradient magnetic field in space multiple times; a receiving coil pair is used to generate an induced voltage; a signal processing module is used to process the voltage signal and extract the triple fundamental frequency harmonic component of the spike signal; and an image reconstruction module is used to reconstruct the distribution image of the magnetic nanoparticle concentration in the target to be measured using a system matrix according to different triple fundamental frequency harmonic components. The present invention can reduce power consumption, improve image quality and expand the imaging field of view.

Description

Technical Field

[0001] The invention belongs to the field of medical imaging, and in particular relates to a magnetic particle imaging system based on a gradient field. Background Art

[0002] At present, clinical medical imaging is mainly divided into two categories: one is structural imaging and the other is functional imaging. Structural imaging mainly shows the internal organs and tissue structures of the human body. Imaging methods include B-mode ultrasound, magnetic resonance imaging and CT (Computed Tomography). Functional imaging shows the functions of blood vessels, organs, tissues and cells. Imaging methods include DSA (Digital subtraction angiography), PET (positron emission tomography), SPECT (Single-Photon Emission Computed Tomography) and CTA (CT angiography). Functional imaging usually requires the injection of tracers into the human body. If the tracer itself is radioactive, imaging can be performed directly with a detector, such as PET and SPECT technology. If the tracer itself is not radioactive, such as iodine-containing contrast agents, imaging must be performed using X-ray scanning equipment, such as CTA and DSA technology. However, the reflective tracers and X-rays in the above-mentioned functional imaging technologies will cause certain ionizing radiation hazards to patients and operating doctors.

[0003] Magnetic Particle Imaging (MPI) is a functional imaging technology without ionizing radiation. MPI uses clinically certified superparamagnetic iron oxide nanoparticles (SPIONs) as tracers. The magnetic core size of the magnetic nanoparticles is in the range of 10-60nm, and high-frequency harmonic signals can be generated as the excitation magnetic field changes. MPI imaging mainly uses the selection field to generate a field free region (Field Free Region, FFR), uses the focusing field to quickly move the field free region, and uses the excitation field (driving field) to excite the magnetic orientation of the magnetic nanoparticles in the field free region to reverse and generate high-frequency harmonic signals. The high-frequency harmonic signals are received by the receiving coil, and the spatial distribution image of the concentration of magnetic nanoparticles in the human body is obtained through image reconstruction. Since the magnetic nanoparticles used in MPI are not radioactive, the imaging process does not require the use of X-rays, so there is no ionizing radiation, which is safer for doctors and patients.

[0004] MPI can be used as a vascular imaging technology to assist in treatment. For example, in the diagnosis and treatment of cardiovascular and cerebrovascular diseases, operations such as stent implantation require reference to vascular imaging. However, conventional vascular imaging requires the injection of iodine or gadolinium contrast agents into the patient. These contrast agents need to be metabolized by the kidneys, which can cause a great burden and harm to patients with weak renal function. The magnetic nanoparticles used in magnetic particle imaging are metabolized by the liver, which does not burden the kidneys and is safer for patients. In addition, MPI does not require digital subtraction processing in DSA and has fewer motion artifacts.

[0005] In order to obtain the signal of a specific point or line, MPI needs to use a gradient coil to generate a small magnetic field free zone. The magnetic field free zone can be a point area (magnetic field free point) or a line area (magnetic field free line). MPI uses point-by-point scanning or line-by-line scanning to continuously move the magnetic field free zone for imaging. The signal collected each time only comes from the magnetic field free zone at a specific position, and the signal strength depends on the concentration of magnetic particles in the magnetic field free zone.

[0006] However, MPI, a magnetic particle imaging technology, still has the following disadvantages:

[0007] 1. High power consumption: MPI usually uses one or more pairs of anti-Helmholtz coils to construct a selection field, forming a magnetic field free zone (point or line) in the middle of the selection field. In order to improve the image resolution, the magnetic field free point needs to be small enough and the magnetic field free line needs to be thin enough. Therefore, high power consumption devices are needed to generate a large enough current to generate a large gradient magnetic field to meet the above requirements, which will result in high power consumption of the device.

[0008] 2. Low resolution: The image resolution of current medical imaging scanning technology can basically reach 0.5 mm. Under a field of view of 20 cm, the image resolution of MPI can only reach 5 mm. The spatial resolution of MPI is determined by the intensity of the gradient magnetic field. The larger the gradient magnetic field, the smaller the range of the magnetic field free zone, and the fewer magnetic nanoparticles that generate signals, which will lead to smaller signal intensity, lower signal-to-noise ratio, and worse image quality. The smaller the range of the magnetic field free zone, the more acquisition points will be required, which will lead to longer scanning time and lower temporal resolution. At the same time, the relaxation effect of magnetic nanoparticles will cause lag and delay in the movement of the magnetic field free zone, making the image blurred, which will further reduce the spatial resolution of the image and reduce the scanning speed.

[0009] 3. Small field of view: The field of view of MPI imaging is determined by the composite magnetic field composed of the superposition of the selection field and the excitation field. Currently, MPI is mainly used for mouse imaging, with an imaging field of view of 1-3 cm and an excitation field strength of 10-30 mT. The scanning field of the human body usually requires 20-50 cm, which requires a very high excitation field strength, so it is difficult to achieve.

[0010] Based on the above shortcomings, it can be seen that if MPI is to meet the requirements of human-sized magnetic nanoparticle imaging, it will inevitably require a very strong selection field and excitation magnetic field, which will lead to very high energy consumption. At the same time, its image resolution is too low and the field of view is too small, making it difficult to expand to clinical human body scanning imaging.

[0011] Therefore, how to propose a new magnetic particle imaging scheme to meet the needs of clinical human scanning imaging is a hot issue worthy of study at this stage. Summary of the invention

[0012] In order to solve the above problems existing in the prior art, an embodiment of the present invention provides a magnetic particle imaging system based on a gradient field. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0013] An excitation magnetic field module, comprising a uniform magnetic field excitation unit, and gradient magnetic field excitation coil pairs in the X direction, the Y direction, and the Z direction; wherein the uniform magnetic field excitation unit is used to provide a constant uniform alternating magnetic field in the Z direction; the two excitation coils of each excitation coil pair are arranged in parallel and spaced relative to each other, and the gradient magnetic field excitation coil pair in each direction is used to provide a gradient magnetic field with varying intensity in the direction after being loaded with a varying reverse alternating current; wherein the target to be measured injected with magnetic nanoparticles is placed in the spatial center area of ​​the excitation magnetic field module, and the long axis is parallel to the Z axis;

[0014] A control module, for selecting, with respect to a target imaging dimension and a target imaging plane, to provide a variable reverse alternating current to a gradient magnetic field excitation coil pair in at least one specific direction, so that the magnitude of the gradient magnetic field in the at least one specific direction changes multiple times, so as to change the direction and magnitude of the total spatial gradient magnetic field multiple times;

[0015] A receiving coil pair, used to generate an induced voltage under the action of all excitation magnetic fields;

[0016] A signal processing module, used for performing signal processing on the voltage signal obtained from the receiving coil pair, and extracting a triple fundamental frequency harmonic component of the peak signal therein;

[0017] An image reconstruction module is used to obtain a distribution image of the magnetic nanoparticle concentration in the target to be measured that conforms to the target imaging dimension and the target imaging plane using a system matrix reconstruction based on different triple fundamental frequency harmonic components obtained by multiple changes in the spatial total gradient magnetic field.

[0018] In the solution provided by the embodiment of the present invention, a non-uniform alternating excitation magnetic field is formed by using an excitation coil pair of a gradient magnetic field in three directions. By changing the current size of the excitation coil of the gradient magnetic field in each direction, the size and direction of the total spatial gradient magnetic field can be arbitrarily changed by superimposing gradient magnetic fields in multiple directions. In a certain direction, the size of the total spatial gradient magnetic field is changed by changing the current loaded by the gradient magnetic field, and one-dimensional spatial encoding of the magnetic particle concentration can be achieved in this direction. On this basis, by continuously changing the direction of the total spatial gradient magnetic field in one or more planes, two-dimensional or three-dimensional spatial encoding of the magnetic particle concentration can be achieved, and voltage signals along multiple directions and multiple gradient sizes can be obtained. The triple fundamental frequency harmonic component of the voltage signal is extracted, and the distribution image of the magnetic particle concentration in different dimensions can be obtained by using the system matrix reconstruction.

[0019] The embodiment of the present invention performs non-uniform excitation on the magnetic nanoparticles in the whole space, and the contribution of the induced voltage comes from all the magnetic nanoparticles in the whole space. Since the selection field is not used to generate the magnetic field free zone, the high-power selection field hardware device of MPI can be avoided, and the purpose of reducing power consumption can be achieved. The embodiment of the present invention does not use the focus field to move the magnetic field free zone, so it can avoid the defects of sparse sampling and low spatial resolution caused by artifacts caused by uneven moving speed of the MPI focus field and uneven spatial sampling caused by irregular moving trajectory. MPI uses the magnetic field free zone to excite the voltage signal, and the voltage signal of a single magnetic field free zone is weak. The embodiment of the present invention adopts the full-area excitation method, and the signal strength is greatly enhanced. Therefore, the signal-to-noise ratio is high, which can improve the image quality and meet the needs of clinical diagnosis. Since the embodiment of the present invention does not adopt the scanning method of moving the magnetic field free zone, but performs non-uniform magnetic field excitation and spatial encoding on the whole space, the scanning area is no longer determined by the selection field gradient and the driving field strength, so it is easy to expand the scanning area and range, so that the imaging field of view can match the human body size, and realize human clinical application. And the embodiment of the present invention does not adopt the moving method of the magnetic field free zone of the Lissajous curve, so that the scanning time of a large area is significantly shortened, which can improve the efficiency of clinical scanning. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic structural diagram of a magnetic particle imaging system based on a gradient field provided by an embodiment of the present invention;

[0021] Figure 2 A schematic diagram of the arrangement of a gradient magnetic field excitation coil pair according to an embodiment of the present invention;

[0022] Figure 3 A schematic structural diagram of a shielding coil assembly according to an embodiment of the present invention;

[0023] Figure 4 It is a schematic diagram of the spherical coordinate system;

[0024] Figure 5 (a) is the original image of the first two-dimensional reconstruction simulation experiment performed in the embodiment of the present invention;

[0025] Figure 5 (b) is a two-dimensional projection image reconstructed using the method of the embodiment of the present invention in the first two-dimensional reconstruction simulation experiment of the embodiment of the present invention;

[0026] Figure 6 (a) is the original image of the second two-dimensional reconstruction simulation experiment performed in the embodiment of the present invention;

[0027] Figure 6 (b) is a two-dimensional projection image reconstructed using the method of the embodiment of the present invention in the second two-dimensional reconstruction simulation experiment of the embodiment of the present invention. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] In order to overcome the problems existing in the existing MPI magnetic particle imaging technology, a magnetic particle imaging solution suitable for clinical application of human body is proposed. An embodiment of the present invention provides a magnetic particle imaging system based on gradient field.

[0030] First, the main structure and functions of the magnetic particle imaging system based on the gradient field of the embodiment of the present invention are introduced. Figure 1 As shown, the magnetic particle imaging system based on the gradient field may include:

[0031] The excitation magnetic field module comprises a uniform magnetic field excitation unit and gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction; wherein the uniform magnetic field excitation unit is used to provide a constant uniform alternating magnetic field in the Z direction; the two excitation coils of each excitation coil pair are arranged in parallel and relatively spaced apart, and the gradient magnetic field excitation coil pair in each direction is used to provide a gradient magnetic field with varying intensity in the direction after being loaded with a varying reverse alternating current; wherein the target to be measured injected with magnetic nanoparticles is placed in the spatial center area of ​​the excitation magnetic field module, and the long axis is parallel to the Z axis.

[0032] The control module is used to select a gradient magnetic field excitation coil pair in at least one specific direction to provide a variable reverse alternating current according to the target imaging dimension and the target imaging plane, so that the magnitude of the gradient magnetic field in the at least one specific direction changes multiple times, so as to change the direction and magnitude of the total spatial gradient magnetic field multiple times.

[0033] The receiving coil pair is used to generate an induced voltage under the action of all excitation magnetic fields.

[0034] The signal processing module is used to process the voltage signal obtained from the receiving coil pair and extract the triple fundamental frequency harmonic component of the peak signal therein.

[0035] The image reconstruction module is used to obtain a distribution image of the magnetic nanoparticle concentration in the target to be measured that conforms to the target imaging dimension and target imaging plane using the system matrix reconstruction based on different triple fundamental frequency harmonic components obtained by multiple changes in the spatial total gradient magnetic field.

[0036] The target to be detected in the embodiment of the present invention may be a human, an animal or the like, which can be injected with magnetic nanoparticles and can generate a signal reflecting the concentration of the magnetic nanoparticles by exciting the magnetic nanoparticles with a magnetic field.

[0037] In order to facilitate understanding of the embodiments of the present invention, the above-mentioned parts are described separately.

[0038] 1) Excitation magnetic field module

[0039] ① Uniform magnetic field excitation unit, used to provide a constant uniform alternating magnetic field in the Z direction. A constant uniform alternating magnetic field is an alternating magnetic field with a constant and uniform intensity in the direction of the magnetic field, and the direction of the magnetic field is the Z direction.

[0040] The uniform magnetic field excitation unit of the embodiment of the present invention generates a magnetic field by electromagnetic means.

[0041] In an optional implementation manner, the uniform magnetic field excitation unit includes:

[0042] A normal conductive electromagnet unit or a superconducting electromagnet unit.

[0043] The constant-conductive electromagnet unit generates a magnetic field using a constant current in the coil conductor, and its magnetic lines of force are parallel to the long axis of the target to be measured, that is, parallel to the Z axis. The embodiments of the present invention can use hollow magnets, iron core magnets, etc. to implement the constant-conductive electromagnet unit. The constant-conductive electromagnet unit may include a pair of coils with a spacing, wherein the axial directions of the two coils coincide and the axial directions face the Z direction. The shape of the coil used may be rectangular, circular, and the like. Generally, it takes several hours of power-on for the magnetic field of the constant-conductive electromagnet unit to reach a stable state. Since a large current flowing through the coil will generate a large amount of heat energy, when using the constant-conductive electromagnet unit, the magnetic particle imaging system of the embodiment of the present invention may be provided with a heat exchanger for water cooling and heat dissipation. The constant-conductive electromagnet unit has the advantages of simple structure, light weight, and easy manufacturing and installation.

[0044] The superconducting electromagnet unit uses a coil made of superconducting material to pass a strong current to generate a strong magnetic field. After the external current is cut off, the current in the superconducting coil remains unchanged. Therefore, the superconducting magnetic field is extremely stable. In addition, in the superconducting state, there is no resistance loss when the current flows through the conductor, so the conductor will not heat up. Compared with a normal conductive coil, a wire of the same diameter can pass a larger current in the superconducting state without being damaged. When using a superconducting electromagnet unit, the system is also equipped with a high vacuum, ultra-low temperature Dewar container to maintain the ultra-low temperature environment of the superconducting coil.

[0045] The uniform magnetic field excitation unit is provided with a power supply interface, and the control module provides current to it and performs power supply signal control.

[0046] In the embodiment of the present invention, the type of the uniform magnetic field excitation unit can be reasonably selected according to the use requirements, and the specific structure of the uniform magnetic field excitation unit can be set, and the specific details will not be described in detail here.

[0047] The role of the constant uniform alternating magnetic field is to increase the magnetic field strength of the final excitation magnetic field, so that the voltage signal generated by the excited magnetic nanoparticles carries high-order harmonic components for imaging. Specifically, the embodiment of the present invention studies the change of the magnetization of magnetic nanoparticles with the magnetic field strength, and the voltage signal induced by the magnetic nanoparticles under the uniform AC excitation magnetic field. If the magnetic field strength is small, the magnetization will be in the linear region, and the voltage signal will only have the same fundamental frequency signal as the oscillation frequency after Fourier transformation, which cannot be used for imaging. Therefore, if high-order harmonic components are to be obtained for imaging, it is necessary to increase the magnetic field strength, push the magnetization from the linear region to the nonlinear region, and make the voltage signal close to the square wave form. In this way, the voltage signal will contain high-order harmonic components such as three times the fundamental frequency harmonic components and five times the fundamental frequency harmonic components after Fourier transformation.

[0048] ② The gradient magnetic field excitation coil pairs in the X, Y and Z directions are used to provide gradient magnetic fields in their respective directions. As the name implies, the gradient magnetic field forms a gradient, which is a magnetic field that changes linearly in space. The gradient magnetic field in a certain direction is achieved by loading the gradient magnetic field excitation coil pair in that direction with the same value of reverse alternating current. The gradient magnetic field in the X, Y and Z directions has the same magnetic field direction as the constant uniform alternating magnetic field.

[0049] Among them, the gradient magnetic field excitation coil pair in the Z direction can be a Maxwell coil pair; the gradient magnetic field excitation coil pair in the X direction and the Y direction can be a saddle coil pair, such as a Golay coil pair. The gradient magnetic field excitation coil pair in each direction is provided with a positive and negative terminal connected to the control module, and the control module controls the current output to it to control the size of the gradient field in that direction. Due to the principle of vector superposition, changing the size of the current loaded on the gradient magnetic field excitation coil pair in any direction can change the gradient magnetic field in that direction. By changing the gradient magnetic fields in three directions, a total spatial gradient magnetic field in any direction can be generated and the size of the total spatial gradient magnetic field can be changed. The total spatial gradient magnetic field and the constant uniform alternating magnetic field together constitute the final excitation magnetic field, and its size and direction change with the total spatial gradient magnetic field.

[0050] As is known to all, magnetic nanoparticles have superparamagnetism. When there is an external magnetic field, the magnetic moment of the magnetic nanoparticles in the liquid will be biased towards the direction of the external magnetic field, causing a change in magnetic flux in the receiving coil, thereby generating a voltage signal. The magnetic nanoparticles of the embodiments of the present invention are not limited to iron oxide magnetic nanoparticles (Fe3O4).

[0051] Therefore, the embodiment of the present invention can orderly change the alternating current magnitude of the gradient magnetic field excitation coil pair in the X direction, Y direction and Z direction, so that the direction of the total spatial gradient magnetic field can change along a certain trajectory in space, and realize traversal of each spatial direction; and in each spatial direction, by loading the two excitation coils of the gradient magnetic field excitation coil pair in the individual direction with a reverse alternating current, the magnitude of the gradient magnetic field loaded with the current can be changed in a linear gradient, and the magnetic nanoparticles are stimulated to generate a plurality of different response magnetic field signals, i.e., voltage signals, so that the concentration of the magnetic nanoparticles in the spatial direction can be encoded in one dimension. It can be understood that when the direction of the total spatial gradient magnetic field changes in a plane, or changes in multiple planes in space, based on one-dimensional spatial encoding, the two-dimensional spatial encoding and three-dimensional spatial encoding of the concentration of the magnetic nanoparticles can be correspondingly realized, and a reconstructed image of the distribution of the concentration of the magnetic nanoparticles can be obtained through corresponding decoding and reconstruction processing. This part of the content will be described in detail later.

[0052] 2) Control module

[0053] The control module is mainly used for current control and can be run on a computer. Specifically, it can include a waveform generator and its corresponding front-end controller. The waveform generator can increase the mains voltage to a certain value of alternating current, convert the boosted alternating current into direct current through rectification, and obtain alternating current at a certain frequency through a frequency converter, such as a frequency of 1.67-50KHz; the front-end controller pre-drives the scanning sequence and then performs power drive, distributing current to each coil under the high-voltage control of the frequency conversion output. In addition, the current applied to each coil can be fed back to the pre-drive through a feedback loop, thereby forming a closed-loop control.

[0054] Therefore, the control module can distribute current to the two coils of the uniform magnetic field excitation unit in the excitation magnetic field module, and provide a constant unidirectional alternating current so that the uniform magnetic field excitation unit generates a cosine alternating uniform magnetic field in the central imaging area of ​​the magnetic particle imaging system of the embodiment of the present invention; at the same time, the two excitation coils of the gradient magnetic field excitation coil pair in any direction can be allocated with a reverse alternating current that changes multiple times, so that the gradient magnetic field excitation coil pair in this direction generates a cosine alternating gradient magnetic field with varying sizes in the central imaging area to oscillate the magnetic nanoparticles.

[0055] Wherein, for the target imaging dimension being one-dimensional, two-dimensional or three-dimensional, and the imaging direction being the X direction, Y direction, Z direction or any other spatial direction in one-dimensional imaging, and the target imaging plane being the XY plane, XZ plane, YZ plane or any other plane in two-dimensional imaging, the gradient magnetic field in the selected specific direction is different, and the reverse alternating current selected and provided by the gradient magnetic field excitation coil in the selected specific directions is also different. The above selection content can be determined by preliminary experiments, so as to be selected according to the needs during imaging.

[0056] Of course, the control module can also perform mechanical control of the system and other signal control, etc., the specific contents will not be described in detail here.

[0057] 3) Receiving coil pair

[0058] As is known to all, magnetic field can induce current, and the direction and magnitude of magnetic field are related to the direction and magnitude of induced current. The change of induced magnetic field can be reflected by the change of voltage in the coil. The receiving coil of the embodiment of the present invention generates a corresponding voltage signal for the change of magnetic flux caused by the magnetization response of the magnetic nanoparticles.

[0059] In the receiving coil pair of the embodiment of the present invention, the axial direction of the two coils is the Z direction and there is a distance between them. The type of the receiving coil pair is not limited here, and any existing coil can be selected as needed.

[0060] 4) Signal processing module

[0061] The signal processing module can be run on a computer and is used to process the voltage signal detected from the receiving coil pair.

[0062] The signal processing module may include an analog processing submodule and a digital processing submodule.

[0063] The analog processing submodule includes a low noise amplifier, a signal correction unit, a mixer, a filter, and an analog-to-digital converter;

[0064] The digital processing submodule includes a Fourier transform unit, a spectrum analysis unit, a baseband elimination unit and a signal parameter extraction unit.

[0065] For the analog processing submodule:

[0066] Low noise amplifiers are used to amplify voltage signals.

[0067] The signal correction unit is used to perform signal correction according to signal attenuation.

[0068] Specifically, the embodiment of the present invention can use a large amount of experimental data to predetermine a signal attenuation model. The signal attenuation model characterizes the correspondence between the original voltage signal, the received voltage signal after the original voltage signal is spatially transmitted and amplified with a certain amount of low noise, and the signal attenuation amount. The signal attenuation amount corresponding to the current low-noise amplified voltage signal can be determined through the signal attenuation model, and the current low-noise amplified voltage signal can be compensated and corrected according to the signal attenuation amount.

[0069] The mixer is used to mix signals and convert the useful signal to a lower intermediate frequency.

[0070] Filters are used to perform high-frequency and low-frequency filtering on signals.

[0071] The analog-to-digital converter (ADC) is used to convert analog signals into digital signals. The analog-to-digital converter has a certain sampling frequency and can sample a certain number of points within half a cosine oscillation period corresponding to the excitation frequency of the current.

[0072] For the digital processing submodule:

[0073] The Fourier transform unit is used to transform the voltage signal from the time domain to the frequency domain.

[0074] The spectrum analysis unit is used to perform frequency analysis on the voltage signal in the frequency domain. For a signal of a complete cosine excitation cycle or half a cosine excitation cycle, after Fourier transform, the spectrum analysis unit decomposes the signal function in a certain way and can express it as a linear combination of sine functions of different frequencies to obtain coefficients of different frequencies.

[0075] The baseband reduction unit is used to eliminate the excitation signal of the baseband. Specifically, the voltage signal in the time domain is usually composed of two overlapping parts, one part of the voltage signal is generated by the excitation magnetic field directly in the receiving coil, and the other part of the voltage signal is generated by the magnetic moment of the magnetic nanoparticles in the target to be measured being excited by the magnetic field, resulting in a change in the direction of the magnetic moment. After Fourier transform, the signal generated by the excitation only exists at one times the baseband frequency, while the signal generated by the magnetic nanoparticles exists at one times the baseband frequency and a higher times the baseband frequency. By setting the coefficient of one times the baseband frequency to 0, the signal of the excitation magnetic field can be reduced, leaving only the harmonic signal emitted by the magnetic nanoparticles, that is, eliminating the peak signal corresponding to the baseband frequency, leaving only the higher harmonic signal of the peak signal.

[0076] The signal parameter extraction unit is used to extract the triple fundamental frequency harmonic component of the peak signal in the voltage signal as the signal parameter for subsequent image reconstruction.

[0077] 5) Image reconstruction module

[0078] The image reconstruction module can run on a computer.

[0079] The embodiment of the present invention is actually a magnetic particle imaging solution based on the triple fundamental frequency harmonic component of the spike signal. In order to facilitate understanding of the role of the image reconstruction module, the imaging principle based on the embodiment of the present invention is briefly described below.

[0080] Depending on the magnitude of the excitation magnetic field strength, the shape and size of the magnetization curve are also different, and the shape and size of the signal peak are also different. The embodiment of the present invention uses a cosine oscillation excitation magnetic field, which is expressed as follows:

[0081] H(t)=-Acos(2πft)

[0082] Where A represents the amplitude of the magnetic field intensity; f represents the excitation frequency; and t represents time.

[0083] The signal strength of triple fundamental frequency is derived as:

[0084]

[0085] in, μ0 represents the vacuum magnetic permeability; m represents the magnetic moment of a single magnetic particle; k B represents the Boltzmann constant; T P Represents the absolute temperature inside the human body, specifically (273+37.5)K=310.5K.

[0086] It can be seen from the aforementioned excitation magnetic field module that the embodiment of the present invention does not use the selection field and focusing field in the existing magnetic particle imaging technology MPI, but adopts the technical solution of full-area non-uniform excitation, so that each point in the entire space is a magnetic field free zone and can be excited by the alternating magnetic field, so they all contribute to the voltage signal. According to the linear properties of the Fourier transform, the three times fundamental frequency harmonic component of the peak signal extracted from the voltage signal each time is equivalent to the superposition of the three times fundamental frequency harmonic components of the magnetic nanoparticles of all points / pixels in the entire space. The embodiment of the present invention has found that the three times fundamental frequency harmonic component has a nonlinear relationship with the magnetic field intensity A and is proportional to the magnetic particle concentration c. Therefore, spatial encoding and cross-sectional imaging can be performed through this relationship, that is, the three times fundamental frequency harmonic component is used as the signal parameter of the embodiment of the present invention to reconstruct the image of the magnetic particle concentration.

[0087] The present invention embodiment studies have found that the spatial encoding based on the excitation magnetic field of the triple fundamental frequency harmonic component includes the following requirements: (1) the uniform excitation magnetic field is >15mT to ensure the occurrence of signal peaks, facilitate the extraction of signal parameters, and filter the iron atoms inside the human body. (2) the excitation magnetic field has a size change along the single direction of XYZ, such as a linear gradient change, and the magnetic field is uniform in the other two directions perpendicular to the single direction to facilitate layer selection, (3) the magnetic field directions of all uniform and non-uniform excitation magnetic fields are the same to facilitate signal reception. Therefore, the present invention embodiment can use the control module to control the loading current of the uniform magnetic field excitation unit to achieve a constant uniform alternating magnetic field size of 15mT to 30mT. The control module is used to load the gradient magnetic field excitation coil in the X direction, Y direction and Z direction with a reverse alternating current to achieve a gradient field with linear changes in the field strength in each direction. The linear change in the field strength of the gradient magnetic field, combined with the nonlinear relationship between the triple fundamental frequency harmonic component and the magnetic field strength, can make each encoding and signal acquisition of the magnetic field spatial distribution independent of each other, so that the unique solution of the magnetic particle concentration matrix can be obtained.

[0088] Regarding the one-dimensional spatial encoding and decoding based on the triple fundamental frequency harmonic component of the spike signal, the discretized magnetic field intensity A, magnetic particle concentration c, and the triple fundamental frequency harmonic component of the spike signal comply with the following relationship:

[0089]

[0090] Among them, u3(t) represents the three times fundamental frequency harmonic component; u3(A(r,t)) represents the three times fundamental frequency harmonic component of the signal peak of magnetic particles of unit concentration under the excitation magnetic field intensity A; s(r) represents the sensitivity of the receiving coil.

[0091] Since in the embodiment of the present invention, time corresponds to current change, the formula is discretized:

[0092]

[0093] Where u3(i) represents the triple fundamental frequency harmonic component; N represents the number of samples encoded in the imaging area, and ΔV in the formula represents the volume size of the voxel at the data sampling point; g(i n ,r n ) are elements of the system matrix g, which is independent of the concentration of magnetic particles. The system matrix is ​​used to characterize the spatial distribution of the three-fold fundamental frequency harmonic component of the response voltage signal generated by magnetic particles of unit concentration under the action of the excitation magnetic field. During its construction, the sensitivity of the receiving coil obtained by actual measurement is used for correction. Based on this system matrix, the magnetic particle concentration corresponding to the different moments (corresponding to different currents) of the change in the magnitude of the total spatial gradient magnetic field in each direction can be inferred, so that imaging can be achieved using the image reconstruction method.

[0094] The operation matrix form of the triple fundamental frequency harmonic component is simplified to:

[0095] gc=u3

[0096] From the above formula, we can know that if the system matrix g at unit concentration is known, the magnetic particle concentration of each encoding point can be calculated:

[0097] c=g -1 u3

[0098] in,

[0099]

[0100] The system matrix of the embodiment of the present invention can be obtained in advance through experiments, and is corrected using the measured receiving coil sensitivity during the construction process. The system matrix is ​​expressed as:

[0101]

[0102] The triple fundamental frequency harmonic component obtained by changing the total spatial gradient magnetic field in the same direction is expressed as:

[0103]

[0104] Then use c = g -1 u3, c can be calculated.

[0105] Among them, i0,i1,…,i N-1 represents the different secondary coil currents that change the magnitude of the total spatial gradient magnetic field in this direction, r0, r1, …, r N-1 represents N position points in this direction; u(i1) represents the triple fundamental frequency harmonic component collected when the coil current is i1; g(i N-1,r0) represents the magnetic particles with unit concentration in current i N-1 The triple fundamental frequency harmonic component generated at the r0th position in this direction under the action of the exciting magnetic field; the meanings of the remaining elements are similar. c represents the one-dimensional reconstruction result, and the elements it contains are the concentrations of magnetic particles at various positions in the imaging area. In actual calculations, due to the large amount of calculations, existing algorithms such as algebraic reconstruction method, joint algebraic reconstruction method, and maximum likelihood expectation maximization algorithm can be used to assist the matrix solution. The solution process using the system matrix is ​​not described in detail here.

[0106] Therefore, based on the signal peak triple fundamental frequency harmonic component of the magnetic nanoparticle response to cosine magnetic field excitation, which is proportional to the magnetic particle concentration and has a nonlinear relationship with the excitation magnetic field strength, multi-directional excitation and spatial encoding are performed by combining the excitation magnetic field with linear gradient changes in the three directions of XYZ. The spatial distribution of the field strength of the excitation magnetic field is not uniform, and the spatial distribution of the field strength is different each time. The image reconstruction module can obtain multiple triple fundamental frequency harmonic component signals, and the one-dimensional distribution map of the magnetic particle concentration can be obtained by performing one-dimensional reconstruction of the spatial distribution of the concentration of magnetic particles through the system matrix. By using the relevant reconstruction method to perform two-dimensional or three-dimensional reconstruction processing on the basis of the one-dimensional reconstruction results in multiple directions, a two-dimensional or three-dimensional concentration spatial distribution image of the magnetic nanoparticles inside the target to be measured can be obtained.

[0107] In the solution provided by the embodiment of the present invention, a non-uniform alternating excitation magnetic field is formed by using an excitation coil pair of a gradient magnetic field in three directions. By changing the current size of the excitation coil of the gradient magnetic field in each direction, the size and direction of the total spatial gradient magnetic field can be arbitrarily changed by superimposing gradient magnetic fields in multiple directions. In a certain direction, the size of the total spatial gradient magnetic field is changed by changing the current loaded by the gradient magnetic field, and one-dimensional spatial encoding of the magnetic particle concentration can be achieved in this direction. On this basis, by continuously changing the direction of the total spatial gradient magnetic field in one or more planes, two-dimensional or three-dimensional spatial encoding of the magnetic particle concentration can be achieved, and voltage signals along multiple directions and multiple gradient sizes can be obtained. The triple fundamental frequency harmonic component of the voltage signal is extracted, and the distribution image of the magnetic particle concentration in different dimensions can be obtained by using the system matrix reconstruction.

[0108] The embodiment of the present invention performs non-uniform excitation on the magnetic nanoparticles in the whole space, and the contribution of the induced voltage comes from all the magnetic nanoparticles in the whole space. Since the selection field is not used to generate the magnetic field free zone, the high-power selection field hardware device of MPI can be avoided, and the purpose of reducing power consumption can be achieved. The embodiment of the present invention does not use the focus field to move the magnetic field free zone, so it can avoid the defects of sparse sampling and low spatial resolution caused by artifacts caused by uneven moving speed of the MPI focus field and uneven spatial sampling caused by irregular moving trajectory. MPI uses the magnetic field free zone to excite the voltage signal, and the voltage signal of a single magnetic field free zone is weak. The embodiment of the present invention adopts the full-area excitation method, and the signal strength is greatly enhanced. Therefore, the signal-to-noise ratio is high, which can improve the image quality and meet the needs of clinical diagnosis. Since the embodiment of the present invention does not adopt the scanning method of moving the magnetic field free zone, but performs non-uniform magnetic field excitation and spatial encoding on the whole space, the scanning area is no longer determined by the selection field gradient and the driving field strength, so it is easy to expand the scanning area and range, so that the imaging field of view can match the human body size, and realize human clinical application. And the embodiment of the present invention does not adopt the moving method of the magnetic field free zone of the Lissajous curve, so that the scanning time of a large area is significantly shortened, which can improve the efficiency of clinical scanning.

[0109] In the second aspect, optional implementation methods and supplementary solutions of individual parts of the gradient field-based magnetic particle imaging system of an embodiment of the present invention are described.

[0110] For the excitation magnetic field module, please refer to the optional structure Figure 2 understand, Figure 2 Schematic diagram of the arrangement of the gradient magnetic field excitation coil pair according to an embodiment of the present invention. Figure 2 The cylinder in the first row in the figure illustrates the space formed by the excitation magnetic field module of the embodiment of the present invention. Taking the target to be measured as a person as an example, the plane where the person lies flat is the XZ plane, and the face faces the positive Y direction. Figure 2 The three rows below respectively illustrate the distribution forms of the gradient magnetic field excitation coil pairs in the X direction, Y direction and Z direction in the space of the excitation magnetic field module.

[0111] The gradient magnetic field excitation coil pair in the X direction or the gradient magnetic field excitation coil pair in the Y direction includes:

[0112] A pair of Golay-type transverse gradient coils symmetrical along the plane, wherein each Golay-type transverse gradient coil includes two Golay coils extending along the Z direction, each Golay coil is distributed on the cylindrical surface in a 120° arc, the opening angle of the near arc is 68.7°, and the opening angle of the far arc is 21.3°;

[0113] The symmetry plane of the gradient magnetic field excitation coil pair in the X direction is the YZ plane; the symmetry plane of the gradient magnetic field excitation coil pair in the Y direction is the XZ plane.

[0114] The Z-direction gradient magnetic field excitation coil pair includes:

[0115] A pair of circular Maxwell coils are axially overlapped, axially oriented in the Z direction, and spaced apart.

[0116] The uniform magnetic field excitation unit uses a constant-conductivity electromagnet unit, specifically including:

[0117] A pair of circular Maxwell coils with overlapping axial directions and axial orientation in the Z direction and with a spacing. The arrangement of a pair of coils in the uniform magnetic field excitation unit is similar to Figure 2 The arrangement of the gradient magnetic field excitation coil pair in the Z direction is not shown in the figure for simplicity. Figure 2 Indicated in the middle.

[0118] Regarding the receiving coil pair, in an optional implementation manner, the receiving coil pair includes:

[0119] A pair of circular Holmholtz coils with axial overlap, axial orientation in the Z direction and a spacing therebetween; and the spacing of the receiving coil pair is greater than the spacing of the gradient magnetic field excitation coil pair in the Z direction.

[0120] That is, the receiving coil pair is parallel to the gradient magnetic field excitation coil pair in the Z direction. One coil in the receiving coil pair is located outside one coil in the gradient magnetic field excitation coil pair in the Z direction, and the other coil in the receiving coil pair is located outside the other coil in the gradient magnetic field excitation coil pair in the Z direction.

[0121] The coils in the excitation magnetic field module and the receiving coil pairs are staggered and arranged in a cylindrical space in the above manner. For the combination of each coil, please refer to the above and Figure 2 For better understanding, no overall diagram is given here.

[0122] Regarding the control module, an optional implementation may include:

[0123] Sequencer, front end controller, gradient controller and gradient amplifier.

[0124] The sequence generator is used to send the timing parameters and amplitude parameters of each coil voltage to the front-end controller according to the sequence parameters set by the computer software. The front-end controller outputs the amplitude parameters to the gradient controller according to the timing, and the gradient controller sends voltage signals to the gradient magnetic field excitation coils in three directions in a given timing according to the obtained parameters. The gradient amplifier is used to amplify the voltage signal. Therefore, it can be understood that the voltage received by the gradient magnetic field excitation coil in each direction constitutes a voltage sequence, and each voltage will drive the corresponding current.

[0125] In another optional implementation manner, the digital processing submodule in the signal processing module further includes:

[0126] Relaxation effect deconvolution correction unit, signal peak correction unit, half-value full width correction unit and magnetic field monitoring unit.

[0127] In this implementation manner, the signal parameter extraction unit is used to extract the signal area and the full width at half maximum of the peak signal in addition to extracting the triple fundamental frequency harmonic component of the peak signal.

[0128] Specifically, in each direction of the total spatial gradient field change, a voltage signal is collected each time the current changes, and the signal area can be extracted by integrating the data collected in the time domain. The half-value full width represents the time domain width corresponding to the peak value of the collected signal dropping to half of the amplitude.

[0129] The relaxation effect deconvolution correction unit is used to perform a deconvolution operation on the voltage signal using the relaxation effect convolution kernel to compensate for the signal error caused by the relaxation effect. Specifically, the peak signal is deconvolved using the relaxation effect convolution kernel to compensate for the signal error caused by the phenomena such as peak amplitude attenuation, signal delay, peak broadening, and peak signal asymmetry caused by the relaxation effect.

[0130] The signal peak correction unit and the half-value full width correction unit are used to correct the signal peak and half-value full width of the voltage signal by comparing the signal areas of multiple voltage signals. Specifically, since the signal area is independent of the magnetic field strength, but is proportional to the concentration of magnetic particles. In the process of the size and direction of the total spatial gradient magnetic field, assuming that the concentration of magnetic particles remains unchanged, the area of ​​the signal peak in the voltage signal collected each time, that is, the signal area, should be a conserved quantity, that is, remain unchanged. Based on this conservation quantity, the signal peak and half-value full width of the voltage signal can be corrected by comparing the signal areas of multiple voltage signals actually collected. For example, individual voltage signals that do not meet the conditions are corrected to be consistent with the remaining normal voltage signals, or the current and other related parameters of the gradient magnetic field in the system are adjusted to re-measure each voltage signal that meets the conditions, etc., to improve the accuracy of subsequent measurements.

[0131] The magnetic field monitoring unit is used to give an alarm when an abnormality occurs by comparing the half-value full width of multiple voltage signals. Specifically, since the half-value full width is independent of the concentration of magnetic particles, but is inversely proportional to the intensity of the gradient magnetic field in three directions, according to this inverse relationship, magnetic field monitoring can be achieved by uniformly comparing the half-value full widths of multiple voltage signals actually collected. Specifically, when it is found that the current half-value full width is different from the theoretical half-value full width obtained by the inverse relationship under the specific intensity of the gradient magnetic field in the three directions, it can be determined that there is an abnormality in the gradient magnetic field in the three directions, thereby outputting a warning signal, such as the possibility that the excitation coil of the gradient magnetic field in a certain direction is faulty. Based on this method, it can be used to correct the size and direction of the gradient magnetic field to ensure the precise change of the excitation magnetic field and the encoding accuracy.

[0132] At the same time, the half-value full width can also be used to verify the correction effect of the relaxation effect deconvolution correction unit, and to remind the system matrix to be recalibrated when an abnormality is detected. The specific process will not be described in detail here.

[0133] In an optional embodiment, the magnetic particle imaging system based on the gradient field further includes:

[0134] A carrying device for placing the object to be measured.

[0135] For example, the bearing device can be in the form of a bed, a bracket, etc., which plays the role of bearing and fixing the object to be measured. The plane of the bearing device is parallel to the XZ plane, and the long axis is parallel to the Z axis.

[0136] Wherein, a shielding coil assembly is arranged inside the carrying device.

[0137] The shielding coil assembly includes a plurality of coils arranged in parallel along the length direction of the carrying device; the coils in the shielding coil assembly that are opposite to the central imaging area of ​​the magnetic particle imaging system are central imaging area coils, and the rest are peripheral area coils. The central imaging area coils cover the projection range of the central imaging area in the XZ plane.

[0138] The type of coil included in the shielding coil assembly is not limited here. In an optional implementation, a rectangular coil can be used, such as Figure 3 As shown, Figure 3 The schematic diagram of the structure of the shielding coil assembly of the embodiment of the present invention. During the imaging process, the peripheral area coil is loaded with current, and the central imaging area coil is not loaded with current, that is, only the peripheral area coil is in an open state to generate a static magnetic field for saturating and confining the magnetic nanoparticles in the peripheral area, so that only the magnetic nanoparticles in the central imaging area are excited by the magnetic field to avoid generating interference signals.

[0139] In an optional embodiment, the magnetic particle imaging system based on the gradient field further includes:

[0140] The load-bearing device control module includes a laser positioning unit and a shielding coil assembly control unit; wherein,

[0141] The laser positioning unit is used to determine the scanning part of the target to be measured by using lasers in horizontal and vertical directions, adjust the position of the carrying device, and align the scanning part with the central imaging area.

[0142] For example, when the target to be measured is the human body and the scanning part is the head, the patient who has been injected with magnetic nanoparticles can lie on his back on a supporting device, which is specifically a bed. Through laser positioning, the bed is pushed to push the patient's head to the central imaging area of ​​the magnetic particle imaging system.

[0143] The shielding coil assembly control unit is used to apply current to the peripheral area coil during imaging.

[0144] In another optional implementation, the functions of the load-bearing device control module may also be implemented by the control module.

[0145] In an optional embodiment, the magnetic particle imaging system based on the gradient field further includes:

[0146] Magnetic nanoparticle injector: The magnetic nanoparticle injector can be arranged outside the space area of ​​the excitation magnetic field module, and is used to inject magnetic nanoparticles into the target to be measured.

[0147] The magnetic nanoparticles can be superparamagnetic iron oxide nanoparticles (Resovist), which are colloidal suspensions with a concentration of 0.5mmol Fe / mL. The injection dose is set according to the weight of the target to be measured, for example, if the patient weighs less than 60kg, the injection dose is 0.9mL (equivalent to 0.45mmol iron); if the patient weighs more than or equal to 60kg, the injection dose is 1.4mL (equivalent to 0.7mmol iron). The magnetic nanoparticles are injected intravenously, either manually by a doctor or automatically by an instrument, etc.

[0148] In an optional embodiment, the magnetic particle imaging system based on the gradient field further includes:

[0149] Closed shell; the closed shell can house the excitation magnetic field module to form a hollow cylindrical accommodating space.

[0150] In an optional embodiment, the magnetic particle imaging system based on the gradient field further includes:

[0151] Image display, laser camera and external storage.

[0152] The image display is used to display the distribution image of the magnetic nanoparticle concentration in the target to be measured, which is convenient for doctors and other personnel to observe. The laser holographic camera is a device that uses laser as coherent light to take holographic photos. It is used for image printing to form a film for diagnosis and is connected to the computer through the DICM interface. The external memory is used to connect to the computer to realize data storage and copying.

[0153] In an optional implementation, the DICM interface is also connected to a PACS-RIS system. PACS refers to a medical image archiving and communication system (PACS), which is a comprehensive system that has developed rapidly in recent years with the advancement of digital imaging technology, computer technology and network technology, and aims to comprehensively solve the acquisition, display, storage, transmission and management of medical images. RIS refers to a radiology information management system (RIS), which is a software system for optimizing the workflow management of the hospital radiology department. A typical process includes registration, appointment, consultation, image generation, film production, report, review, and film issuance.

[0154] As before, by changing the gradient magnetic field in three directions, a spatial total gradient magnetic field in any direction can be generated and the size of the spatial total gradient magnetic field can be changed. Figure 4 Schematic diagram of the spherical coordinate system. The relationship between the gradient magnetic field in three directions and the total gradient magnetic field in space, including:

[0155]

[0156]

[0157]

[0158] Among them, G x Indicates the magnitude of the gradient magnetic field in the X direction; G y Indicates the magnitude of the gradient magnetic field in the Y direction; G z represents the magnitude of the gradient magnetic field in the Z direction; G represents the magnitude of the total gradient magnetic field in space, θ and is the angle of the spherical coordinate system. The two angles jointly determine the direction of the total gradient magnetic field in space. If either angle changes, the direction of the total gradient magnetic field in space will change. arctan(·) represents the inverse tangent function; arccos(·) represents the inverse cosine function.

[0159] Alternatively, the above relationship can also be expressed as:

[0160]

[0161] Gx , G y and G z are the projection components of G on each coordinate axis. Therefore, it can be understood that by adjusting G x , G y and G z The size of can be combined to obtain the required total spatial gradient magnetic field size G, as well as the characterization of the total spatial gradient magnetic field direction

[0162] The embodiment of the present invention can determine a certain regulation rule through preliminary experiments, and purposefully change G x , G y and G z The size of each θ and In the spatial direction of the composition, the size of the total spatial gradient magnetic field G is changed multiple times, so that one-dimensional spatial encoding can be realized in this direction. The concentration distribution of magnetic particles in this direction can be obtained by reconstructing the multiple triple fundamental frequency harmonic components obtained by one-dimensional spatial encoding using the system matrix, that is, a one-dimensional distribution map. On the basis of one-dimensional spatial encoding, by purposefully changing G x , G y and G z The size of each θ and Angle, so that the direction of the spatial total gradient magnetic field is traversed in space, so as to obtain one-dimensional spatial encoding in multiple directions, through the one-dimensional spatial encoding in multiple spatial directions, a two-dimensional or three-dimensional reconstructed image of the magnetic particle concentration distribution is obtained by using a reconstruction method.

[0163] The following briefly describes the functions of the control module and the image reconstruction module when imaging in different dimensions. Please understand the other modules together, and no detailed description will be given here.

[0164] (1) When the target imaging dimension is one-dimensional,

[0165] The control module is specifically used to determine, in a predetermined corresponding relationship between imaging and gradient magnetic fields, a gradient magnetic field in a specific direction corresponding to a target imaging direction during one-dimensional imaging, and an alternating voltage sequence of a gradient magnetic field excitation coil pair in the specific direction; using a voltage drive to generate current, provide the two excitation coils of the gradient magnetic field in the specific direction with the same increasing alternating current sequence or the same decreasing alternating current sequence, so that the magnetic field size of the total spatial gradient magnetic field in the specific direction is changed multiple times; wherein the steps between the current values ​​in any current sequence in the increasing alternating current sequence and the decreasing alternating current sequence are the same.

[0166] The image reconstruction module is specifically used to perform one-dimensional reconstruction using the system matrix based on multiple different triple fundamental frequency harmonic components obtained according to the change in the magnetic field size of the total spatial gradient magnetic field in this specific direction, thereby obtaining a one-dimensional reconstruction result including the concentration information of multiple layers of magnetic nanoparticles in this direction.

[0167] The target imaging direction may be the X direction, the Y direction, or any other spatial direction. The one-dimensional reconstruction result is expressed in the form of an image, which is a one-dimensional distribution diagram of the magnetic particle concentration in the target imaging direction.

[0168] (2) When the target imaging dimension is two-dimensional,

[0169] The control module is specifically used to determine, in a predetermined corresponding relationship between imaging and gradient magnetic fields, at least two specific directions of gradient magnetic field corresponding to the target imaging plane, and respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the at least two specific directions; using a voltage drive to generate current, provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the at least two specific directions, so that the magnitudes of the gradient magnetic fields in the at least two specific directions change multiple times, causing the direction of the spatial total gradient magnetic field to change within a specific plane, and by providing the same increasing alternating current sequence or the same decreasing alternating current sequence for the two excitation coils of the gradient magnetic field in each specific direction, the magnitude of the magnetic field of the spatial total gradient magnetic field changes multiple times in each direction of the change; wherein, the steps between the current values ​​in any current sequence in the increasing alternating current sequence and the decreasing alternating current sequence are the same; and the specific plane is determined according to the target imaging plane.

[0170] The image reconstruction module is specifically used to perform one-dimensional reconstruction using the system matrix according to multiple different triple fundamental frequency harmonic components obtained by the change of the magnetic field magnitude in the same direction of the total spatial gradient magnetic field, and obtain a one-dimensional reconstruction result including the concentration information of multiple layers of magnetic nanoparticles in the direction; perform two-dimensional filtering back projection on all one-dimensional reconstruction results obtained by the change of the direction of the total spatial gradient magnetic field, and obtain a two-dimensional projection map for the target imaging plane, and the two-dimensional projection map represents the distribution image of the concentration of magnetic nanoparticles in the target to be measured in the target imaging plane. Among them, the target imaging plane can be an XY plane, an XZ plane, a YZ plane, and any other plane.

[0171] The corresponding relationship between imaging and gradient magnetic field is predetermined by experimental data, including the gradient magnetic field in a specific direction to be adjusted under the target imaging dimension and the target imaging plane, and the alternating voltage sequence of the gradient magnetic field excitation coil in a specific direction, the alternating current sequence generated by the corresponding drive, and the sequence of changes in the magnitude of the magnetic field under the corresponding alternating current sequence of the gradient magnetic field in the specific direction to be adjusted, and the θ and The sequence of angle changes and the sequence of changes in the total spatial gradient magnetic field magnitude G.

[0172] Specifically, for 2D imaging, θ and One of the angles is fixed and the other angle is traversed, so that the direction of the total spatial gradient magnetic field changes within a specific plane corresponding to the fixed angle, and in the direction formed by each traversal angle, multiple changes in the size of the total spatial gradient magnetic field are achieved in that direction. Each change in the size of the total spatial gradient magnetic field can obtain a triple fundamental frequency harmonic component.

[0173] In two-dimensional imaging, θ and The numerical range of is determined according to the imaging plane, and the traversal step of the traversal angle and the number of changes in the total spatial gradient magnetic field in a certain direction are determined according to the imaging resolution requirements. The smaller the traversal step of the traversal angle and the more the number of changes in the total spatial gradient magnetic field in a certain direction, the higher the imaging resolution. It can be understood that the data dimension of the obtained triple fundamental frequency harmonic component = the number of changes in the traversal angle × the number of changes in the total spatial gradient magnetic field in a certain direction.

[0174] In a certain direction, a changing alternating current is provided to the gradient magnetic field excitation coil pair in a specific direction so that the magnitude of the gradient magnetic field in the specific direction changes multiple times. This can be achieved by providing the same increasing alternating current sequence to the two excitation coils of the gradient magnetic field excitation coil pair in the specific direction, or by providing the same decreasing alternating current sequence to the two excitation coils, so that the magnitude of the gradient magnetic field in the specific direction changes multiple times according to the corresponding current sequence.

[0175] The mathematical principle of the filtered back-projection reconstruction method is Radon transform, which is commonly used in CT imaging reconstruction. For the specific transformation process, please refer to the relevant prior art and will not be described in detail here.

[0176] (3) When the target imaging dimension is three-dimensional,

[0177] The control module is specifically used to determine the respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in the predetermined corresponding relationship between the imaging and the gradient magnetic field, and provide the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions by means of voltage drive to generate current, so that the magnitudes of the gradient magnetic fields in the three directions are changed multiple times, causing the direction of the spatial total gradient magnetic field to change in space, and by providing the same increasing alternating current sequence or the same decreasing alternating current sequence for the two excitation coils of the gradient magnetic field in each direction in at least two directions of the gradient magnetic field, so that the magnetic field magnitude of the spatial total gradient magnetic field in each direction of the change is changed multiple times; wherein the steps between the current values ​​in any current sequence in the increasing alternating current sequence and the decreasing alternating current sequence are the same.

[0178] The image reconstruction module is specifically used to perform one-dimensional reconstruction using a system matrix based on multiple different triple fundamental frequency harmonic components obtained by the change of the magnetic field size of the total spatial gradient magnetic field in the same direction, so as to obtain a one-dimensional reconstruction result including the concentration information of multiple layers of magnetic nanoparticles in this direction; perform two-dimensional filtering back-projection on all one-dimensional reconstruction results obtained by the change of the direction of the total spatial gradient magnetic field in the same plane to obtain a two-dimensional projection map; perform three-dimensional reconstruction on the two-dimensional projection map obtained by the change of the direction of the total spatial gradient magnetic field in each plane to obtain a three-dimensional imaging map; the three-dimensional imaging map represents the distribution image of the concentration of magnetic nanoparticles in the target to be measured in three-dimensional space.

[0179] Specifically, for 3D imaging, θ and One of the angles is fixed, that is, it is used as a fixed angle, and the other angle is used as a traversal angle to traverse within a certain range with corresponding steps, and in the direction formed by each fixed angle + traversal angle, the gradient magnetic field current is changed to achieve multiple changes in the total spatial gradient magnetic field size in this direction.

[0180] When the traversal angle is completed, the fixed angle is changed by one step, and the traversal angle is traversed again according to the above method until the traversal angle is completed.

[0181] The above process is repeated while changing the fixed angle until the fixed angle reaches its traversal upper limit.

[0182] In the above process, first θ and There is no limit on which angle is used as the fixed angle to start the traversal, as long as the traversal process of all angles in the space can be realized. The numerical range of is determined according to the three-dimensional imaging requirements, and the traversal step of the traversal angle and the number of changes in the total spatial gradient magnetic field in a certain direction are determined according to the imaging resolution requirements.

[0183] It is understandable that the

[0184] Among them, the three-dimensional reconstruction is to calculate the distribution image of the magnetic particle concentration in the target to be measured in three-dimensional space based on the data information in the two-dimensional magnetic particle concentration distribution image projected in different directions. The methods used can be tomographic synthesis, filtered back projection reconstruction, iterative reconstruction or artificial intelligence reconstruction. The specific processes of these methods are not described in detail here.

[0185] In the solution provided by the embodiment of the present invention, a non-uniform alternating excitation magnetic field is formed by using an excitation coil pair of a gradient magnetic field in three directions. By changing the current size of the excitation coil of the gradient magnetic field in each direction, the size and direction of the total spatial gradient magnetic field can be arbitrarily changed by superimposing gradient magnetic fields in multiple directions. In a certain direction, the size of the total spatial gradient magnetic field is changed by changing the current loaded by the gradient magnetic field, and one-dimensional spatial encoding of the magnetic particle concentration can be achieved in this direction. On this basis, by continuously changing the direction of the total spatial gradient magnetic field in one or more planes, two-dimensional or three-dimensional spatial encoding of the magnetic particle concentration can be achieved, and voltage signals along multiple directions and multiple gradient sizes can be obtained. The triple fundamental frequency harmonic component of the voltage signal is extracted, and the distribution image of the magnetic particle concentration in different dimensions can be obtained by using the system matrix reconstruction.

[0186] The embodiment of the present invention performs non-uniform excitation on the magnetic nanoparticles in the whole space, and the contribution of the induced voltage comes from all the magnetic nanoparticles in the whole space. Since the selection field is not used to generate the magnetic field free zone, the high-power selection field hardware device of MPI can be avoided, and the purpose of reducing power consumption can be achieved. The embodiment of the present invention does not use the focus field to move the magnetic field free zone, so it can avoid the defects of sparse sampling and low spatial resolution caused by artifacts caused by uneven moving speed of the MPI focus field and uneven spatial sampling caused by irregular moving trajectory. MPI uses the magnetic field free zone to excite the voltage signal. The voltage signal of a single magnetic field free zone is weak, while the embodiment of the present invention adopts the full-area excitation method, and the signal strength is greatly enhanced. Therefore, the signal-to-noise ratio is very high, which can improve the image quality and meet the needs of clinical diagnosis. Since the embodiment of the present invention does not adopt the scanning method of moving the magnetic field free zone, but performs non-uniform magnetic field excitation and spatial encoding on the whole space, the scanning area is no longer determined by the selection field gradient and the driving field strength, so it is easy to expand the scanning area and range, so that the imaging field of view can match the human body size, and realize human clinical application. And the embodiment of the present invention does not adopt the moving method of the magnetic field free zone of the Lissajous curve, so that the scanning time of a large area is significantly shortened, which can improve the efficiency of clinical scanning. At the same time, by providing a variety of auxiliary equipment, the clinical use function of the embodiment of the present invention is enhanced.

[0187] In the third aspect, the scanning imaging process is described in conjunction with a specific structure of a magnetic particle imaging system based on a gradient field according to an embodiment of the present invention. Figure 2 It should be noted that the parameter values ​​involved below are not intended to limit the embodiments of the present invention, but are only used as an example of an implementation method to facilitate understanding of the solution. In actual use, appropriate values ​​can be specifically selected as needed.

[0188] The diameter of the two circular Maxwell coils of the uniform magnetic field excitation unit is 40 cm, the thickness and width are both 5 cm, the number of coil turns is 200, and the distance between the two coils is 40 cm. The two coils are loaded with unidirectional alternating current, and the maximum current value range is 20-60 amperes, generating a 10-20mT cosine alternating uniform magnetic field in the central imaging area of ​​the magnetic particle imaging system, and the excitation frequency is 1.67-5.0 kHz.

[0189] A pair of Golay-type transverse gradient coils in the gradient magnetic field excitation coil pair in the X direction are applied with reverse alternating currents, and the excitation frequency is 1.67-5.0 kHz. The axial magnetic field component is distributed as a linear gradient along the x direction and uniformly distributed in the yz plane. The variation range of the magnetic field intensity within 20 cm of the central imaging area is less than 5%, ensuring that the isomagnetic field surface is a plane, not a curved surface. The size of the gradient magnetic field is changed by increasing the current of the two gradient magnetic field excitation coils at the same time. For example, if the current loaded on the coil is changed 256 times, the size of the gradient magnetic field changes 256 times, and the intensity changes from -25mT / m to 25mT / m, with each change of 0.195mT / m.

[0190] A pair of Golay-type transverse gradient coils in the Y-direction gradient magnetic field excitation coil pair are applied with reverse alternating currents, and the excitation frequency is 1.67-5.0 kHz. The axial magnetic field component is distributed as a linear gradient along the y direction and uniformly distributed in the xz plane. The variation range of the magnetic field intensity within 20 cm of the central imaging area is less than 5%, ensuring that the isomagnetic field surface is a plane, not a curved surface. The size of the gradient magnetic field is changed by increasing the current of the two gradient magnetic field excitation coils at the same time. For example, if the current loaded on the coil is changed 256 times, the size of the gradient magnetic field changes 256 times, and the intensity changes from -25mT / m to 25mT / m, with each change of 0.195mT / m.

[0191] In the two circular Maxwell coils of the gradient magnetic field excitation coil pair in the Z direction, each coil has a diameter of 40 cm, a thickness and width of 5 cm, 200 turns of the coil, and a spacing of 40 cm between the two coils. The two coils apply reverse alternating currents, the maximum current ranges from 20-60 amperes, and the excitation frequency is 1.67-5.0 kHz. The axial magnetic field component changes in a linear gradient magnetic field along the z direction and is uniformly distributed in the xy plane. The size of the gradient magnetic field is changed by increasing the current of the two gradient magnetic field excitation coils at the same time. For example, if the current loaded on the coil is changed 256 times, the size of the gradient magnetic field changes 256 times, and the intensity changes from -25mT / m to 25mT / m, with each change of 0.195mT / m.

[0192] The two circular Holmholtz coils of the receiving coil pair are used to receive the magnetization vector changes in the z direction. Each coil has a diameter of 40 cm, a thickness and width of 5 cm, and a spacing of 50 cm between the two coils.

[0193] The uniform magnetic field excitation unit, the three-directional gradient magnetic field excitation coil pair and the receiving coil pair are encapsulated in the shell to form a hollow cylindrical structure, for example, the inner diameter of the hollow cylindrical structure is 30-75 cm, the outer diameter is 50-200 cm, and the length is 50-100 cm. The maximum load-bearing capacity of the bed as the carrier device is 500 pounds. Under the control of the laser positioning unit of the carrier device control module, the scanning part of the target to be measured placed on the bed is controlled to be placed in the center of the hollow cylindrical structure, that is, the central imaging area during imaging.

[0194] The shielding coil assembly inside the bed includes 15 rectangular coils arranged along the length of the bed, with a width of 10 cm and a length of 30 cm. Each coil has 200 turns and the DC current loaded on the coil is 30 amperes. Figure 3 Understand that during imaging, 2-5 central imaging area coils are turned off, so that the magnetic nanoparticles in the central imaging area can be excited by the coils to oscillate and generate signals. The remaining peripheral area coils are turned on to generate a 30mT static magnetic field to saturate and constrain the magnetic nanoparticles in the peripheral area to avoid generating interference signals.

[0195] The control module controls the current of the gradient magnetic field excitation coil pair in a specific direction, and changes the size of the gradient magnetic field in a specific direction to perform spatial encoding of different dimensions and imaging planes. The voltage signal is obtained by receiving the coil pair. The signal sampling frequency of the signal processing module can be 16.5MHz. By continuously extracting the triple fundamental frequency harmonic component of the peak signal in the voltage signal to the image reconstruction module, the image reconstruction module can use the triple fundamental frequency harmonic component of multiple angles and multiple gradient sizes to reconstruct the image of the magnetic particle concentration.

[0196] (I) On One-Dimensional Space Coding and Reconstruction

[0197] When the signal sampling frequency is f c , under the setting of excitation frequency f. First, the one-dimensional spatial encoding and reconstruction process is explained by taking the x direction as an example. After the scanning part of the target to be measured is located in the central imaging area, in the predetermined corresponding relationship between imaging and gradient magnetic field, the alternating voltage sequence of the two excitation coils of the gradient magnetic field excitation coil pair in the x direction is determined.

[0198] The gradient magnetic field G in the X direction x That is the total spatial gradient magnetic field G. According to the initial value of the alternating current sequence generated by the alternating voltage sequence drive, G x exist In the initial spatial direction, the magnetic field magnitude is, for example, -25 mT / m. The gradient magnetic field magnitudes in the other two directions are always 0.

[0199] The current changes according to the alternating current sequence, so that the magnitude of the gradient magnetic field in the X direction changes after each half cycle of cosine oscillation, and a total of 256 half cosine oscillations are completed, so that the magnitude of the gradient magnetic field in the X direction changes from -25mT / m to 25mT / m.

[0200] Therefore, in In this direction, 256 triple fundamental frequency harmonic component signals can be obtained. The triple fundamental frequency harmonic components of the 256 peak signals are reconstructed in one dimension using the corresponding system matrix, and the magnetic particle concentration of each of the 256 layers along this direction can be obtained, that is, One-dimensional reconstruction results in the direction.

[0201] In the embodiment of the present invention, the signal sampling frequency is f c The excitation frequency f can be 16.5 MHz, and the excitation frequency f can be 3.3 KHz. Of course, other values ​​can also be selected as needed.

[0202] (II) About 2D Space Coding and Reconstruction

[0203] Regarding two-dimensional imaging, the imaging plane can be the XY plane, XZ plane, YZ plane, or any other plane. The following uses the XY plane as an example to illustrate the spatial encoding and imaging process. Specifically, when the target imaging dimension is two-dimensional and the target imaging plane is the XY plane:

[0204] The control module is specifically used to determine the gradient magnetic fields in the X direction and the Y direction corresponding to the XY plane, and the respective alternating voltage sequences of the gradient magnetic field excitation coils in the X direction and the Y direction in the predetermined corresponding relationship between the imaging and the gradient magnetic field, and provide the respective alternating current sequences for the gradient magnetic field excitation coils in the X direction and the Y direction by using the voltage drive to generate current, so as to change the magnitude of the gradient magnetic field in the X direction and the Y direction multiple times, causing the direction of the total gradient magnetic field in space to change from Start in the YZ plane and change along the θ direction in steps of 1° to And in each θ direction, the magnitude of the total spatial gradient magnetic field is changed 256 times;

[0205] The image reconstruction module is specifically used to convert the total spatial gradient magnetic field into The 256 different triple fundamental frequency harmonic components obtained by the change of magnetic field magnitude in the direction are reconstructed in one dimension using the system matrix to obtain The one-dimensional reconstruction result of the direction; the 180 one-dimensional reconstruction results obtained by changing θ∈[0°,180°] are back-projected with two-dimensional filtering to obtain a two-dimensional projection map for the XY plane.

[0206] Specifically, after the scanning part of the target to be measured is located in the central imaging area, in the predetermined corresponding relationship between imaging and gradient magnetic field, the alternating voltage sequences of the two excitation coils of the gradient magnetic field excitation coil pair in the X direction and the Y direction are determined to drive and generate corresponding alternating current sequences.

[0207] The gradient magnetic fields in the X and Y directions are superimposed to form the total spatial gradient magnetic field G. According to the alternating current sequence, G The initial spatial direction of the magnetic field is, for example, -25 mT / m. The magnitude of the gradient magnetic field in the Z direction is always 0.

[0208] The current changes in an alternating current sequence, so that the magnitude of the gradient magnetic field in the X direction changes once after each half cycle of cosine oscillation, and a total of 256 half cosine oscillations are completed, so that the magnitude of G changes from -25mT / m to 25mT / m 256 times. In the direction, 256 triple fundamental frequency harmonic component signals can be obtained. The triple fundamental frequency harmonic components of the 256 peak signals are reconstructed in one dimension using the corresponding system matrix, and the following can be obtained: The one-dimensional reconstruction result in the direction contains 256 magnetic particle concentration values.

[0209] Keep unchanged, θ increases by 1°, at each In the direction, the current of the gradient magnetic field excitation coil pair in the X direction and the Y direction is changed 256 times according to the corresponding alternating current sequence to obtain 256 triple fundamental frequency harmonic component signals, and the corresponding system matrix is ​​used for one-dimensional reconstruction to obtain the one-dimensional reconstruction result in this direction. Repeat the above process until The 180 one-dimensional reconstruction results are back-projected using two-dimensional filtering to obtain a two-dimensional projection image for the XY plane.

[0210] It can be understood that during the two-dimensional imaging process, a total of 256×180 half cosine oscillation excitations are completed to encode the signal, that is, the data dimension of the triple fundamental frequency harmonic component is 256×180. Calculated based on the signal sampling frequency of 16.5MHz and the excitation frequency of 3.3KHz, the number of sampling points in half an excitation oscillation cycle is 5000, and a total of 256×180 half oscillation cycles are required, which takes a total of 6.98 seconds.

[0211] Regarding XY plane imaging, the magnitudes of the gradient magnetic fields in three directions can also be adjusted simultaneously using the corresponding alternating current sequence, but the current sequence loaded on the gradient magnetic field excitation coil in the Z direction makes the magnitude of the gradient magnetic field in the Z direction always zero.

[0212] Similarly, XZ plane imaging can be achieved by adjusting the current of the gradient magnetic field in the X and Z directions; YZ plane imaging can be achieved by adjusting the current of the gradient magnetic field in the Y and Z directions. The specific process will not be described in detail.

[0213] Two simulation experiments were conducted on the two-dimensional space encoding and reconstruction process, and the results were as follows: Figure 5 and Figure 6 As shown, Figure (a) in each figure is the original image of the simulation experiment; only the white area in the original image corresponds to the magnetic nanoparticles. Figure (b) in each figure is the two-dimensional image reconstructed by the method of the embodiment of the present invention in the simulation experiment; wherein the two-dimensional filtered back projection is obtained by Radon inverse transformation. Figure 5 (a) is the maximum intensity projection image of the human head blood vessel image obtained by nuclear magnetic resonance. It can be seen from the simulation results that the two-dimensional image reconstructed using the method of the embodiment of the present invention can clearly show the original magnetic particle distribution of the target to be measured.

[0214] 3. About 3D space encoding and reconstruction

[0215] The control module is specifically used to determine the respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in the predetermined corresponding relationship between the imaging and the gradient magnetic field, and provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions by means of voltage drive to generate current, so that the magnitudes of the gradient magnetic fields in the three directions change multiple times, causing the total spatial gradient magnetic field to change from Direction starts, The preset steps are up to 180°, at each At the angle, θ changes from 0° to 180° in steps of 1°, and at each angle θ, the magnitude of the total spatial gradient magnetic field is changed 256 times;

[0216] The image reconstruction module is specifically used to Angle execution: For each angle θ, the 256 different triple fundamental frequency harmonic components obtained by the change of the magnetic field magnitude of the total spatial gradient magnetic field in the direction corresponding to the angle θ are reconstructed in one dimension using the system matrix to obtain the one-dimensional reconstruction result in that direction; the 180 one-dimensional reconstruction results obtained by θ∈[0°,180°] are back-projected by two-dimensional filtering to obtain the The two-dimensional projection diagram corresponding to the angle; when After the angle is executed, all The two-dimensional projection image corresponding to the angle is reconstructed into three dimensions using the tomographic synthesis method to obtain a three-dimensional imaging image.

[0217] Specifically, after the scanning part of the target to be measured is located in the central imaging area, in the predetermined corresponding relationship between imaging and gradient magnetic field, the alternating voltage sequences of the two excitation coils of the gradient magnetic field excitation coil pairs in three directions are determined to drive and generate corresponding alternating current sequences.

[0218] The gradient magnetic fields in three directions are superimposed to form the total spatial gradient magnetic field G. According to the alternating current sequence. In the initial spatial direction, the magnetic field magnitude is, for example, -25mT / m.

[0219] The current changes in an alternating current sequence, so that the magnitude of the gradient magnetic field in the X direction changes once after each half cycle of cosine oscillation, and a total of 256 half cosine oscillations are completed, so that the magnitude of G changes from -25mT / m to 25mT / m 256 times. In the direction, 256 triple fundamental frequency harmonic component signals can be obtained. The triple fundamental frequency harmonic components of the 256 peak signals are reconstructed in one dimension using the corresponding system matrix, and the following can be obtained: The one-dimensional reconstruction result in the direction contains 256 magnetic particle concentration values.

[0220] Keep unchanged, θ increases by 1°, at each In each direction, the current of the gradient magnetic field excitation coil pair is changed 256 times according to the corresponding alternating current sequence to obtain 256 triple fundamental frequency harmonic component signals, and the corresponding system matrix is ​​used for one-dimensional reconstruction to obtain the one-dimensional reconstruction results in each direction. Repeat the above process until The 180 one-dimensional reconstruction results are back-projected using two-dimensional filtering to obtain the one-dimensional reconstruction results for The corresponding two-dimensional projection diagram.

[0221] Next, The step increment is 12°, keeping unchanged, according to the above method, θ changes from 0° to 180°, and at each θ, the current of the gradient magnetic field excitation coil pair in each direction is changed 256 times according to the corresponding alternating current sequence... Finally, the The corresponding two-dimensional projection diagram.

[0222] Repeat the above process until you get the Corresponding two-dimensional projection images. All the obtained two-dimensional projection images are synthesized by tomosynthesis to obtain a distribution image of the concentration of magnetic nanoparticles in the target to be measured in three-dimensional space.

[0223] It can be understood that during the three-dimensional imaging process, the signal encoding caused by 256×180×15 half cosine oscillation excitations is completed, and the data dimension of the triple fundamental frequency harmonic component is 256×180×15. Calculated based on the signal sampling frequency of 16.5 MHz, the excitation frequency of 3.3 kHz, and the number of internal sampling points of 5000, a total of 256*180*15 half oscillation cycles are required, and the total time required is 1.75 minutes.

[0224] In the solution provided by the embodiment of the present invention, a non-uniform alternating excitation magnetic field is formed by using an excitation coil pair of a gradient magnetic field in three directions. By changing the current size of the excitation coil of the gradient magnetic field in each direction, the size and direction of the total spatial gradient magnetic field can be arbitrarily changed by superimposing gradient magnetic fields in multiple directions. In a certain direction, the size of the total spatial gradient magnetic field is changed by changing the current loaded by the gradient magnetic field, and one-dimensional spatial encoding of the magnetic particle concentration can be achieved in this direction. On this basis, by continuously changing the direction of the total spatial gradient magnetic field in one or more planes, two-dimensional or three-dimensional spatial encoding of the magnetic particle concentration can be achieved, and voltage signals along multiple directions and multiple gradient sizes can be obtained. The triple fundamental frequency harmonic component of the voltage signal is extracted, and the distribution image of the magnetic particle concentration in different dimensions can be obtained by using the system matrix reconstruction.

[0225] The embodiment of the present invention performs non-uniform excitation on the magnetic nanoparticles in the whole space, and the contribution of the induced voltage comes from all the magnetic nanoparticles in the whole space. Since the selection field is not used to generate the magnetic field free zone, the high-power selection field hardware device of MPI can be avoided, and the purpose of reducing power consumption can be achieved. The embodiment of the present invention does not use the focus field to move the magnetic field free zone, so it can avoid the defects of sparse sampling and low spatial resolution caused by artifacts caused by uneven moving speed of the MPI focus field and uneven spatial sampling caused by irregular moving trajectory. MPI uses the magnetic field free zone to excite the voltage signal, and the voltage signal of a single magnetic field free zone is weak. The embodiment of the present invention adopts the full-area excitation method, and the signal strength is greatly enhanced. Therefore, the signal-to-noise ratio is high, which can improve the image quality and meet the needs of clinical diagnosis. Since the embodiment of the present invention does not adopt the scanning method of moving the magnetic field free zone, but performs non-uniform magnetic field excitation and spatial encoding on the whole space, the scanning area is no longer determined by the selection field gradient and the driving field strength, so it is easy to expand the scanning area and range, so that the imaging field of view can match the human body size, and realize human clinical application. And the embodiment of the present invention does not adopt the moving method of the magnetic field free zone of the Lissajous curve, so that the scanning time of a large area is significantly shortened, which can improve the efficiency of clinical scanning.

[0226] At the same time, the existing nuclear magnetic resonance imaging technology carries information about tissues such as muscles and bones, which interferes with the observation of blood vessels. The embodiment of the present invention utilizes the characteristic that magnetic nanoparticles only exist in the blood, and imaging does not require digital subtraction, has fewer motion artifacts, and can be used for targeted imaging. Compared with the existing PET and SPECT imaging technologies, the embodiment of the present invention has higher sensitivity and image resolution, and has no ionizing radiation, and the production and storage of tracers are also relatively easy. The two-dimensional reconstruction scheme of the embodiment of the present invention can replace the existing DSA angiography technology, and can provide quick and effective reference information for the diagnosis and treatment of vascular diseases.

[0227] The above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A magnetic particle imaging system based on a gradient field, characterized in that: include: The excitation magnetic field module comprises a uniform magnetic field excitation unit and a gradient magnetic field excitation coil pair in the X direction, the Y direction and the Z direction; wherein the uniform magnetic field excitation unit is used to provide a constant uniform alternating magnetic field in the Z direction; the two excitation coils of each excitation coil pair are arranged in parallel and spaced relative to each other, and the gradient magnetic field excitation coil pair in each direction is used to provide a gradient magnetic field with varying intensity in the direction after being loaded with a varying reverse alternating current; wherein the target to be measured injected with magnetic nanoparticles is placed in the spatial center area of ​​the excitation magnetic field module, and the long axis is parallel to the Z axis; the uniform magnetic field excitation unit comprises: a pair of circular Maxwell coils with axial overlap, axial orientation in the Z direction and a spacing; the gradient magnetic field in the Z direction A field excitation coil pair, comprising: a pair of circular Maxwell coils that overlap in the axial direction, face in the Z direction in the axial direction, and have a spacing; the gradient magnetic field excitation coil pair in the X direction or the gradient magnetic field excitation coil pair in the Y direction comprises: a pair of Golay-type transverse gradient coils that are symmetrical along a plane, wherein each Golay-type transverse gradient coil comprises two Golay coils extending along the Z direction, and each Golay coil is distributed on a cylindrical surface in a 120° arc, with an opening angle of the near arc being 68.7° and an opening angle of the far arc being 21.3°; wherein the symmetry plane of the gradient magnetic field excitation coil pair in the X direction is the YZ plane; and the symmetry plane of the gradient magnetic field excitation coil pair in the Y direction is the XZ plane; A control module, for selecting, with respect to a target imaging dimension and a target imaging plane, to provide a variable reverse alternating current to a gradient magnetic field excitation coil pair in at least one specific direction, so that the magnitude of the gradient magnetic field in the at least one specific direction changes multiple times, so as to change the direction and magnitude of the total spatial gradient magnetic field multiple times; A receiving coil pair, used to generate an induced voltage under the action of all excitation magnetic fields; A signal processing module, used for performing signal processing on the voltage signal obtained from the receiving coil pair, and extracting a triple fundamental frequency harmonic component of the peak signal therein; An image reconstruction module is used to obtain a distribution image of the magnetic nanoparticle concentration in the target to be measured that conforms to the target imaging dimension and the target imaging plane using a system matrix reconstruction based on different triple fundamental frequency harmonic components obtained by multiple changes in the spatial total gradient magnetic field.

2. The magnetic particle imaging system based on gradient field according to claim 1, characterized in that: The receiving coil pair comprises: A pair of circular Holmholtz coils with axial overlap, axial orientation in the Z direction and with a spacing; and the spacing of the receiving coil pair is greater than the spacing of the gradient magnetic field excitation coil pair in the Z direction.

3. The magnetic particle imaging system based on gradient field according to claim 1, characterized in that: The relationship between the gradient magnetic field in three directions and the total gradient magnetic field in space includes: Among them, G x Indicates the magnitude of the gradient magnetic field in the X direction; G y Indicates the magnitude of the gradient magnetic field in the Y direction; G z represents the magnitude of the gradient magnetic field in the Z direction; G represents the magnitude of the total gradient magnetic field in space, θ and is the angle of the spherical coordinate system. The two angles jointly determine the direction of the total gradient magnetic field in space. If either angle changes, the direction of the total gradient magnetic field in space will change. arctan(·) represents the inverse tangent function; arccos(·) represents the inverse cosine function.

4. The magnetic particle imaging system based on gradient field according to claim 1 or 3, characterized in that: When the target imaging dimension is two-dimensional, The control module is specifically used to determine, in a predetermined corresponding relationship between imaging and gradient magnetic field, at least two specific directions of gradient magnetic field corresponding to the target imaging plane, and respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the at least two specific directions; using a voltage drive to generate current, provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the at least two specific directions, so that the magnitudes of the gradient magnetic fields in the at least two specific directions change multiple times, causing the direction of the spatial total gradient magnetic field to change within a specific plane, and by providing the same increasing alternating current sequence or the same decreasing alternating current sequence for the two excitation coils of the gradient magnetic field in each specific direction, the magnitude of the magnetic field of the spatial total gradient magnetic field changes multiple times in each direction of the change; wherein the steps between the current values ​​in any current sequence of the increasing alternating current sequence and the decreasing alternating current sequence are the same; the specific plane is determined according to the target imaging plane; The image reconstruction module is specifically used to perform one-dimensional reconstruction using a system matrix according to multiple different triple fundamental frequency harmonic components obtained by the change in magnetic field magnitude in the same direction of the spatial total gradient magnetic field, so as to obtain a one-dimensional reconstruction result including the concentration information of multiple layers of magnetic nanoparticles in this direction; perform two-dimensional filtering back-projection on all one-dimensional reconstruction results obtained by the change in the direction of the spatial total gradient magnetic field, so as to obtain a two-dimensional projection image for the target imaging plane, wherein the two-dimensional projection image represents a distribution image of the concentration of magnetic nanoparticles in the target to be measured in the target imaging plane.

5. The magnetic particle imaging system based on gradient field according to claim 4, characterized in that: When the target imaging dimension is two-dimensional and the target imaging plane is the XY plane, The control module is specifically used to determine the gradient magnetic fields in the X direction and the Y direction corresponding to the XY plane, and the respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction and the Y direction in a predetermined corresponding relationship between imaging and the gradient magnetic field, and provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the X direction and the Y direction by means of voltage driving to generate current, so as to change the magnitudes of the gradient magnetic fields in the X direction and the Y direction multiple times, thereby causing the direction of the total spatial gradient magnetic field to change from (θ=0°, ) starts in the YZ plane and changes along θ in steps of 1° to (θ=180°, ), and in each θ direction, the magnitude of the spatial total gradient magnetic field is changed 256 times; The image reconstruction module is specifically used to, for each θ, transform the total spatial gradient magnetic field into The 256 different triple fundamental frequency harmonic components obtained by the change of magnetic field magnitude in the direction are reconstructed in one dimension using the system matrix to obtain The one-dimensional reconstruction result of the direction; the 180 one-dimensional reconstruction results obtained by changing θ∈[0°,180°] are back-projected with two-dimensional filtering to obtain a two-dimensional projection map for the XY plane.

6. The magnetic particle imaging system based on gradient field according to claim 1 or 5, characterized in that: When the target imaging dimension is three-dimensional, The control module is specifically used to determine the respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in the predetermined corresponding relationship between the imaging and the gradient magnetic field, and provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions by means of voltage drive to generate current, so that the magnitudes of the gradient magnetic fields in the three directions are changed multiple times, causing the direction of the spatial total gradient magnetic field to change in space, and by providing the same increasing alternating current sequence or the same decreasing alternating current sequence for the two excitation coils of the gradient magnetic field in each direction in at least two directions of the gradient magnetic field, so that the magnetic field magnitude of the spatial total gradient magnetic field in each direction of the change is changed multiple times; wherein the steps between the current values ​​in any current sequence of the increasing alternating current sequence and the decreasing alternating current sequence are the same; The image reconstruction module is specifically used to perform one-dimensional reconstruction using a system matrix according to multiple different triple fundamental frequency harmonic components obtained by the change of the magnetic field size of the spatial total gradient magnetic field in the same direction, so as to obtain a one-dimensional reconstruction result including the concentration information of multiple layers of magnetic nanoparticles in this direction; perform two-dimensional filtering back-projection on all one-dimensional reconstruction results obtained by the change of the direction of the spatial total gradient magnetic field in the same plane to obtain a two-dimensional projection map; perform three-dimensional reconstruction on the two-dimensional projection map obtained by the change of the direction of the spatial total gradient magnetic field in each plane to obtain a three-dimensional imaging map; the three-dimensional imaging map represents the distribution image of the concentration of magnetic nanoparticles in the target to be measured in three-dimensional space.

7. The magnetic particle imaging system based on gradient field according to claim 6, characterized in that: When the target imaging dimension is three-dimensional, The control module is specifically used to determine the respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in the predetermined corresponding relationship between the imaging and the gradient magnetic field, and provide respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions by means of voltage driving to generate current, so that the magnitudes of the gradient magnetic fields in the three directions change multiple times, causing the total spatial gradient magnetic field to change from Direction starts, The preset steps are up to 180°, at each At the angle, θ changes from 0° to 180° in steps of 1°, and at each angle θ, the magnitude of the spatial total gradient magnetic field is changed 256 times; The image reconstruction module is specifically used to, for each Angle execution: For each angle θ, the 256 different triple fundamental frequency harmonic components obtained by the change of the magnetic field magnitude of the total spatial gradient magnetic field in the direction corresponding to the angle θ are reconstructed in one dimension using the system matrix to obtain the one-dimensional reconstruction result in this direction; the 180 one-dimensional reconstruction results obtained by θ∈[0°,180°] are back-projected by two-dimensional filtering to obtain the The two-dimensional projection diagram corresponding to the angle; when After the angle is executed, all The two-dimensional projection image corresponding to the angle is reconstructed into three dimensions using the tomographic synthesis method to obtain a three-dimensional imaging image.

Images