A magnetic particle imaging method based on frequency-mixing magnetic field excitation

Abstract

The present invention belongs to the field of magnetic particle imaging technology and relates to a magnetic particle imaging method based on frequency-mixing magnetic field excitation. The method comprises the following steps: obtaining a point-like magnetic nanoparticle sample and an object to be imaged; applying a frequency-mixing excitation magnetic field in an imaging mode to each of the point-like magnetic nanoparticle sample and the object to be imaged; obtaining a system matrix and a mixed frequency domain magnetization response spectrum signal based on the frequency-mixing magnetic field excitation; inputting the system matrix and the mixed frequency domain magnetization response spectrum signal into a concentration spatial distribution inversion model to invert the concentration spatial distribution of the magnetic nanoparticles, obtain the concentration spatial distribution of the magnetic nanoparticles in the object to be imaged, and perform magnetic particle concentration distribution imaging. The present invention can achieve real-time, high-resolution magnetic particle imaging.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic particle imaging, and in particular to a magnetic particle imaging method based on frequency mixing magnetic field excitation. Background Art

[0002] Magnetic particle imaging is a novel medical imaging technique that uses magnetic nanoparticles as tracers. By measuring the magnetization response and spectrum of magnetic nanoparticles under alternating magnetic field excitation, it can provide three-dimensional quantitative imaging of their distribution in vivo. It offers advantages such as real-time, high sensitivity, high spatial resolution, no tissue depth restrictions, no radioactivity, and no interference from tissue background signals. Since its public release, magnetic particle imaging technology has been rapidly developing and has shown broad application prospects in research such as cell tracing, drug delivery, angiography, and tumor imaging.

[0003] In magnetic particle imaging, the presence and concentration of superparamagnetic nanoparticles are determined by their magnetization response signals under an excitation magnetic field. By constructing a field-free point (FFP) and performing spatial encoding, magnetic particle concentration distribution imaging is achieved. Currently, magnetic nanoparticle imaging methods are primarily based on analytical or algebraic methods, using x-space (spatial domain) information of the FFP position or k-space (frequency domain) spectral information of the magnetic nanoparticle magnetization response to perform imaging inversion. Analytical x-space imaging methods are widely adopted due to their simplicity and ease of implementation, but their spatial resolution is relatively low. In contrast, algebraic x-space imaging methods, while offering higher spatial resolution, require a lengthy system matrix acquisition process.

[0004] Therefore, a new magnetic nanoparticle imaging method with both high resolution and real-time performance is urgently needed to advance the clinical application of magnetic nanoparticle technology. This new imaging method should simplify system complexity, reduce costs, and increase imaging speed without sacrificing image quality, making it more suitable for clinical applications. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to propose a magnetic particle imaging method based on mixed-frequency magnetic field excitation, which fully utilizes the mixed-frequency magnetic field to excite rich mixed-frequency magnetization response spectrum information, and realizes real-time high-resolution imaging of the spatial distribution of magnetic nanoparticles, which is of great significance for the clinical application of magnetic particle imaging.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A magnetic particle imaging method based on frequency-mixing magnetic field excitation, comprising:

[0008] Obtaining a point-shaped magnetic nanoparticle sample and an object to be imaged, applying a frequency mixing excitation magnetic field in an imaging mode to the point-shaped magnetic nanoparticle sample and the object to be imaged, respectively, to obtain a system matrix based on the frequency mixing magnetic field excitation and a mixed frequency domain magnetization response spectrum signal;

[0009] The system matrix and the mixed frequency domain magnetization response spectrum signal are input into a concentration spatial distribution inversion model to perform concentration spatial distribution inversion of magnetic nanoparticles, obtain the concentration spatial distribution of magnetic nanoparticles in the object to be imaged, and perform magnetic particle concentration distribution imaging.

[0010] Optionally, after obtaining the dot-shaped magnetic nanoparticle sample and the object to be imaged, the method includes:

[0011] Set the imaging mode and imaging parameters:

[0012] The imaging modes include: a first mode and a second mode, the first mode is a three-dimensional mixed frequency domain dimension, and the second mode is to set any one dimension as a spatial domain dimension and the remaining dimensions as mixed frequency domain dimensions;

[0013] The imaging parameters include: imaging area pixel size and mixing excitation magnetic field parameters.

[0014] Optionally, setting the imaging parameters includes:

[0015] The imaging field of view area is divided into target pixel numbers along the x-axis, y-axis, and z-axis directions, and the size of the pixels in the x-axis, y-axis, and z-axis directions is further set to obtain the relationship between the volume of the imaging field of view area and the imaging field of view size in each dimension:

[0016]

[0017] in, is the volume of the imaging field of view, 、 、 is the imaging field size of each dimension;

[0018] Based on the relationship between the imaging field of view area volume and the imaging field of view size of each dimension, the relationship between the imaging field of view size of each dimension and the pixel size is obtained:

[0019]

[0020] in, 、 、 is the size of the pixel in the x-axis, y-axis and z-axis directions, 、 、 is the number of target pixels for each segmentation along the x-axis, y-axis, and z-axis directions;

[0021] Set the excitation magnetic field amplitudes in the x-axis, y-axis, and z-axis directions, as well as the magnetic field gradients in each dimension, and determine the constraint relationship between the excitation magnetic field amplitude and the magnetic field gradient:

[0022]

[0023] Among them, H x 、H y 、H z is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, G x , G y , G z is the magnetic field gradient in each dimension.

[0024] Optionally, the frequency mixing excitation magnetic field of the imaging mode is expressed as:

[0025]

[0026] Among them, H x 、H y 、H z is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, f x 、f y 、f z is the excitation frequency of the excitation magnetic field in each dimension, is the phase of the excitation magnetic field in each dimension.

[0027] Optionally, obtaining the system matrix includes:

[0028] placing the point-shaped magnetic nanoparticle sample at an initial position within an imaging field of view, applying a three-dimensional frequency mixing excitation magnetic field to the point-shaped magnetic nanoparticle sample at the initial position, and acquiring a mixed frequency domain magnetization response at the initial position;

[0029] In the process of obtaining the mixed frequency domain magnetization response at the initial position, the high frequency magnetic field excites the magnetic particles to generate magnetization response harmonics at the high frequency multiples, and the low frequency magnetic field excites the magnetic particles to generate mixed frequency magnetization response harmonics on both sides of each high frequency multiple;

[0030] The position of the point-like magnetic nanoparticle sample within the imaging field of view is adjusted, and a three-dimensional frequency mixing excitation magnetic field of a target time is applied until all pixel positions are traversed, and the mixed frequency domain magnetization response spectrum at all pixel positions is obtained. Based on the mixed frequency domain magnetization response spectrum at all pixel positions, the system matrix is ​​constructed.

[0031] Optionally, obtaining mixed frequency-domain magnetization response spectra at all pixel positions includes:

[0032]

[0033] in, is the mixed frequency domain magnetization response spectrum information at all pixel positions, For magnetic nanoparticles The mth frequency mixing magnetization response complex harmonic at position.

[0034] Optionally, obtaining the mixed frequency-domain magnetization response spectrum signal includes:

[0035]

[0036] in, represents the mth frequency-mixing magnetization response complex harmonic of the object to be imaged, Represents the mixing response signal u(x i ,y j ,z k ,t)’s mth frequency-mixed magnetization response complex harmonic.

[0037] Optionally, the concentration spatial distribution inversion model expression is:

[0038]

[0039] in, is the spatial distribution of magnetic particle concentration, is the mixed frequency domain magnetization response spectrum signal of the object to be imaged, is the system matrix based on mixed-frequency magnetic field excitation.

[0040] Optionally, obtaining the spatial distribution of the concentration of the magnetic nanoparticles includes:

[0041]

[0042] in, is the spatial distribution of the concentration of magnetic nanoparticles.

[0043] The beneficial effects of the present invention are:

[0044] The present invention fully utilizes the abundant frequency-mixing magnetization response spectrum information under the excitation of the frequency-mixing magnetic field to realize the real-time high-resolution imaging of the spatial distribution of magnetic nanoparticles. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1This is a flow chart of a magnetic particle imaging method based on frequency-mixing magnetic field excitation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0048] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0049] like Figure 1 As shown, this embodiment discloses a magnetic particle imaging method based on mixed-frequency magnetic field excitation, including: obtaining a point-like magnetic nanoparticle sample and an object to be imaged, applying a mixed-frequency excitation magnetic field of an imaging mode to the point-like magnetic nanoparticle sample and the object to be imaged, respectively, obtaining a system matrix and a mixed-frequency domain magnetization response spectrum signal based on the mixed-frequency magnetic field excitation; inputting the system matrix and the mixed-frequency domain magnetization response spectrum signal into a concentration spatial distribution inversion model to perform concentration spatial distribution inversion of magnetic nanoparticles, obtaining the concentration spatial distribution of magnetic nanoparticles in the object to be imaged, and performing magnetic particle concentration distribution imaging.

[0050] Specifically, this embodiment discloses a magnetic particle imaging method based on frequency-mixing magnetic field excitation, comprising:

[0051] Step S10: Pixel segmentation is performed in the imaging field of view. The proposed method performs magnetic particle imaging based on the mixed frequency domain magnetization response information of the magnetic particles under the mixed frequency magnetic field excitation. For imaging, the three-dimensional mixed frequency domain information can be used to image the three-dimensional space, i.e., a three-dimensional mixed frequency domain imaging mode; or one dimension can be set as the spatial domain dimension, and the remaining two dimensions as the mixed frequency domain dimensions for layer-by-layer imaging; the imaging mode, pixel size, mixed frequency excitation magnetic field parameters, etc. are set;

[0052] Step S20: Placing a point-shaped magnetic nanoparticle sample within the imaging field of view, and applying mutually orthogonal excitation magnetic fields of different frequencies in the mixed frequency domain dimension to form a three-dimensional frequency mixing excitation magnetic field. The spatial position of the magnetic nanoparticle sample is changed so that it traverses the pixel positions in the imaging field of view, and the harmonics of the frequency mixing magnetization response at different positions are measured to form a system matrix A based on frequency mixing magnetic field excitation.

[0053] Step S30: placing the object to be imaged within the imaging field of view, exciting the object to be imaged using a three-dimensional frequency-mixing magnetic field, and measuring a mixed frequency-domain magnetization response spectrum signal U;

[0054] Step S40: Using the mixed frequency spectrum U measured in step S30 and the system matrix A obtained in step S20, a mathematical model of the system matrix A, the spatial distribution of magnetic particle concentration C, and the mixed frequency domain magnetization response spectrum signal U is constructed, which can be expressed as:

[0055]

[0056] By solving the above equation, the concentration spatial distribution C of the magnetic nanoparticles can be inverted, thereby realizing the imaging of the magnetic particle concentration distribution.

[0057] Furthermore, after obtaining the point-like magnetic nanoparticle sample, the method includes: setting the imaging mode and imaging parameters: the imaging mode includes: a first mode and a second mode, the first mode is a three-dimensional mixed frequency domain dimension, and the second mode is to set any one dimension as a spatial domain dimension, and the remaining dimensions as mixed frequency domain dimensions; the imaging parameters include: imaging area pixel size and mixing excitation magnetic field parameters.

[0058] Furthermore, setting imaging parameters includes:

[0059] The imaging field of view is divided into target pixels along the x-axis, y-axis, and z-axis directions, and the size of the pixels along the x-axis, y-axis, and z-axis directions are further set to obtain the relationship between the volume of the imaging field of view and the size of the imaging field of view in each dimension:

[0060]

[0061] in, is the volume of the imaging field of view, 、 、 is the imaging field size of each dimension;

[0062] Based on the relationship between the imaging field of view area volume and the imaging field of view size of each dimension, the relationship between the imaging field of view size of each dimension and the pixel size is obtained:

[0063]

[0064] in, 、 、 is the size of the pixel in the x-axis, y-axis and z-axis directions, 、 、 is the number of target pixels for each segmentation along the x-axis, y-axis, and z-axis directions;

[0065] Set the excitation magnetic field amplitudes in the x-axis, y-axis, and z-axis directions, as well as the magnetic field gradients in each dimension, and determine the constraint relationship between the excitation magnetic field amplitude and the magnetic field gradient:

[0066]

[0067] Among them, H x 、H y 、H z is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, G x , G y , G z is the magnetic field gradient in each dimension.

[0068] Specifically, taking the mode of performing imaging using three-dimensional mixed frequency domain information as an example, step S10 includes:

[0069] Step S11: setting the imaging mode, using three-dimensional mixed frequency domain information for magnetic particle imaging, dividing the imaging area into I, J, and K pixels along the x-axis, y-axis, and z-axis directions respectively, with a total spatial pixel number of I×J×K;

[0070] Step S12: Set the pixel size in the x / y / z axis direction to be l x / l y / l z , then the imaging field area volume V Fov FOV of imaging in each dimension x / FOV y / FOV z The relationship is:

[0071]

[0072] The relationship between the imaging field size and pixel size in each dimension is:

[0073]

[0074] Step S13: Set the excitation magnetic field amplitude in the x / y / z axis direction to H x / H y / H z , the magnetic field gradient in each dimension is G x / G y / G z , the relationship between the excitation magnetic field amplitude and the magnetic field gradient constraint is:

[0075]

[0076] Furthermore, the mixing excitation magnetic field expression of the imaging mode is:

[0077]

[0078] Among them, H x 、H y 、Hz is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, f x 、f y 、f z is the excitation frequency of the excitation magnetic field in each dimension, is the phase of the excitation magnetic field in each dimension.

[0079] Furthermore, obtaining the system matrix includes: placing the point-like magnetic nanoparticle sample at an initial position within the imaging field of view, applying a three-dimensional frequency mixing excitation magnetic field to the point-like magnetic nanoparticle sample at the initial position, and obtaining a mixed frequency domain magnetization response at the initial position; in the process of obtaining the mixed frequency domain magnetization response at the initial position, the high-frequency magnetic field excites the magnetic particles to generate magnetization response harmonics at high-frequency multiples, and the low-frequency magnetic field excites the magnetic particles to generate mixed frequency magnetization response harmonics on both sides of each high-frequency multiple; adjusting the position of the point-like magnetic nanoparticle sample within the imaging field of view, applying a three-dimensional frequency mixing excitation magnetic field for a target time, until all pixel positions are traversed, obtaining the mixed frequency domain magnetization response spectrum at all pixel positions, and constructing a system matrix based on the mixed frequency domain magnetization response spectrum at all pixel positions.

[0080] Specifically, step S20 includes:

[0081] Step S21: Place the dot-shaped magnetic nanoparticle sample at the initial position (x1, y1, z1), and set the three-dimensional alternating frequency mixing excitation magnetic field to:

[0082]

[0083] where f x / f y / f z is the excitation frequency of the excitation magnetic field in each dimension, is the phase of the excitation magnetic field in each dimension, where the frequency is f x The magnetic field is a high-frequency magnetic field with a frequency of f x , f y The magnetic field is a low-frequency magnetic field;

[0084] Step S22: applying a three-dimensional frequency-mixing excitation magnetic field to the magnetic nanoparticles at the initial position (x1, y1, z1) to obtain the mixed frequency-domain magnetization response u(x1, y1, z1, t) of the magnetic nanoparticles;

[0085] Step S23: Adjust the position of the magnetic nanoparticle point sample in the imaging field (x i ,y j ,z k ), apply a three-dimensional frequency mixing excitation magnetic field for a time of T, and obtain the magnetic nanoparticle mixed frequency domain magnetization response signal u(x i ,y j ,yk ,t), until all pixel positions (x1,y1,z1)~(x I ,y J ,z K );

[0086] Step S24: Obtain (x1, y1, z1)~(x I ,y J ,z K ) position, and compose the system matrix A based on the spectrum.

[0087] Furthermore, obtaining the mixed frequency domain magnetization response spectrum at all pixel positions includes:

[0088]

[0089] in, is the mixed frequency domain magnetization response spectrum information at all pixel positions, For magnetic nanoparticles The mth frequency mixing magnetization response complex harmonic at position.

[0090] Specifically, step S24 includes:

[0091] Step S241: The mixed frequency domain magnetization response signal u(x i ,y j ,z k ,t) Perform Fourier transform to obtain the mixed frequency domain magnetization response spectrum information :

[0092]

[0093] The high-frequency magnetic field excites the magnetic particles to generate the magnetization response harmonics at the high-frequency multiples, and the low-frequency magnetic field excites the magnetic particles to generate the mixed-frequency magnetization response harmonics on both sides of each high-frequency multiple. M is the total number of mixed-frequency harmonics. Indicates that the magnetic nanoparticles are i ,y j ,z k ) position.

[0094] Furthermore, obtaining a mixed frequency domain magnetization response spectrum signal includes:

[0095]

[0096] in, represents the mth frequency-mixing magnetization response complex harmonic of the object to be imaged, Represents the mixing response signal u(xi ,y j ,z k ,t)’s mth frequency-mixed magnetization response complex harmonic.

[0097] Furthermore, the inversion model expression of concentration spatial distribution is:

[0098]

[0099] in, is the spatial distribution of magnetic particle concentration, is the mixed frequency domain magnetization response spectrum signal of the object to be imaged, is the system matrix based on mixed-frequency magnetic field excitation.

[0100] Furthermore, obtaining the spatial distribution of the concentration of magnetic nanoparticles includes:

[0101]

[0102] in, is the spatial distribution of the concentration of magnetic nanoparticles.

[0103] Specifically, step S30 includes:

[0104] Step S31: Place the object to be imaged in the imaging field of view, and set the three-dimensional alternating frequency mixing excitation magnetic field to:

[0105]

[0106] Step S32: applying a three-dimensional frequency-mixing excitation magnetic field for a time period of T to the object to be imaged, and obtaining a mixed frequency-domain magnetization response signal u(t) of the object to be imaged within the time period of T.

[0107] Step S33: Calculate the mixed frequency domain magnetic response spectrum matrix U of the object to be imaged using the magnetic response signal u(t) of the object to be imaged.

[0108] Step S331: Perform Fourier transform on the magnetization response signal u(t) of the object to be imaged under the excitation of the three-dimensional mixed-frequency magnetic field to obtain the mixed-frequency domain magnetization response spectrum of the object to be imaged:

[0109]

[0110] in represents the mth frequency-mixed magnetization response complex harmonic of the object to be imaged, the harmonic components mainly including the magnetization response harmonics at the high-frequency magnetic field frequency multiples and the frequency-mixed magnetization response harmonics on both sides of each high-frequency multiple;

[0111] Step S40 includes performing image inversion based on the mixed-frequency magnetization response spectrum U of the object to be imaged obtained in step S30 and the imaging system matrix A obtained in step S20 to obtain the spatial distribution of the concentration of magnetic nanoparticles:

[0112]

[0113] in, is the spatial distribution of the concentration of magnetic nanoparticles.

[0114] The magnetic particle imaging method based on frequency-mixing magnetic field excitation proposed in the present invention can achieve real-time high-resolution magnetic particle imaging, which can meet the needs of biomedical applications such as cell tracing, angiography, and targeted drug delivery.

[0115] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A magnetic particle imaging method based on frequency mixing magnetic field excitation, characterized in that: include: Obtaining point-shaped magnetic nanoparticle samples and objects to be imaged; After obtaining the dot-shaped magnetic nanoparticle sample and the object to be imaged, the method includes: Set the imaging mode and imaging parameters: The imaging modes include: a first mode and a second mode, the first mode is a three-dimensional mixed frequency domain dimension, and the second mode is to set any one dimension as a spatial domain dimension and the remaining dimensions as mixed frequency domain dimensions; The imaging parameters include: pixel size of the imaging field of view and mixing excitation magnetic field parameters; Applying a mixed frequency excitation magnetic field in an imaging mode to the dot-shaped magnetic nanoparticle sample and the object to be imaged, respectively, to obtain a system matrix based on the mixed frequency magnetic field excitation and a mixed frequency domain magnetization response spectrum signal; The frequency mixing excitation magnetic field expression of the imaging mode is: Among them, H x 、H y 、H z is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, f x 、f y 、f z is the excitation frequency of the excitation magnetic field in each dimension, Excite the magnetic field phase for each dimension; Obtaining the system matrix includes: placing the point-shaped magnetic nanoparticle sample at an initial position within an imaging field of view, applying a three-dimensional frequency mixing excitation magnetic field to the point-shaped magnetic nanoparticle sample at the initial position, and acquiring a mixed frequency domain magnetization response at the initial position; In the process of obtaining the mixed frequency domain magnetization response at the initial position, the high frequency magnetic field excites the magnetic particles to generate magnetization response harmonics at the high frequency multiples, and the low frequency magnetic field excites the magnetic particles to generate mixed frequency magnetization response harmonics on both sides of each high frequency multiple; Adjusting the position of the dot-shaped magnetic nanoparticle sample within the imaging field of view, applying a three-dimensional frequency mixing excitation magnetic field for a target time until all pixel positions are traversed, obtaining mixed frequency domain magnetization response spectra at all pixel positions, and constructing the system matrix based on the mixed frequency domain magnetization response spectra at all pixel positions; The system matrix and the mixed frequency domain magnetization response spectrum signal are input into a concentration spatial distribution inversion model to perform concentration spatial distribution inversion of magnetic nanoparticles, obtain the concentration spatial distribution of magnetic nanoparticles in the object to be imaged, and perform magnetic particle concentration distribution imaging.

2. The magnetic particle imaging method based on frequency mixing magnetic field excitation according to claim 1, characterized in that: Setting the imaging parameters includes: The imaging field of view area is divided into target pixel numbers along the x-axis, y-axis, and z-axis directions, and the size of the pixels in the x-axis, y-axis, and z-axis directions is further set to obtain the relationship between the volume of the imaging field of view area and the imaging field of view size in each dimension: in, is the volume of the imaging field of view, 、 、 is the imaging field size of each dimension; Based on the relationship between the imaging field of view area volume and the imaging field of view size of each dimension, the relationship between the imaging field of view size of each dimension and the pixel size is obtained: in, 、 、 is the size of the pixel in the x-axis, y-axis and z-axis directions, 、 、 is the number of target pixels for each segmentation along the x-axis, y-axis, and z-axis directions; Set the excitation magnetic field amplitudes in the x-axis, y-axis, and z-axis directions, as well as the magnetic field gradients in each dimension, and determine the constraint relationship between the excitation magnetic field amplitude and the magnetic field gradient: Among them, H x 、H y 、H z is the excitation magnetic field amplitude in the x-axis, y-axis and z-axis directions, G x , G y , G z is the magnetic field gradient in each dimension.

3. The magnetic particle imaging method based on frequency mixing magnetic field excitation according to claim 1, characterized in that: Obtaining the mixed frequency domain magnetization response spectrum at all pixel locations includes: in, is the mixed frequency domain magnetization response spectrum information at all pixel positions, For magnetic nanoparticles The mth frequency mixing magnetization response complex harmonic at position.

4. The magnetic particle imaging method based on frequency mixing magnetic field excitation according to claim 1, characterized in that: Acquiring the mixed frequency domain magnetization response spectrum signal includes: in, represents the mth frequency-mixing magnetization response complex harmonic of the object to be imaged, Represents the mixed frequency magnetization response signal u(x i ,y j ,z k )’s mth frequency mixing magnetization response complex harmonic, For Spatial distribution of magnetic nanoparticle concentration at the location.

5. The magnetic particle imaging method based on frequency mixing magnetic field excitation according to claim 1, characterized in that: The concentration spatial distribution inversion model expression is: in, is the mixed frequency domain magnetization response spectrum signal of the object to be imaged, is the spatial distribution of the concentration of magnetic nanoparticles, is the system matrix based on mixed-frequency magnetic field excitation.

6. The magnetic particle imaging method based on frequency mixing magnetic field excitation according to claim 5, characterized in that: Obtaining the concentration spatial distribution of the magnetic nanoparticles includes: in, For Spatial distribution of magnetic nanoparticle concentration at the location.

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