CN117518052A - Planar echo diffusion weighted imaging method and system based on reacquired navigator echo - Google Patents
Planar echo diffusion weighted imaging method and system based on reacquired navigator echo Download PDFInfo
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Abstract
The invention discloses a plane echo diffusion weighted imaging method and a plane echo diffusion weighted imaging system based on a re-acquisition navigator echo, wherein the method comprises the following steps: first acquisition of first imaging echo data d of a magnetic resonance apparatus img Acquiring the echo data d of the second imaging nav The method comprises the steps of carrying out a first treatment on the surface of the Processing the two echo data to obtain first imaging echo k-space data D img And second imaging echo k-space data D nav The method comprises the steps of carrying out a first treatment on the surface of the Performing matrix construction and iteration on the two types of space data to obtain imaging echo k-space data D poc The method comprises the steps of carrying out a first treatment on the surface of the For the acquired imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m . The system comprises an imaging echo data acquisition module, a re-acquisition navigation echo acquisition module, a k-space data construction module, a low-rank iteration module and a k-space data synthesis module. The plane echo diffusion weighted imaging method and system of the invention have the function of scanningThe imaging efficiency is high, and the imaging system is less sensitive to image distortion of the navigator echo, so that the artifact removal effect is good.
Description
Technical Field
The invention relates to a telemedicine technology, in particular to a plane echo diffusion weighted imaging method and a plane echo diffusion weighted imaging system based on a reacquired navigator echo.
Background
The magnetic resonance imaging MRI (MagneticResonanceImaging) is a new technology of the latest medical image by utilizing the nuclear magnetic resonance principle, and has excellent diagnosis function on brain, thyroid, liver, gall bladder, spleen, kidney, pancreas, adrenal gland, uterus, ovary, prostate and other solid organs, heart and large blood vessels. Compared with other auxiliary examination means, the nuclear magnetic resonance imaging method has the advantages of multiple imaging parameters, high scanning speed, high tissue resolution, clearer images, no harm to human bodies and the like, can help doctors to 'see' the early lesions which are not easy to perceive, and has become a sharp tool for early screening of tumors, heart diseases and cerebrovascular diseases.
The EPI (echo imaging sequence) is a pulse sequence for acquiring echo signals in a planar echo imaging manner, is one of common sequences for MRI scanning, has extremely high imaging speed, has extremely high requirements on a magnetic resonance gradient system, is extremely sensitive to magnetic field uniformity, and can cause serious distortion of images. A special sequence in DWI (diffusion weighting imaging) nuclear magnetic resonance, the basis of which is water molecule movement; the EPI sequence is often adopted to realize the detection, is very sensitive to the diffusion of object molecules, and can be used for detecting human bleeding; DWI requires very stringent hardware for the magnetic resonance system, often requiring extensive pre-correction and post-processing, and can also be implemented based on multi-shot EPI in order to reduce image distortion, a technique known as multi-shot DWI imaging.
At present, EPI-DWI is widely applied in many directions, however, obvious artifacts and distortion appear in imaging results due to mismatching among odd-even echoes, eddy current influence and field nonuniformity, the quality of images is greatly influenced, and the accuracy of diagnosis is further influenced.
Firstly, in order to solve the problem of artifacts caused by mismatch between parity echoes, a plurality of echo data which are not subjected to phase encoding can be additionally scanned, so that the phase errors of the parity echoes are obtained by processing the echo data, and the phase errors are compensated and enter the imaging echoes, so that the effect of suppressing the artifacts is achieved.
Second, the distortion in the image is due to the long echo spacing of the EPI/DWI sequence, which makes its phase direction particularly sensitive to the field uniformity of the magnetic resonance. The most straightforward way to reduce distortion is therefore to reduce the echo spacing of the imaging sequence, and one more sophisticated technique is the multiple excitation EPI/DWI technique. Echo spacing can be reduced by multiple times of scanning data and interlacing k-space filling, so that image distortion is reduced by multiple times, and image signal-to-noise ratio and resolution are effectively improved. Although multiple excitation techniques are very advantageous for improving imaging quality, since diffusion imaging is very sensitive to motion, the long-term high intensity gradients applied during imaging can cause phase differences in the echoes of each excitation, resulting in artifacts. It is therefore desirable to use parallel imaging techniques to remove such artifacts.
The existing method at present adopts an image reconstruction technology of image space sampling to reconstruct the EPI/DWI excited by high order. The basic idea comprises the following aspects: (1) Based on the current spin echo EPI/DWI sequence, continuously applying 180-degree pulse to acquire complete k-space center data as a navigation signal after the acquisition of imaging echo is finished; (2) Closing phase encoding, collecting pre-scanning data without phase encoding, and correcting mismatching of odd-even echo for imaging echo and re-collecting navigation echo after processing; (3) Processing the navigation signal to obtain a phase diagram of each excitation, and obtaining a phase error by performing conjugate difference to compensate in the reconstruction of the imaging echo so as to achieve the effect of removing the artifact; (4) Finally, the data obtained by each excitation are processed by the steps and then are subjected to Fourier transformation to obtain images, and then the square sum of the amplitude values of the data in the image domain is taken to combine all the images to obtain a final image.
In summary, the existing method still needs to additionally acquire the pre-scan data to perform the odd-even echo correction in the imaging process, which increases the scanning time to a certain extent, and the pre-scan data cannot completely eliminate the artifact in some poor scenes, such as when the signal is weak. In addition, the echo time interval of the acquired navigation echo signal is inconsistent with the imaging echo interval, so that the image distortion conditions reconstructed by the two echoes are different, and the phase correction effect in the later reconstruction process can be influenced. Thus, when corrected based on parallel imaging techniques, the final image may still have residual motion artifacts.
Disclosure of Invention
The invention provides a plane echo diffusion weighted imaging method and a plane echo diffusion weighted imaging system based on the reacquired navigator echo, which are used for avoiding the defects existing in the prior art, so as to remove the artifacts generated in the plane echo imaging process on the premise of not increasing the scanning time.
The invention adopts the following technical scheme for solving the technical problems.
The invention relates to a plane echo diffusion weighted imaging method based on a re-acquisition navigator echo, which comprises the following steps:
step 1: first acquisition of first imaging echo data d of a magnetic resonance apparatus img Acquiring the echo data d of the second imaging nav ；
Step 2: for the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
Step 3:for first imaging echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
Step 4: for the acquired imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
The planar echo diffusion weighted imaging method based on the re-acquisition navigator echo is also characterized by comprising the following steps of:
further, in the step 1, the first imaging echo data d is acquired img A 180-degree pulse structure is added to the first magnetic resonance spin echo EPI/DWI sequence to acquire the acquired second imaging echo data d nav A second magnetic resonance spin echo EPI/DWI sequence.
Further, in the step 2, the echo data d is imaged for the second time nav Performing zero padding operation to obtain second imaging echo data d nav Dimension W of (2) nav Extend to and primary imaging echo data d img Dimension W of (2) img Likewise, second imaging echo k-space data D is obtained nav 。
Further, in the step 2, the first imaging echo data d img Is divided according to the parity echo type, the separated and removed data position is processed by zero padding operation to obtain first imaging echo k-space data D img 。
Further, in the step 3, the positive operation matrix H is used to perform the first imaging of the echo k-space data D img And second imaging echo k-space data D nav The matrix is constructed to obtain a matrix H (D img ) Sum matrix H (D nav )。
Further, in the step 3, the matrix H (D nav ) Singular value decomposition is performed to obtain a matrix H (D nav ) Is a signal subspace V of (2) SPs And noise subspace V SPn 。
Further, in the step 4, the process of data synthesis includes fourier transform, phase analysis, and complex summation.
The invention also discloses a system of the planar echo diffusion weighted imaging method based on the re-acquisition navigator echo, which comprises an imaging echo data acquisition module, a re-acquisition navigator echo acquisition module, a k-space data construction module, a low-rank iteration module and a k-space data synthesis module;
the imaging echo data acquisition module is used for acquiring first imaging echo data d of the magnetic resonance equipment img And echo data d of the first imaging img Transmitting to the k-space data construction module;
the re-acquisition navigator echo acquisition module adds a 180-degree pulse on the first magnetic resonance spin echo EPI/DWI sequence of the imaging echo data acquisition module to serve as a second magnetic resonance spin echo EPI/DWI sequence, and re-acquires second imaging echo data d nav And imaging echo data d for the second time nav Transmitting to the k-space data construction module;
the k-space data construction module is used for carrying out the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
The low-rank iteration module is used for carrying out first imaging on echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
The k-space data synthesis module is used for obtaining imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
The invention also discloses an electronic device, which comprises at least one processor and a memory in communication connection with the at least one processor; wherein the memory stores instructions executable by the at least one processor to enable the at least one processor to perform the re-acquisition navigator echo based planar echo diffusion weighted imaging method.
The invention also discloses a computer readable storage medium, which stores a computer program; the computer program when executed by the processor implements the planar echo diffusion weighted imaging method based on the re-acquired navigator echo.
Compared with the prior art, the invention has the beneficial effects that:
the invention discloses a plane echo diffusion weighted imaging method and a plane echo diffusion weighted imaging system based on a re-acquisition navigator echo, wherein the method comprises the following steps: first acquisition of first imaging echo data d of a magnetic resonance apparatus img Acquiring the echo data d of the second imaging nav The method comprises the steps of carrying out a first treatment on the surface of the For the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav The method comprises the steps of carrying out a first treatment on the surface of the For first imaging echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc The method comprises the steps of carrying out a first treatment on the surface of the The obtained imaging echo k-space data Dpoc is subjected to data synthesis processing to obtain final image data I m . The planar echo diffusion weighted imaging system based on the reacquired navigator echo comprises an imaging echo data acquisition module, a reacquired navigator echo acquisition module, a k-space data construction module, a low-rank iteration module and a k-space data synthesis module.
The planar echo diffusion weighted imaging method and system based on the re-acquisition navigator echo have the following technical characteristics.
1. The scanning imaging efficiency is improved by correcting the parity echo phase without additional parity echo data which is not subjected to phase encoding;
2. no additional scan data is required to achieve parallel imaging;
3. the image distortion of the navigator echo is less sensitive, and a better artifact removal effect can be obtained.
The plane echo diffusion weighted imaging method and system have the advantages of high scanning imaging efficiency, better artifact removal effect due to insensitivity to image distortion of navigation echo, and the like.
Drawings
FIG. 1 is a flow chart of a planar echo diffusion weighted imaging method based on reacquiring navigator echoes according to the present invention.
FIG. 2 is a schematic diagram of two echo EPI/DWI sequences of a planar echo diffusion weighted imaging method based on re-acquisition navigator echoes of the present invention.
Fig. 3 is a schematic diagram of k-space structure data construction of a planar echo diffusion weighted imaging method based on re-acquisition navigator echo of the present invention.
Fig. 4 is a schematic diagram of a low rank matrix construction operator of the present invention.
Fig. 5 is a schematic diagram of the summation of parity echo images of the present invention.
The invention is further described below by means of specific embodiments in connection with the accompanying drawings.
Detailed Description
In the present invention, several technical terms are explained as follows.
Magnetic resonance echo: one type of magnetic resonance signal is typically characterized by peaks in the middle of a segment of the signal, essentially a signal obtained by fourier transforming the spin signal in space;
magnetic resonance signal phase: the magnetic resonance signals are in fact complex data, i.e. contain real and imaginary information, and the relative strength of the real and imaginary parts generally implies important information.
k-space: the acquired magnetic resonance signals are constructed and combined in a certain order into a matrix called k-space data, the space in which the k-space data is located is called k-space, and the spatial frequency in the magnetic resonance image is actually represented.
And acquiring navigation echo: imaging echo signals are acquired during magnetic resonance imaging to fill k-space in order to fourier transform the acquired echo signals to obtain a magnetic resonance image. In addition to acquiring imaging echo signals when performing magnetic resonance imaging, if additional echo signals are acquired for other operations, this portion of the echo signals is defined herein as re-acquired navigator echoes;
magnetic resonance gradient polarity: the magnetic resonance imaging requires the application of gradients in space, positive and negative values of gradient strength are gradient polarities, and in the method, the echoes obtained by applying positive gradient to the magnetic resonance imaging are odd echoes, and the echoes obtained by applying negative gradient are even echoes;
magnetic resonance gradient eddy currents: magnetic resonance produces residual currents, i.e. eddy currents, in the surrounding conductors when gradients are applied, due to the changing magnetic field;
artifacts: due to the imperfections of the magnetic resonance imaging system, partial deviations exist in the acquired data, which are finally reflected in the image, such as winding artifacts, fat artifacts, signal interference and the like, wherein the artifacts mainly refer to the winding artifacts;
plane echo imaging sequence: the EPI sequence, a special magnetic resonance imaging sequence, has extremely high imaging speed, extremely high requirements on a magnetic resonance gradient system, extremely high sensitivity on magnetic field uniformity, and serious distortion of an image caused by poor magnetic field uniformity, can be realized in a mode of multiple excitation for reducing the image distortion, but increases the data correction difficulty;
diffusion weighted imaging sequence: the DWI sequence is often realized by adopting an EPI sequence, is very sensitive to the diffusion of object molecules, and can be used for checking human bleeding. The hardware requirements on the magnetic resonance system are very strict, a large amount of pre-correction and post-processing are often required, and the image distortion is reduced, the image distortion can be realized based on multi-excitation EPI, and the technology is called a multi-excitation DWI imaging technology;
low rank theoretical reconstruction: the magnetic resonance data is constructed into a specific structure, the structure is decomposed into a signal subspace and a noise subspace by the low-rank theory, and the signal-to-noise ratio of imaging can be improved and the artifact suppression effect of the image can be improved by applying the structure to undersampled data recovery during magnetic resonance image reconstruction.
Referring to fig. 1, the planar echo diffusion weighted imaging method based on the reacquired navigator echo of the present invention includes the following steps:
step 1: first acquisition of first imaging echo data d of a magnetic resonance apparatus img Acquiring the echo data d of the second imaging nav The method comprises the steps of carrying out a first treatment on the surface of the Second imaging echo data d nav I.e. navigator echo data.
Step 2: for the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
Step 3: for first imaging echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
Step 4: for the acquired imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
In the specific implementation, in the step 1, the first imaging echo data d is acquired img A 180-degree pulse structure is added to the first magnetic resonance spin echo EPI/DWI sequence to acquire the acquired second imaging echo data d nav A second magnetic resonance spin echo EPI/DWI sequence.
The basic structure of the first magnetic resonance spin echo EPI/DWI sequence comprises a fat suppression structure, 90-degree pulse excitation, 180-degree pulse excitation and gradient coding structure so as to realize basic imaging echo acquisition and diffusion functions. The data acquired by the imaging echo data acquisition module is d img The specific structure is shown in fig. 2.
Based on the basic structure of the first magnetic resonance spin echo EPI/DWI sequence, a 180-degree pulse structure is additionally added to obtain a second magnetic resonance spin echo EPI/DWI sequence. And continuing to acquire navigator echo data by adopting a second magnetic resonance spin Echo (EPI)/DWI sequence. Because the navigation echo data is acquired again in each excitation process, the measurement of the parity echo error and the detection of the motion phase error can be realized. Re-acquisition navigationThe data acquired by the echo acquisition module is d nav The specific structure of the method is shown in fig. 2, and parity echoes are acquired for each phase code during acquisition.
In the specific implementation, in the step 2, the echo data d is imaged for the second time nav Performing zero padding operation to obtain second imaging echo data d nav Dimension W of (2) nav Extend to and primary imaging echo data d img Dimension W of (2) img Likewise, second imaging echo k-space data D is obtained nav 。
In the specific implementation, in the step 2, the first imaging echo data d img Is divided according to the parity echo type, the separated and removed data position is processed by zero padding operation to obtain first imaging echo k-space data D img 。
The step 2 comprises the following steps: first, echo data d is imaged for the second time nav Performing zero filling operation to obtain second imaging echo data d nav Dimension W of (2) nav Expanding to echo data d of first imaging img Dimension W of (2) img The same; second, the echo data d of the first imaging is processed img Is divided by parity echo type, and the separated and removed data positions are processed by performing zero padding operation. The first imaging echo k-space data D with completely consistent dimension can be obtained by expanding the acquired navigation echo data to the dimension same as the imaging echo data in a zero filling way, separating the data acquired in each imaging echo data according to the odd-even echo type and executing zero filling operation on the separated and removed data position img And second imaging echo k-space data D nav . As shown in fig. 3.
In particular, in step 3, the positive operation matrix H is used to image the echo k-space data D for the first time img And second imaging echo k-space data D nav The matrix is constructed to obtain a matrix H (D img ) Sum matrix H (D nav )。
In the specific implementation, in the step 3, the matrix H (D nav ) Singular value decomposition is carried out to obtain momentArray H (D) nav ) Is a signal subspace V of (2) SPs And noise subspace V SPn 。
As shown in fig. 4, two operators are contained in the low-rank iteration module: a positive operation matrix H and an inverse operation matrix H; the echo k-space data D is imaged for the first time by the two operator pairs img And second imaging echo k-space data D nav The forward operation and the reverse operation of the matrix construction are performed. The specific operation process comprises the following steps:
step 31: first, a positive operation of matrix construction is performed to obtain a matrix H (D img ) Sum matrix H (D nav )；
H (·) is a matrix operator that can construct the acquired imaging echo k-space data into a set matrix form, and H (·) is the inverse operator thereof, i.e. the set matrix form data is restored to the original k-space data, the two operators are constructed as shown in fig. 4, fig. 4 is a graph of H (D) img ) For example, matrix H (D nav ) Similar thereto. In FIG. 4, D odd Representing first imaging echo k-space data D img Odd echo of D even Representing first imaging echo k-space data D img Is a constant, even echo. The k-space data coil 1-coil 4 of 4 channels can be obtained assuming a number of receiving channels of 4 in the magnetic resonance imaging system. H and H are difficult to express in a conventional manner as operators, so that a corresponding matrix structure can be constructed by the illustration of fig. 4 in the present invention.
Step 32: then in matrix H (D nav ) Singular value decomposition is performed as a reference, resulting in its signal subspace SPs and noise subspace SPn.
For matrix H (D nav ) SVD (singular value decomposition) operations are performed, i.e., using three matrices U, S, V for matrix H (D nav ) Expression was carried out to give H (D) nav )＝USV * . Suppose H (D) nav ) For a p×q-order matrix, then U is a p×p-order unitary matrix; s is a half positive definite p multiplied by q order diagonal matrix; and V, i.e., the conjugate transpose of V, is a qxq unitary matrix. Only diagonal elements in the S matrix are non-zero, and only diagonal elements in the S matrix are H (D nav ) Is a singular value of (c). Diagonal representationMatrix H (D) nav ) And decreases from the upper left corner to the lower right corner, the signal subspace and the noise subspace can be distinguished according to the singular value size.
Assume that the first n rows of the V matrix are constructed as signal subspaces V with the nth singular value as the distinction SPs While the remaining rows are used as noise subspaces V SPn 。
Step 33: then by combining the signal subspaces V SPs And noise subspace V SPn Sum matrix H (D img ) Substituting the low-rank constraint into iteration, and stopping until the iteration termination condition is reached;
the key to performing low rank constraint is to exploit noise subspace V SPn And matrix H (D img ) Multiplication constructs a constraint solving problem as shown below.
In the formula (1), the argmin function represents a variable value when the objective function f (x) is made to take a minimum value. M (·) represents D img The part which is not filled with zero is extracted, and lambda is a constraint term penalty factor for controlling low-rank constraint intensity. Solving the above formula, i.e. D to be solved by gradient descent method img Iterate, when D img Results D of two iterations before and after the m-1 st and m-th iterations img,m-1 、D img,m When the change in (2) is smaller than the set threshold co, the expression is expressed by a binary norm, that is, the following expression (2) is satisfied.
||D img,m-1 -D img,m || 2 ≤ò (2)
The final result was H (D' img )。
Step 34: then performing the inverse operation of the matrix construction to obtain final processed imaging echo k-space data D poc 。
Obtaining H (D' img ) Then, the matrix is subjected to inverse operation of H, and referring to fig. 4, matrix H (D 'can be obtained by using the inverse operation of H #' img ) K-space data D restored to final imaging echo poc The calculation formula is as follows (3).
D poc ＝H*[H(D′ img )] (3)
In specific implementation, in the step 4, the data synthesis process includes fourier transform, phase analysis and complex summation.
As shown in fig. 5, the k-space data synthesis module obtains imaging echo k-space data D for the low rank iteration module poc And (5) performing synthesis treatment. The process of the treatment comprises the following steps:
step 41: performing fourier transform on all k-space parity echo data separated in the step 2;
step 42: then, the phase analysis is carried out on the image obtained by the data reconstructed by each excitation echo to obtain the phase main component
Step 43: the final image data I is obtained by complex summation of all images after removal of the additional phase components m 。
The analysis of the phase between the images to be synthesized can be additionally increased when the image synthesis is finally performed. Combining all images by analyzing the principal components of each image phase can achieve better artifact removal than doing the sum of the squares of the amplitudes directly in the image domain.
Acquired imaging echo k-space data D poc Odd echo imaging k-space data D comprising two sets of data multi-channels odd And even echo imaging k-space data D even Respectively carrying out Fourier transform on the image domain data to obtain multichannel image domain data Im odd And Im even I.e. satisfying the following formula. The foot code 'odd and even' corresponds to odd-even echo imaging, the foot code 'even' corresponds to even echo imaging, and the foot code 'odd' corresponds to odd echo imaging.
Im odd,even ＝FFT2[D odd,even ] (4)
In equation (4), FFT2 (·) is a two-dimensional fourier transform. Since there is still a possible phase difference between the two sets of image domain data, phase analysis is performed.
Firstly, conjugate multiplication is carried out on the odd-even echo imaging data to obtain a phase difference psi, and the following formula (5) is satisfied.
Ψ＝Im odd ×conj(Im even ) (5)
In equation (5), conj (·) represents principal component analysis of complex conjugate ψ to obtain smoothed phase difference ψ' by compensating the phase into even image data Im even Then the even image Im even And odd image Im odd And carrying out complex summation to obtain a final reconstruction result Im, wherein the final reconstruction result Im satisfies the following formula (6).
Im＝Im odd +Im even ×e iΨ′ (6)
In the formula (6), i is an imaginary unit, and e is a natural constant. A specific flow of the calculation is shown in fig. 5.
The invention also discloses a system of the planar echo diffusion weighted imaging method based on the re-acquisition navigator echo, which comprises an imaging echo data acquisition module, a re-acquisition navigator echo acquisition module, a k-space data construction module, a low-rank iteration module and a k-space data synthesis module;
the imaging echo data acquisition module is used for acquiring first imaging echo data d of the magnetic resonance equipment img And echo data d of the first imaging img Transmitting to the k-space data construction module;
the re-acquisition navigator echo acquisition module adds a 180-degree pulse on the first magnetic resonance spin echo EPI/DWI sequence of the imaging echo data acquisition module to serve as a second magnetic resonance spin echo EPI/DWI sequence, and re-acquires second imaging echo data d nav And imaging echo data d for the second time nav Transmitting to the k-space data construction module;
the imaging echo data acquisition module and the re-acquisition navigator echo acquisition module are performed in parallel, i.e. the imaging echo data and the re-acquisition navigator echo data may be considered to be simultaneously ready, the resulting data dimg and dnav being sent to the k-space data construction module. The key point in the acquisition module of the reacquiring navigation echo is as follows: whether the acquired parity echo data respectively constitute the central part of the fully sampled k-space data.
In the case of a reacquiring navigator echo acquisition, the echo length of each acquisition is smaller than the length acquired by the imaging echo. The operation mode shortens the interval time of adjacent echoes of the k-space data of the reacquired navigator echo, so that the image reconstructed by the reacquired navigator echo can better match the image distortion of the imaging echo, and further, other errors are avoided.
The k-space data construction module is used for carrying out the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
k-space data construction module pair data d img And d nav Processing to obtain constructed k-space data D img And D nav . The key point of the k-space data construction module is that the form of matrix construction is shown in fig. 2, and the key point is whether to separate the parity of the re-acquired navigator echo data, zero padding at the periphery and separate the parity of the imaging echo data, and zero padding is performed on the removed data, so that the size dimensions of the imaging echo and the re-acquired navigator echo data are the same.
The low-rank iteration module is used for carrying out first imaging on echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
Structured k-space data D img And D nav The low-rank constraint reconstruction is carried out by the low-rank iteration module, and the processed data D is returned after the loop termination condition is reached poc . The low rank iteration module focuses on: whether or not a matrix H (D nav ) To limit the constructed matrix H (D img ) Is subjected to iterative processing.
The k-space data synthesis module is used for obtaining imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
k-spaceThe data synthesis module receives the processed data D poc Data D poc Respectively performing Fourier transform to obtain multiple images, and synthesizing into one image I m Finally returning to the software platform to carry out image I m And displaying to a user.
The invention also discloses an electronic device, which comprises at least one processor and a memory in communication connection with the at least one processor; wherein the memory stores instructions executable by the at least one processor to enable the at least one processor to perform the re-acquisition navigator echo based planar echo diffusion weighted imaging method.
The invention also discloses a computer readable storage medium, which stores a computer program; the computer program when executed by the processor implements the planar echo diffusion weighted imaging method based on the re-acquired navigator echo.
The plane echo diffusion weighted imaging method and system based on the re-acquisition navigator echo are modified based on the spin echo EPI/DWI sequence, namely a re-acquisition navigator echo acquisition module is added after an imaging echo acquisition module of a traditional imaging sequence, and the re-acquisition navigator echo acquisition module is shown in figure 1. In the present invention, the re-acquisition navigator echo differs from the conventional scheme in that the echo covers only the central part of k-space as shown in fig. 1 and 2, and the acquired parity echoes respectively constitute a complete set of k-space data as shown in fig. 2.
And processing the acquired imaging echo and the acquired k-space data of the acquired navigator echo, performing zero padding processing on the k-space data of the acquired navigator echo to match the data dimension of the imaging echo, and then performing matrix construction on the k-space data of the imaging echo and the acquired navigator echo according to the same rule to obtain a new data matrix form as shown in fig. 2.
After imaging echo data and re-acquisition navigation echo k-space data form a new matrix form, reconstructing by an algorithm based on a low-rank theory, continuously restricting the rank of an imaging echo matrix in the reconstruction iteration process, and terminating the cycle after a set step size is reached or matrix update change is smaller than a set threshold value. And (3) restoring the calculated matrix into the original matrix form again according to the reverse rule in the step (2) to obtain processed imaging echo data. The invention has the greatest characteristic that the rank of the imaging echo matrix is effectively restrained so as to achieve better image reconstruction effect.
The planar echo diffusion weighted imaging method and system based on the re-acquisition navigator echo have the following innovation points.
1. The odd-even echo data collected by the navigation echo collection module can form complete k-space data instead of combining the data.
2. key points in the k-space data construction module are: the acquired data is constructed in a matrix form, the specific form of which is shown in fig. 2. The imaging echo k-space data and the re-acquisition navigator echo k-space data will be constructed in the form of fig. 2 to be of the same size.
3. The key points of the low-rank iteration module are as follows: the rank of the matrix constructed from the imaging echo k-space data is limited based on the matrix constructed from the re-navigator echo k-space data to achieve a low rank iterative process.
4. The key points in the k-space data synthesis module are: the input image data is not directly square sum-of-squares in the image domain, but needs to analyze the principal components of each image phase, then reject the secondary phase components, and finally sum all the excited images by complex numbers to obtain the final image.
5. During imaging, no extra scanning of the parity echo data which is not subjected to phase encoding is needed to correct the parity echo phase.
The planar echo diffusion weighted imaging method and system based on the re-acquisition navigator echo have the following technical advantages.
1. The parity echoes respectively construct fully sampled k-space data when acquiring and re-acquiring the navigator echoes, so that extra parity echoes are not needed to correct the parity echo phase difference, and the scanning time is saved to a certain extent.
2. Secondly, since a group of fully sampled k-space data is acquired for the parity echoes, the correction data required for additional scanning parallel imaging is not actually required, and therefore a certain scanning time is saved.
3. The processing of the imaging echo k-space data is iterated using a low rank theory based approach, the low rank constraint of which results from re-acquisition of navigator echo k-space data. This way of limiting the matrix rank allows better recovery of the imaging echo data making it more robust to mismatch between imaging echo and re-acquired navigator echo k-space data.
The planar echo diffusion weighted imaging method and system based on the re-acquisition navigator echo can be used for removing artifacts caused by parity echo errors of planar echo imaging due to magnetic resonance gradient polarity or magnetic resonance gradient eddy currents; and for diffusion weighted imaging of multiple excitations, the artifact caused by the phase difference between different excitations can be restrained while the artifact caused by the original plane echo imaging is restrained. In addition, the invention further modifies the diffusion weighted imaging sequence, continuously collects a small amount of echoes as reference data in a short time after the reading part is finished, and then reconstructs the images by an algorithm based on a low-rank theory, and additionally does not need to scan. Since no extra time is required for pre-scanning, the resolution of diffusion weighted imaging can be significantly improved, the distortion rate of imaging can be reduced, and insensitivity to mismatch between the reacquired navigator echo and the imaging echo can be achieved.
In the invention, when acquiring and re-acquiring navigator echoes, a central part is acquired in both the phase encoding and frequency encoding directions to reduce the time interval between adjacent phase encoding echoes to control image distortion, and parity echoes respectively form a complete k-space data. Based on this characteristic, no mismatch term for the parity echo will be contained in the full sampling space of the re-navigation data, so no additional pre-scan will be required to correct for the re-navigation data, increasing the artifact removal effect as a whole. After the artifact-free re-navigation data are obtained, errors introduced in reconstruction of image distortion of the re-navigation data can be more stably and effectively avoided through an imaging method based on a low-rank theory, and the actually sampled re-navigation data comprise echo data with two polarities, so that odd-even echo errors in imaging echoes and phase errors caused by motion can be corrected simultaneously.
In summary, the planar echo diffusion weighted imaging method and the planar echo diffusion weighted imaging system have the advantages of high scanning imaging efficiency, better artifact removal effect due to insensitivity to image distortion of navigator echo, and the like.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present disclosure describes embodiments, not every embodiment is provided with a separate embodiment, and that this description is provided for clarity only, and that the disclosure is not limited to the embodiments described in detail below, and that the embodiments described in the examples may be combined as appropriate to form other embodiments that will be apparent to those skilled in the art.
Claims (10)
1. A planar echo diffusion weighted imaging method based on re-acquisition navigator echo is characterized by comprising the following steps:
step 1: first acquisition of first imaging echo data d of a magnetic resonance apparatus img Acquiring the echo data d of the second imaging nav ；
Step 2: for the first imaging echo data d img And second imaging echo data d nav Proceeding withProcessing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
Step 3: for first imaging echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
Step 4: for the acquired imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
2. The method of claim 1, wherein in step 1, the first imaging echo data d is acquired img A 180-degree pulse structure is added to the first magnetic resonance spin echo EPI/DWI sequence to acquire the acquired second imaging echo data d nav A second magnetic resonance spin echo EPI/DWI sequence.
3. The method of claim 1, wherein in step 2, the second imaging echo data d is obtained by nav Performing zero padding operation to obtain second imaging echo data d nav Dimension W of (2) nav Extend to and primary imaging echo data d img Dimension W of (2) img Likewise, second imaging echo k-space data D is obtained nav 。
4. The method of claim 1, wherein in step 2, the first imaging echo data d is obtained by img Is divided according to the parity echo type, the separated and removed data position is processed by zero padding operation to obtain first imaging echo k-space data D img 。
5. A according to claim 1The planar echo diffusion weighted imaging method based on the re-acquisition navigator echo is characterized in that in the step 3, positive operation matrix H is adopted to perform first imaging on echo k-space data D img And second imaging echo k-space data D nav The matrix is constructed to obtain a matrix H (D img ) Sum matrix H (D nav )。
6. The method according to claim 5, wherein in the step 3, the matrix H (D nav ) Singular value decomposition is performed to obtain a matrix H (D nav ) Is a signal subspace V of (2) SPs And noise subspace V SPn 。
7. The method according to claim 1, wherein in the step 4, the process of data synthesis includes fourier transform, phase analysis and complex summation.
8. A system of the re-acquisition navigator echo-based planar echo diffusion weighted imaging method according to claims 1 to 7, comprising an imaging echo data acquisition module, a re-acquisition navigator echo acquisition module, a k-space data construction module, a low-rank iteration module and a k-space data synthesis module;
the imaging echo data acquisition module is used for acquiring first imaging echo data d of the magnetic resonance equipment img And echo data d of the first imaging img Transmitting to the k-space data construction module;
the re-acquisition navigator echo acquisition module adds a 180-degree pulse on the first magnetic resonance spin echo EPI/DWI sequence of the imaging echo data acquisition module to serve as a second magnetic resonance spin echo EPI/DWI sequence, and re-acquires second imaging echo data d nav And imaging echo data d for the second time nav Transmitting to the k-space data construction module;
the k-space data construction moduleFor the first imaging echo data d img And second imaging echo data d nav Processing to obtain k-space data D of first imaging echo with completely consistent dimension img And second imaging echo k-space data D nav ；
The low-rank iteration module is used for carrying out first imaging on echo k-space data D img And second imaging echo k-space data D nav Performing matrix construction and iteration to obtain imaging echo k-space data D poc ；
The k-space data synthesis module is used for obtaining imaging echo k-space data D poc Data synthesis processing is carried out to obtain final image data I m 。
9. An electronic device comprising at least one processor and a memory communicatively coupled to the at least one processor; wherein the memory stores instructions executable by the at least one processor to enable the at least one processor to perform the re-acquired navigator echo based planar echo diffusion weighted imaging method of any one of claims 1 to 7.
10. A computer readable storage medium storing a computer program, wherein the computer program when executed by a processor implements the re-acquired navigator echo based planar echo diffusion weighted imaging method of any one of claims 1 to 7.
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