CN111077486A - Three-dimensional positive contrast magnetic resonance imaging method, device, equipment and storage medium - Google Patents
Three-dimensional positive contrast magnetic resonance imaging method, device, equipment and storage medium Download PDFInfo
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Abstract
The embodiment of the invention discloses a three-dimensional positive contrast magnetic resonance imaging method, a device, equipment and a storage medium, wherein the method comprises the following steps: acquiring post-shift data and pre-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter based on a SPACE sequence with a variable flip angle; calculating the phase difference of the real phase corresponding to the data after the shift and the data before the shift of each body layer; determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer; and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field. The technical problem that the three-dimensional alignment ratio magnetic resonance imaging based on the rapid spin echo sequence consumes longer time is solved, and the technical effect of shortening the three-dimensional alignment ratio magnetic resonance imaging method is achieved.
Description
Technical Field
The embodiment of the invention relates to the field of image processing, in particular to a three-dimensional positive contrast magnetic resonance imaging method, a three-dimensional positive contrast magnetic resonance imaging device, three-dimensional positive contrast magnetic resonance imaging equipment and a storage medium.
Background
Magnetic Resonance Imaging (MRI) is based on the Imaging of hydrogen protons in human tissue, and generates a Resonance signal by applying a radio frequency pulse signal with the same frequency as the hydrogen proton spin. Since the magnetic compatible interventional/implanted device itself is free of hydrogen protons, it shows signal loss in the actual location area of the interventional/implanted device in conventional MRI images, and the interventional/implanted device is usually made of metal and is magnetized in the MRI magnetic field to generate a local magnetic field, which causes interference, i.e., susceptibility artifact, to the tissue area around the interventional/implanted device, thus showing a large black hole in the area around the interventional/implanted device, which is much larger than the size of the interventional/implanted device itself, i.e., negative contrast image. The negative contrast of the images makes it difficult for the diagnostician to distinguish the aforementioned black hole from the tissue cavity, and thus does not allow for accurate positioning and evaluation of the device.
In order to clearly show the position of the interventional/implanted device in the magnetic resonance image, the position of the interventional/implanted device is usually shown by means of a positive (bright) contrast, i.e. by means of a positive contrast magnetic resonance image. However, in the existing three-dimensional positive contrast magnetic resonance imaging method, a fast spin echo (GRE) is used as a data acquisition basis, a local field image is obtained after background field removal processing is performed on phase information, a magnetic susceptibility image is reconstructed by combining a quantitative magnetic susceptibility imaging method, and finally the position of metal particles is displayed in a positive contrast mode. However, the fast spin echo is used to realize three-dimensional data acquisition, and the data acquisition usually needs a long time to complete, so that the application of quantitative magnetic susceptibility imaging in clinic is limited, and further the application of three-dimensional positive contrast magnetic resonance imaging in clinic is limited.
Disclosure of Invention
The embodiment of the invention provides a three-dimensional positive contrast magnetic resonance imaging method, a device, equipment and a storage medium, which aim to solve the technical problem that the three-dimensional positive contrast magnetic resonance imaging method in the prior art consumes longer time.
In a first aspect, an embodiment of the present invention provides a three-dimensional direct contrast magnetic resonance imaging method, including:
acquiring post-shift data and pre-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter based on a SPACE sequence with a variable flip angle;
calculating the phase difference of the real phase corresponding to the data after the shift and the data before the shift of each body layer;
determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer;
and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field.
In a second aspect, an embodiment of the present invention further provides a three-dimensional direct contrast magnetic resonance imaging apparatus, including:
the data acquisition module is used for acquiring data after offset and data before offset corresponding to echo readout gradients of each body layer of the target object containing the metal foreign matters based on the SPACE sequence with the variable flip angle;
the phase difference determining module is used for solving the phase difference of the real phase corresponding to the data after the deviation and the data before the deviation of each body layer;
the local magnetic field determining module is used for determining a local magnetic field formed by the magnetic field intensity of the metal foreign bodies on each body layer according to the phase difference of each body layer;
and the direct magnetic resonance image module is used for solving a three-dimensional direct magnetic resonance image of the target object according to the local magnetic field.
In a third aspect, an embodiment of the present invention further provides a computer device, where the computer device includes:
one or more processors;
storage means for storing one or more programs;
when executed by the one or more processors, cause the one or more processors to implement the three-dimensional direct contrast magnetic resonance imaging method of the first aspect.
In a fourth aspect, embodiments of the present invention also provide a storage medium containing computer executable instructions for performing the three-dimensional direct contrast magnetic resonance imaging method according to the first aspect when executed by a computer processor.
According to the technical scheme of the three-dimensional positive contrast ratio magnetic resonance imaging method, after-shift data and before-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter are acquired based on a SPACE sequence with a variable flip angle; calculating the phase difference of the real phase corresponding to the data after the deviation and the data before the deviation of each body layer; determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer; and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field. The tissue signals can be kept in a stable state in most of the time of an echo chain through a SPACE sequence with a variable flip angle, the length of the echo chain reaches 100-200, the data acquisition efficiency of a T2 weighted image is greatly improved, the phase of an echo readout gradient is shifted, the phase difference between the data before the phase shift of the echo readout gradient and the data after the phase shift is determined, then a local magnetic field image generated by metal to surrounding tissues is determined according to the phase difference, and a three-dimensional positive contrast magnetic resonance image is further determined.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a flow chart of a three-dimensional positive contrast ratio magnetic resonance imaging method according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a SPACE sequence provided in the first embodiment of the present invention;
fig. 3A is a magnitude diagram of data reconstruction before offset based on SPACE sequence acquisition according to an embodiment of the present invention;
FIG. 3B is a three-dimensional positive contrast MR image according to an embodiment of the present invention;
fig. 4 is a block diagram of a three-dimensional positive contrast ratio magnetic resonance imaging apparatus according to a second embodiment of the present invention;
fig. 5 is a block diagram of a computer device according to a third embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described through embodiments with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example one
Fig. 1 is a flowchart of a three-dimensional positive contrast magnetic resonance imaging method according to an embodiment of the present invention. The technical scheme of the embodiment is suitable for the situation of positioning the metal foreign body implanted into the target object. The method can be executed by the three-dimensional positive contrast magnetic resonance imaging device provided by the embodiment of the invention, and the device can be realized in a software and/or hardware manner and is configured to be applied in a processor. The method specifically comprises the following steps:
s101, acquiring data after offset and data before offset corresponding to echo readout gradient of each body layer of a target object containing metal foreign matters based on a SPACE sequence with a variable flip angle.
The SPACE sequence is developed on the basis of a fast spin echo sequence, which is developed on the basis of a spin echo sequence, the flip angle of a conventional fast spin echo sequence is usually a constant, such as 180 degrees, the tissue signal decays exponentially and the decay speed is fast, and when an echo train reaches about 20 degrees, the echo signal is basically not used for imaging. After the variable flip angle is adopted, the tissue signals can be kept in a stable state in most of the time of the echo chain by selecting the appropriate flip angle, the length of the echo chain can reach 100-200, and the acquisition efficiency of the T2 weighted image is greatly improved. The method includes the steps that a layer selection excitation pulse is generated, a fast spin echo sequence with a variable flip angle is combined with the layer selection excitation pulse to obtain a SPACE sequence with the variable flip angle, the FSE can be used for precisely selecting layers and can be applied to different body parts, and based on the sequence, the SPACE sequence is used as an acquisition sequence of magnetic resonance measurement data.
As shown in fig. 2, it can be seen from the timing diagram of SPACE: the first pulse of the rf pulse is a 90 degree slice select excitation pulse, the second pulse is a 180 degree refocusing pulse, followed by a variable flip angle refocusing pulse. Wherein, 180 degrees and variable flip angle all adopt non-selective echo pulse to shorten echo signal's interval, thereby improve magnetic resonance data's collection efficiency.
To accommodate the longer duration of the 90 degree slice selection excitation pulse, the Echo Spacing (ESP) of the 180 degree echo pulses is extended, called ESP1, followed by an echo spacing of variable flip angle, called EP2, with EP2 being as short as possible and determined by the gradient system, resolution and bandwidth. After 180-degree echo pulse, complete echo is generated on the transverse plane through ESP1/2 time, and the variable flip angle extends the time of the complete echo on the transverse plane, so that the echo chain is prolonged, and the acquisition efficiency of magnetic resonance data is improved.
It should be noted that, since the pulse echo interval between the variable flip angles is ESP2, the time between the first variable flip angle and the 180-degree pulse interval is ESP1/2+ ESP 2/2. Of course, the variable flip angle of the SPACE sequence also needs to satisfy the condition of CPMG (Carr-Purcell-Meiboom-Gill).
It can also be seen from fig. 2 that the SPACE sequence of the present embodiment also shifts the position of the echo readout gradient by a shift time TshiftAnd is short, preferably but not limited to 0.3-0.8ms, so that the effective echo time is short. In order to eliminate the phase shift caused by the difference in magnetic susceptibility of the magnetic resonance system itself, the present embodiment acquires post-shift data and pre-shift data corresponding to the echo readout gradient of each slice of the target object containing the metallic foreign matter by the SPACE sequence.
S102, phase difference of real phases corresponding to the data after the shift and the data before the shift of each body layer is obtained.
Because the metal foreign object of the target object affects the magnetic field information of the original tissue of the target object to change the magnetic field information, and the phase information of the magnetic resonance measurement data contains abundant tissue magnetic susceptibility change information, the position of the metal foreign object can be determined by using a phase image containing the phase information.
When determining the real phase corresponding to the data after the offset and the data before the offset of each body layer, considering that the phase value generated by the magnetic resonance system is limited in the range of (-pi, pi), the phase value is different from the real phase by 2k pi, so that the actually obtained phase value and the real phase value have different degrees of phase cycle ambiguity. The process of obtaining the true phase from the wrapped phase, i.e. the phase value actually obtained, is called phase unwrapping.
In the embodiment, the data after the offset and the data before the offset of each body layer are unwrapped in space to obtain the real phase of the data after the offset and the real phase of the data before the offset of each body layer; the difference between the true phase of the data after the shift and the true phase of the data before the shift for each slice is taken as the phase difference for each slice to obtain the phase change caused by the difference in the susceptibility of the tissue itself (local magnetic field).
S103, according to the phase difference of each layer, a local magnetic field formed by the magnetic field intensity of the metal foreign matter in each layer is determined.
The difference in the magnetic susceptibility of the tissue itself, i.e., the local magnetic field, is expressed as: Δ B ═ Δ θ/γ B0Tshift. Wherein Δ B represents a local magnetic field formed by the metal foreign matter in each body layer, that is, a local field pattern, Δ θ represents a phase difference, and γ represents a gyromagnetic ratio, which is a constant; b is0Representing the main magnetic field of magnetic resonance imaging, TshiftRepresenting the shift time of the echo readout gradient.
And S104, obtaining a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field.
Since the positive contrast magnetic resonance image is generated based on the magnetic susceptibility image, the present embodiment first reconstructs the magnetic susceptibility image by the quantitative magnetic sensitivity imaging method, and then determines the positive contrast magnetic resonance image based on the magnetic susceptibility image.
In order to reconstruct a magnetic susceptibility image, the embodiment first determines a relational expression of magnetic susceptibility and a magnetic field in a fourier domain, specifically:
wherein k is a Fourier domain coordinate,
kx，kyand kzB (k) is the magnetic field in the fourier domain, determined by the phase shift in the echo time, reflecting the phase distribution determined by the susceptibility distribution, belonging to the global characteristics of the phase, and x (k) is the susceptibility distribution in the fourier domain.
As can be seen from the above formula, the susceptibility distribution X (k) is required to be avoided
It is therefore ill-suited to solve any susceptibility distribution with the commonly obtained magnetic field. To this end, this is trueThe embodiment introduces L1 norm constraint to solve the optimal solution of the ill-posed problem, and simultaneously, two constraint matrixes, namely a weighting matrix W and a mask matrix M, are added to improve the quality of image reconstruction in consideration of the characteristic that the magnetic susceptibility of the metal foreign bodies is far greater than that of human tissues. The reconstruction formula is:
wherein, λ is a regularization parameter to ensure the consistency and sparsity of data, χ is a magnetic susceptibility, Δ B represents a local magnetic field generated by a metal foreign body in a target region to surrounding tissues, namely a local graph, Δ θ represents a phase difference, γ represents a gyromagnetic ratio and is a constant, B0Representing the main magnetic field of magnetic resonance imaging, TshiftAnd the gradient operator in three directions in a three-dimensional space is represented by G.
In order to improve the solving speed and accuracy of f (χ, λ), the present embodiment utilizes the primitive dual algorithm to iteratively solve f (χ, λ), which is used to solve the primitive minimization problem as follows:
the dual problem corresponding to the original minimization problem is as follows:
using Legendre transformation to obtain
Substituting the formula into the original minimization problem expression, and then transforming the minimization problem into a saddle point problem
Then, approximate Mapping (Proximal Mapping) is adopted to respectively solve the original problem and the dual problem so as to obtain the target objectMatrix C and matrix G are determined.
After the matrix C and the matrix G are determined, reconstructing a susceptibility image by adopting a first-order original dual algorithm, such as a chambole-Pock first-order original dual algorithm, specifically comprising the following steps:
1、
2、χ0，p0，q0are initialized to 0.
3、
4. And sequentially iterating according to the following formula until N is larger than or equal to N.
χn+1＝χn-τCTWTpn+1+τGTMTqn+1
n＝n+1
After the iteration is completed, a magnetic susceptibility image is obtained, and the magnetic susceptibility image is a final three-dimensional positive contrast magnetic resonance image. Because the first-order original dual algorithm has higher operation efficiency, the reconstruction speed of the magnetic susceptibility image can be improved, and further the reconstruction speed of the three-dimensional opposite magnetic resonance image is improved, thereby being convenient for clinical popularization. It should be noted that, in the present embodiment, the magnetic susceptibility image is converted into the three-dimensional positive contrast magnetic resonance image by using the prior art.
To better illustrate that the three-dimensional alignment described in this embodiment has higher image quality than an image formed by a magnetic resonance imaging method, this embodiment also provides a simulation experiment and an experimental result, specifically: a puncture needle made of titanium and having a diameter of 2mm was placed in an experimental flask of 1mg/ml copper sulfate solution, and gradient echo pre-shift data and post-shift data of the experimental flask were collected based on a SPACE sequence. Magnetic resonance image reconstruction is performed on the pre-offset data by a prior art magnetic resonance imaging method to obtain a magnetic resonance image based on the amplitude information, as shown in fig. 3A. The three-dimensional direct contrast magnetic resonance imaging method according to this embodiment performs image reconstruction on the pre-shift data and the post-shift data to obtain a three-dimensional direct contrast magnetic resonance image based on phase information, as shown in fig. 3B.
As can be seen by comparing FIG. 3A with FIG. 3B, the puncture needle of FIG. 3A produces a significant artifact (black hole), as seen where the arrows are located; the opposite magnetic resonance image in fig. 3B shows the specific position of the puncture needle, so that the opposite magnetic resonance image reconstructed by the three-dimensional opposite magnetic resonance imaging method according to this embodiment can clearly show the accurate position of the metallic foreign object of the target object, which is beneficial to clinical popularization and improves the accuracy of clinical diagnosis.
According to the technical scheme of the three-dimensional positive contrast ratio magnetic resonance imaging method, after-shift data and before-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter are acquired based on a SPACE sequence with a variable flip angle; calculating the phase difference of the real phase corresponding to the data after the deviation and the data before the deviation of each body layer; determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer; and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field. The tissue signals can be kept in a stable state in most of the time of an echo chain through a SPACE sequence with a variable flip angle, the length of the echo chain reaches 100-200, the data acquisition efficiency of a T2 weighted image is greatly improved, the phase of an echo read gradient is shifted, the phase difference between the data before the phase shift of the echo read gradient and the data after the phase shift is determined through the echo read gradient, then a local magnetic field is determined according to the phase difference, and a three-dimensional direct contrast magnetic resonance image is further determined.
Example two
Fig. 4 is a block diagram of a three-dimensional positive contrast mri apparatus according to a second embodiment of the present invention. The apparatus is used for executing the three-dimensional positive contrast magnetic resonance imaging method provided by any of the above embodiments, and the apparatus can be implemented by software or hardware. The device includes:
the data acquisition module 11 is configured to acquire post-shift data and pre-shift data corresponding to echo readout gradients of each slice of the target object including the metallic foreign object based on the SPACE sequence with the variable flip angle;
a phase difference determining module 12, configured to obtain a phase difference between real phases corresponding to the post-offset data and the pre-offset data of each slice;
a local magnetic field determining module 13, configured to determine, according to the phase difference of each body layer, a local magnetic field formed by the magnetic field intensity of the metal foreign object in each body layer;
and the direct magnetic resonance image module 14 is configured to obtain a three-dimensional direct magnetic resonance image of the target object according to the local magnetic field.
According to the technical scheme of the three-dimensional positive contrast ratio magnetic resonance imaging device, the data acquisition module is used for acquiring post-shift data and pre-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter based on a variable flip angle SPACE sequence; the phase difference of the real phase corresponding to the data after the deviation and the data before the deviation of each body layer is obtained through a phase difference determining module; determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer through a local magnetic field determination module according to the phase difference of each body layer; and solving a three-dimensional opposite magnetic resonance image of the target object according to the local magnetic field through the opposite magnetic resonance image module. The tissue signals can be kept in a stable state in most of the time of an echo chain through a SPACE sequence with a variable flip angle, the length of the echo chain reaches 100-200, the data acquisition efficiency of a T2 weighted image is greatly improved, the phase of an echo read gradient is shifted, the phase difference between the data before the phase shift of the echo read gradient and the data after the phase shift is determined through the data before the phase shift of the echo read gradient, then a local magnetic field is determined according to the phase difference, and an opposite magnetic resonance image is further determined.
The three-dimensional positive contrast ratio magnetic resonance imaging device provided by the embodiment of the invention can execute the three-dimensional positive contrast ratio magnetic resonance imaging method provided by any embodiment of the invention, and has corresponding functional modules and beneficial effects of the execution method.
EXAMPLE III
Fig. 5 is a block diagram of a computer apparatus according to a third embodiment of the present invention, as shown in fig. 5, the computer apparatus includes a processor 201, a memory 202, an input device 203, and an output device 204; the number of the processors 201 in the computer device may be one or more, and one processor 201 is taken as an example in fig. 5; the processor 201, the memory 202, the input device 203 and the output device 204 in the apparatus may be connected by a bus or other means, and fig. 5 illustrates the connection by a bus as an example.
The memory 202, as a computer-readable storage medium, may be used for storing software programs, computer-executable programs, and modules, such as program instructions/modules corresponding to the three-dimensional contrast magnetic resonance imaging method in the embodiment of the present invention (for example, the data acquisition module 11, the phase difference determination module 12, the local magnetic field determination module 13, and the contrast magnetic resonance imaging module 14). The processor 201 executes various functional applications of the apparatus and data processing, i.e. implements the three-dimensional positive contrast magnetic resonance imaging method described above, by running software programs, instructions and modules stored in the memory 202.
The memory 202 may mainly include a storage program area and a storage data area, wherein the storage program area may store an operating system, an application program required for at least one function; the storage data area may store data created according to the use of the terminal, and the like. Further, the memory 202 may include high speed random access memory, and may also include non-volatile memory, such as at least one magnetic disk storage device, flash memory device, or other non-volatile solid state storage device. In some examples, the memory 202 may further include memory located remotely from the processor 201, which may be connected to the device over a network. Examples of such networks include, but are not limited to, the internet, intranets, local area networks, mobile communication networks, and combinations thereof.
The input device 203 may be used to receive input numeric or character information and generate key signal inputs related to user settings and function controls of the apparatus.
The output device 204 may include a display device such as a display screen, for example, of a user terminal.
Example four
A fourth embodiment of the present invention further provides a storage medium containing computer-executable instructions, which when executed by a computer processor, are configured to perform a method for three-dimensional face-to-face ratio magnetic resonance imaging, the method comprising:
acquiring post-shift data and pre-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter based on a SPACE sequence with a variable flip angle;
calculating the phase difference of the real phase corresponding to the data after the shift and the data before the shift of each body layer;
determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer;
and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field.
Of course, the embodiment of the present invention provides a storage medium containing computer-executable instructions, and the computer-executable instructions are not limited to the operations of the method described above, and can also perform the operations related to the three-dimensional direct contrast magnetic resonance imaging method provided by any embodiment of the present invention.
From the above description of the embodiments, it is obvious for those skilled in the art that the present invention can be implemented by software and necessary general hardware, and certainly, can also be implemented by hardware, but the former is a better embodiment in many cases. Based on such understanding, the technical solutions of the present invention may be embodied in the form of a software product, which may be stored in a computer-readable storage medium, such as a floppy disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a FLASH Memory (FLASH), a hard disk or an optical disk of a computer, and includes several instructions for enabling a computer device (which may be a personal computer, a server, or a network device) to execute the three-dimensional positive contrast mri method according to the embodiments of the present invention.
It should be noted that, in the embodiment of the three-dimensional direct contrast mri apparatus, the included units and modules are only divided according to functional logic, but are not limited to the above division as long as the corresponding functions can be realized; in addition, specific names of the functional units are only for convenience of distinguishing from each other, and are not used for limiting the protection scope of the present invention.
It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.
Claims (10)
1. A three-dimensional positive contrast magnetic resonance imaging method, comprising:
acquiring post-shift data and pre-shift data corresponding to echo readout gradients of each body layer of a target object containing a metal foreign matter based on a SPACE sequence with a variable flip angle;
calculating the phase difference of the real phase corresponding to the data after the shift and the data before the shift of each body layer;
determining a local magnetic field formed by the magnetic field intensity of the metal foreign matters on each body layer according to the phase difference of each body layer;
and solving a three-dimensional positive contrast magnetic resonance image of the target object according to the local magnetic field.
2. The method of claim 1, wherein the determining a phase difference between the true phases of the post-offset data and the pre-offset data for each slice comprises:
spatially unwrapping the post-migration data and the pre-migration data for each volume layer to obtain a true phase of the post-migration data and a true phase of the pre-migration data for each volume layer;
and taking the difference value of the real phase of the data after the offset and the real phase of the data before the offset of each body layer as the phase difference of each body layer.
3. The method of claim 1, wherein the local magnetic field has an expression of B ═ Δ θ/γ B0TshiftWherein Δ B represents a local magnetic field formed by the metal foreign matter in each body layer, Δ θ represents a phase difference, and γ represents a gyromagnetic ratio, which is a constant; b is0Representing the main magnetic field of magnetic resonance imaging, TshiftRepresenting the shift time of the echo readout gradient.
4. The method of claim 3, wherein said deriving a three-dimensional positive contrast magnetic resonance image of the target object from the local magnetic field comprises:
and according to the local magnetic field, carrying out magnetic resonance image reconstruction by a quantitative magnetic sensitivity imaging method to obtain a three-dimensional positive contrast magnetic resonance image.
5. The method of claim 4, wherein the performing magnetic resonance image reconstruction by quantitative magnetic sensitivity imaging based on the local magnetic field to obtain a three-dimensional positive contrast magnetic resonance image comprises:
determining a relational expression of the magnetic susceptibility and the magnetic field in a Fourier domain;
carrying out regularization constraint reconstruction on the relational expression by using a local magnetic field to obtain a magnetic susceptibility image of a target object;
and determining a three-dimensional positive contrast magnetic resonance image according to the magnetic susceptibility image.
6. The method of claim 5, wherein the regularized constrained reconstruction of the relational expression using local magnetic fields to obtain a susceptibility image of a target object comprises:
the expression for carrying out regularization constraint reconstruction on the relational expression by using the local magnetic field is as follows:
wherein, λ is regularization parameter, χ is magnetic susceptibility, Δ B represents local magnetic field generated by metal foreign body in target region to surrounding tissue, i.e. local field pattern, θ represents phase difference, γ represents gyromagnetic ratio and is constant, B0Representing the main magnetic field of magnetic resonance imaging, TshiftRepresenting echo read gradient offset time, wherein W is a weighting matrix, M is a mask matrix, C is a polarization kernel matrix obtained by discretizing a convolution kernel, and G represents gradient operators in three directions in a three-dimensional space;
f (χ, λ) is solved by a first order primal dual algorithm to obtain a susceptibility image.
7. The method of claim 1, wherein the echo readout gradient is shifted by a time TshiftIn the range of 0.3-0.8 ms.
8. A three-dimensional face-to-face ratio magnetic resonance imaging apparatus, comprising:
the data acquisition module is used for acquiring data after offset and data before offset corresponding to echo readout gradients of each body layer of a target object containing the metal foreign matters by using a SPACE sequence based on a variable flip angle;
the phase difference determining module is used for solving the phase difference of the real phase corresponding to the data after the deviation and the data before the deviation of each body layer;
the local magnetic field determining module is used for determining a local magnetic field formed by the magnetic field intensity of the metal foreign bodies on each body layer according to the phase difference of each body layer;
and the direct magnetic resonance image module is used for solving a three-dimensional direct magnetic resonance image of the target object according to the local magnetic field.
9. A computer device, characterized in that the computer device comprises:
one or more processors;
storage means for storing one or more programs;
when executed by the one or more processors, cause the one or more processors to implement a three-dimensional positive contrast magnetic resonance imaging method as claimed in any one of claims 1-7.
10. A storage medium containing computer executable instructions for performing the three dimensional direct contrast magnetic resonance imaging method of any one of claims 1 to 7 when executed by a computer processor.
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