CN114376550A

CN114376550A - Magnetic particle imaging system based on gradient field

Info

Publication number: CN114376550A
Application number: CN202111525777.6A
Authority: CN
Inventors: 贾广; 黄力宇; 田捷; 惠辉; 苗启广; 李檀平; 席力; 王颖; 王泽�; 胡凯
Original assignee: Qingdao Institute Of Computing Technology Xi'an University Of Electronic Science And Technology; Xidian University
Current assignee: Qingdao Institute Of Computing Technology Xi'an University Of Electronic Science And Technology; Xidian University
Priority date: 2021-12-14
Filing date: 2021-12-14
Publication date: 2022-04-22

Abstract

The invention discloses a magnetic particle imaging system based on a gradient field, which comprises: the excitation magnetic field module comprises a uniform magnetic field excitation unit and three-direction gradient magnetic field excitation coil pairs; the gradient magnetic field excitation coil pairs in all directions are used for providing gradient magnetic fields with variable strength after being loaded with variable reverse alternating current; the control module is used for selecting to provide changed reverse alternating current for at least one gradient magnetic field excitation coil pair in a specific direction according to a target imaging dimension and a target imaging plane so as to change the magnetic field size of the gradient magnetic field excitation coil pair for multiple times, so that the direction and size of the total spatial gradient magnetic field are changed for multiple times; the receiving coil pair is used for generating induced voltage; the signal processing module is used for processing the voltage signal and extracting triple fundamental frequency harmonic components of the spike signal; and the image reconstruction module is used for utilizing the system matrix to reconstruct according to different triple fundamental frequency harmonic components to obtain a distribution image of the magnetic nanoparticle concentration in the target to be detected. The invention can reduce power consumption, improve image quality and enlarge imaging visual field.

Description

Magnetic particle imaging system based on gradient field

Technical Field

The invention belongs to the field of medical imaging, and particularly relates to a magnetic particle imaging system based on a gradient field.

Background

Currently, clinical medical imaging is largely divided into two categories: one is structural imaging and one is functional imaging. Structural imaging is mainly to show the structures of organs and tissues inside the human body, and imaging methods include B-mode ultrasound, magnetic resonance, CT (Computed Tomography), and the like. Functional imaging is to show the function of blood vessels, organs, tissues and cells, and imaging methods include DSA (Digital subtraction angiography), PET (positron Emission Computed Tomography), SPECT (Single-Photon Emission Computed Tomography), CTA (CT angiography), and other techniques. Functional imaging typically requires the injection of a tracer into the body. If the tracer itself is radioactive, imaging can be performed directly with detectors, such as PET and SPECT techniques. If the tracer itself does not have radioactivity, such as iodine-containing contrast agents, imaging by X-ray scanning equipment, such as CTA and DSA techniques, is necessary. However, the aforementioned functional imaging techniques involve reflective tracers and X-rays which pose a certain risk of ionizing radiation to the patient and the operating physician.

Magnetic Particle Imaging (MPI) is a functional Imaging technique without ionizing radiation. MPI uses clinically certified superparamagnetic iron oxide nanoparticles (SPIONs) as tracers. The magnetic nano-particle has a magnetic core size in the range of 10-60nm, and can generate a high-frequency harmonic signal along with the change of an excitation magnetic field. MPI imaging mainly utilizes a selection Field to generate a magnetic Field Free Region (FFR), utilizes a focusing Field to rapidly move the magnetic Field Free Region, utilizes an excitation Field (driving Field) to excite the magnetic orientation of magnetic nanoparticles in the magnetic Field Free Region to generate a high-frequency harmonic signal, utilizes a receiving coil to receive the high-frequency harmonic signal, and obtains a spatial distribution image of the concentration of the magnetic nanoparticles in a human body through image reconstruction. Because the magnetic nanoparticles used in MPI do not have radioactivity, the imaging process does not need to use X-rays, so that no ionizing radiation exists, and the safety of doctors and patients is higher.

MPI can be used as an auxiliary treatment of a blood vessel imaging technology, for example, in the diagnosis and treatment process of cardiovascular and cerebrovascular diseases, the operation of implanting a stent and the like needs to be referred to blood vessel imaging. Conventional vascular imaging, however, requires the injection of iodine or gadolinium contrast agents into the patient, which require metabolism through the kidneys, and can be a significant burden and hazard for patients with reduced renal function. The magnetic nanoparticles used for magnetic particle imaging are metabolized through the liver, so that the kidney is not burdened, and the magnetic particle imaging method is safer for patients. Further, MPI does not require digital subtraction processing in DSA, and has fewer motion artifacts.

MPI to obtain signals at specific points or lines, it is necessary to use gradient coils to generate a small free region of magnetic field, which can be either a point region (free point of magnetic field) or a line region (free line of magnetic field). MPI adopts a point-by-point scanning or line-by-line scanning mode, a magnetic field free area is continuously moved for imaging, signals acquired each time only come from the magnetic field free area at a specific position, and the signal intensity depends on the concentration of magnetic particles in the magnetic field free area.

However, the magnetic particle imaging technique MPI still has the following disadvantages:

1. the power consumption is large: MPI usually uses one or more pairs of anti-helmholtz coils to construct the selection field, and a free magnetic field region (point or line) is formed in the middle of the selection field, and in order to improve the image resolution, the free magnetic field point needs to be small enough, and the free magnetic field line needs to be thin enough, so that a large power consumption device is needed to generate a large enough current, and thus a large gradient magnetic field is generated to meet the above requirements, which results in large power consumption of the device.

2. The resolution is low: the image resolution of the current medical imaging scanning technology can basically reach 0.5mm, and the image resolution can only reach 5mm under the MPI field of view of 20 cm. The spatial resolution of MPI is determined by the strength of the gradient magnetic field, the larger the gradient magnetic field, the smaller the extent of the free region of the magnetic field, and the fewer magnetic nanoparticles that generate the signal, which results in smaller signal strength, lower signal-to-noise ratio, and poorer image quality. The smaller the extent of the free region of the magnetic field, the more acquisition points are required, which results in longer scanning time and lower time resolution. Meanwhile, the relaxation effect of the magnetic nanoparticles can cause the movement of the free region of the magnetic field to lag and delay, so that the image becomes blurred, the spatial resolution of the image can be further reduced, and the scanning speed is reduced.

3. The visual field is small; the MPI imaging field of view size is determined by the composite magnetic field formed by the superposition of the selection field and the excitation field. Currently, MPI is mainly applied to mouse imaging, the imaging field of view is 1-3 centimeters, and the required excitation field strength is 10-30 mT. The scanning field of view of the human body usually needs 20-50 cm, which requires high excitation field strength and is therefore difficult to realize.

In summary, MPI requires strong selection field and excitation magnetic field to satisfy the requirement of magnetic nanoparticle imaging of human body size, which results in very large energy consumption. Meanwhile, the image resolution is too low, the visual field is too small, and the extension to clinical human body scanning imaging is difficult.

Therefore, how to propose a new magnetic particle imaging scheme to meet the requirement of clinical human body scanning imaging is a hot issue worth of research at present.

Disclosure of Invention

In order to solve the above problems in the prior art, embodiments of the present invention provide a magnetic particle imaging system based on gradient fields. The technical problem to be solved by the invention is realized by the following technical scheme:

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; the uniform magnetic field excitation unit is used for providing a constant uniform alternating magnetic field in the Z direction; the two exciting coils of each exciting coil pair are arranged in parallel and oppositely at intervals, and the gradient magnetic field exciting coil pair in each direction is used for providing a gradient magnetic field with strength changing in the direction after being loaded with changed reverse alternating current; the target to be measured, into which the magnetic nanoparticles are injected, is placed in the spatial central region of the excitation magnetic field module, and the long axis of the target is parallel to the Z axis;

the control module is used for selecting to provide changed reverse alternating current for at least one gradient magnetic field excitation coil pair in a specific direction according to a target imaging dimension and a target imaging plane, so that the magnitude of the gradient magnetic field in the specific direction is changed for multiple times, and the direction and the magnitude of the total spatial gradient magnetic field are changed for multiple times;

the receiving coil pair is used for generating induced voltage under the action of all the excitation magnetic fields;

the signal processing module is used for carrying out signal processing on the voltage signal obtained from the receiving coil pair and extracting triple fundamental frequency harmonic components of spike signals;

and the image reconstruction module is used for reconstructing to obtain a distribution image of the magnetic nano particle concentration in the target to be detected according to the target imaging dimension and the target imaging plane by utilizing a system matrix according to different triple fundamental frequency harmonic components obtained by changing the spatial total gradient magnetic field for multiple times.

In the scheme provided by the embodiment of the invention, the non-uniform alternating excitation magnetic field is formed by using the excitation coil pairs of the gradient magnetic fields in three directions, and the size and the direction of the total spatial gradient magnetic field can be randomly changed in a multidirectional gradient magnetic field superposition mode by changing the current of the excitation coil of the gradient magnetic field in each direction. In a certain direction, the size of the total gradient magnetic field in space is changed through the change of the loading current of the gradient magnetic field, and one-dimensional space encoding of the magnetic particle concentration in the direction can be realized. On the basis, the direction of the total spatial gradient magnetic field is continuously changed in one or more planes, so that two-dimensional or three-dimensional spatial coding of the magnetic particle concentration can be realized, and voltage signals along multiple directions and multiple gradient magnitudes can be obtained. Three times of fundamental frequency harmonic components of the voltage signals are extracted, and distribution images of the magnetic particle concentration in different dimensions can be obtained by utilizing system matrix reconstruction.

The embodiment of the invention carries out non-uniform excitation on the magnetic nano particles in the whole space, the contribution of the induced voltage comes from all the magnetic nano particles in the whole space, and a magnetic field free area is generated because a selection field is not used, so that MPI high-power-consumption selection field hardware equipment can be avoided, and the purpose of reducing power consumption is realized. The embodiment of the invention does not use a free area of a moving magnetic field of the focusing field, thereby avoiding the defects of sparse sampling and low spatial resolution caused by artifacts caused by non-uniform moving speed of the MPI focusing field, non-uniform spatial sampling caused by irregular moving track and the like. MPI adopts a magnetic field free area to excite a voltage signal, the voltage signal of a single magnetic field free area is weak, and the embodiment of the invention adopts a full-area excitation mode to greatly enhance the signal intensity, so the signal-to-noise ratio is higher, the image quality can be improved, and the requirement of clinical diagnosis can be met. Because the embodiment of the invention does not adopt the scanning mode of the free area of the moving magnetic field, but carries out non-uniform magnetic field excitation and space coding on the whole space, and the scanning area is not determined by the selection field gradient and the driving field strength together any more, the scanning area and the scanning range are easily expanded, the imaging visual field can be matched with the size of the human body, and the clinical application of the human body is realized. In addition, the embodiment of the invention does not adopt a movement mode of a magnetic field free region of a Lissajous curve, so that the large-region scanning time is obviously shortened, and the clinical scanning efficiency can be improved.

Drawings

FIG. 1 is a schematic structural diagram of a magnetic particle imaging system based on gradient field according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of an arrangement of pairs of gradient magnetic field excitation coils according to an embodiment of the present invention;

FIG. 3 is a schematic structural diagram of a shield coil assembly according to an embodiment of the present invention;

FIG. 4 is a schematic view of a spherical coordinate system;

FIG. 5(a) is an original image of a first two-dimensional reconstruction simulation experiment performed according to an embodiment of the present invention;

FIG. 5(b) is a two-dimensional projection diagram reconstructed by using the method of the embodiment of the present invention in a first two-dimensional reconstruction simulation experiment performed by the embodiment of the present invention;

fig. 6(a) is an original image of a second two-dimensional reconstruction simulation experiment performed according to an embodiment of the present invention;

fig. 6(b) is a two-dimensional projection diagram reconstructed by using the method of the embodiment of the present invention in a second two-dimensional reconstruction simulation experiment performed by the embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In order to solve the problems of the existing MPI magnetic particle imaging technology and provide a magnetic particle imaging scheme suitable for human clinical application, the embodiment of the invention provides a magnetic particle imaging system based on a gradient field.

In a first aspect, the main structure and function of a magnetic particle imaging system based on gradient field according to an embodiment of the present invention are described, as shown in fig. 1, the magnetic particle imaging system based on gradient field may include:

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; the uniform magnetic field excitation unit is used for providing a constant uniform alternating magnetic field in the Z direction; the two exciting coils of each exciting coil pair are arranged in parallel and oppositely at intervals, and the gradient magnetic field exciting coil pair in each direction is used for providing a gradient magnetic field with strength changing in the direction after being loaded with changed reverse alternating current; the target to be measured, into which the magnetic nanoparticles are injected, is placed in the spatial central region of the excitation magnetic field module, and the long axis is parallel to the Z axis.

And the control module is used for selecting to provide the changed reverse alternating current for the gradient magnetic field excitation coil pair in at least one specific direction according to the target imaging dimension and the target imaging plane, so that the magnitude of the gradient magnetic field in at least one specific direction is changed for multiple times, and the direction and the magnitude of the total spatial gradient magnetic field are changed for multiple times.

And the receiving coil pair is used for generating induced voltage under the action of all the excitation magnetic fields.

And the signal processing module is used for carrying out signal processing on the voltage signals obtained from the receiving coil pair and extracting triple fundamental frequency harmonic components of spike signals.

And the image reconstruction module is used for obtaining a distribution image of the magnetic nano particle concentration in the target to be detected according to the target imaging dimension and the target imaging plane by utilizing system matrix reconstruction according to different triple fundamental frequency harmonic components obtained by changing the spatial total gradient magnetic field for multiple times.

The target to be detected in the embodiment of the invention can be a human body, an animal and the like which can be injected with the magnetic nano particles, and the magnetic nano particles can be excited by utilizing the magnetic field to generate an object reflecting the concentration signal of the magnetic nano particles.

The above-mentioned parts are respectively explained for facilitating the understanding of the embodiments of the present invention.

1) Excitation magnetic field module

The uniform magnetic field excitation unit is used for providing a constant uniform alternating magnetic field in the Z direction. The constant uniform alternating magnetic field is an alternating magnetic field, the intensity of which is constant and uniform in the magnetic field direction, and the magnetic field direction is the Z direction.

The uniform magnetic field excitation unit of the embodiment of the invention generates a magnetic field in an electromagnetic mode.

In an alternative embodiment, the uniform magnetic field excitation unit includes:

a normally conductive electromagnet unit or a superconducting electromagnet unit.

The normally conductive electromagnet unit generates a magnetic field by using constant current in a coil wire, and the magnetic line of force of the magnetic field is parallel to the long axis of the target to be measured, namely parallel to the Z axis. The embodiment of the invention can realize the normally conducting electromagnet unit by utilizing a hollow magnet, an iron core magnet and the like. The normally conductive electromagnet unit may comprise a pair of spaced coils, wherein the axial directions of the two coils are coincident and axially face in the Z-direction. The coil shape used may be rectangular, circular, etc. Generally, several hours after the power is applied, the magnetic field of the normally conductive magnet unit can reach a steady state. Since a large amount of heat energy is generated by a large current flowing through the coil, the magnetic particle imaging system according to the embodiment of the present invention may be provided with a heat exchanger for water cooling heat dissipation when a normally conductive magnet unit is used. The normally conductive electromagnet unit has the advantages of simple structure, light weight and simple and convenient manufacture and installation.

The superconducting electromagnet unit is formed by connecting a coil made of superconducting material with a strong current to generate a strong magnetic field, and the current in the superconducting coil is kept unchanged after the external current is cut off. The superconducting magnetic field is thus extremely stable. In addition, in the superconducting state, there is no resistance loss when a current flows through the conductor, so that the conductor is not heated. Compared with a normally conducting coil, a wire with the same diameter can pass larger current in a superconducting state without being damaged. When using superconducting electromagnet units, the system is also provided with a high vacuum, ultra-low temperature dewar vessel for maintaining the ultra-low temperature environment of the superconducting coils.

The uniform magnetic field excitation unit is provided with a power interface, and the control module provides current for the uniform magnetic field excitation unit and controls power signals.

The embodiment of the invention can reasonably select the type of the uniform magnetic field excitation unit according to the use requirement, and set the specific structure of the uniform magnetic field excitation unit, and the specific details are not described in detail herein.

The effect 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 higher harmonic components for imaging. Specifically, the embodiment of the invention researches the change situation of the magnetization of the magnetic nanoparticles along with the magnetic field strength and the situation of the voltage signal induced by the magnetic nanoparticles in a uniform alternating-current excitation magnetic field, if the magnetic field strength is small, the magnetization will be in a linear region, and the voltage signal after fourier transform only has a fundamental frequency signal with the same oscillation frequency and cannot be used for imaging. Therefore, if the higher harmonic component is to be obtained for imaging, the magnetic field intensity needs to be increased, and the magnetization amount is pushed from the linear region to the non-linear region, so that the voltage signal is close to the square wave form, and thus, the voltage signal contains the higher harmonic components such as the harmonic component of the triple fundamental frequency, the harmonic component of the quintuple fundamental frequency and the like after Fourier transformation.

And the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction are used for providing gradient magnetic fields in respective directions. The gradient magnetic field is a magnetic field which is linearly changed in space and forms a gradient as the name suggests. The gradient magnetic field in a certain direction is realized by loading a reverse alternating current with the same value to a gradient magnetic field excitation coil pair in the direction. The gradient magnetic fields in the X direction, the Y direction and the Z direction are the same as the magnetic field direction of the constant uniform alternating magnetic field.

The gradient magnetic field excitation coil pair in the Z direction can be a Maxwell coil pair; the X-direction and Y-direction gradient magnetic field excitation coil pairs may be saddle coil pairs, such as Golay coil pairs. The gradient magnetic field exciting coil pair in each direction is provided with a positive electrode connector and a negative electrode connector which are connected with the control module, and the control module controls the current to be output to the control module so as to control the magnitude of the gradient field in the direction. Due to the principle of vector superposition, the gradient magnetic field in any direction can be changed by changing the current loaded on the gradient magnetic field excitation coil pair in any direction, and the total spatial gradient magnetic field in any direction can be generated and the change of the total spatial gradient magnetic field can be realized by changing the gradient magnetic fields in three directions. The total gradient magnetic field and the constant uniform alternating magnetic field form a final excitation magnetic field, and the size and the direction of the final excitation magnetic field change along with the total gradient magnetic field.

It is well known that magnetic nanoparticles have superparamagnetism, when presentWhen a magnetic field is applied, the magnetic moments of the magnetic nanoparticles in the liquid are biased towards the direction of the applied magnetic field, and magnetic flux changes are generated in the receiving coil, so that a voltage signal is generated. The magnetic nanoparticles of embodiments of the present invention are not limited to iron oxide magnetic nanoparticles (Fe)₃O₄)。

Therefore, the embodiment of the invention can change the alternating current of the gradient magnetic field exciting coil pairs in the X direction, the Y direction and the Z direction in order, so that the direction of the total spatial gradient magnetic field can change along a certain track in space, and the traversal of each spatial direction is realized; in each space direction, the two exciting coils of the gradient magnetic field exciting coil pair in the individual direction are loaded with the changed reverse alternating current, so that the magnitude of the gradient magnetic field loaded with the current is changed in a linear gradient manner, the magnetic nanoparticles are excited to generate a plurality of different response magnetic field signals, namely voltage signals, and the one-dimensional space encoding of the concentration of the magnetic nanoparticles in the space direction can be realized. It can be understood that when the direction of the spatial total gradient magnetic field changes in a plane or changes in multiple planes of a space, two-dimensional space encoding and three-dimensional space encoding of the magnetic nanoparticle concentration can be correspondingly realized based on one-dimensional space encoding, and a reconstructed image of the magnetic nanoparticle concentration distribution can be obtained through corresponding decoding reconstruction processing. Details of this section will be described later.

2) Control module

The control module is mainly used for current control and can be operated on a computer. The system can specifically comprise a waveform generator and a front-end controller corresponding to the waveform generator, wherein the waveform generator can boost the mains supply voltage to alternating current with a certain numerical value, convert the boosted alternating current into direct current through rectification, and obtain alternating current with a certain frequency through a frequency converter, such as the frequency of 1.67-50 KHz; the front-end controller pre-drives the scanning sequence, further performs power driving, and distributes current to each coil under the high-voltage control of variable-frequency output. In addition, the current applied to each coil can be fed back to the front of the pre-driving through a feedback loop, so that closed-loop control is formed.

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 constant equidirectional 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 in the embodiment of the invention; meanwhile, the two exciting coils of the gradient magnetic field exciting coil pair in any direction can be distributed with reversely alternating currents which change for many times, so that the gradient magnetic field exciting coil pair in the direction generates cosine alternating gradient magnetic fields with changed sizes in the central imaging area to oscillate the magnetic nano particles.

The gradient magnetic field in the selected specific direction is different, and the gradient magnetic field exciting coils in the selected specific directions have differences in the selected variable reverse alternating currents. The above-mentioned selection contents can be determined by experiments in advance to be selected as needed in imaging.

Of course, the control module may perform mechanical control of the system, remaining signal control, and the like, and the specific contents thereof will not be described in detail herein.

3) Receiving coil pair

It is known that a magnetic field induces a current, and the direction and magnitude of the magnetic field are related to the direction and magnitude of the induced current. The change in the induced magnetic field can be reflected by a change in the voltage in the coil. The receiving coil of the embodiment of the invention is used for receiving the change of magnetic flux caused by the magnetization response of the magnetic nano particles and generating corresponding voltage signals.

In the receiving coil pair of the embodiment of the invention, the axial direction of the two coils is Z direction and has a distance. The type of the receiving coil pair is not limited, and any one of the existing coils can be selected according to the requirement.

4) Signal processing module

The signal processing module may be run on a computer. For processing the voltage signals detected from the pair of receiving coils.

The signal processing module may include an analog processing sub-module and a digital processing sub-module.

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

the digital processing sub-module comprises a Fourier transform unit, a frequency spectrum analysis unit, a fundamental frequency reduction unit and a signal parameter extraction unit.

For the simulation processing submodule:

the low noise amplifier is used for amplifying the voltage signal.

And the signal correction unit is used for performing signal correction according to the signal attenuation.

Specifically, the embodiment of the present invention may use a large amount of experimental data to determine a signal attenuation model in advance. The signal attenuation model represents the original voltage signal, the received voltage signal which is subjected to space transmission and certain low-noise amplification of the original voltage signal and the corresponding relation of the signal attenuation, the signal attenuation corresponding to the current low-noise amplified voltage signal can be determined through the signal attenuation model, and compensation correction is carried out on the current low-noise amplified voltage signal according to the signal attenuation.

The mixer is used for signal mixing to convert the useful signal to a lower intermediate frequency.

The filter is used for filtering the signal at high frequency and low frequency.

An Analog-to-Digital Converter (ADC) is used to convert an Analog signal into a Digital signal, and the ADC has a certain sampling frequency, and can sample a number of points within a half cosine oscillation period corresponding to an excitation frequency of a current.

For the digital processing sub-module:

the Fourier transform unit is used for converting the voltage signal from a time domain to a frequency domain.

The frequency spectrum analysis unit is used for carrying out frequency analysis on the voltage signal of the frequency domain. After Fourier transform is performed on a signal in a complete cosine excitation period or a half cosine excitation period, a frequency spectrum analysis unit decomposes a signal function to a certain degree, and can express the signal function into a linear combination form of sine functions with different frequencies to obtain coefficients with different frequencies.

The fundamental frequency reduction unit is used for eliminating the excitation signal of the fundamental frequency. Specifically, the voltage signal in the time domain is usually generated by overlapping two parts, one part of the voltage signal is generated by exciting the magnetic field directly at the receiving coil, and the other part of the voltage signal is generated by exciting the magnetic moment of the magnetic nanoparticles in the target to be detected by the magnetic field to change the direction of the magnetic moment. After Fourier transformation, the signals generated by excitation exist only on one-time fundamental frequency, and the signals generated by the magnetic nano particles exist on one-time fundamental frequency and high-time fundamental frequency. The coefficient of the one-time fundamental frequency is set to be 0, so that the signals of the excitation magnetic field can be reduced, only harmonic signals emitted by the magnetic nanoparticles are left, namely spike signals corresponding to the fundamental frequency are eliminated, and only higher harmonic signals of the spike signals are left.

The signal parameter extraction unit is used for extracting triple fundamental frequency harmonic components of spike signals in the voltage signals as signal parameters of subsequent image reconstruction.

5) Image reconstruction module

The image reconstruction module may be run on a computer.

The embodiment of the invention is actually a magnetic particle imaging scheme based on the harmonic component of three times fundamental frequency of a peak signal. In order to facilitate understanding of the function of the image reconstruction module, a brief description of the imaging principle on which the embodiments of the present invention are based follows.

The shape and size of the magnetization curve also differ according to the intensity of the excitation magnetic field, and the shape and size of the signal spikes also differ. The embodiment of the invention adopts an excitation magnetic field of cosine oscillation, and is expressed as follows:

H(t)＝-Acos(2πft)

wherein A represents the magnitude of the magnetic field strength; f represents the excitation frequency; t represents time.

The signal strength at three times the fundamental frequency is derived as:

wherein the content of the first and second substances,

μ₀represents the vacuum permeability; m represents the magnetic moment of a single magnetic particle; k is a radical of_BRepresents the boltzmann constant; t is^PSpecifically, (273+37.5) K is 310.5K, which indicates the absolute temperature inside the human body.

As can be seen from the aforementioned excitation magnetic field module, the embodiment of the present invention does not use the selection field and the focusing field in the existing magnetic particle imaging technology MPI, but adopts a technical scheme of full-area non-uniform excitation, so that each point in the whole space is a free magnetic field region, can be excited by an alternating magnetic field, and therefore contributes to a voltage signal. According to the linear property of Fourier transform, the triple fundamental frequency harmonic component of the spike signal extracted from the voltage signal every time is equal to the superposition of the triple fundamental frequency harmonic components of the magnetic nano-particles of all points/pixels in the whole space. The embodiment of the invention finds that the harmonic component of the triple fundamental frequency has a nonlinear relation with the magnetic field intensity A and a proportional relation with the magnetic particle concentration c. Therefore, spatial encoding and cross-sectional imaging can be performed through the relationship, namely, the harmonic component of the fundamental frequency three times is used as a signal parameter of the embodiment of the invention to reconstruct an image of the concentration of the magnetic particles.

The research of the embodiment of the invention finds that the space coding of the excitation magnetic field based on the triple fundamental frequency harmonic component comprises the following requirements: (1) the uniform excitation magnetic field is more than 15mT to ensure that signal peak appears, facilitate signal parameter extraction, and simultaneously can filter the iron atoms in the human body. (2) An excitation field having a magnitude varying along a single direction of XYZ, such as a linear gradient, and a uniform magnetic field in the other two directions perpendicular to the single direction, to facilitate slice selection, (3) all of the uniform and non-uniform excitation fields have the same magnetic field direction, to facilitate signal reception. Therefore, the embodiment of the invention can utilize the control module to control the loading current of the uniform magnetic field excitation unit, and realize that the size of the constant uniform alternating magnetic field meets 15 mT-30 mT and the like. The control module is used for exciting the coils to load the changed reverse alternating current to the gradient magnetic field in the X direction, the Y direction and the Z direction, so that the gradient field with the field intensity in each direction changing linearly is realized. The linear change of the field intensity of the gradient magnetic field is matched with the nonlinear relation between the triple fundamental frequency harmonic component and the magnetic field intensity, so that each encoding and signal acquisition of the magnetic field spatial distribution are independent, and the unique solution of the magnetic particle concentration matrix can be obtained.

With respect to one-dimensional spatial encoding and decoding based on the three fundamental frequency harmonic component of the spike signal, the discretized magnetic field strength a, the magnetic particle concentration c, and the three fundamental frequency harmonic component of the spike signal obey the following relationship:

wherein u is₃(t) represents a triple fundamental frequency harmonic component; u. of₃(a (r, t)) represents the harmonic component of the fundamental frequency of the signal spike of a unit concentration of magnetic particles at the excitation magnetic field strength a; s (r) represents the receive coil sensitivity.

In the embodiment of the invention, the time corresponds to the current change, and the formula is discretized:

wherein u is₃(i) Representing the triple fundamental frequency harmonic components; n represents the encoding number of samples in the imaging area, and in the formula, Delta V represents the volume size of the voxel of the data sampling point; g (i)_n,r_n) The system matrix g is an element of the system matrix g, and is independent of the magnetic particle concentration. The system matrix is used for representing the spatial distribution of triple fundamental frequency harmonic components of response voltage signals generated by magnetic particles with unit concentration under the action of an excitation magnetic field, and correction is realized by using the sensitivity of the receiving coil obtained by actual measurement in the construction process. Based on the system matrix, the method can reversely deduce the corresponding time (corresponding to different currents) of the change of the size of the total gradient magnetic field in space in each directionThereby realizing imaging by using an image reconstruction method.

The operation matrix form of the triple fundamental frequency harmonic component is simplified as follows:

gc＝u₃

from the above equation, if the system matrix g in unit concentration is known, the magnetic particle concentration of each encoded dot can be calculated:

c＝g^-1u₃

wherein the content of the first and second substances,

the system matrix of the embodiment of the invention can be obtained in advance through experiments, and is corrected by utilizing the actually measured sensitivity of the receiving coil in the construction process, and the system matrix is expressed as follows:

the harmonic component of three fundamental frequencies resulting from the change in the magnitude of the total gradient magnetic field in space in the same direction is expressed as:

then use c ═ g^-1u₃C can be calculated.

Wherein i₀,i₁,…,i_N-1Representing different sub-coil currents, r, which cause a change in the magnitude of the total gradient magnetic field in space in that direction₀,r₁,…,r_N-1Represents N position points in the direction; u (i)₁) Indicates a coil current of i₁Then, the harmonic component of the collected triple fundamental frequency is obtained; g (i)_N-1,r₀) Magnetic particles representing unit concentration at a current i_N-1Under the action of the excited magnetic field, at the r-th direction₀Triple harmonic components of the fundamental frequency generated by each location point; the meaning of the remaining elements are as suchAnd (6) pushing. c denotes the one-dimensional reconstruction result, which contains elements of the concentration of magnetic particles at each position point in the imaging region. In actual calculation, because the calculated amount is large, the matrix can be solved by using the existing algorithm auxiliary matrix such as an algebraic reconstruction method, a combined algebraic reconstruction method, a maximum likelihood expectation maximization algorithm and the like. The solving process using the system matrix will not be described in detail.

Therefore, according to the three times fundamental frequency harmonic component of the signal peak of the magnetic nano particle response excited by the cosine magnetic field, the harmonic component is in direct proportion to the concentration of the magnetic particles and the nonlinear relation with the intensity of the exciting magnetic field, multi-directional excitation and space encoding are carried out by combining the excitation magnetic field with linear gradient change in three directions of XYZ, the field intensity space distribution of the excitation magnetic field is not uniform, and the field intensity spatial distribution of each time is different, the image reconstruction module can obtain a plurality of triple fundamental frequency harmonic component signals, the one-dimensional distribution map of the magnetic particle concentration can be obtained by performing one-dimensional reconstruction on the concentration space distribution of the magnetic particles through the system matrix, and a related reconstruction method is utilized to carry out two-dimensional or three-dimensional reconstruction processing on the basis of the one-dimensional reconstruction results in a plurality of directions, then a two-dimensional or three-dimensional concentration spatial distribution image of the magnetic nanoparticles in the target to be measured can be obtained.

In a second aspect, an alternative implementation and a complementary approach of individual parts of a gradient field based magnetic particle imaging system according to embodiments of the present invention are described.

Referring to fig. 2, it should be understood that an alternative structure of the excitation magnetic field module is shown, and fig. 2 is a schematic diagram of an arrangement manner of the gradient magnetic field excitation coil pair according to the embodiment of the present invention. The cylinder in the first row in fig. 2 is a schematic view of the space formed by the excitation magnetic field modules according to the embodiment of the present invention, and is exemplified by a human being, the plane on which the human being lies is an XZ plane, and the face faces in the positive Y direction. The lower three rows in fig. 2 illustrate the spatial distribution of the pairs of gradient magnetic field excitation coils in the X, Y and Z directions, respectively, in the excitation magnetic field module. Wherein:

an X-direction gradient magnetic field excitation coil pair or a Y-direction gradient magnetic field excitation coil pair, comprising:

a pair of Golay-type lateral gradient coils symmetric along a plane, wherein each Golay-type lateral gradient coil comprises two Golay coils extending along the Z-direction, each Golay coil is distributed on the cylindrical surface in a 120 ° circular arc, the opening angle of the near circular arc is 68.7 °, and the opening angle of the far circular arc is 21.3 °;

wherein, the symmetrical plane of the X-direction gradient magnetic field exciting coil pair is a YZ plane; the symmetry plane of the Y-direction gradient magnetic field excitation coil pair is the XZ plane.

A Z-direction gradient magnetic field excitation coil pair comprising:

and the pair of circular Maxwell coils are axially overlapped, axially face to the Z direction and have intervals.

The normally conductive electromagnet unit adopted by the uniform magnetic field excitation unit specifically comprises:

and the pair of circular Maxwell coils are axially overlapped, axially face to the Z direction and have intervals. The arrangement of the pair of coils of the uniform magnetic field excitation unit is similar to that of the pair of gradient magnetic field excitation coils in the Z direction in fig. 2. For simplicity, this is not illustrated in fig. 2.

With respect to the receiving coil pair, in an alternative embodiment, the receiving coil pair includes:

a pair of circular Homholtz coils which are axially overlapped, axially face in the Z direction and have a distance; and the spacing of the receiving coil pairs is larger than that of the gradient magnetic field excitation coil pairs in the Z direction.

That is, the receive coil pair excites the coil pair with a gradient magnetic field parallel to the Z direction. One coil in the receiving coil pair is positioned 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 positioned outside the other coil in the gradient magnetic field excitation coil pair in the Z direction.

The coils and the receiving coil pairs in the excitation magnetic field module are distributed in a staggered manner to surround a cylindrical space in the above-mentioned manner, and the combination form of the coils is understood by referring to the above and fig. 2, which is not described in the whole figure.

Regarding the control module, in an optional implementation manner, the method may include:

a sequencer, a front-end controller, a gradient controller and a gradient amplifier.

And the sequence generator is used for sending the time sequence parameters and the amplitude parameters of the voltage of each coil to the front-end controller according to the sequence parameters set by the computer software. The front-end controller outputs the amplitude parameter to the gradient controller according to the time sequence, and the gradient controller respectively sends voltage signals to gradient magnetic field exciting coils in three directions in the given time sequence according to the obtained parameter. The gradient amplifier is used for amplifying the voltage signal. Therefore, it can be understood that the voltages received by the gradient magnetic field excitation coils in each direction constitute a voltage sequence, and each voltage is driven to generate a corresponding current.

In another optional embodiment, the digital processing sub-module in the signal processing module further includes:

the device comprises a relaxation effect deconvolution correction unit, a signal spike correction unit, a full width half maximum correction unit and a magnetic field monitoring unit.

In this embodiment, the signal parameter extraction unit is configured to extract the signal area and the full width at half maximum of the spike signal in addition to the fundamental frequency triple harmonic component of the spike signal.

Specifically, in each direction of the change of the total gradient field in space, a voltage signal is collected every time the current changes, and the signal area extraction may be performed by integrating data collected in a time domain. Full width at half maximum represents the corresponding time domain width for the acquired signal when its peak value drops to half the amplitude.

The relaxation effect deconvolution correction unit is used for performing deconvolution operation on the voltage signal by using a relaxation effect convolution kernel so as to compensate signal errors caused by the relaxation effect. Specifically, the deconvolution operation is performed on the peak signal by using the relaxation effect convolution kernel, so that signal errors caused by phenomena such as peak amplitude attenuation, signal delay, peak broadening, peak signal asymmetry and the like caused by the relaxation effect can be compensated.

The signal peak correction unit and the full width at half maximum correction unit are used for respectively correcting the signal peak and the full width at half maximum of the voltage signal by comparing the signal areas of the voltage signal for multiple times. Specifically, the signal area is independent of the magnetic field intensity, but is proportional to the magnetic particle concentration. During the change of the magnitude and direction of the spatial total gradient magnetic field, assuming that the magnetic particle concentration remains unchanged, the area of the signal peak in the voltage signal acquired each time, i.e. the signal area, should be a conservative amount, i.e. remain unchanged. According to the conservation quantity, the signal peak and the full width at half maximum of the voltage signal can be corrected by comparing the signal areas of the voltage signals which are actually collected for multiple times. For example, the voltage signal of each time which does not meet the condition is corrected to be consistent with the normal voltage signal of the other times, or the related parameters such as the current of the gradient magnetic field in the system are adjusted to obtain the voltage signal of each time which meets the condition by re-measurement, and the like, so that the accuracy of the subsequent measurement is improved.

The magnetic field monitoring unit is used for warning when abnormality occurs by comparing the full width at half maximum of the voltage signals for multiple times. Specifically, since the full width at half maximum is independent of the magnetic particle concentration, but has an inverse relationship with the intensities of the gradient magnetic fields in the three directions, the magnetic field monitoring can be realized by uniformly comparing the full widths at half maximum of the voltage signals acquired for a plurality of times in practice according to the inverse relationship. Specifically, when the current full width at half maximum is found to be different from the theoretical full width at half maximum obtained from the inverse relationship under the specific strengths of the gradient magnetic fields in the three directions, it can be determined that there is an abnormality in the gradient magnetic fields in the three directions, and a warning signal is output, for example, there may be a failure in the excitation coil of the gradient magnetic field in a certain direction. Based on the mode, the size and the direction of the gradient magnetic field can be corrected, and the precise change of the excitation magnetic field and the encoding accuracy are ensured.

Meanwhile, the full width at half maximum can also be used for verifying the correction effect of the relaxation effect deconvolution correction unit, reminding the system matrix to be corrected again when detecting abnormity, and the like. The specific process is not described in detail herein.

In an alternative embodiment, the magnetic particle imaging system based on gradient field further comprises:

and the bearing device is used for placing the target to be detected.

For example, the carrying device can be in the form of a bed body, a support and the like, and plays a role in carrying and fixing the target to be measured. The plane of the carrier is parallel to the XZ plane and the long axis is parallel to the Z axis.

Wherein, the interior of the bearing device is provided with a shielding coil component.

The shielding coil assembly comprises a plurality of coils which are arranged in parallel along the length direction of the bearing device; the coil of the shield coil assembly opposite the central imaging region of the magnetic particle imaging system is the central imaging region coil, and the remainder are the peripheral region coils. Wherein the central imaging area coil covers the projection range of the central imaging area in the XZ plane.

The type of the coil included in the shielding coil assembly is not limited herein, and in an alternative implementation, the shielding coil assembly may be implemented by using a rectangular coil, as shown in fig. 3, where fig. 3 is a schematic structural diagram of the shielding coil assembly according to the embodiment of the present invention. In the imaging process, the peripheral region coil is loaded with current, the central imaging region coil is not loaded with current, namely, only the peripheral region coil is in an open state, so that a static magnetic field is generated to saturate and restrict the magnetic nano particles in the peripheral region, and the magnetic nano particles only in the central imaging region are excited by the magnetic field to avoid generating interference signals.

the bearing device control module comprises a laser positioning unit and a shielding coil assembly control unit; wherein the content of the first and second substances,

and the laser positioning unit is used for determining the scanning part of the target to be detected by utilizing the lasers in the horizontal and vertical directions, adjusting the position of the bearing device and aligning the scanning part to the central imaging area.

For example, for the target that awaits measuring for the human body, the scanning position is the head, can let the patient who has injected into magnetic nano particles lie on the face of one's back specifically for the load bearing device of the bed body, through laser positioning, promotes the bed body and promotes the central imaging region of this magnetic particle imaging system with patient's head.

And the shielding coil assembly control unit is used for loading current to the coils in the peripheral area in the imaging process.

In an alternative embodiment, the functions of the control module of the carrying device can also be realized by the control module.

a magnetic nanoparticle injector. The magnetic nanoparticle injector can be arranged outside the space region of the excitation magnetic field module and used for injecting magnetic nanoparticles to a target to be measured.

The magnetic nanoparticles can be superparamagnetic iron oxide nanoparticles (Resovist), are colloidal suspensions, and have a concentration of 0.5mmol Fe/mL. The injection dose is set according to the weight of the target to be measured, for example, the weight of a patient is less than 60kg, and the injection dose is 0.9mL (equivalent to 0.45mmol of iron); the patient weighed 60kg or more and the injection dose was 1.4mL (equivalent to 0.7mmol of iron). The magnetic nanoparticles are injected intravenously, manually by a doctor, automatically by an instrument, and the like.

a closed housing; the closed shell can embed the excitation magnetic field module therein to form a hollow cylindrical accommodating space.

image display, laser camera and external memory.

The image display is used for displaying a distribution image of the magnetic nanoparticle concentration in the target to be detected, and is convenient for doctors and other personnel to observe. A laser holographic camera is a device for taking a hologram by using laser as coherent light, is used for image printing to form a film for diagnosis, and is connected with a computer through a DICM interface. The external memory is used for connecting the computer to realize data storage and copying.

In an optional embodiment, the DICM interface is further connected to a PACS-RIS system. The PACS refers to a Picture Archiving and Communication System (PACS), which is a comprehensive system that has been rapidly developed in recent years with the progress of digital imaging technology, computer technology, and network technology and aims to comprehensively solve the problems of acquisition, display, storage, transmission, and management of medical images. The RIS is a radiology information management system (RIS), which is a software system for optimizing the workflow management of the radiology department of a hospital, and a typical flow includes links such as registration appointment, diagnosis, image generation, film production, report, audit, film distribution and the like.

As before, by changing the gradient magnetic field in three directions, it is possible to generate a spatial total gradient magnetic field in an arbitrary direction and to realize a change in the magnitude of the spatial total gradient magnetic field. Specifically, please refer to the schematic diagram of the spherical coordinate system in fig. 4. The relation of gradient magnetic fields in three directions and the total gradient magnetic field in space comprises:

wherein G is_xRepresents the magnitude of the gradient magnetic field in the X direction; g_yRepresents the magnitude of the gradient magnetic field in the Y direction; g_zRepresents the magnitude of the gradient magnetic field in the Z direction; g represents the magnitude of the total gradient magnetic field in space, theta and

the direction of the total gradient magnetic field in space is determined by the two angles of the spherical coordinate system, and the direction of the total gradient magnetic field in space is changed when any angle is changed; arctan (·) represents an arctangent function; arccos (·) represents an inverse cosine function.

Alternatively, the above relationship may also be expressed as:

G_x、G_yand G_zThe projection components of G on the coordinate axes. Thus, it can be understood that by adjusting G_x、G_yAnd G_zCan be combined to obtain a desired total gradient magnetic field magnitude G in space, and a direction characterizing the total gradient magnetic field in space

The embodiment of the invention can determine a certain regulation rule through experiments in advance and change G purposefully_x、G_yAnd G_zAt each of θ and

in the space direction of the composition, the size G of the total gradient magnetic field in the space is changed for many times, so that one-dimensional space encoding can be realized in the direction, and the concentration distribution of the magnetic particles in the direction can be obtained by reconstructing a plurality of triple fundamental frequency harmonic components obtained by the one-dimensional space encoding by using a system matrix, namely a one-dimensional distribution map. On the basis of one-dimensional spatial coding, by purposefully varying G_x、G_yAnd G_zOf each theta sum

And (3) traversing the directions of the spatial total gradient magnetic field in the space by using the angle so as to obtain one-dimensional space codes in a plurality of directions, and obtaining a two-dimensional or three-dimensional reconstruction image of the concentration distribution of the magnetic particles by using a reconstruction method through the one-dimensional space codes in the plurality of spatial directions.

The functions of the control module and the image reconstruction module in the case of imaging in different dimensions are briefly described below. The rest of the modules are understood in combination and not described in detail herein. In particular, the method comprises the following steps of,

(1) when the imaging dimension of the target is one-dimensional,

the control module is specifically used for determining a gradient magnetic field in a specific direction corresponding to a target imaging direction in one-dimensional imaging and an alternating voltage sequence of a gradient magnetic field excitation coil pair in the specific direction in a predetermined corresponding relation between the imaging and the gradient magnetic field; providing the same ascending alternating current sequence or the same descending alternating current sequence for the two exciting coils of the gradient magnetic field in the specific direction by utilizing a voltage-driven current generation mode, so that the magnetic field size of the total spatial gradient magnetic field in the specific direction is changed for multiple times; wherein, the stepping is the same between each current value in any current sequence of the ascending alternating current sequence and the descending alternating current sequence.

The image reconstruction module is specifically used for performing one-dimensional reconstruction by using a system matrix according to a plurality of different triple fundamental frequency harmonic components obtained by the magnetic field size change of the spatial total gradient magnetic field in the specific direction to obtain a one-dimensional reconstruction result including the multilayer magnetic nanoparticle concentration information in the direction.

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

(2) When the imaging dimension of the target is two-dimensional,

the control module is specifically used for determining gradient magnetic fields in at least two specific directions corresponding to a target imaging plane in a predetermined corresponding relation between imaging and the gradient magnetic fields, and respective alternating voltage sequences of gradient magnetic field excitation coils in the at least two specific directions; providing respective alternating current sequences for the at least two gradient magnetic field excitation coil pairs in the specific direction by using a voltage-driven current generation mode so as to change the magnitude of the gradient magnetic fields in the at least two specific directions for multiple times, thereby causing the direction of the total spatial gradient magnetic field to change in a specific plane, and 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 so as to change the magnitude of the magnetic field in each direction of change for multiple times; wherein, the stepping is the same between each current value in any current sequence in the ascending alternating current sequence and the descending alternating current sequence; the specific plane is determined according to the target imaging plane.

The image reconstruction module is specifically used for performing one-dimensional reconstruction by using a system matrix according to a plurality of different triple fundamental frequency harmonic components obtained by the magnetic field size change of the spatial total gradient magnetic field in the same direction to obtain a one-dimensional reconstruction result comprising the multilayer magnetic nanoparticle concentration information in the direction; and performing two-dimensional filtering back projection on all one-dimensional reconstruction results obtained by the direction change of the total spatial gradient magnetic field to obtain a two-dimensional projection diagram for the target imaging plane, wherein the two-dimensional projection diagram shows a distribution image of the concentration of the magnetic nanoparticles in the target to be detected on the target imaging plane. The target imaging plane can be an XY plane, an XZ plane, a YZ plane, and any other planes.

The corresponding relation between the imaging and the gradient magnetic field is predetermined by using experimental data, and comprises a target imaging dimension, a gradient magnetic field in a specific direction to be adjusted under a target imaging plane, an alternating voltage sequence of a gradient magnetic field excitation coil in the specific direction, an alternating current sequence generated by corresponding driving, a change sequence of the magnetic field size of the gradient magnetic field in the specific direction to be adjusted under the corresponding alternating current sequence, and theta representing the total gradient magnetic field direction in space

A sequence of changes in angle, and a sequence of changes in the magnitude G of the total gradient magnetic field in space.

Specifically, for two-dimensional imaging, θ and

one angle is fixed, the other angle is traversed, the direction of the space total gradient magnetic field is changed in a specific plane corresponding to the fixed angle, and the space total gradient in the direction is realized in the direction formed by each traversal angleAnd the harmonic component of the fundamental frequency three times can be obtained by changing the size of the total gradient magnetic field in each time.

Sum of theta in two-dimensional imaging

The numerical range of the magnetic field is determined according to an imaging plane, and the traversing stepping of the traversing angle and the changing times of the size of the total spatial gradient magnetic field in a certain direction are determined according to the requirements of imaging resolution. The smaller the traversal step of the traversal angle is, the more the change times of the size of the spatial total gradient magnetic field in a certain direction are, and the higher the imaging resolution is. It will be appreciated that the data dimension for the harmonic components of the triple fundamental frequency obtained is the number of changes in the angle of traversal x the number of changes in the magnitude of the total gradient magnetic field in space in a direction.

In a certain direction, the gradient magnetic field excitation coil pair in a specific direction is provided with a changing alternating current, so that the magnitude of the gradient magnetic field in the specific direction is changed for a plurality of times, the same increasing alternating current sequence is provided for two excitation coils of the gradient magnetic field excitation coil pair in the specific direction, or the same decreasing alternating current sequence is provided for two excitation coils, so that the magnitude of the gradient magnetic field in the specific direction is changed for a plurality of times according to the corresponding current sequence.

The mathematical principle of the filtered back projection reconstruction method is radon transform, and the method is commonly used in CT imaging reconstruction. For the specific transformation, please refer to the related prior art, which is not described herein.

(3) When the imaging dimension of the target is three-dimensional,

the control module is specifically used for determining respective alternating voltage sequences of gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in a predetermined corresponding relation between imaging and gradient magnetic fields, providing the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions by utilizing a voltage drive current generation mode, so that the magnitude of the gradient magnetic fields in the three directions is changed for multiple times, the direction of a total spatial gradient magnetic field is caused to be changed in space, and providing the same ascending alternating current sequence or the same descending alternating current sequence for two excitation coils of the gradient magnetic field in each direction in the gradient magnetic fields in at least two directions, so that the magnetic field magnitude of the total spatial gradient magnetic field in each changed direction is changed for multiple times; wherein, the stepping is the same between each current value in any current sequence of the ascending alternating current sequence and the descending alternating current sequence.

The image reconstruction module is specifically used for performing one-dimensional reconstruction by using a system matrix according to a plurality of different triple fundamental frequency harmonic components obtained by the magnetic field size change of the spatial total gradient magnetic field in the same direction to obtain a one-dimensional reconstruction result comprising the multilayer magnetic nanoparticle concentration information in the direction; carrying out two-dimensional filtering back projection on all one-dimensional reconstruction results obtained by changing the direction of the total spatial gradient magnetic field in the same plane to obtain a two-dimensional projection image, and carrying out three-dimensional reconstruction on the two-dimensional projection image obtained by changing the direction of the total spatial gradient magnetic field in each plane to obtain a three-dimensional imaging image; the three-dimensional imaging graph represents a distribution image of the magnetic nanoparticle concentration in a three-dimensional space of the target to be measured.

Specifically, for three-dimensional imaging, θ and

one angle is fixed, namely, the angle is used as a fixed angle, the other angle is used as a traversal angle to traverse in a corresponding step within a certain range, and in the direction formed by each fixed angle and the traversal angle, the gradient magnetic field current is changed to realize multiple changes of the size of the spatial total gradient magnetic field in the direction.

And after the traversal of the traversal angle is finished, changing the fixed angle by one step, and traversing the traversal angle again according to the mode until the traversal of the traversal angle is finished.

And changing the fixed angle to repeatedly execute the process until the fixed angle reaches the traversal upper limit value of the fixed angle.

In the above process, first, the sum of theta

Which angleThe fixed angle is started to traverse without limitation, and the traversing process of each angle in space can be realized. Theta and

the numerical range of the magnetic field is determined according to the three-dimensional imaging requirement, and the traversing stepping of the traversing angle and the changing times of the size of the total spatial gradient magnetic field in a certain direction are determined according to the imaging resolution requirement.

It is understood that what is obtained

The three-dimensional reconstruction is to calculate and obtain a distribution image of the concentration of the magnetic particles in the target to be measured in a three-dimensional space according to data information in the two-dimensional magnetic particle concentration distribution images projected along different directions. The adopted method can be chromatographic synthesis, filtering back projection reconstruction, iterative reconstruction or artificial intelligence reconstruction and the like. The specific procedures of these methods are not described in detail herein.

The embodiment of the invention carries out non-uniform excitation on the magnetic nano particles in the whole space, the contribution of the induced voltage comes from all the magnetic nano particles in the whole space, and a magnetic field free area is generated because a selection field is not used, so that MPI high-power-consumption selection field hardware equipment can be avoided, and the purpose of reducing power consumption is realized. The embodiment of the invention does not use a free area of a moving magnetic field of the focusing field, thereby avoiding the defects of sparse sampling and low spatial resolution caused by artifacts caused by non-uniform moving speed of the MPI focusing field, non-uniform spatial sampling caused by irregular moving track and the like. MPI adopts a magnetic field free area to excite a voltage signal, the voltage signal of a single magnetic field free area is weak, and the embodiment of the invention adopts a full-area excitation mode to greatly enhance the signal intensity, so that the signal-to-noise ratio is high, the image quality can be improved, and the requirement of clinical diagnosis can be met. Because the embodiment of the invention does not adopt the scanning mode of the free area of the moving magnetic field, but carries out non-uniform magnetic field excitation and space coding on the whole space, and the scanning area is not determined by the selection field gradient and the driving field strength together any more, the scanning area and the scanning range are easily expanded, the imaging visual field can be matched with the size of the human body, and the clinical application of the human body is realized. In addition, the embodiment of the invention does not adopt a movement mode of a magnetic field free region of a Lissajous curve, so that the large-region scanning time is obviously shortened, and the clinical scanning efficiency can be improved. Also, by providing a variety of ancillary devices, the clinical utility of embodiments of the present invention is enhanced.

In a third aspect, a scanning imaging process of the magnetic particle imaging system based on the gradient field is described in conjunction with a specific structure of the magnetic particle imaging system based on the gradient field. As understood with reference to fig. 2. It should be noted that the parameter values mentioned below are not intended to limit the embodiments of the present invention, but are merely an example of an implementation manner to facilitate understanding of the scheme, and in practical use, suitable values may be specifically selected according to needs.

The diameter of two circular Maxwell coils of the uniform magnetic field excitation unit is 40 cm, the thickness and the width are both 5 cm, the number of turns of the coils is 200 turns, and the distance between the two coils is 40 cm. The two coils are loaded and applied with equidirectional alternating current, the maximum current value ranges from 20 to 60 amperes, a cosine alternating uniform magnetic field of 10 to 20mT is generated in the central imaging area of the magnetic particle imaging system, and the excitation frequency is 1.67 to 5.0 kilohertz.

A pair of Golay-type transverse gradient coils of an X-direction gradient magnetic field excitation coil pair are applied with reverse alternating currents at an excitation frequency of 1.67-5.0 kHz. The axial magnetic field components are distributed in a linear gradient mode along the x direction and are uniformly distributed on a yz plane, the variation range of the magnetic field intensity within the range of 20 cm of the central imaging area is less than 5%, and the constant magnetic field surface is ensured to be a plane instead of a curved surface. The magnitude of the gradient magnetic field is changed by simultaneously increasing the currents of the two gradient magnetic field excitation coils. For example, if the current applied to the coil is changed 256 times, the magnitude of the gradient magnetic field changes 256 times, and the strength changes from-25 mT/m to 25mT/m, each time the magnitude changes to 0.195 mT/m.

A pair of Golay-type transverse gradient coils of a Y-direction gradient magnetic field excitation coil pair are applied with reverse alternating currents at an excitation frequency of 1.67-5.0 kHz. The axial magnetic field components are distributed in a linear gradient mode along the y direction, are uniformly distributed on an xz plane, and the variation range of the magnetic field intensity within the range of 20 cm of the central imaging area is smaller than 5%, so that the isomagnetic field surface is a plane instead of a curved surface. The magnitude of the gradient magnetic field is changed by simultaneously increasing the currents of the two gradient magnetic field excitation coils. For example, if the current applied to the coil is changed 256 times, the magnitude of the gradient magnetic field changes 256 times, and the strength changes from-25 mT/m to 25mT/m, each time the magnitude changes to 0.195 mT/m.

In two circular Maxwell coils of the gradient magnetic field excitation coil pair in the Z direction, the diameter of each coil is 40 centimeters, the thickness and the width of each coil are both 5 centimeters, the number of turns of each coil is 200 turns, and the distance between the two coils is 40 centimeters. The two coils are supplied with alternating currents in opposite directions, the maximum current having a value in the range of 20 to 60 amperes and an excitation frequency of 1.67 to 5.0 kilohertz. The axial magnetic field component is a linear gradient magnetic field change along the z direction and is uniformly distributed on the xy plane. The magnitude of the gradient magnetic field is changed by simultaneously increasing the currents of the two gradient magnetic field excitation coils. For example, if the current applied to the coil is changed 256 times, the magnitude of the gradient magnetic field changes 256 times, and the strength changes from-25 mT/m to 25mT/m, each time the magnitude changes to 0.195 mT/m.

And two circular Homholtz coils of the receiving coil pair are used for receiving the magnetization vector change in the z direction. Each coil had a diameter of 40 cm, a thickness and width of 5 cm, and a spacing of 50 cm between the two coils.

The uniform magnetic field excitation unit, the gradient magnetic field excitation coil pairs in three directions and the receiving coil pairs are packaged in a 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 cylinder length is 50-100 cm. The maximum load of the bed body serving as the bearing device can be 500 pounds, and the scanning part of the target to be detected on the bed body is controlled under the control of the laser positioning unit of the bearing device control module and is arranged in the center of the hollow cylindrical structure, namely the central imaging area during imaging.

The shielding coil assembly in the bed body comprises 15 rectangular coils which are arranged along the length direction of the bed body, the width of each rectangular coil is 10 centimeters, the length of each rectangular coil is 30 centimeters, the number of turns of each rectangular coil is 200 turns, and the direct current loaded by each rectangular coil is 30 amperes. As can be appreciated with reference to FIG. 3, during imaging, the 2-5 central imaging region coils are turned off so that the magnetic nanoparticles in the central imaging region can be oscillated by the excitation coil to generate a signal. And the coils of the other peripheral regions are opened to generate a static magnetic field of 30mT for saturation constraint of the magnetic nano particles in the peripheral regions to avoid generating interference signals.

The control module controls the current of the gradient magnetic field excitation coil pair in a specific direction, changes the size of the gradient magnetic field in the specific direction and carries out spatial coding of different dimensions and imaging planes. A voltage signal is obtained by the pair of receiving coils. The signal sampling frequency of the signal processing module can be 16.5MHz, and the image reconstruction module can reconstruct the image of the magnetic particle concentration by continuously extracting the triple fundamental frequency harmonic component of the peak signal in the voltage signal to the image reconstruction module and utilizing the triple fundamental frequency harmonic component with a plurality of angles and a plurality of gradient sizes.

(one) with respect to one-dimensional spatial encoding and reconstruction

At a signal sampling frequency f_cAt the setting of the excitation frequency f. First, a one-dimensional spatial encoding and reconstruction process is described by taking the x direction as an example. At the scanning position of the object to be measuredAfter being located in the central imaging region, the respective alternating voltage sequences of the two excitation coils of the X-direction gradient magnetic field excitation coil pair are determined in the predetermined correspondence of imaging and gradient magnetic field.

Gradient magnetic field G in X direction_xI.e. the total gradient magnetic field in space G. Initial value of alternating current sequence generated by driving according to alternating voltage sequence, G_xIn that

For example-25 mT/m. The gradient magnetic field in the other two directions is always 0.

The current changes according to the alternating current sequence, so that after each cosine oscillation of the gradient magnetic field in the X direction is performed for half a period, the magnetic field changes once, and half cosine oscillation is completed for 256 times in total, so that the gradient magnetic field in the X direction changes from-25 mT/m to 25 mT/m.

Thus, in

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 subjected to one-dimensional reconstruction by utilizing a corresponding system matrix, the concentration of magnetic particles in each of 256 layers along the direction can be obtained, and the concentration of the magnetic particles can be obtained

And (5) one-dimensional reconstruction results in the direction.

In the embodiment of the invention, the signal sampling frequency is f_cMay be 16.5MHz and the excitation frequency may be 3.3 KHz. Of course, other values may be selected as desired.

(II) two-dimensional spatial encoding and reconstruction

With respect to two-dimensional imaging, the imaging plane may be an XY plane, an XZ plane, a YZ plane, and an arbitrary plane. The spatial encoding and imaging process is described below using the XY plane as an example. Specifically, when the target imaging dimension is two-dimensional and the target imaging plane is an XY plane:

control moduleThe method is specifically used for determining gradient magnetic fields in the X direction and the Y direction corresponding to an XY plane and respective alternating voltage sequences of gradient magnetic field excitation coil pairs in the X direction and the Y direction in a predetermined corresponding relation between imaging and the gradient magnetic fields, providing the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the X direction and the Y direction in a mode of generating current by voltage driving, and inducing the direction of a spatial total gradient magnetic field from the direction of the spatial total gradient magnetic field by changing the magnitude of the gradient magnetic fields in the X direction and the Y direction for multiple times

Starting in the YZ plane, changing to 1 along a theta step

And in each theta direction, the size of the total gradient magnetic field in the space is changed 256 times;

the image reconstruction module is specifically configured to sum the total gradient magnetic field in space for each theta

256 different triple fundamental frequency harmonic components obtained by the change of the magnetic field in the direction are subjected to one-dimensional reconstruction by utilizing a system matrix to obtain

One-dimensional reconstruction results of the direction; theta is formed to be 0 DEG and 180 DEG]And performing two-dimensional filtering back projection on the 180 one-dimensional reconstruction results obtained by changing to obtain a two-dimensional projection diagram aiming at the XY plane.

Specifically, after the scanning part of the target to be measured is located in the central imaging area, in the predetermined corresponding relationship between the imaging and the gradient magnetic field, the respective 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 the corresponding alternating current sequences.

The gradient magnetic fields in the X direction and the Y direction are superposed to form a total spatial gradient magnetic field G. According to an alternating current sequence, G is in

For example-25 mT/m. The magnitude of the gradient magnetic field in the Z direction is always 0.

The current changes according to the alternating current sequence, so that after the gradient magnetic field in the X direction oscillates for half a period every cosine, the size of the magnetic field changes once, and half cosine oscillation is completed for 256 times in total, so that the size of G changes from-25 mT/m to 25mT/m for 256 times. Thus, in

In the direction, 256 triple fundamental frequency harmonic component signals can be obtained, and the triple fundamental frequency harmonic components of the 256 peak signals are subjected to one-dimensional reconstruction by using the corresponding system matrix, so that the signals can be obtained

The one-dimensional reconstruction of the direction results, which contains 256 values of the magnetic particle concentration.

Holding

Constant, θ increases along 1 °, at each

In the direction, the currents of the gradient magnetic field excitation coil pairs in the X direction and the Y direction are changed for 256 times according to the corresponding alternating current sequence to obtain 256 triple fundamental frequency harmonic component signals, and the corresponding system matrix is utilized to perform one-dimensional reconstruction to obtain a one-dimensional reconstruction result in the direction. Repeating the above process until obtaining

And (5) one-dimensional reconstruction results in the direction. And (4) utilizing two-dimensional filtering back projection to obtain a two-dimensional projection diagram aiming at the XY plane by using 180 one-dimensional reconstruction results.

It can be understood that the 256 × 180 times half of the cosine oscillation excitation induced signal encoding is performed in the two-dimensional imaging process, i.e. the data dimension of the triple fundamental frequency harmonic component is 256 × 180. The signal sampling frequency is 16.5MHz, the excitation frequency is 3.3KHz, the number of sampling points in a half excitation oscillation period is 5000, 256 multiplied by 180 half oscillation periods are needed, and the time is 6.98 seconds.

With respect to XY plane imaging, it is also possible to adjust the magnitude of the gradient magnetic fields in the three directions simultaneously with corresponding alternating current sequences, but the current sequence in which the Z-direction gradient magnetic field excitation coil pair is loaded is such that the magnitude of the Z-direction gradient magnetic field is always 0.

Similarly, with respect to XZ plane imaging, this can be achieved by adjusting the currents of the gradient magnetic fields in the X and Z directions; regarding YZ plane imaging, it can be realized by adjusting currents of gradient magnetic fields in Y and Z directions, and a specific process is not described in detail.

Carrying out two simulation experiments on the two-dimensional space coding and reconstruction process, wherein the obtained results are respectively shown in fig. 5 and fig. 6, and a graph (a) in each graph is an original image of the simulation experiment; only the white areas in the original image correspond to the magnetic nanoparticles. The graph (b) in each graph is a two-dimensional image reconstructed by the simulation experiment by using the method in the embodiment of the invention; wherein the two-dimensional filtered back projection is obtained by inverse Radon transform. Wherein, fig. 5(a) is the maximum intensity projection diagram of the blood vessel image of the human head obtained by nuclear magnetic resonance. As can be seen from the simulation result, the two-dimensional image reconstructed by the method of the embodiment of the invention can clearly display the original magnetic particle distribution condition of the target to be measured.

(III) METHOD AND APPARATUS FOR THREE-DIMENSIONAL SPACE ENCODING AND RECONSTRUCTION

The control module is specifically used for determining 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 relation between the imaging and the gradient magnetic field, and providing the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions in a voltage driving current generation mode, so that the magnitude of the gradient magnetic field in the three directions is changed for multiple times to trigger the total spatial gradient magnetic field to follow

The direction is started in the beginning of the direction,

in preset steps to 180 deg., each of which

At angles theta is changed from 0 DEG to 180 DEG in steps of 1 DEG, and the size of the total gradient magnetic field in space is changed 256 times at each theta angle;

the image reconstruction module is particularly adapted to, for each

Angle execution: for each theta angle, carrying out one-dimensional reconstruction on 256 different triple fundamental frequency harmonic components obtained by changing the magnetic field size of the spatial total gradient magnetic field in the direction corresponding to the theta angle by using a system matrix to obtain a one-dimensional reconstruction result in the direction; theta is formed to be 0 DEG and 180 DEG]Two-dimensional filtering back projection is carried out on the 180 one-dimensional reconstruction results to obtain the three-dimensional reconstruction method

Two-dimensional projection drawings corresponding to the angles; when in use

After the angle execution is finished, all the angles are processed

And (4) performing three-dimensional reconstruction on the two-dimensional projection image corresponding to the angle by using a tomography synthesis method to obtain a three-dimensional imaging image.

Specifically, after the scanning part of the target to be measured is located in the central imaging region, in the predetermined corresponding relationship between the imaging and the gradient magnetic field, the respective alternating voltage sequences of the two excitation coils of the gradient magnetic field excitation coil pair in the three directions are determined so as to drive and generate the corresponding alternating current sequences.

The gradient magnetic fields in the three directions are superposed to form a spatial total gradient magnetic field G. According to an alternating current sequence. G is at

For example-25 mT/m.

Holding

Constant, θ increases along 1 °, at each

In the direction, the current of the gradient magnetic field excitation coil pair in each direction is changed for 256 times according to the corresponding alternating current sequence to obtain 256 triple fundamental frequency harmonic component signals, and the corresponding system matrix is utilized to carry out one-dimensional reconstruction to obtain a one-dimensional reconstruction result in each direction. Repeating the above process until obtaining

And (5) one-dimensional reconstruction results in the direction. The 180 one-dimensional reconstruction results are subjected to two-dimensional filtering back projection to obtain the point pairs

A corresponding two-dimensional projection map.

Then, in the following

Step by step of 12 DEGIncrease, maintain

The theta is changed from 0 DEG to 180 DEG in the above-mentioned mode, and the currents of the gradient magnetic field excitation coil pairs in each theta are changed for 256 times … according to the corresponding alternating current sequence to finally obtain the theta-gradient magnetic field excitation coil pair

A corresponding two-dimensional projection map.

Repeating the above process until the target is obtained

A corresponding two-dimensional projection map. And carrying out chromatography synthesis on all the obtained two-dimensional projection images to obtain a distribution image of the magnetic nanoparticle concentration in the target to be detected in a three-dimensional space.

It can be understood that the data dimension of the triple fundamental frequency harmonic component is 256 × 180 × 15, which is the common completion of 256 × 180 × 15 signal encoding caused by 256 × 180 × 15 half cosine oscillation excitation in the three-dimensional imaging process. The number of internal sampling points is 5000, and the total time is 256 × 180 × 15 half oscillation periods, and the total time is 1.75 minutes, which is calculated by that the signal sampling frequency is 16.5mhz and the excitation frequency is 3.3 khz.

Meanwhile, the existing nuclear magnetic resonance imaging technology carries tissue information such as muscles and bones, and has a certain interference item for observing blood vessels. The embodiment of the invention utilizes the characteristic that the magnetic nanoparticles only exist in blood, digital subtraction is not needed in imaging, less motion artifacts exist, and the magnetic nanoparticle magnetic resonance imaging method can be used for targeted imaging. Compared with the existing PET and SPECT imaging technologies, the embodiment of the invention has higher sensitivity and image resolution, no ionizing radiation and easier production and storage of the tracer. The two-dimensional reconstruction scheme of the embodiment of the invention can replace the existing DSA angiography technology, and can provide quick and effective reference information for diagnosis and treatment of vascular diseases.

The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims

1. A gradient field based magnetic particle imaging system, comprising:

2. The gradient field-based magnetic particle imaging system of claim 1, wherein the uniform magnetic field excitation unit comprises:

3. A gradient field based magnetic particle imaging system according to claim 1, wherein said Z-direction gradient magnetic field excitation coil pair comprises:

4. A gradient field based magnetic particle imaging system according to claim 1, wherein said X-direction gradient magnetic field excitation coil pair or said Y-direction gradient magnetic field excitation coil pair comprises:

the symmetry plane of the X-direction gradient magnetic field excitation coil pair is a YZ plane; and the symmetry plane of the gradient magnetic field excitation coil pair in the Y direction is an XZ plane.

5. A gradient field based magnetic particle imaging system according to claim 1 or 3, wherein the receive coil pair comprises:

a pair of circular Homholtz coils which are axially overlapped, axially face in the Z direction and have a distance; and the distance between the receiving coil pairs is larger than that between the gradient magnetic field excitation coil pairs in the Z direction.

6. A gradient field based magnetic particle imaging system as claimed in claim 1 wherein the relationship of the three directional gradient magnetic fields to the total spatial gradient magnetic field comprises:

7. A gradient field based magnetic particle imaging system according to claim 1 or 6, wherein when the target imaging dimension is two-dimensional,

the control module is specifically configured to determine, in a predetermined correspondence relationship between imaging and gradient magnetic fields, gradient magnetic fields in at least two specific directions corresponding to the target imaging plane, and respective alternating voltage sequences of gradient magnetic field excitation coils in the at least two specific directions; providing respective alternating current sequences for the at least two gradient magnetic field excitation coil pairs in the specific direction by using a voltage-driven current generation mode so as to change the magnitude of the gradient magnetic fields in the at least two specific directions for multiple times, thereby causing the direction of the total spatial gradient magnetic field to change in a specific plane, and providing the same ascending alternating current sequence or the same descending alternating current sequence for the two excitation coils of the gradient magnetic field in each specific direction so as to change the magnitude of the magnetic field in each direction of change for multiple times; wherein the current values in any one of the increasing alternating current sequence and the decreasing alternating current sequence are same in step; the specific plane is determined according to the target imaging plane;

the image reconstruction module is specifically used for performing one-dimensional reconstruction by using a system matrix according to a plurality of different triple fundamental frequency harmonic components obtained by the magnetic field size change of the spatial total gradient magnetic field in the same direction to obtain a one-dimensional reconstruction result comprising the multilayer magnetic nanoparticle concentration information in the direction; and performing two-dimensional filtering back projection on all one-dimensional reconstruction results obtained by the direction change of the spatial total gradient magnetic field to obtain a two-dimensional projection image aiming at the target imaging plane, wherein the two-dimensional projection image represents a distribution image of the concentration of the magnetic nanoparticles in the target to be detected on the target imaging plane.

8. The gradient field based magnetic particle imaging system of claim 7, wherein the target imaging dimension is two-dimensional and the target imaging plane is an XY plane,

the control module is specifically used for determining gradient magnetic fields in the X direction and the Y direction corresponding to the XY plane and respective alternating voltage sequences of gradient magnetic field excitation coil pairs in the X direction and the Y direction in a predetermined corresponding relation between imaging and the gradient magnetic fields, providing the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the X direction and the Y direction in a mode of generating current by voltage driving, and triggering the direction of the total spatial gradient magnetic field to follow the direction of the total spatial gradient magnetic field by changing the magnitude of the gradient magnetic fields in the X direction and the Y direction for multiple times

Starting in the YZ plane, changing to 1 along a theta step

And the magnitude of the spatial total gradient magnetic field is changed 256 times in each theta direction;

the image reconstruction module is specifically configured to, for each theta, map the total gradient magnetic field in space at

9. A gradient field based magnetic particle imaging system according to claim 1 or 8, wherein when the target imaging dimension is three-dimensional,

the control module is specifically configured to determine, in a predetermined correspondence relationship between imaging and gradient magnetic fields, respective alternating voltage sequences of gradient magnetic field excitation coil pairs in an X direction, a Y direction, and a Z direction, provide the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions in a manner of generating a current by voltage driving, so that the magnitudes of the gradient magnetic fields in the three directions are changed many times, and the direction of the total spatial gradient magnetic field is changed in space, and the magnitude of the total spatial gradient magnetic field is changed many times in each direction by providing, for the gradient magnetic fields in at least two directions, the same increasing alternating current sequence or the same decreasing alternating current sequence to the two excitation coils of the gradient magnetic field in each direction; wherein the current values in any one of the increasing alternating current sequence and the decreasing alternating current sequence are same in step;

the image reconstruction module is specifically used for performing one-dimensional reconstruction by using a system matrix according to a plurality of different triple fundamental frequency harmonic components obtained by the magnetic field size change of the spatial total gradient magnetic field in the same direction to obtain a one-dimensional reconstruction result comprising the multilayer magnetic nanoparticle concentration information in the direction; carrying out two-dimensional filtering back projection on all one-dimensional reconstruction results obtained by changing the direction of the spatial total gradient magnetic field in the same plane to obtain a two-dimensional projection image, and carrying out three-dimensional reconstruction on the two-dimensional projection image obtained by changing the direction of the spatial total gradient magnetic field in each plane to obtain a three-dimensional imaging image; and the three-dimensional imaging graph represents a distribution image of the magnetic nano particle concentration in the target to be detected in a three-dimensional space.

10. The gradient field-based magnetic particle imaging system of claim 9, wherein when the target imaging dimension is three-dimensional,

the control module is specifically configured to determine respective alternating voltage sequences of the gradient magnetic field excitation coil pairs in the X direction, the Y direction and the Z direction in a predetermined correspondence relationship between imaging and gradient magnetic fields, and provide the respective alternating current sequences for the gradient magnetic field excitation coil pairs in the three directions in a manner of generating current by voltage driving, so that the magnitudes of the gradient magnetic fields in the three directions are changed for multiple times, and the total spatial gradient magnetic field is triggered to follow

The direction is started in the beginning of the direction,

in preset steps to 180 deg., each of which

At angles theta changes from 0 deg. in steps of 1 deg. to 180 deg., and at each of these theta angles the magnitude of the spatial total gradient magnetic field is changed 256 times;

the image reconstruction module is particularly adapted to, for each

Two-dimensional projection drawings corresponding to the angles; when in use

After the angle execution is finished, all the angles are processed