CN114587327A

CN114587327A - Magnetic particle tomography method based on full-space coding

Info

Publication number: CN114587327A
Application number: CN202210039183.2A
Authority: CN
Inventors: 李檀平; 贾广; 胡凯; 黄力宇; 田捷; 惠辉; 苗启广; 张艺飞
Original assignee: Xidian University
Current assignee: Xidian University
Priority date: 2022-01-13
Filing date: 2022-01-13
Publication date: 2022-06-07

Abstract

The invention discloses a magnetic particle tomography method based on full-space coding, which comprises the following steps: executing the coil attitude regulation and control step for multiple times, executing the magnetic field distribution regulation and control step for multiple times after executing the coil attitude regulation and control step for each time, and executing the signal acquisition step after executing the magnetic field distribution regulation and control step for each time to obtain scanning data of full-space coding; the signal acquisition step comprises: applying an alternating excitation field with spatially non-uniform and non-linear distribution to an imaging area by using a pair of excitation coils, simultaneously acquiring a response signal of an imaging target responding to the alternating excitation field, and extracting target characteristics of the imaging target; the response signal is formed by superposing signals excited by all magnetic particles in the imaging area; the magnetic field distribution regulating and controlling step comprises the following steps: adjusting the amplitude of the alternating current on the excitation coil to adjust the spatial distribution state of the alternating excitation field; the coil attitude regulating and controlling step comprises the following steps: the spatial attitude of the excitation coil relative to the imaging target is adjusted. The invention can realize scanning imaging on a large-size imaging target.

Description

Magnetic particle tomography method based on full-space coding

Technical Field

The invention belongs to the technical field of magnetic particle functional imaging, and particularly relates to a magnetic particle tomography method based on full-space coding.

Background

Magnetic particle imaging is a functional imaging technique without ionizing radiation. The magnetic particles are superparamagnetic iron oxide nanoparticles, and the size range of the magnetic particles is 10 nm-60 nm. The magnetic particles are metabolized by the liver, no burden is caused to the kidney, and the tracer is safer. And, magnetic particle imaging does not need to be assisted by X-rays, and is free from the harm of ionizing radiation. The basic principle of magnetic particle imaging is that magnetic particles distributed in an imaging target can generate high-frequency harmonic signals along with the change of an external excitation magnetic field; the signals are collected by the receiving coil, and a spatial distribution image of the magnetic particle concentration can be obtained by using an image reconstruction method, and the image can display an internal image of an imaging target.

In the conventional magnetic particle imaging technology, a point-like or linear magnetic field free region is often generated in an imaging target having a magnetic particle concentration distribution by using a gradient coil, an additional excitation magnetic field is applied to the magnetic field free region, magnetic particles located in the magnetic field free region inside the imaging target are driven to generate high-frequency harmonic signals, and these signals are induced and received by using a receiving coil, so that an induced voltage can be obtained, and the magnetic particle concentration of one point or one line in the imaging target can be obtained based on the induced voltage. By continuously moving the magnetic field free region for scanning, the distribution information of the magnetic particle concentration of the whole imaging target can be acquired, thereby realizing magnetic particle imaging for the imaging target based on the distribution information.

However, in order to ensure the resolution of the imaging image, in the existing magnetic particle imaging technology, the mode of scanning the imaging target by moving the magnetic field free area needs to set the magnetic field free area small enough, so that the whole imaging target needs to be scanned many times, the scanning is performed by almost taking the resolution of the image as a step, and the scanning efficiency is low. In addition, the relaxation effect of the magnetic particles can be increased by long-time scanning, so that the movement of a free area of a magnetic field is delayed and delayed, the final imaging image becomes blurred, and the possibility of scanning and imaging a large-size imaging target by the existing magnetic particle imaging technology is restricted. Therefore, the image resolution of the existing magnetic particle imaging technology can only reach 5mm under a field of view of 20cm, and the existing magnetic particle imaging technology can only be applied to imaging of a mouse-like target (3-5 cm).

Disclosure of Invention

In order to enable the magnetic particle imaging technology to realize scanning imaging on a large-size imaging target, the invention provides a magnetic particle tomography method based on full-space coding.

The technical problem to be solved by the invention is realized by the following technical scheme:

a magnetic particle tomography method based on full-space encoding comprises the following steps:

executing a coil attitude regulating step for multiple times, executing a magnetic field distribution regulating step for multiple times after executing the coil attitude regulating step each time, and executing a signal acquisition step after executing the magnetic field distribution regulating step each time to obtain scanning data of full-space coding for magnetic particle imaging; wherein the content of the first and second substances,

the signal acquisition step comprises: applying an alternating excitation field with spatially non-uniform and non-linear distribution to an imaging region where an imaging target is located by using a pair of excitation coils, acquiring a response signal of the imaging target responding to the alternating excitation field through a pair of receiving coils, and extracting a target feature of the response signal; wherein the response signal is formed by superposing signals of all excited magnetic particles in the imaging target; the target features include: spike amplitude and/or 3 fundamental harmonic components;

the magnetic field distribution regulating step comprises the following steps: adjusting the amplitude of the alternating current applied to the excitation coil to adjust the spatial distribution state of the alternating excitation field;

the coil posture regulating and controlling step comprises the following steps: adjusting a spatial pose of the excitation coil relative to the imaging target.

Optionally, the pair of excitation coils includes: a pair of circular Helmholtz transmitting coils; the pair of receiving coils includes: a pair of circular Helmholtz receiving coils; the two Homholtz transmitting coils are respectively and symmetrically arranged close to the two Homholtz receiving coils, and the axial directions of the two Homholtz transmitting coils and the axial directions of the four Homholtz receiving coils are overlapped to form two groups of receiving and transmitting coils with opposite positions; the imaging target is positioned at the midpoint of the two groups of transceiver coils, the two groups of transceiver coils can move in different circular tracks by taking the center of the imaging target as the center of a circle, and the movement process in each circular track comprises a plurality of stay point positions;

the changing the spatial pose of the excitation coil relative to the imaging target includes:

and changing the stop point positions of the two groups of transceiving coils when the two groups of transceiving coils move along the current circular track, or switching to the next circular track when traversing each stop point position in the current circular track.

Optionally, the pair of excitation coils is one of a plurality of pairs of parallel excitation wires; the multiple pairs of parallel excitation wires are attached to a cylindrical structure and distributed in a dispersed mode, and the plane where each pair of parallel excitation wires is located penetrates through the center line of the cylindrical structure; the cylindrical structure can rotate around the center of the imaging target at different angles so as to drive the plurality of pairs of parallel excitation wires to be distributed according to different circumferential tracks; the pair of receiving coils includes: a pair of circular Homholtz coils respectively positioned on two bottom surfaces of the cylindrical structure;

and switching the pair of parallel excitation wires which are opened when the plurality of pairs of parallel excitation wires are distributed according to the current circumferential track, or switching to the next circumferential track when each pair of parallel excitation wires under the current circumferential track is traversed.

Optionally, the circumferential trajectory comprises:

the three-dimensional coordinate system comprises a first circumferential track, a second circumferential track, a third circumferential track and a fourth circumferential track, wherein the plane of the first circumferential track is parallel to the first plane of the three-dimensional coordinate system, the second circumferential track forms an included angle with the first plane, the third circumferential track forms an included angle with the second plane of the three-dimensional coordinate system, and the fourth circumferential track forms an included angle with the first plane and the second plane; the first plane is a YZ plane, an XZ plane or an XY plane under a three-dimensional coordinate system, and the second plane is a plane which is not the first plane in the YZ plane, the XZ plane and the XY plane.

Optionally, the applying, by using a pair of excitation coils, a spatially non-uniform, non-linearly distributed alternating excitation field to an imaging region in which an imaging target is located includes: applying a homodromous alternating current to a pair of excitation coils;

the adjusting the amplitude of the alternating current applied to the excitation coil comprises:

and increasing the current amplitude in one excitation coil once according to a preset adjustment step, synchronously reducing the current amplitude in the other excitation coil once, and maintaining a half cosine oscillation period after the current amplitude is adjusted.

Optionally, the method of performing magnetic particle imaging on the imaging target according to the scanning data includes:

for each stopping point position in each circumferential track, reconstructing one-dimensional projection distribution data which corresponds to the stopping point position and contains the magnetic particle concentration information of the imaging target according to the target characteristics extracted from the stopping point position and the system matrix; wherein the system matrix is used for representing the spatial distribution of the target characteristics of the response signals of the magnetic particles with unit concentration under the action of the alternating excitation field;

for each circular track, reconstructing a two-dimensional magnetic particle concentration distribution image projected by the imaging target along the corresponding imaging direction by using a filtering back projection reconstruction method according to the one-dimensional projection distribution data of each stop point position in the circular track; and the imaging direction corresponding to each circumferential track is perpendicular to the plane of the circumferential track.

for a pair of parallel excitation wires opened in each circumferential track, reconstructing one-dimensional projection distribution data which correspond to the pair of parallel excitation wires and contain the magnetic particle concentration information of the imaging target according to the target characteristics extracted when the pair of parallel excitation wires are opened and the system matrix; wherein the system matrix is used for representing the spatial distribution of the target characteristics of the response signals of the magnetic particles with unit concentration under the action of the alternating excitation field;

for each circumferential track, reconstructing a two-dimensional magnetic particle concentration distribution image projected by the imaging target along the corresponding imaging direction by using a filtering back projection reconstruction method according to the one-dimensional projection distribution data corresponding to each pair of parallel excitation wires in the circumferential track; and the imaging direction corresponding to each circumferential track is vertical to the plane of the circumferential track.

Optionally, the method of performing magnetic particle imaging on the imaging target according to the scanning data further includes:

and reconstructing a magnetic particle concentration distribution image of the imaging target in a three-dimensional space by utilizing a chromatography synthesis method according to the two-dimensional magnetic particle concentration distribution image of the imaging target along each imaging direction.

Optionally, the magnetic particle tomography method further comprises:

when the signal acquisition step is performed, a shield magnetic field is generated using a shield coil to saturation-confine magnetic particles present in a region outside the imaging region.

Optionally, the magnetic particle tomography method further comprises: in the process of performing the signal acquisition step each time, before extracting the target feature, deconvoluting processing is performed on the acquired response signal to reduce signal deformation caused by the magnetic particle relaxation effect.

Optionally, the magnetic particle tomography method further comprises:

extracting and recording the signal area of the acquired response signal in the process of executing the signal acquisition step each time; correcting a target feature in the scan data based on the recorded signal area.

Optionally, the magnetic particle tomography method further comprises:

extracting and recording the full width at half maximum of the acquired response signal during each execution of the signal acquisition step; determining from the recorded full width at half maximum whether an anomaly is present in the alternating excitation field during scanning.

In the magnetic particle tomography method based on the full-space coding, an alternating excitation field which is spatially non-uniform and nonlinearly distributed is applied to an imaging region in the signal acquisition step, so that the magnetic field intensity sensed by magnetic particles at different positions in an imaging target is different; under the excitation of the alternating excitation field, the peak of the signal of the response of the magnetic particles is in direct proportion to the concentration of the magnetic particles and the intensity of the excitation magnetic field, and 3 times of fundamental frequency harmonic component is in a certain nonlinear relation to the intensity of the magnetic field and is in direct proportion to the concentration of the magnetic particles, so that the two target characteristics of the response signal can be used as effective parameters for imaging of the magnetic particles. On the basis, the response signals acquired in the signal acquisition step are formed by superposing signals excited by all magnetic particles in the imaging target, namely all the magnetic particles in the full space where the imaging target is positioned contribute to the acquired response signals; the one-dimensional space coding can be realized by correspondingly acquiring the response signals by regulating the spatial distribution state of the magnetic field, and the two-dimensional or even three-dimensional space coding can be realized by correspondingly acquiring the response signals by regulating the spatial attitude of the exciting coil relative to the imaging target; therefore, all response signals acquired in the whole scanning process can be used as the scanning data of the full space coding of the magnetic particle imaging, and therefore the magnetic particle imaging is realized. In summary, in the scanning manner of the present invention, there is no need to provide a magnetic field free region, and there is no need to change the position of the magnetic field free region; therefore, the imaging visual field is not limited by the size and the moving range of the magnetic field free area as in the prior art, so that the imaging visual field can be improved, the magnetic particle imaging technology can realize scanning imaging on a large-size imaging target, and the scanning visual field of 20 cm-50 cm required for imaging a human body target is met. In addition, the coils required for constructing the selection field and the focusing field and the corresponding consumed power consumption can be omitted without arranging the magnetic field free area, so that the scale and the power consumption of the imaging device can be reduced. In addition, compared with the mode of executing scanning by almost taking the resolution of an imaging image as stepping in the prior art, the method only needs to regulate and control the non-uniform distribution state of the whole alternating excitation field for several times and adjust the space posture of the excitation coil relative to the imaging target for several times, the time consumed by the scanning mode is far shorter than that of the prior art, the timeliness is higher, the relaxation effect of magnetic particles can be effectively reduced, and the imaging result is clearer.

The present invention will be described in further detail with reference to the accompanying drawings.

Drawings

FIG. 1 is a flow chart of a magnetic particle tomography method based on full-space encoding according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a scanning environment for implementing a magnetic particle tomography method provided by an embodiment of the invention;

FIG. 3 is a plurality of circular motion trajectories followed by two sets of transceiver coils in motion in the scanning environment shown in FIG. 2;

FIG. 4(a) is a flow chart for two-dimensional imaging based on scan data acquired in the scanning environment shown in FIG. 2;

FIG. 4(b) is a flow chart for three-dimensional imaging based on scan data acquired in the scanning environment shown in FIG. 2;

FIG. 5(a) is a schematic diagram of another scanning environment for implementing the magnetic particle tomography method provided by the embodiment of the invention;

FIG. 5(b) illustrates various angles at which the drum structure rotates in the scanning environment of FIG. 5 (a);

FIG. 6(a) is a flow chart for two-dimensional imaging based on scan data acquired in the scanning environment shown in FIG. 5 (a);

FIG. 6(b) is a flow chart for three-dimensional imaging based on scan data acquired in the scanning environment shown in FIG. 5 (a);

FIG. 7 is a schematic diagram of a circular track driving two sets of transceiver coils to move in the scanning environment shown in FIG. 2;

fig. 8 is a schematic diagram of a stage that can be used for scanning by the magnetic particle tomography method according to the embodiment of the present invention;

FIG. 9 is a diagram of an effect of two-dimensional imaging realized by the magnetic particle tomography method provided by the embodiment of the invention under the scanning environment shown in FIG. 2;

FIG. 10 is a diagram of another effect of two-dimensional imaging achieved by the magnetic particle tomography method provided by the embodiment of the invention in the scanning environment shown in FIG. 2;

fig. 11 is a diagram showing an effect of two-dimensional imaging realized by the magnetic particle tomography method provided in the embodiment of the present invention in the scanning environment shown in fig. 5 (a).

Detailed Description

The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto.

In order to enable the magnetic particle imaging technology to realize scanning imaging on a large-size imaging target, the embodiment of the invention provides a magnetic particle tomography method based on full-space encoding. In particular, when the magnetic particle tomography method is performed, the performing process can be as shown in fig. 1, and includes performing the coil posture regulating step (M times in fig. 1) a plurality of times, performing the magnetic field distribution regulating step (N times in fig. 1) a plurality of times after each time of performing the coil posture regulating step, and performing a signal acquisition step after each time of performing the magnetic field distribution regulating step. Full spatial encoded scan data for magnetic particle imaging can thus be obtained, which scan data essentially consists of the target features of the response signals acquired in the respective signal acquisition steps. The following is a description of these several key steps separately.

A signal acquisition step: a pair of excitation coils is utilized to apply an alternating excitation field with spatially non-uniform and non-linear distribution to an imaging region where an imaging target is located, meanwhile, a pair of receiving coils is utilized to acquire a response signal of the imaging target responding to the alternating excitation field, and target characteristics of the response signal are extracted.

Wherein the response signal is formed by superposing signals of all excited magnetic particles in the imaging target; the target characteristics of the response signal include: the peak amplitude and/or 3 times the fundamental harmonic component of the response signal.

In the embodiment of the invention, the theoretical basis on which magnetic particle imaging is realized based on the peak amplitude and/or 3 times fundamental frequency harmonic component of a signal is as follows: according to the intensity of the excitation magnetic field, the shape and the size of the magnetization curve of unit concentration magnetic particles are different, and the shape and the amplitude of signal peaks are different. The inventor finds that the signal peak u generated by the magnetic particles under the excitation of the magnetic particles is generated by adopting an excitation magnetic field H (t) ═ -Acos (2 pi ft) of alternating cosine oscillation_peak3 times fundamental frequency harmonic component u in proportion to the intensity A of the exciting magnetic field and in proportion to the concentration c of magnetic particles₃Is in a nonlinear relationship with the excitation magnetic field intensity A and is in direct proportion to the magnetic particle concentration c. Equations (1) and (2) show a simple proof of the theoretical basis:

where f denotes the frequency and m denotes the magnetic moment of a single magnetic particle. Mu.s₀Denotes the vacuum permeability, k_BDenotes the Boltzmann constant, T^PRepresenting the absolute temperature of the imaged object.

In the signal acquisition step, there are various pairs of excitation coils that can apply an alternating excitation field to the imaging region, such as circular helmholtz coils or parallel excitation wires. Keeping a certain distance between the two exciting coils, and applying equidirectional alternating currents with different amplitudes to the two exciting coils through a current exciting device, so that an alternating exciting field with nonuniform space and nonlinear distribution can be generated between the two exciting coils; in this way, an imaging region is provided between the two excitation coils, in which region magnetic particles at different positions of the imaging target experience different magnetic field strengths. The current excitation device can be a digital alternating current power supply or a waveform generator, and the current excitation device can be provided with a corresponding software and hardware control module to regulate and control the amplitude, waveform, loading time and the like of the current.

The receiving coil is preferably a circular helmholtz coil, although not limited thereto. The acquisition of the response signals by the receiver coils can be carried out by means of a data acquisition unit. Specifically, all the magnetic particles in the imaging target jointly emit a superposed response signal under the excitation of the alternating excitation field, the response signal can be induced by the receiving coil, so that an induced voltage is generated on the receiving coil, and the response signal can be acquired by acquiring the induced voltage by using the data acquisition unit. The data acquisition device may be integrated with an ADC (Analog-to-Digital Converter), so as to convert the induced voltage on the receiving coil into a Digital signal, so as to extract the target feature by using a data processing method.

In an optional implementation mode, before data acquisition is performed on the induced voltage on the winding coil, low-noise amplification and preliminary correction processing can be performed on the induced voltage; mixing is then performed to filter out low and high frequency noise, followed by analog to digital conversion. Wherein the preliminary correction process includes: correcting according to a signal attenuation model that a radio frequency signal emitted by the magnetic particles reaches a receiving coil after penetrating through an imaging target; the signal attenuation model can be obtained in advance through testing or simulation means.

Magnetic field distribution regulation and control: and adjusting the amplitude of the alternating current applied to the excitation coil to adjust the spatial distribution state of the alternating excitation field.

Specifically, the current amplitude in one excitation coil is increased once, the current amplitude in the other excitation coil is synchronously decreased once, and a half cosine oscillation period is maintained after the current amplitude is adjusted. It is understood that the cosine oscillation period as referred to herein refers to the oscillation period of the alternating current, and accordingly, the current amplitude refers to the maximum value of the alternating current within the oscillation period. Therefore, in the signal acquisition step, the target feature is extracted from the response signal, and specifically, fourier transform and spectrum analysis may be performed on the response signal acquired in each cosine oscillation period, so as to extract the peak amplitude and/or triple fundamental frequency harmonic component of the signal from the analysis result.

In a preferred implementation, in order to make the encoding uniform in the one-dimensional spatial encoding, during the application of the alternating current with varying amplitude to the two excitation coils, the current amplitude in one excitation coil may be increased step by step according to a preset adjustment step, and the current amplitude in the other excitation coil may be decreased synchronously, and also the half period of the cosine oscillation may be maintained after each current amplitude adjustment.

Coil attitude regulation and control: the spatial attitude of the excitation coil relative to the imaging target is changed.

It is understood that any relative position relationship of the two excitation coils in the three-dimensional space relative to the imaging target can be regarded as a different spatial posture. If the space postures changed for many times do not exceed one two-dimensional plane, the finally obtained scanning data can be used for two-dimensional imaging, and if the space postures changed for many times are changed in a three-dimensional space, the finally obtained scanning data can be used for three-dimensional imaging. In other words, the magnetic particle tomography method based on the full-space encoding provided by the embodiment of the invention can realize the magnetic particle imaging of a two-dimensional plane or a three-dimensional space. The specific imaging process is mainly based on the acquired response signal target characteristics and the system matrix, and the imaging is carried out by using an image reconstruction method. For clarity of the description, the imaging mode will be further described with reference to different types of scan data.

In the magnetic particle tomography method based on the full-space coding provided by the embodiment of the invention, an alternating excitation field which is spatially non-uniform and nonlinearly distributed is applied to an imaging region in the signal acquisition step, so that the magnetic field intensity sensed by magnetic particles at different positions in an imaging target is different; under the excitation of the alternating excitation field, the peak of the signal of the response of the magnetic particles is in direct proportion to the concentration of the magnetic particles and the intensity of the excitation magnetic field, and 3 times of fundamental frequency harmonic component is in a certain nonlinear relation to the intensity of the magnetic field and is in direct proportion to the concentration of the magnetic particles, so that the two target characteristics of the response signal can be used as effective parameters for imaging of the magnetic particles. On this basis, the response signals acquired in the signal acquisition step in the embodiment of the present invention are obtained by superimposing signals excited by all the magnetic particles in the imaging target, that is, all the magnetic particles in the full space where the imaging target is located contribute to the acquired response signals; the one-dimensional space coding can be realized by regulating the non-uniform distribution state of the excitation magnetic field and correspondingly acquiring the response signal, and the two-dimensional or even three-dimensional space coding can be realized by regulating the spatial attitude of the excitation coil relative to the imaging target and correspondingly acquiring the response signal; therefore, all response signals acquired in the whole scanning process can be used as the scanning data of the full space coding of the magnetic particle imaging, and therefore the magnetic particle imaging is realized. In summary, in the scanning manner in the embodiment of the present invention, it is not necessary to provide a magnetic field free region, and it is also not necessary to change the position of the magnetic field free region; therefore, the imaging visual field is not limited by the size and the moving range of the magnetic field free area as in the prior art, so that the imaging visual field can be improved, the magnetic particle imaging technology can realize scanning imaging on a large-size imaging target, and the scanning visual field of 20 cm-50 cm required for imaging a human body target is met. And the coil required for constructing the magnetic field free area and the corresponding consumed power consumption can be omitted without arranging the magnetic field free area, so that the scale and the power consumption of the imaging device can be reduced. In addition, compared with the mode of executing scanning by almost taking the resolution of an imaging image as stepping in the prior art, the embodiment of the invention only needs to regulate and control the non-uniform distribution state of the whole alternating excitation field for several times and adjust the spatial posture of the excitation coil relative to an imaging target for several times, the time consumed by the scanning mode is far shorter than that of the prior art, the timeliness is higher, the relaxation effect of magnetic particles can be effectively reduced, and the imaging result is clearer.

In a preferred implementation, referring to fig. 2, a pair of excitation coils used in the magnetic particle tomography method described above may include: a pair of circular Helmholtz transmitting coils; the pair of receiving coils used may include: a pair of circular Helmholtz receiving coils; the two Homholtz transmitting coils are respectively and symmetrically arranged close to the two Homholtz receiving coils, and the axial directions of the two Homholtz transmitting coils and the axial directions of the four Homholtz receiving coils are overlapped to form two groups of receiving and transmitting coils with opposite positions; the imaging target is positioned at the middle point of two groups of receiving and transmitting coils, the two groups of receiving and transmitting coils can move in different circular tracks by taking the center of the imaging target as the center of a circle, and the moving process in each circular track comprises a plurality of stay point positions; fig. 2 shows an exemplary schematic diagram of the movement of two sets of transceiver coils along a circular trajectory, wherein the two sets of transceiver coils are about to move from the dwell position 1 to the dwell position 2.

It can be understood that, because of the position symmetry of the two sets of transceiver coils, the magnetic field generated by the two sets of transceiver coils is also symmetrical, so that the two sets of transceiver coils can cover the whole circular track by moving half a circle along the circular track; for example, two sets of transceiver coils move 180 ° along the circular track by 1 ° step, so as to cover the entire circular track.

The various circular tracks along which the two sets of transceiver coils move may include one or more of the following circular tracks:

the three-dimensional coordinate system comprises a first circumferential track, a second circumferential track, a third circumferential track and a fourth circumferential track, wherein the plane of the first circumferential track is parallel to the first plane of the three-dimensional coordinate system, the second circumferential track forms an included angle with the first plane, the third circumferential track forms an included angle with the second plane of the three-dimensional coordinate system, and the fourth circumferential track forms an included angle with the first plane and the second plane; the first plane is a YZ plane, an XZ plane or an XY plane under a three-dimensional coordinate system, and the second plane is any one of the YZ plane, the XZ plane and the XY plane which is not the first plane.

For example, assuming that the first plane is a YZ plane in a three-dimensional coordinate system and the second plane is an XZ plane in the three-dimensional coordinate system, the various circular trajectories may include: the three-dimensional coordinate system comprises a first circumferential track, a second circumferential track, a third circumferential track and a fourth circumferential track, wherein the first circumferential track is parallel to a YZ plane under the three-dimensional coordinate system, the second circumferential track forms an included angle between the plane and the YZ plane, the third circumferential track forms an included angle between the plane and an XZ plane under the three-dimensional coordinate system, and the fourth circumferential track forms an included angle between the plane and the YZ plane and between the plane and the XZ plane. In fig. 3, the Y axis is perpendicular to fig. 3, and each dotted frame of the perfect circles represents a virtual sphere existing in space under the three-dimensional coordinate system, and the center of the sphere is where the imaging target is located, i.e., the small black dots in fig. 3. It will be appreciated that the second circumferential track, the third circumferential track and the fourth circumferential track each comprise a plurality of tracks in addition to the first circumferential track, one of which is shown in fig. 3 by way of example only.

Accordingly, the changing the spatial orientation of the excitation coil with respect to the imaging target in the coil orientation adjusting step may include:

It is understood that the switching to the next circular track described herein specifically refers to the movement of the two sets of transceiver coils to a certain stop point of the next circular track. Changing the space attitude of the exciting coil relative to the imaging target in a period of time, namely changing the stop point positions of the two groups of receiving and transmitting coils when moving along the new circular track until the circular track is traversed; and so on until the scanning is finished.

In the scanning environment shown in fig. 2, the axial components of the alternating excitation field generated between the two excitation coils are linearly decreased and then linearly increased, and are distributed in a V shape, and each plane perpendicular to the axial direction of the coils is an isomagnetic field surface. With the magnetic field distribution regulating step performed a plurality of times, the "V" shaped magnetic field will be shifted in position along the axial direction of the coil, thereby achieving one-dimensional spatial encoding.

Referring to FIG. 4, based on scan data obtained under this "V" shaped magnetic field excitation, the manner of magnetic particle imaging for the imaging target includes:

s401: and reconstructing one-dimensional projection distribution data which corresponds to each stop point position and contains the magnetic particle concentration information of the imaging target according to the target characteristics extracted from the stop point position and the system matrix aiming at each stop point position in each circular track.

The system matrix is used for representing the spatial distribution of target characteristics of response signals of magnetic particles with unit concentration under the action of an alternating excitation field; based on the system matrix, the magnetic particle concentration of each point at each moment can be reversely deduced, so that the imaging is realized by using an image reconstruction method.

It should be noted that, in the system matrix of the existing MPI (Magnetic Particle Imaging) system, each column of elements includes a set of fourier components of a signal generated by a Magnetic Particle sample with a known concentration at a certain position in an Imaging region. I.e. a column of elements almost covers the harmonics of the signal generated at that location. In the embodiment of the present invention, each element of the system matrix is a peak amplitude or 3 times fundamental frequency harmonic component of a signal generated by magnetic particles with unit concentration at a certain position, which is different from the system matrix of the existing MPI system.

The specific reconstruction process can be represented by the following formula:

c＝g^-1u；(3)

wherein i₀,i₁,…,i_N+1Representing N different magnitudes of current, r, applied to two excitation coils₀,r₁,…,r_N+1N position points representing an imaging area into which an imaging target is located; u represents a target feature at a point of dwell, where the element u (i)₁) Indicating that a current i is applied to the exciting coil₁Half cosine oscillation ofIn the period, the target characteristics of the collected response signals, the meanings of the rest elements and the like are analogized. g is the system matrix, where the lower left element g (i)_N-1,r₀) Magnetic particles representing unit concentration at current i_N-1Under the action of the excited magnetic field, the target characteristic of the generated response signal is distributed in the r-th area of the imaging area₀A component of each location point; the meaning of the remaining elements is analogized. c represents the reconstructed one-dimensional projection distribution data, which contains elements of the magnetic particle concentrations at the respective position points in the imaging region.

In practical applications, if the system matrix is not very large, the system matrix may be directly inverted according to the formula (3) shown above, and then the inverted system matrix g is inverted^-1And the vector u is multiplied to realize data reconstruction. If the system matrix is huge and direct inversion is difficult, the elements in c can be used as variables x to be solved, and a set of equations u (i) is constructed_n)＝g(i_n,r₀)x+g(i_n,r₁)x+…+g(i_n,r₁)x，n∈[0,N-1]And solving the set of equations in an iterative mode, thereby realizing data reconstruction according to the solving result. The iterative method is, for example, a common algebraic reconstruction method, a joint algebraic reconstruction method, a maximum likelihood expectation-maximization algorithm or an ordered subset expectation-maximization algorithm, etc.

S402: and for each circular track, reconstructing a two-dimensional magnetic particle concentration distribution image projected by the imaging target along the corresponding imaging direction by using a filtering back projection reconstruction method according to the one-dimensional projection distribution data of each stop point position in the circular track.

Here, the two-dimensional magnetic particle concentration distribution image is a two-dimensional imaging result projected in an imaging direction for an imaging target. The imaging direction corresponds to the circumferential tracks, and the imaging direction corresponding to each circumferential track is perpendicular to the plane of the circumferential track. It can be understood that if only two-dimensional imaging is required along one imaging direction, only one corresponding circumferential track needs to be scanned; if two-dimensional imaging is required to be performed along a plurality of imaging directions, a plurality of circumferential tracks are correspondingly scanned.

In step S402, the filtered back-projection reconstruction method is commonly used in CT (Computed Tomography) imaging reconstruction, and the mathematical principle behind the method is radon transform. The method for reconstructing the magnetic particle concentration distribution image by using the filtered back projection reconstruction method in the embodiments of the present invention is basically the same, and therefore, the description thereof is omitted.

On the basis of two-dimensional imaging, the embodiment of the present invention may further implement three-dimensional imaging for the imaging target, and at this time, multiple scans may be performed according to four circumferential trajectories shown in fig. 2. Accordingly, referring to fig. 4(b), the three-dimensional imaging process may further add step S403 after step S402.

S403: and reconstructing a magnetic particle concentration distribution image of the imaging target in a three-dimensional space by utilizing a chromatography synthesis method according to the two-dimensional magnetic particle concentration distribution image of the imaging target along each imaging direction.

The reconstruction process based on tomosynthesis is to obtain a magnetic particle concentration distribution image of an imaging target in a three-dimensional space through inversion calculation based on data information in the two-dimensional magnetic particle concentration distribution images projected along different directions, and the specific imaging process is similar to CT imaging, which is not repeated in the embodiment of the invention.

In another implementation, referring to fig. 5(a), the pair of excitation coils used in performing scanning in the embodiment of the present invention may be one of a plurality of pairs of parallel excitation wires; the parallel excitation wires are attached to a cylindrical structure and distributed in a dispersed mode, and the plane where each pair of parallel excitation wires is located penetrates through the center line of the cylindrical structure; the cylinder structure can rotate around the center of an imaging target at different angles, so that the parallel excitation wires are driven to be distributed according to different circumferential tracks by taking the center of the imaging target as a circle center; in addition, a pair of receiving coils used when performing scanning may include: and the pair of circular Homholtz coils are respectively positioned on the two bottom surfaces of the cylindrical structure.

The postures of the cylindrical structure after rotating by various angles with the center of the imaging target as the center of the sphere can be seen from fig. 5(b), including inclining to the left or right by a certain angle, and inclining on the basis of a certain inclination angle, and the like, and all the postures are not completely shown in fig. 5 (b). Thus, under the driving of the cylindrical structure, the circular tracks along which the parallel excitation wires are distributed according to the circular tracks can be seen in fig. 3, and each excitation wire is perpendicular to the circular tracks.

Correspondingly, the changing the spatial attitude of the excitation coil relative to the imaging target in the coil attitude adjusting step includes:

and switching a pair of parallel excitation wires which are opened when the parallel excitation wires are distributed according to the current circumferential track, or switching to the next circumferential track when traversing each pair of parallel excitation wires under the current circumferential track.

It will be understood that switching to the next circular track as referred to herein means in particular that the cylindrical structure is rotated until the cross-section of its barrel coincides with the next circular track and a certain pair of parallel excitation wires is opened accordingly. Changing the spatial posture of the exciting coil relative to the imaging target within a period of time, namely switching a pair of parallel exciting leads which are opened by the cylinder structure under the condition of maintaining the current posture until all pairs of parallel exciting leads are traversed; and so on until the scanning is finished.

Under the scanning environment shown in fig. 5(a), the alternating excitation field generated between each pair of parallel excitation wires decreases linearly first and increases linearly later along the direction of the vertical line connecting the two, and is distributed in a V shape.

Accordingly, after the scanning data obtained by performing the scanning in the scanning environment shown in fig. 5(5), the manner of performing magnetic particle imaging for the imaging target based on the scanning data is shown in fig. 6(a), and includes:

s601: and reconstructing one-dimensional projection distribution data which correspond to the pair of parallel excitation wires and contain magnetic particle concentration information of the imaging target according to the target characteristics extracted when the pair of parallel excitation wires are opened and the system matrix aiming at the pair of parallel excitation wires opened in each circumferential track.

Wherein the system matrix is used for characterizing the spatial distribution of the target characteristics of the response signals of the magnetic particles of unit concentration under the action of the generated alternating excitation field. The specific reconstruction process is the same as the reconstruction process in the scanning environment shown in fig. 2, and is not described here again.

S602: and for each circumferential track, reconstructing a two-dimensional magnetic particle concentration distribution image projected by an imaging target along a corresponding imaging direction by using a filtering back projection reconstruction method according to the one-dimensional projection distribution data corresponding to each pair of parallel excitation wires in the circumferential track.

Here, the two-dimensional magnetic particle concentration distribution image is a two-dimensional imaging result projected in an imaging direction for an imaging target. The imaging direction corresponds to the circumferential tracks, and the imaging direction corresponding to each circumferential track is perpendicular to the plane of the circumferential track. Similar to the reconstruction process in the scanning environment shown in fig. 2, if only two-dimensional imaging is required along one imaging direction, only one corresponding circular track needs to be scanned; if two-dimensional imaging is required to be performed along a plurality of imaging directions, a plurality of circumferential tracks are correspondingly scanned.

On the basis of two-dimensional imaging, three-dimensional imaging can also be achieved using the scanning environment shown in fig. 5 (a). Specifically, step S603 is added after step S602, as shown in fig. 6 (b).

S603: and reconstructing a magnetic particle concentration distribution image of the imaging target in a three-dimensional space by utilizing a chromatography synthesis method according to the two-dimensional magnetic particle concentration distribution image of the imaging target along each imaging direction.

The specific implementation manner of step S603 is similar to step S403, and is not described again.

In an alternative implementation manner, the magnetic particle tomography method based on full-space encoding provided by the embodiment of the present invention may further include: when the signal acquisition step is performed, a shield magnetic field is generated using a shield coil to saturate magnetic particles present in a region outside the confined imaging region.

Specifically, a mounting table may be disposed below an imaging target, and a plurality of rectangular shield coils may be arranged in parallel inside the mounting table. For example, when scanning and imaging the human body, the table may be a bed. When an alternating excitation field is applied, shielding coils corresponding to the upper part and the lower part of the position of an imaging target are closed, and direct current is loaded to the rest shielding coils. In this way, a shielding magnetic field can be generated in a non-imaging area around the imaging target, thereby reducing the influence of external interference on the accuracy of the scanning data. For example, when the magnetic particle device is located in an environment where the shielding effect is poor, turning on the shielding coil may effectively saturation-confine magnetic particles present in a region other than the non-imaging region, so that only the magnetic particles located in the imaging region are excited by the exciting coil.

Preferably, in an implementation manner, the magnetic particle tomography method based on full-space encoding provided by the embodiment of the present invention may further include: in the process of executing the signal acquisition step each time, before extracting the target characteristic, deconvolution processing is carried out on the acquired response signal so as to reduce signal deformation caused by the magnetic particle relaxation effect. The signal deformation caused by the magnetic particle relaxation effect mainly includes the reduction of the signal amplitude, the broadening and the delay of the time domain, asymmetry and the like. Through deconvolution operation, the acquired signals can be corrected, signal deformation is reduced, and finally extracted target features can be better and more accurate.

In practical applications, magnetic particles of large size (30nm to 100nm) are more likely to produce relaxation effects, and therefore, if the size of magnetic particles in the imaging target is large, signal deformation can be mitigated by performing the step of deconvolution processing.

In addition, in order to further reduce signal deformation and thus extract an accurate target feature, the magnetic particle tomography method based on full-space encoding provided by the embodiment of the present invention may further include:

extracting and recording the signal area of the acquired response signal in the process of executing the signal acquisition step each time; the target feature in the scan data is corrected based on the recorded signal area.

The signal area refers to an area under a time domain curve of a signal, and can be obtained by integrating data acquired in a time domain.

In the process of implementing the invention, the inventor finds that the signal area is independent of the magnetic field intensity and is in direct proportion to the concentration of magnetic particles. Therefore, no matter how to adjust the spatial distribution of the magnetic field or adjust the spatial attitude of the excitation coil relative to the imaging target, the area of the response signal acquired each time is actually a conservative value, provided that the magnetic particle concentration distribution of the imaging target remains unchanged. Considering the situation that the magnetic particle concentration distribution of the actual imaging target is not changed in a short time and may be changed in a long time, the embodiment of the present invention preferably adopts a scheme of performing target feature correction according to the signal area by taking the stay point/parallel excitation conductor pair as a unit, so that the finally extracted target feature can be better and more accurate.

Specifically, extracting the signal area may specifically be performing integration processing on data acquired in a time domain. Acquiring a response signal once when the current amplitude is converted once at each stop point position or each pair of parallel excitation wires is started, and extracting the signal area of the response signal; when the process of current amplitude adjustment is finished, namely after the scanning on the stay point position/parallel excitation wire pair is finished, the signal areas of all the collected response signals are compared, and the response signals with abnormal signal areas are corrected, wherein the specific correction modes are various. For example, the corresponding current may be reapplied to the excitation coil to re-acquire; or to correct the abnormal response signal by acquiring a response signal adjacent in time, etc., which is reasonable and realizable.

In an alternative implementation manner, the magnetic particle tomography method based on full-space encoding provided by the embodiment of the present invention may further include:

extracting and recording the full width at half maximum of the acquired response signal in the process of executing the signal acquisition step each time; determining from the recorded full width at half maximum whether an anomaly of the alternating excitation field is present during the scan.

The full width at half maximum refers to the width of the corresponding time domain when the amplitude of the signal decreases to half. The full width at half maximum can be used to help verify the performance of relaxation effect deconvolution, and excitation field monitoring can also be performed.

The inventor in the process of realizing the invention finds that the full width at half maximum of the response signal is independent of the magnetic particle concentration, but has an inverse relation with the excitation magnetic field strength; therefore, the method can be used for checking the stability of the excitation magnetic field by uniformly comparing the full width at half maximum of the actually acquired response signals, and finding possible anomalies in the invisible magnetic field so as to ensure that the data depended on by the final imaging result is true and effective. Generally, the occurrence of excitation magnetic field anomalies may be related to external disturbances; after the excitation magnetic field is found to be abnormal, higher-level shielding measures can be taken, such as opening shielding coils outside the imaging area, and the like, measuring the system matrix again, and then scanning imaging is performed again. Considering that the comparison efficiency of the full width at half maximum of the signals collected in the whole scanning process is low, the embodiment of the invention adopts a scheme of comparing the full width at half maximum by taking the stay point/parallel excitation wire pair as a unit.

Specifically, when each pair of parallel excitation wires is opened or stopped at each stopping point, a signal is collected once when the current amplitude is converted once, and the full width at half maximum of the signal is extracted; when the process of current amplitude adjustment is finished, the full widths at half maximum of all the collected response signals are compared, so that an abnormal full width at half maximum is found therefrom. In addition, the manner of outputting the excitation magnetic field abnormality indication may include display output, sound output, or the like.

In an embodiment, the magnetic particle tomography method based on the full-space encoding provided by the embodiment of the invention can simultaneously perform imaging by using the peak amplitude and 3 times of fundamental frequency harmonic components respectively, and fuse the two obtained imaging results. Specifically, the spike amplitude and 3-fold fundamental frequency harmonic components are extracted each time a target feature is extracted from the response signal. And then, imaging by respectively utilizing the peak amplitude and the 3-time fundamental frequency harmonic component to obtain two imaging results. Then, the two imaging results are fused, that is, the two imaged images are subjected to image fusion, so that the reconstruction effect is further improved. The specific image fusion mode may include weighted fusion of pixels at the same position between images or other common image fusion modes.

In a preferred example, and referring to the scanning environment shown in FIG. 2, both excitation coils are 40cm in diameter, 5cm in thickness and width, 200 turns, and the applied current is 20A-40A in amplitude. The distance between the two excitation coils is 40cm, namely the diameter of the circular track is 40cm, the two excitation coils are applied with equidirectional alternating current, a cosine alternating excitation field of 15 mT-30 mT can be generated in the central imaging area of the two groups of receiving and transmitting coils, and the alternating current frequency is 25 KHz-35 KHz; the magnetic field intensity is more than 10mT to ensure that the signal has amplitude peak and can filter iron in the human body in medical application. During the process of applying current excitation, the current of one excitation coil is gradually increased from 20A to 256 times by 0.78A in each time until the current is increased to 40A; meanwhile, the current of the other exciting coil is synchronously reduced from 40A, and is reduced by 0.78A each time for 256 times until the current is reduced to 20A; therefore, the shape and the position of the V-shaped magnetic field are changed 256 times independently, namely 256-bit one-dimensional space coding is realized. After the current amplitude is adjusted each time, the current amplitude is kept for 0.017ms, and excitation and corresponding signal acquisition within a half cosine oscillation period are carried out. Thus, a total of 4.267ms is required for the scan of each dwell point. Accordingly, after one-dimensional data reconstruction is subsequently performed, information on the concentration distribution of magnetic particles at 256 slices along the axial direction of the excitation coil is obtained. The two receiving coils are both 40cm in diameter and 5cm in thickness and width. The distance between the two coils is 50cm, and the two coils are respectively close to the inner sides of the two exciting coils to form two groups of transmitting and receiving coils with opposite positions.

Two groups of transmitting and receiving coils are driven by a circular track to move along a plurality of circular tracks around the center of an imaging target. The radius of the circular track is 40cm, and two groups of transceiving coils are respectively fixed on the circular track and symmetrically arranged by taking the circle center of the track as a midpoint (as shown in fig. 7). The circular track rotates around the circle center to drive the two groups of receiving and transmitting coils to rotate 180 degrees around the center of the imaging target, the two groups of receiving and transmitting coils stay 4.267ms every 1 degree, the exciting coils and the receiving coils are opened within the stay time, 256 times of signal acquisition at the angle is completed, then the next 1 degree is reached, and the like. Due to the symmetry of the V-shaped magnetic field, only signals of 0-180 degrees need to be acquired, and 181-1 degrees are repeated without repeated acquisition. The signal acquisition from 0 ° to 180 ° was performed 180 times in total, taking 0.768 seconds. After the scanning is finished under each circular track, the next circular track can be switched to continue the scanning.

Specifically, it is assumed that the initial position of the circular orbit is located in the YZ plane under the three-dimensional coordinate system; moreover, the circular tracks can be respectively inclined by 1-7 degrees towards the left and right directions, and 15 inclination angles are provided (the inclined mode refers to a second circular track in fig. 3); in addition, at each inclination angle, the circular track can also be respectively deviated from the front and back directions by 1 to 7 degrees and 15 side angles (the deviation manner when the inclination angle is 0 degree is referred to as a third circular track in fig. 3, and the deviation manner when the inclination angle is not 0 degree is referred to as a fourth circular track in fig. 3). Therefore, when the circular track is located at the initial position, the circular track moves along the first circular track, and the consumed scanning time is 0.768 seconds; then, setting the inclination angle to be 1 °, traversing 15 side angles, and consuming the scanning time of 0.768 × 15 ═ 11.52 seconds; setting the inclination angle to be 2 degrees, traversing 15 side angles, and consuming the scanning time of 0.768 × 15 to 11.52 seconds; and repeating the steps until the inclination angle of-7 degrees and the side angle of-7 degrees are completely traversed, and consuming the scanning time of 2.88 minutes. The sampling frequency of the response signal during data acquisition is 15MHz, and the number of sampling points in a half cosine oscillation period is 250. Since data acquisition is performed in a total of 256 × 180 × 15 half oscillation cycles, the total number of signal points acquired is 14.4M. In addition, in order to enhance the shielding effect, a shielding coil can be used in the scanning process; specifically, an imaging target was placed on a table, and 15 rectangular coils (as shown in fig. 8) having a width of 10cm and a length of 30cm were placed inside the table, the number of turns of the coils was 200 turns, and a direct current was applied thereto at 30A. During scanning, 2-5 coils in the area, used for imaging, in the center are closed, and the coils at the rest positions are opened to generate a shielding magnetic field of 30mT for saturation constraint of magnetic particles in the peripheral area, so that an interference signal is avoided.

Two-dimensional imaging is performed based on this scanning environment, and the imaging effect is shown in fig. 9 and 10.

In fig. 9 and 10, the imaging target is a sample in which magnetic particles are distributed in a two-dimensional plane, the magnetic particle distribution is the original image as shown in the figure, the white area is an area with magnetic particle distribution, and the black area is an area without magnetic particle distribution; the reconstructed two-dimensional image can be seen to clearly show the original magnetic particle distribution of the imaging target.

In another preferred example, referring to the scanning environment shown in fig. 5, the cylinder structure has a diameter of 40cm and a length of 60cm, whereby the distance between each pair of parallel excitation wires is 40cm and the length is 60cm, 360 parallel wires are distributed around the cylinder structure, and two parallel wires capable of dividing the cylinder structure into two from the circular bottom surface are selected as a pair of parallel excitation wires each time. The length of each pair of parallel excitation wires is 40cm, and the wire diameter is 2 cm. Applying equidirectional alternating current to any pair of parallel exciting wires can generate a cosine alternating exciting field with the frequency of 25 KHz-35 KHz and the range of 15 mT-30 mT in the central area of the cylinder structure. During the process of applying current excitation, the current on one excitation wire is gradually increased from 20A to 256 times by 0.78A in each time until the current is increased to 40A; meanwhile, the current of the other excitation wire is synchronously reduced from 40A, and is reduced by 0.78A each time for 256 times until the current is reduced to 20A; therefore, the shape and the position of the V-shaped magnetic field are changed 256 times independently, and 256-bit one-dimensional space encoding is realized. After the current amplitude is adjusted each time, the current amplitude is kept for 0.017ms, and excitation and corresponding signal acquisition within a half cosine oscillation period are carried out. Thus, it takes 4.267ms for each open pair of parallel excitation wires to scan. Correspondingly, after one-dimensional data reconstruction is subsequently carried out, the magnetic particle concentration distribution information of 256 layers along the vertical connecting line direction of the excitation lead pair is obtained. The two receiving coils are 60cm in diameter, 5cm in thickness and width and abut against the two circular bottom surfaces of the cylindrical structure respectively. The cylinder structure can be rotated around the central imaging area at various angles, and each time the cylinder structure stops at an angle for 4.267ms, one pair of parallel excitation wires and receiving coils are opened to complete 256 signal acquisitions at the angle, then the next pair of parallel excitation wires is selected and still stops at 4.267ms, and the like. After the acquisition is finished, the angle of the cylinder structure is adjusted, and the signal acquisition at other angles is continued.

Specifically, referring to fig. 5(b), the cylindrical structures can be inclined respectively from 1 ° to 7 ° in the left-right direction for 15 inclination angles; wherein, under each inclination angle, the cylinder structure can also deviate from the left or right side by 1 to 7 degrees respectively, and 15 side angles are totally formed. Thus, assuming that the initial posture of the cylindrical structure is as shown in the middle area in fig. 5(b), at this time, each pair of parallel excitation wires is distributed according to the first circumferential trajectory, and the scanning time taken to traverse each pair of parallel excitation wires is 0.768 seconds; then, setting the inclination angle to be 1 °, traversing 15 side angles, and consuming the scanning time of 0.768 × 15 to 11.52 seconds; setting the inclination angle to be 2 degrees, traversing 15 side angles, and consuming the scanning time of 0.768 × 15 ═ 11.52 seconds; and repeating the steps until the inclination angle of-7 degrees and the side angle of-7 degrees are completely traversed, and consuming the scanning time of 2.88 minutes. The sampling frequency of the response signal during data acquisition is 15MHz, and the number of sampling points in a half cosine oscillation period is 250. Since data acquisition is performed in a total of 256 × 180 × 15 half oscillation cycles, the total number of signal points acquired is 14.4M. In addition, in order to enhance the shielding effect, a shielding coil can be used as well, and the description is omitted here.

Two-dimensional imaging is performed based on this scanning environment, and the imaging effect is shown in fig. 11. In fig. 11, the imaging target is the head of a patient, and the original image is the maximum intensity projection of the magnetic resonance blood vessel image of the head of the patient taken by the magnetic resonance device; it can be seen that the magnetic resonance imaging contains images of other tissues inside the cranium, which are superimposed with the images of the vascular tissues; in contrast, in the two-dimensional image reconstructed from the data scanned by the magnetic particle tomography method provided by the embodiment of the present invention, only the vascular tissue with the magnetic particle distribution is shown.

The embodiment of the invention does not use a selection field and a focusing field in the existing magnetic particle imaging technology, and each point of the whole imaging space is a magnetic field free area and can be excited by a cosine alternating excitation field. The magnetic particle concentration distribution image of the imaging target is reconstructed by carrying out space coding on the whole space and utilizing a system matrix and an image reconstruction method, so that the magnetic particle imaging with low power consumption, large visual field and high resolution is realized.

The magnetic particle tomography method based on the full-space coding provided by the embodiment of the invention can be applied to the fields of medical imaging, industrial defect detection and the like. The magnetic particle tomography method provided by the embodiment of the invention can be used in medicine, including but not limited to cardiovascular and cerebrovascular imaging, tumor imaging, and targeted imaging such as stem cell tracking, red blood cell marking, immune cell marking, inflammatory cell monitoring, and the like. Compared with the existing blood vessel imaging technology, the embodiment of the invention does not need to perform digital subtraction when performing magnetic particle imaging, and has less motion artifacts. Compared with the existing imaging technologies of PET (Positron Emission Computed Tomography) and SPECT (Single Photon Emission Computed Tomography), the embodiment of the invention has higher sensitivity and image resolution, and can meet the requirements of clinical application; the embodiment of the invention has no ionizing radiation, and the production and storage of the tracer are easy.

The magnetic particle tomography method based on the full-space encoding provided by the embodiment of the invention can be applied to magnetic particle imaging equipment/system. For the magnetic particle imaging apparatus/system, the internal structure thereof may include other modules, such as a communication interface, a display module, a printing module, an auxiliary module, and the like, besides the software and hardware modules for implementing the method provided by the embodiment of the present invention, which is not limited in the embodiment of the present invention.

It should be noted that the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more features. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.

In the description of the specification, reference to the description of the term "one embodiment", "some embodiments", "an example", "a specific example", or "some examples", etc., means that a particular feature or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples described in this specification can be combined and combined by those skilled in the art.

While the present application has been described in connection with various embodiments, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed application, from a review of the drawings, the disclosure, and the appended claims.

The foregoing is a further detailed description of the invention in connection with specific preferred embodiments and it is not intended to limit the invention to the specific embodiments described. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims

1. A magnetic particle tomography method based on full-space encoding is characterized by comprising the following steps:

2. The magnetic particle tomography method according to claim 1, wherein the pair of excitation coils includes: a pair of circular Helmholtz transmitting coils; the pair of receiving coils includes: a pair of circular Helmholtz receiving coils; the two Homholtz transmitting coils are respectively and symmetrically arranged close to the two Homholtz receiving coils, and the axial directions of the two Homholtz transmitting coils and the axial directions of the four Homholtz receiving coils are overlapped to form two groups of receiving and transmitting coils with opposite positions; the imaging target is positioned at the middle point of the two groups of the receiving and transmitting coils, the two groups of the receiving and transmitting coils can do different circular tracks by taking the center of the imaging target as the circle center, and the motion process in each circular track comprises a plurality of stop point positions;

3. The magnetic particle tomography method according to claim 1, wherein the pair of excitation coils is one of a plurality of pairs of parallel excitation wires; the multiple pairs of parallel excitation wires are attached to a cylindrical structure and distributed in a dispersed mode, and the plane where each pair of parallel excitation wires is located penetrates through the center line of the cylindrical structure; the cylindrical structure can rotate around the center of the imaging target at different angles so as to drive the plurality of pairs of parallel excitation wires to be distributed according to different circumferential tracks; the pair of receiving coils includes: a pair of circular Homholtz coils respectively positioned on two bottom surfaces of the cylindrical structure;

4. A magnetic particle tomography method as claimed in claim 2 or 3, characterized in that the circumferential trajectory comprises one or more of the following circumferential trajectories:

the three-dimensional coordinate system comprises a first circumferential track, a second circumferential track, a third circumferential track and a fourth circumferential track, wherein the plane of the first circumferential track is parallel to the first plane under the three-dimensional coordinate system, the second circumferential track forms an included angle with the first plane, the third circumferential track forms an included angle with the second plane under the three-dimensional coordinate system, and the fourth circumferential track forms an included angle with the first plane and the second plane; the first plane is a YZ plane, an XZ plane or an XY plane under a three-dimensional coordinate system, and the second plane is a plane which is not the first plane among the YZ plane, the XZ plane and the XY plane.

5. A magnetic particle tomography method as claimed in any one of claims 1 to 3, characterized in that the application of a spatially non-uniform, non-linearly distributed alternating excitation field to the imaging region in which the imaging target is located by means of a pair of excitation coils comprises: applying a homodromous alternating current to a pair of excitation coils;

6. The magnetic particle tomography method of claim 2, wherein the manner of performing magnetic particle imaging for the imaging target based on the scan data comprises:

for each stop point position in each circular track, reconstructing one-dimensional projection distribution data which corresponds to the stop point position and contains the magnetic particle concentration information of the imaging target according to the target characteristics extracted from the stop point position and the system matrix; wherein the system matrix is used for representing the spatial distribution of the target characteristics of the response signals of the magnetic particles with unit concentration under the action of the alternating excitation field;

for each circular track, reconstructing a two-dimensional magnetic particle concentration distribution image projected by the imaging target along the corresponding imaging direction by using a filtering back projection reconstruction method according to the one-dimensional projection distribution data of each stop point position in the circular track; and the imaging direction corresponding to each circumferential track is vertical to the plane of the circumferential track.

7. The magnetic particle tomography method of claim 3, wherein the manner of magnetic particle imaging for the imaging target based on the scan data comprises:

for a pair of parallel excitation wires opened in each circumferential track, reconstructing one-dimensional projection distribution data which correspond to the pair of parallel excitation wires and contain magnetic particle concentration information of the imaging target according to the target characteristics extracted when the pair of parallel excitation wires are opened and the system matrix; wherein the system matrix is used for representing the spatial distribution of the target characteristics of the response signals of the magnetic particles with unit concentration under the action of the alternating excitation field;

8. The magnetic particle tomography method according to claim 7 or 8, wherein the manner of performing magnetic particle imaging for the imaging target according to the scan data further comprises:

9. The magnetic particle tomography method according to claim 1, further comprising:

10. The magnetic particle tomography method according to claim 1, further comprising: in the process of performing the signal acquisition step each time, before extracting the target feature, deconvoluting processing is performed on the acquired response signal to reduce signal deformation caused by the magnetic particle relaxation effect.