WO2023087246A1 - Multi-contrast imaging method and device for low-field magnetic resonance - Google Patents

Abstract

A multi-contrast imaging method and device for low-field magnetic resonance. The method comprises: sequentially applying a set first radio frequency pulse sequence and second radio frequency pulse sequence to a target imaging area; collecting a first group of echo signals for the first radio frequency pulse sequence, and collecting a second group of echo signals for the second radio frequency pulse sequence, both the first group of echo signals and the second group of echo signals comprising a plurality of echoes; and performing modulation and undersampling on the obtained echo signals for reconstruction to obtain multi-contrast magnetic resonance images of the target imaging area. Multi-contrast images can be generated and the image generation time is significantly shortened by means of a single scanning process, and the definition of reconstructed images is enhanced.

Description

A multi-contrast imaging method and device for low-field magnetic resonance technical field

The present invention relates to the technical field of magnetic resonance imaging, in particular to a multi-contrast imaging method for low-field magnetic resonance.

Background technique

In conventional magnetic resonance imaging (MRI) systems, only one contrast-weighted image can be reconstructed in one scan. If it is a quantitative image, it is necessary to set different repetition time (TR) and echo time (TE) for multiple scans to fit a quantitative image. These different qualitative weighted maps and quantitative images usually consume a lot of scanning time and may result in wrongly registered images. Furthermore, T1-weighted map (T1W) images of GRE sequences with different flip angles (FA) yield effective gray matter (GM) and white matter (WM) contrast and have inherent variability in signal response from radiofrequency transmit (B1) and receive coils. Image non-uniformity. Insufficient GM/WM contrast and RF inhomogeneity add to the complexity of T1W automatic brain structure segmentation of the signal. In addition, compared with the conventional GRE (gradient refocusing echo imaging) sequence without any magnetization preparation, T1W is generated by using gradient echo rapid acquisition (T1MPRAGE), but the smaller FA is used in T1MPRAGE than the larger one. The GRE sequence of FA has more inhomogeneity of the B1 field, and finally the QSM (quantitative susceptibility imaging) of long echo time TE may have phase aliasing. Since data with long echo times are more sensitive to small changes in sensitivity, shorter TEs are needed to better quantify sensitivity.

Due to the ease of data collection, the variable flip angle (VFA) method has gained popularity for the evaluation of T1 quantification map (T1map) and PD (proton density). T1 and PD were quantified appropriately. However, the VFA method requires accurate knowledge of the RF emission field or flip angle. For the accurate acquisition of proton density and T1map, the former depends on the RF receptive field and the latter depends on the RF receptive field, the RF receptive field can be calculated pixel by pixel without any constraints by using three different FAs, although this The method requires a larger flip angle, but PD and T1 can still be calculated, otherwise it will become an ill-posed problem. Recent studies have shown that bias field correction algorithms can be used for radiofrequency emission field mapping, but this is difficult to implement in the clinic. In the MRI equipment whose main magnetic field strength is from 0.2T to 7T, the relaxation time of T1 (longitudinal relaxation time) increases with the increase of the magnetic field, while the relaxation time of T2/T2* decreases with the increase of the magnetic field, This opens up opportunities for simultaneous imaging of more parameters at the same time relative to high field. However, there is no relatively complete multi-contrast imaging method for low-field magnetic resonance in the prior art.

Contents of the invention

The object of the present invention is to overcome the defects of the above-mentioned prior art, and provide a multi-contrast imaging method and equipment for low-field magnetic resonance.

According to a first aspect of the invention there is provided a method of multi-contrast imaging for low field magnetic resonance. The method includes the following steps:

sequentially applying the set first radio frequency pulse sequence and second radio frequency pulse sequence to the target imaging area, wherein the first radio frequency pulse sequence adopts the first flip angle, and the second radio frequency pulse sequence adopts the second flip angle;

Collecting a first group of echo signals for the first radio frequency pulse sequence, and collecting a second group of echo signals for the second radio frequency pulse sequence, wherein both the first group of echo signals and the second group of echo signals contain a plurality of echoes ;

The obtained echo signals are modulated and under-sampled, and then the magnetic resonance images of various contrasts of the target imaging region are reconstructed.

According to a second aspect of the invention there is provided a multi-contrast imaging apparatus for low field magnetic resonance. The equipment includes:

Scanning unit: used to sequentially apply the set first radio frequency pulse sequence and second radio frequency pulse sequence to the target imaging area, wherein the first radio frequency pulse sequence adopts the first flip angle, and the second radio frequency pulse sequence adopts the second flip angle;

Signal collection unit: used for collecting the first group of echo signals for the first radio frequency pulse sequence, and collecting the second group of echo signals for the second radio frequency pulse sequence, wherein the first group of echo signals and the second group of echo signals Both contain multiple echoes;

An image reconstruction unit: used for modulating and under-sampling the obtained echo signals, and then reconstructing magnetic resonance images of various contrasts of the target imaging region.

Compared with the prior art, the advantage of the present invention is that, compared with the high-field multi-contrast image generation method, the present invention can effectively utilize the longer T2/T2* (transverse relaxation time) in low-field magnetic resonance, and realize While changing the flip angle to T1mapping (measuring the T1 value of the tissue), the gap between the two flip angles is effectively utilized for T2 image generation. In addition, an encoding method based on wave gradient fields is proposed to modulate and undersample the signal to perform high-quality reconstruction of highly undersampled data using an image reconstruction algorithm based on multi-measurement compressed sensing, thereby reducing the condition number of the system And increase the signal-to-noise ratio of the reconstructed image. The imaging sequence provided by the present invention can realize the generation and reconstruction of multiple contrast images in a single scan.

Other features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings.

Description of drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

1 is a flowchart of a multi-contrast imaging method for low-field magnetic resonance according to an embodiment of the present invention;

Fig. 2 is a schematic diagram of a fast multi-contrast magnetic resonance imaging sequence according to one embodiment of the present invention.

Detailed ways

Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that the relative arrangements of components and steps, numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless specifically stated otherwise.

The following description of at least one exemplary embodiment is merely illustrative in nature and in no way taken as limiting the invention, its application or uses.

Techniques, methods and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such techniques, methods and devices should be considered part of the description.

In all examples shown and discussed herein, any specific values should be construed as exemplary only, and not as limitations. Therefore, other instances of the exemplary embodiment may have different values.

It should be noted that like numerals and letters denote like items in the following figures, therefore, once an item is defined in one figure, it does not require further discussion in subsequent figures.

Compared with high-field and ultra-high-field, the T2/T2* value of tissue under low-field magnetic resonance is longer, and the longer T2/T2* value slows down tissue signal attenuation, providing a longer time window for signal acquisition . Therefore, in order to make full use of the characteristic of slow signal attenuation under low-field magnetic resonance, the present invention combines the principles of multi-echo GRE sequence and FSE (fast spin echo pulse sequence) sequence to design a Fast multi-contrast MRI sequences and corresponding multi-contrast imaging methods.

Referring to FIG. 1 , the provided multi-contrast imaging method for low-field magnetic resonance includes the following steps.

In step S110, two radio frequency pulse scans are performed on the target imaging region, and a fast multi-contrast magnetic resonance imaging sequence is designed by taking advantage of the increased transverse relaxation time in low-field magnetic resonance.

Taking the flip angle of the radio frequency pulse in the first scan as α ₁ and the flip angle of the radio frequency pulse in the second scan as α ₂ as an example, the designed multi-contrast MRI sequence is shown in Fig. 2 . After the asymmetric radio frequency pulse with a flip angle of _α1 excites the tissue in the target imaging area, first use the double-echo GRE pair to read out two sets of gradient echo signals (marked as echo #1 and #2). Then, two sets of spin echo signals (echo #3 and #4) are read out by using the FSE with an echo chain length of 2. Then, the tissue was excited again with an asymmetric radiofrequency pulse with a flip angle of _α2 , and two sets of gradient echo signals (echo #5 and #6) were collected using double-echo GRE. In order to shorten the echo time TE to obtain a high-signal echo, the embodiment in FIG. 2 uses an asymmetric radio frequency pulse to excite the tissue.

After collecting the above multiple sets of echoes, for the generation of magnetic resonance images, in one embodiment, echo 1 and echo 2 are used to generate T2*map by adopting a nonlinear fitting method; echo 3 and Echo 4 uses nonlinear fitting to generate T2map; echo 5 and echo 6 are used to generate T1map. It should be noted that in the middle of α1 and α2 radio frequency excitation, it cannot be used in high field. However, for low field magnetic resonance, T2 will be relatively much larger than that of high field magnetic resonance, so there is an inherent Taking advantage of the variable length of T2, T2map images can be further generated. Moreover, T1-weighted images (T1W) and T2-weighted images (T2W) can be generated by Fourier transform in echo 1 and echo 3, and T2*weighted images (T2*W) can be generated in echo 1 and echo 2 ), a total of 6 images are generated at this time. It should be understood that the phases of α1 and α2 may be the same, but are not limited to the same phase. The 180° hard pulses are out of phase by π, and the first hard pulse and α1 have a phase difference of π.

Specifically, by solving the Bloch equation, we can get:

Among them, S represents the overall signal intensity of fast spin echo (FSE), _ρ0 represents the proton density, TR represents the repetition time, T1 represents the longitudinal relaxation time, T2 represents the transverse relaxation time, and TE represents the echo time. A T2W image can be obtained by setting long TR and long TE. In one embodiment, combined with the basic principle of FSE, the time of echo 3 and echo 4 can be adjusted to generate a T2W image. When only T2W images remain, formula (1) becomes:

S'=ρ ₀ *e ^-TE/T2 (2)

By setting different TE values, different signal loudness (strength) is obtained, and then the T2map image is obtained by linear fitting. In the present invention, different times of echo 3 and echo 4 represent different TE values, so the T2W images obtained by echo 3 and echo 4 can be directly used to fit the T2map.

In gradient echo, by adjusting the long TR and long TE values, a signal model similar to that shown in Equation (2) can be obtained, expressed as:

S″=ρ ₀ *e ^-TE/T2 (3)

For different TE values, different T2* weighted images can be obtained, and the corresponding T2*map images can be obtained after linear fitting through different T2* weighted images.

Further, through two flip angles α ₁ and α ₂ , the proton density weighted map (PD) and the true proton density weighted map (truePD, tPD) can be generated, expressed as:

Among them, ρ ₀ represents the spatial distribution image of PD, ρ _i represents the PDW image, PD map can be obtained by linear fitting from echo 1 and echo 5, T2 ^* represents the effective transverse relaxation time, and i represents different gradient echoes.

In the case of inhomogeneous radio frequency field, the signal of radio frequency attenuation gradient echo data acquisition can be used as a function of variable flip angle θ, for example, obtained by the following formula:

Among them, TR represents the repetition time, bias represents the bias combined with coil sensitivity, PD represents the proton density, TE represents the echo time, and k represents the scaling factor of the emission field. In order to make the calculation of T1map more direct, the formula (5) is simplified as :

Among them, E1=e ^-TR/T1 ,

By collecting excitation data with different flip angles, the transformed data can be fitted to linearity, where the slope is E1 and the intercept is PD _eff * (1-E1). In order to solve the T1map and PD map, at least two flips are required horn.

For the ideal steady-state RF damage gradient GRE data acquisition, the Ernst equation of the flip angle θ is:

Wherein, ρ ₀ is the proton density, E ₁ =e ^-TR/T1 ,

TR is repetition time, TE is echo time, T1 and T2* are longitudinal and transverse relaxation times, respectively. Combining with formula (7), it can be seen that due to the change of radio frequency emission field and receiving field, a given tissue may display different signal strengths at different positions. From formula (7), it can be obtained that the FA ratio of the Ernst angle smaller than that of white matter is greater than that of Ernst FA withstands more variations in the RF emission field. In one embodiment, the enhanced T1 weighted map (aT1W) can be obtained by subtracting the signal with a smaller flip angle from the echo signal with a larger flip angle, expressed as:

aT1=s(θ ₂ ,TE _n )-λs(θ ₁ ,TE _n ) (8)

Wherein, θ ₂ represents α ₂ in the present invention, for example, θ ₂ = 24° provides an aT1W image, θ ₁ = 6° provides a PDW image, θ ₁ represents α ₁ , and aT1 represents the measured enhanced T1-weighted signal. It should be understood that the flip angles of the two RFs are not limited to 6° and 24°,

k is the change of the extracted radio frequency emission field, and TE _n is the nth echo. Since the two scans are double-echo scans, the final aT1 is composed of the first and second (here, the first and the second Secondary refers to the resulting echo of two excitations of _α2 and _α1 ) The echo is given as the average between the two T1W images calculated according to equation (8). In the GRE sequence, it can be known from the Bloch equation and the Ernst equation that the T1W image is given by the angle FA greater than the Ernst angle.

For RF pulses with different flip angles (α ₁ and α ₂ ), the following two gradient echoes, a total of four gradient echoes (echo 1, echo 2, echo 5 and echo 6) can be selected As a weighted map for calculating Susceptibility Sensitivity Map (QSM). For example, a 3D phase unwrapping algorithm is used to solve QSM weighted images. In the embodiment of the present invention, using echo 1 and echo 2, echo 5 and echo 6, the TE time difference such as 1.25ms (not limited to 1.25ms) can effectively prevent phase aliasing, at echo 2 Can be effectively dealiased, for example by the following formula:

in,

represents the unfolded phase,

For the raw phase at each echo, by adding the unwrapped phase at echo 1 and echo 2 to echo 5, one can get the phase at 1.25ms before echo 5 (17.5ms), which is 16.25ms the phase at. Finally, the phase at echo 6 can be developed by combining the phase at echo 5 with the phase developed by formula (9), and the phase at echo 6 can be expanded by using the corresponding mask (mask) of the brain to generate the brain phase Unfold the graph for masking. QSM images were subsequently generated for each echo using a truncated k-space segmentation algorithm.

Step S120 , for the proposed fast multi-contrast magnetic resonance imaging sequence, the echo signal is modulated and under-sampled by using the wave gradient magnetic field and the phase-shifted gradient magnetic field, so as to perform high-quality image reconstruction.

In one embodiment, by adding the Wave-CAIPI encoding gradient field and phase shift strategy in the two directions of phase and layer selection, by causing aliasing in the readout direction and causing phase shift in the two directions of phase, Reduce the condition number of the reconstruction system and increase the signal-to-noise ratio (SNR) of the reconstructed image. The signal model after adding wave is:

Among them, S represents the coil sensitivity used in parallel reconstruction, m[x,y,z] is the image to be solved, Psf is the point spread function, M represents the sampling template, and k _x represents the Fourier transform of Psf along the readout direction Transform, F _x represents Fourier transform, the reconstruction can use the traditional SENSE reconstruction algorithm, in the present invention, by using the wave-CAIPI mode, can increase the (SNR) of the reconstructed image, so that the various qualitative and quantitative images in front solution is more accurate. The specific image reconstruction method may be known in the art, and will not be repeated here.

In this step, the echo signals are modulated and under-sampled using the wave-gradient magnetic field and phase-shifted gradient magnetic field (wave-CAIPI) technique for the proposed fast multi-contrast MRI sequence in order to utilize the multi-measurement compressive sensing based image The reconstruction algorithm performs high-quality reconstruction on highly undersampled data.

Correspondingly, the present invention also provides a multi-contrast imaging device for low-field magnetic resonance, which is used to realize one or more aspects of the above method. For example, the device includes: a scanning unit, which is used to sequentially apply a set first radio frequency pulse sequence and a second radio frequency pulse sequence to the target imaging area, wherein the first radio frequency pulse sequence adopts a first flip angle, and the second radio frequency pulse sequence The sequence adopts the second flip angle; the signal acquisition unit is used to collect the first group of echo signals for the first radio frequency pulse sequence, and collect the second group of echo signals for the second radio frequency pulse sequence, wherein the first group of echo signals Both the echo signals and the second group of echo signals include a plurality of echoes; the image reconstruction unit is configured to modulate and under-sample the obtained echo signals, and then reconstruct magnetic resonance images of various contrasts of the target imaging region.

To sum up, the magnetic resonance imaging sequence diagram designed by the present invention can utilize the inherent advantages of T2/T2* in low-field magnetic resonance relative to the increase in high-field, and make full use of the idle time of the sequence (such as echo 2 and echo 5 Between) to generate T2W images, and perform linear fitting to obtain T2map images, then realize T1 images and aT1 images through variable flip angle RF excitation pulses, and then obtain T2, T2*, and PD parameter images by linear fitting.

In summary, the single-sequence multi-contrast image generation method for low-field magnetic resonance provided by the present invention utilizes the characteristics of T2/T2* lengthening in low-field magnetic resonance, and realizes enhancing T1-weighted images at the same time. Insert the spin echo gradient to generate T2 weighted map and quantitative map; generate T1 weighted and T1map images by using different flip angles; use Wave-CAIPI coding method to make the signal-to-noise ratio of the reconstructed image lower. In addition, the imaging sequence designed by the present invention includes any single-sequence multi-contrast image generation method using the advantages of T2/T2* lengthening in low-field magnetic resonance, in addition to the above scheme. Using the multi-contrast imaging sequence and the corresponding image generation method provided by the present invention, at least 10 kinds of contrast images can be generated through a single scan combined with calculations, including but not limited to T1, T2, T2* and PD parameter images, and T1, T1 enhanced , T2*, T2, PD and QSM weighted images, etc., and significantly shorten the time of traditional qualitative and quantitative image generation. In addition, the signal-to-noise ratio of quantitative and qualitative images is further improved by introducing wave-CAIPI technology, and the clarity of reconstructed images is enhanced.

The present invention can be a system, method and/or computer program product. A computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the invention.

A computer readable storage medium may be a tangible device that can retain and store instructions for use by an instruction execution device. A computer readable storage medium may be, for example, but is not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer diskettes, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), or flash memory), static random access memory (SRAM), compact disc read only memory (CD-ROM), digital versatile disc (DVD), memory stick, floppy disk, mechanically encoded device, such as a printer with instructions stored thereon A hole card or a raised structure in a groove, and any suitable combination of the above. As used herein, computer-readable storage media are not to be construed as transient signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., pulses of light through fiber optic cables), or transmitted electrical signals.

Computer-readable program instructions described herein may be downloaded from a computer-readable storage medium to a respective computing/processing device, or downloaded to an external computer or external storage device over a network, such as the Internet, a local area network, a wide area network, and/or a wireless network. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and/or edge servers. A network adapter card or a network interface in each computing/processing device receives computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in a computer-readable storage medium in each computing/processing device .

Computer program instructions for carrying out operations of the present invention may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or Source or object code written in any combination, including object-oriented programming languages—such as Smalltalk, C++, Python, etc., and conventional procedural programming languages—such as the “C” language or similar programming languages. Computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer, or entirely on the remote computer or server implement. In cases involving a remote computer, the remote computer can be connected to the user computer through any kind of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computer (such as via the Internet using an Internet service provider). connect). In some embodiments, an electronic circuit, such as a programmable logic circuit, field programmable gate array (FPGA), or programmable logic array (PLA), can be customized by utilizing state information of computer-readable program instructions, which can Various aspects of the invention are implemented by executing computer readable program instructions.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It should be understood that each block of the flowcharts and/or block diagrams, and combinations of blocks in the flowcharts and/or block diagrams, can be implemented by computer-readable program instructions.

These computer-readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine such that when executed by the processor of the computer or other programmable data processing apparatus , producing an apparatus for realizing the functions/actions specified in one or more blocks in the flowchart and/or block diagram. These computer-readable program instructions can also be stored in a computer-readable storage medium, and these instructions cause computers, programmable data processing devices and/or other devices to work in a specific way, so that the computer-readable medium storing instructions includes An article of manufacture comprising instructions for implementing various aspects of the functions/acts specified in one or more blocks in flowcharts and/or block diagrams.

It is also possible to load computer-readable program instructions into a computer, other programmable data processing device, or other equipment, so that a series of operational steps are performed on the computer, other programmable data processing device, or other equipment to produce a computer-implemented process , so that instructions executed on computers, other programmable data processing devices, or other devices implement the functions/actions specified in one or more blocks in the flowcharts and/or block diagrams.

The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in a flowchart or block diagram may represent a module, a portion of a program segment, or an instruction that includes one or more Executable instructions. In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks in succession may, in fact, be executed substantially concurrently, or they may sometimes be executed in the reverse order, depending upon the functionality involved. It should also be noted that each block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by a dedicated hardware-based system that performs the specified function or action , or may be implemented by a combination of dedicated hardware and computer instructions. It is well known to those skilled in the art that implementation by means of hardware, implementation by means of software, and implementation by a combination of software and hardware are all equivalent.

Having described various embodiments of the present invention, the foregoing description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and alterations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principle of each embodiment, practical application or technical improvement in the market, or to enable other ordinary skilled in the art to understand each embodiment disclosed herein. The scope of the invention is defined by the appended claims.

Claims (10)

1. A multi-contrast imaging method for low-field magnetic resonance comprising the steps of:

   Step S1: sequentially applying the set first radio frequency pulse sequence and second radio frequency pulse sequence to the target imaging area, wherein the first radio frequency pulse sequence adopts the first flip angle, and the second radio frequency pulse sequence adopts the second flip angle;

   Step S2: collecting a first group of echo signals for the first radio frequency pulse sequence, and collecting a second group of echo signals for the second radio frequency pulse sequence, wherein both the first group of echo signals and the second group of echo signals contain multiple echo;

   Step S3: Modulating and under-sampling the obtained echo signals, and then reconstructing magnetic resonance images of various contrasts of the target imaging region.
2. The method according to claim 1, wherein collecting the first group of echo signals for the first radio frequency pulse sequence comprises: using a double-echo gradient refocusing echo imaging sequence to read out two groups of gradient echo signals, marking are the first echo and the second echo; two sets of spin echo signals are read out using a fast spin echo pulse sequence with an echo chain length of 2, marked as the third echo and the fourth echo;

   Acquiring a second set of echo signals for the second radio frequency pulse sequence includes: acquiring two sets of gradient echo signals, labeled fifth echo and sixth echo, using a dual-echo gradient refocusing echo imaging sequence.
3. The method according to claim 2, characterized in that, for the first echo and the second echo, a non-linear fitting method is used to generate the T2* image; for the third echo and the fourth echo, a non-linear fitting method is used Generate _T2 images; the fifth and sixth echoes are used to generate T1 images; for the first and third echoes, T1 weighted images and T2 weighted images are generated by Fourier transform; for the first echo The T2*-weighted image is generated by means of the Fourier transform of the wave and the second echo.
4. The method according to claim 3, further comprising: using the first flip angle and the second flip angle, by generating a proton density weighted map and a true proton density map from the first echo and the fifth echo Density weighted plot.
5. The method according to claim 3, further comprising: using the first echo and the fifth echo to obtain an enhanced T1 weighted map, expressed as:

   aT1=s(θ ₂ ,TE _n )-λs(θ ₁ ,TE _n )

   Wherein, θ ₂ is the flip angle corresponding to the second radio frequency pulse sequence, θ ₁ represents the flip angle corresponding to the first radio frequency pulse sequence,

   

   k is the change of the extracted radio frequency emission field, and TE _n is the nth echo.
6. The method according to claim 3, wherein when the first flip angle and the second flip angle are set to different values, using the first echo, the second echo, and the fifth echo and the sixth echo using the 3D phase unwrapping algorithm to calculate the susceptibility-sensitive weighted map QSM, where the demixing formula at the second echo is expressed as:

   

   in,

   

   represents the unfolded phase,

   

   is the original phase at each echo, and TE _n is the nth echo.
7. The method according to claim 1, characterized in that, in step S3, by adding phase offset gradient magnetic field encoding gradient field and phase offset in two directions of phase and layer selection, the signal model is expressed as:

   

   Among them, S represents the coil sensitivity used in parallel reconstruction, m[x,y,z] is the image to be solved, Psf is the point spread function, M represents the sampling template, and k _x represents the Fourier transform of Psf along the readout direction Transform, F _x means Fourier transform.
8. The method according to claim 1, wherein the first flip angle and the second flip angle are set to the same or different values.
9. A multi-contrast imaging device for low-field magnetic resonance comprising:

   Scanning unit: used to sequentially apply the set first radio frequency pulse sequence and second radio frequency pulse sequence to the target imaging area, wherein the first radio frequency pulse sequence adopts the first flip angle, and the second radio frequency pulse sequence adopts the second flip angle;

   Signal collection unit: used for collecting the first group of echo signals for the first radio frequency pulse sequence, and collecting the second group of echo signals for the second radio frequency pulse sequence, wherein the first group of echo signals and the second group of echo signals Both contain multiple echoes;

   An image reconstruction unit: used for modulating and under-sampling the obtained echo signals, and then reconstructing magnetic resonance images of various contrasts of the target imaging region.
10. A computer-readable storage medium, on which a computer program is stored, wherein, when the program is executed by a processor, the steps of the method according to any one of claims 1 to 8 are realized.

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