CN116973823B - Line scanning magnetic resonance imaging method and system based on full steady-state balance precession - Google Patents
Line scanning magnetic resonance imaging method and system based on full steady-state balance precession Download PDFInfo
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Abstract
The application relates to a line scanning magnetic resonance imaging method and system based on full steady-state equilibrium precession, wherein the method comprises the following steps: scanning an object to be scanned based on the acquired gradient echo; in the scanning process, phase encoding is sequentially carried out on repeated periods of the gradient echo by utilizing line scanning, a group of phase encoding line elements corresponding to different moments in each period are collected, the phase encoding line elements are sequentially and transversely filled into phase encoding lines of each frame based on the number of frames of a K space of a magnetic resonance imaging sequence, so that magnetic resonance signals of an object to be scanned are obtained, and when the gradient echo of each period is finished, the residual transverse magnetization intensity is subjected to phase gathering based on refocusing pulses, so that residual magnetization signals are generated, and gradient echo signals of the next period are enhanced; finally, based on magnetic resonance signals, a magnetic resonance image of an object to be scanned is generated, and by adopting the method, magnetic resonance imaging with rapid periodic changes can be realized, and the signal-to-noise ratio and the image contrast of the acquired magnetic resonance image are improved.
Description
Technical Field
The present application relates to the field of magnetic resonance imaging sequences, and in particular, to a line scanning magnetic resonance imaging method and system based on full steady-state equilibrium precession.
Background
In conventional magnetic resonance imaging technology, good spatial resolution is often a great advantage of magnetic resonance imaging, and tissues and organs are precisely imaged clinically through the magnetic resonance imaging technology with high precision. However, since the magnetic resonance imaging process includes multiple phase direction codes and frequency direction codes, it is technically difficult to achieve rapid imaging to acquire images with high time resolution, and thus the need to rely on higher time resolution, such as for cardiac pacing imaging, diaphragm muscle imaging during uniform breathing, neural activity imaging, etc., is a challenge for magnetic resonance imaging techniques.
In the field of magnetic resonance rapid imaging, we generally employ variants of the sequences gradient echo imaging (GRE, gradient recalled echo), planar echo imaging (EPI, echo planar imaging), etc. to achieve higher imaging speeds. These sequences can produce smaller echoes and thus can reduce the time required for a single phase encoding to a greater extent. Meanwhile, by using unconventional K space filling technology, such as spiral sampling, radial sampling, blade sampling and other modes, the frequency of phase encoding is reduced. However, with these magnetic resonance imaging methods, under the condition of greatly sacrificing imaging quality, only the imaging speed of about 100ms per frame can be achieved, and still the purpose of multi-frame images in a shorter period of time cannot be achieved. For example, if it is desired to observe magnetic resonance signal changes over a neural activity period, then more than 10 scan imaging passes must be completed within 50ms (a common neural activity period). In the existing magnetic resonance imaging strategy for neural activity, a small flip angle gradient echo sequence FLASH (fast low-angle shot) is adopted, so that each phase encoding can be completed quickly, but the signal to noise ratio of the obtained magnetic resonance imaging is still low, and the imaging contrast is poor.
Disclosure of Invention
In view of the foregoing, it is desirable to provide a line scanning magnetic resonance imaging method and system based on a steady-state equilibrium precession that can acquire a magnetic resonance image with a high signal-to-noise ratio and a high contrast when performing magnetic resonance imaging.
In a first aspect, the present application provides a line scan magnetic resonance imaging method based on full steady state balanced precession, the method comprising:
scanning an object to be scanned based on the acquired gradient echo, wherein the gradient echo is generated by a magnetic resonance imaging sequence;
in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different times in each period along the period repetition direction of the gradient echo, and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame in sequence based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, carrying out phase focusing on the residual transverse magnetization based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period;
A magnetic resonance image of the object to be scanned is generated based on the magnetic resonance signals.
In one embodiment, the sequentially performing phase encoding on the repetition period of the gradient echo, and collecting a set of phase encoding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo includes:
sequentially performing phase encoding on the repetition period of the gradient echo based on the phase gradient pulse of the magnetic resonance imaging sequence in the phase encoding direction;
and acquiring a group of phase encoding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo.
In one embodiment, the generating the residual magnetization signal at the end of the gradient echo of each period based on a refocusing pulse of the residual transverse magnetization of the gradient echo comprises:
generating refocusing pulses of the residual transverse magnetization in the phase encoding direction of the magnetic resonance imaging sequence at the end of the gradient echo of each cycle, the refocusing pulses being equal in power and opposite in polarity to the phase gradient pulses of the sequence;
And based on the refocusing pulse, carrying out phase-focusing on the residual transverse magnetization to generate the residual magnetization signal.
In one embodiment, the generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals comprises:
acquiring magnetic resonance data of the object to be scanned based on the magnetic resonance signals;
generating a data matrix of the magnetic resonance image based on the magnetic resonance data;
reconstructing the magnetic resonance image using a discrete fourier transform based on the data matrix.
In one embodiment, the reconstructing the magnetic resonance image using a discrete fourier transform based on the data matrix comprises:
determining first information of a K space of the magnetic resonance imaging sequence based on the data matrix, wherein the first information comprises a dimensional length and a dimensional complex unit root of the K space;
and generating second information of an image domain corresponding to the data matrix by using the discrete Fourier transform based on the first information, and reconstructing the magnetic resonance image.
In one embodiment, the acquiring the gradient echo includes, before scanning the object to be scanned:
And generating a gradient echo of the magnetic resonance imaging sequence by utilizing a dephasing gradient pulse and a dephasing gradient pulse in the frequency coding direction of the magnetic resonance imaging sequence based on the excitation pulse of the magnetic resonance imaging sequence, wherein the dephasing gradient pulse and the dephasing gradient pulse have the same width, the same amplitude and opposite polarities.
In one embodiment, the generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals further comprises:
based on the magnetic resonance signals, a corresponding signal strength is calculated and based on the signal strength, an imaging contrast of the magnetic resonance image is determined.
In a second aspect, the present application also provides a line scan magnetic resonance imaging apparatus based on full steady state balanced precession, the apparatus comprising:
the gradient echo module is used for scanning the object to be scanned based on the acquired gradient echo, and the gradient echo is generated by a full steady-state equilibrium precession magnetic resonance imaging sequence;
the signal acquisition module is used for sequentially carrying out phase coding on the repetition period of the gradient echo in the scanning process, acquiring a group of phase coding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo, and sequentially transversely filling the phase coding line elements into the phase coding lines of the K space of each frame based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, the residual transverse magnetization is subjected to phase focusing based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period;
A magnetic resonance imaging module for generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals.
In a third aspect, the present application also provides a magnetic resonance imaging system comprising a scanning device for acquiring a magnetic resonance imaging sequence and a line scan magnetic resonance imaging device based on a steady-state balanced precession as described in the second aspect above.
In a fourth aspect, the present application also provides a computer readable storage medium having stored thereon a computer program which when executed by a processor implements the above-described aspects.
According to the line scanning magnetic resonance imaging method and system based on the full steady-state equilibrium precession, the object to be scanned is scanned through the gradient echo generated by the full steady-state equilibrium precession magnetic resonance imaging sequence; in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different times in each period along the period repetition direction of the gradient echo, and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame in sequence based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, carrying out phase focusing on the residual transverse magnetization based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period; based on the magnetic resonance signals, a magnetic resonance image of the object to be scanned is generated, so that magnetic resonance imaging with rapid periodic changes is realized, and the signal-to-noise ratio and the image contrast of the acquired magnetic resonance image are improved.
Drawings
FIG. 1 (a) is a block diagram of a magnetic resonance imaging system of a line scan magnetic resonance imaging method based on steady-state equilibrium precession in one embodiment;
FIG. 1 (b) is a schematic diagram of a line scan performed by the acquisition encoding module 103 in one embodiment;
FIG. 2 is a flow diagram of a line scan magnetic resonance imaging method based on steady-state equilibrium precession in one embodiment;
FIG. 3 is a flowchart illustrating the step S204 in one embodiment;
FIG. 4 is a flowchart illustrating the step S204 in another embodiment;
FIG. 5 is a flow chart of step S206 in one embodiment;
FIG. 6 is a flowchart illustrating the step S506 in one embodiment;
FIG. 7 is a schematic diagram of a TrueFISP-based line scan magnetic resonance imaging in an example embodiment;
FIG. 8 is a line graph of T2 contrast in an example embodiment;
fig. 9 is an internal structural diagram of a computer device in one embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the present application will be further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the present application.
Unless defined otherwise, technical or scientific terms used herein should be given the ordinary meaning as understood by one of ordinary skill in the art to which this application belongs. The terms "comprising," "including," "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, article, or apparatus that comprises a list of steps or modules (elements) is not limited to only those steps or elements but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus. The term "plurality" as used herein refers to two or more. "and/or" describes an association relationship of an association object, meaning that there may be three relationships, e.g., "a and/or B" may mean: a exists alone, A and B exist together, and B exists alone. The terms "first," "second," "third," and the like, as used herein, are merely distinguishing between similar objects and not representing a particular ordering of objects.
Line scan magnetic resonance imaging is an effective method of achieving high temporal resolution imaging in uniform periodic motion or variation of an object being imaged, for example in one-dimensional line scan magnetic resonance imaging, rapid imaging can be achieved by confining the field of view (FOV) to a one-dimensional region. While dynamic line scanning extends to multidimensional imaging, repeated motion or changes of the scanned object are induced in a reliable manner to allow multiple k-space lines to be collected at each motion phase (frame), thereby achieving an increase in total sampling time to achieve extremely high time resolution imaging in a small period while preserving imaging quality as much as possible. This technique can be used for cardiac imaging under drug controlled stable beating of the heart, trained respiratory motion imaging, even direct imaging of living local neural activity, etc.
However, in the line scan imaging, we still consider the imaging speed of single phase encoding, because even with reliable periodic motion, there is a small difference, and this difference causes a large imaging artifact in the line scan magnetic resonance imaging process. Meanwhile, because multiple phase encodings need to be acquired, the total scanning time length is relatively longer, and therefore, the faster single phase encoding imaging speed is required to reduce the total time length of magnetic resonance scanning, thereby reducing the discomfort and the scanning cost of a scanned object.
The line scanning magnetic resonance imaging method provided by the embodiment of the application can be applied to a magnetic resonance imaging system as shown in fig. 1 (a), wherein the system comprises a scanning device 11 and a line scanning magnetic resonance imaging device 12 for acquiring a magnetic resonance imaging sequence, and the system specifically comprises the following modules: a variable angle excitation module 101, a gradient echo module 102, an acquisition encoding module 103, a residual intensity refocusing module 104, and a magnetic resonance imaging module 105. The scanning device 11 for acquiring the magnetic resonance imaging sequence comprises the variable angle excitation module 101, the gradient echo module 102, the acquisition encoding module 103 and the residual intensity refocusing module 104; the line scan magnetic resonance imaging apparatus 12 includes a gradient echo module 102, a signal acquisition module 106, and a magnetic resonance imaging module 105, the signal acquisition module 106 being composed of an acquisition encoding module 103 and a residual intensity refocusing module 104.
The variable angle excitation module 101 is configured to generate a pulse signal with a small angle. Conventional spin echo imaging sequences typically use a 180 degree dump pulse to generate the echo, not supporting a small angle excitation pulse. While a small angle excitation pulse is advantageous for fast phase encoding. In the embodiment of the application, the echo is formed by means of gradient inversion and does not depend on the dumping pulse of 180 degrees, so that the gradient echo can be generated in the frequency coding direction of the magnetic resonance imaging sequence by utilizing the pulse signal of a small angle.
The gradient echo module 102 is configured to generate a dephasing gradient pulse in the frequency encoding direction of the magnetic resonance imaging sequence based on the excitation pulse generated by the variable angle excitation module 101 to dephas the magnetization of the hydrogen atom, and then generate a dephasing gradient pulse with the same width, the same amplitude and opposite polarity as the dephasing gradient pulse to dephas the magnetization of the hydrogen atom, thereby obtaining the gradient echo of the magnetic resonance imaging sequence.
The acquisition encoding module 103 is configured to, as shown in fig. 1 (b), sequentially perform phase encoding on the repetition period of the gradient echo in the scanning process, acquire a group of phase encoding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo, and sequentially fill the phase encoding line elements into the phase encoding lines of the K space of each frame based on the frame number of the K space of the magnetic resonance imaging sequence, so as to obtain the magnetic resonance signal of the object to be scanned. Wherein Kx and Ky respectively represent an x-layer selection direction and a y-layer selection direction of the sequence, 1 and 2 are the frame numbers corresponding to different acquisition moments in a gradient echo period, a group of phase coding line elements acquired in a first period are sequentially filled in a first row from a 1 st frame to an n-th frame, a group of phase coding line elements acquired in a second gradient echo period are sequentially filled in a second row from the 1 st frame to the n-th frame, and the steps are repeatedly executed until the K space is completely filled.
A residual intensity refocusing module 104, configured to generate a refocusing pulse of a residual transverse magnetization of the gradient echo at the end of the gradient echo in each period, perform phase-focusing on the residual transverse magnetization, and generate a residual magnetization signal, where the residual magnetization signal is used to perform signal enhancement on the gradient echo in the next period. Wherein the refocusing pulse is the same width, the same amplitude but opposite polarity as the excitation pulse.
A magnetic resonance imaging module 105 for reconstructing a magnetic resonance image using a fourier transform based on magnetic resonance signals generated during a scan of the magnetic resonance imaging sequence.
The various modules in the magnetic resonance imaging system described above may be implemented in whole or in part by software, hardware, and combinations thereof. The above modules may be embedded in hardware or may be independent of a processor in the computer device, or may be stored in software in a memory in the computer device, so that the processor may call and execute operations corresponding to the above modules.
The embodiment of the application provides a line scanning magnetic resonance imaging method based on full steady-state equilibrium precession, as shown in fig. 2, the method is applied to a magnetic resonance system as shown in fig. 1 (a), and specifically comprises the following steps:
S202, scanning an object to be scanned based on the acquired gradient echo.
Wherein the gradient echo is generated by a full steady state equilibrium precession magnetic resonance imaging sequence. The full steady state balanced precession magnetic resonance imaging sequence is in particular a full steady state balanced precession (TrueFISP) based line scan imaging sequence.
In one embodiment, S202 further includes the following steps before scanning the object to be scanned based on the acquired gradient echo:
s201, generating a gradient echo of the magnetic resonance imaging sequence by using an dephasing gradient pulse and a dephasing gradient pulse in a frequency encoding direction of the magnetic resonance imaging sequence based on an excitation pulse of the magnetic resonance imaging sequence.
The dephasing gradient pulse and the phase accumulating gradient pulse have the same width, the same amplitude and opposite polarities.
Specifically, based on the small-angle excitation pulse, a dephasing gradient pulse is firstly generated in the frequency coding direction of the magnetic resonance imaging sequence to make the magnetization intensity of the hydrogen atomic nucleus dephased, and then a dephasing gradient pulse with the same width, the same amplitude and opposite polarity as the dephasing gradient pulse is generated to make the magnetization intensity of the hydrogen atomic nucleus dephased, so that the gradient echo of the magnetic resonance imaging sequence is obtained. The magnetic dipole moment of the nuclei therefore undergoes a phase-first-phase-second-phase-convergence process, the divergent phases being completely compensated for to form echo peaks at the instant t=te (echo interval).
S204, in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo, and sequentially and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame based on the frame number of the K space of the magnetic resonance imaging sequence to obtain the magnetic resonance signal of the object to be scanned.
And at the end of the gradient echo of each period, carrying out phase aggregation on the residual transverse magnetization intensity based on a refocusing pulse of the residual transverse magnetization intensity of the gradient echo to generate a residual magnetization signal, wherein the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period.
Specifically, the number of frames of the K space is determined by the resolution of the magnetic resonance image and the imaging field of view, and different moments of phase encoding acquisition in one gradient echo period are determined based on the resolution of the magnetic resonance image and the imaging field of view, and each moment corresponds to one frame.
In one embodiment, as shown in fig. 3, in S204, the repetition period of the gradient echo is sequentially phase-coded, and a group of phase-coded line elements corresponding to different moments in each period are acquired along the period repetition direction of the gradient echo, which specifically includes the following steps:
S302, carrying out phase encoding on the repetition period of the gradient echo in sequence based on the phase gradient pulse of the magnetic resonance imaging sequence in the phase encoding direction.
S304, a group of phase coding line elements corresponding to different moments in each period are acquired along the period repetition direction of the gradient echo.
Specifically, based on gradient echo, phase gradient pulses are synchronously generated in the phase coding direction of a magnetic resonance imaging sequence, the repeated period of the gradient echo is subjected to phase coding, and simultaneously, a group of phase coding line elements corresponding to different moments in each period are acquired along the period repeated direction of the gradient echo to fill K space.
Specifically, each gradient echo period of the conventional gradient echo imaging represents one frame of the acquired magnetic resonance image, so that gradient echo needs to be continuously performed to fill a complete K space, and a conventional gradient echo scanning mode is adopted, so that the scanning time of the magnetic resonance imaging is longer. The line scanning magnetic resonance imaging is an effective method for realizing high-time resolution imaging in uniform periodic motion or change of an imaged object, phase codes at different moments of a first gradient echo are acquired in repeated scanning periods of the gradient echo to obtain first line phase code line elements, the first line phase code line elements are transversely filled into first lines of phase codes of all frames in sequence based on the frame number of a K space and the different acquisition moments of one gradient echo, and the second gradient echo is operated in the same way as the first gradient echo, so the operation is repeated until the phase code lines of all frames of the K space are filled.
In one embodiment, as shown in fig. 4, S204, at the end of the gradient echo of each period, performs phase-focusing on the residual transverse magnetization of the gradient echo based on a refocusing pulse of the residual transverse magnetization, and generates a residual magnetization signal, and specifically includes the following steps:
s402, at the end of the gradient echo of each cycle, generating a refocusing pulse of the residual transverse magnetization in the phase encoding direction of the magnetic resonance imaging sequence.
S404, carrying out phase aggregation on the residual transverse magnetization intensity based on the refocusing pulse to generate the residual magnetization signal.
In particular, in conventional small flip angle gradient echo imaging sequences (FLASH), the residual transverse magnetization remaining after each gradient echo has ended is treated by applying a gradient pulse in the selected layer direction to destroy the transverse magnetization that has not decayed to zero. In the application, the residual transverse magnetization intensity is subjected to phase aggregation by using the refocusing pulse, so that a unified phase coherent residual magnetization signal is reestablished, and the gradient echo of the next period is subjected to signal enhancement, so that the obtained magnetic resonance signal has a higher signal-to-noise ratio after the periodic gradient echo is completed.
S206, generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals.
Specifically, the magnetic resonance imaging sequence acquires phase encoding line elements of the K space in a line scanning mode, and adds a refocusing pulse to the phase encoding direction to carry out signal enhancement on the gradient echo of the next period when each repetition period of the gradient echo is finished, so that the acquired complete magnetic resonance signal has higher signal-to-noise ratio and time resolution, and a magnetic resonance image with high signal-to-noise ratio, high time resolution and high imaging contrast is generated based on the magnetic resonance signal.
In the line scanning magnetic resonance imaging method based on the full steady-state equilibrium precession, the object to be scanned is scanned through the gradient echo generated by the full steady-state equilibrium precession magnetic resonance imaging sequence; in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different times in each period along the period repetition direction of the gradient echo, and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame in sequence based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, carrying out phase focusing on the residual transverse magnetization based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period; based on the magnetic resonance signals, a magnetic resonance image of the object to be scanned is generated, so that magnetic resonance imaging with rapid periodic changes is realized, and the signal-to-noise ratio and the image contrast of the acquired magnetic resonance image are improved.
In one embodiment, as shown in fig. 5, S206 generates a magnetic resonance image of the object to be scanned based on the magnetic resonance signals, and specifically includes the following steps:
s502, acquiring magnetic resonance data of the object to be scanned based on the magnetic resonance signals.
S504, generating a data matrix of the magnetic resonance image based on the magnetic resonance data.
S506, reconstructing the magnetic resonance image by using discrete Fourier transform based on the data matrix.
Specifically, after obtaining a magnetic resonance signal based on the method, extracting acquired magnetic resonance data from a magnetic resonance imaging system, obtaining N corresponding complete cycle frames (N represents the frame number corresponding to different moments of phase encoding acquisition in a cycle change) according to a line scanning filling mode of a K space, generating a data matrix of a magnetic resonance image based on the N complete cycle frames, and then reconstructing by using discrete Fourier change to obtain the magnetic resonance image.
In one embodiment, as shown in fig. 6, S506 reconstructs the magnetic resonance image by using discrete fourier transform based on the data matrix, and specifically includes the following steps:
s602, determining first information of K space of the magnetic resonance imaging sequence based on the data matrix.
Wherein the first information includes a dimension length and a dimension complex unit root of the K space.
And S604, generating second information of an image domain corresponding to the data matrix by utilizing the discrete Fourier transform based on the first information, and reconstructing the magnetic resonance image.
Specifically, based on the dimensional length and the dimensional complex unit root of the K space in the data matrix, discrete Fourier transform is carried out on the data matrix, and the data matrix is converted into image domain information, so that the magnetic resonance image is obtained.
Illustratively, the formula for performing a discrete fourier transform on the data matrix K of the two-dimensional K space is as follows:
wherein,information representing a kth dimension in the image domain; />Information representing a kth dimension in K space; />Representing the first dimension length, +.>Representing a second dimension length; />Representing the complex unit root of the first dimension,representing a second dimension complex unit root, where i is an imaginary unit.
In one embodiment, in step S204, the method further comprises: based on the magnetic resonance signals, a corresponding signal strength is calculated and based on the signal strength, an imaging contrast of the magnetic resonance image is determined.
In particular, the ion metabolism by neural activity mainly causes weak changes in T2 contrast in magnetic resonance imaging. In a scene in which magnetic resonance scanning is achieved using gradient echo, however, it is generally only possible to obtain Contrast ratio, using bloch principle, signal intensity calculation is carried out on magnetic resonance signals generated by magnetic resonance imaging by using flip angle gradient echo sequence FLASH combined line scanning in the prior art, and the formula is as follows:
wherein,is the magnetic resonance signal strength, beta isThe flip angle of the excitation pulse, TR, is the repetition of the excitation cycle time, TE is the echo time of the gradient echo, T1 is the longitudinal relaxation time of the gradient echo, +.>Is the transverse relaxation time of the gradient echo, NH]Is the hydrogen atom density.
It follows that the echo time TE controls the transverse relaxation time when imaging using a FLASH sequence in combination with line scanningContrast, when echo time TE is large, +.>The contrast is dominant; the flip angle beta and the repetition excitation period time TR control the T1 contrast, wherein a small flip angle increases the proton density term and +.>Contrast, a large flip angle increases the T1 contrast, decreasing the repetitive firing cycle time TR increases the T1 contrast. But->Contrast is not only related to the T2 contrast but also to the homogeneity of the magnetic field strength, the more inhomogeneous the magnetic field the faster the transverse relaxation is +.>The smaller the contrast value. Therefore, due to the influence of the magnetic field inhomogeneity, there will be +.>The response of the contrast to the T2 contrast is much weaker, resulting in poor imaging contrast of the magnetic resonance image.
Specifically, the signal intensity calculation is performed on the magnetic resonance signals obtained by the line scanning magnetic resonance imaging method in the embodiment of the application by using the bloch principle, and the formula is as follows:
when the echo time TE of the gradient echo is equal to half the repetitive excitation period time TR (i.e. when te=tr/2),
where S is the magnetic resonance signal strength, α is the flip angle of the excitation pulse, TE is the echo time of the gradient echo, T1 is the longitudinal relaxation time of the gradient echo, T2 is the transverse relaxation time of the gradient echo, N [ H ]]Is the hydrogen atom density. Since the refocusing pulse refocuses the residual transverse magnetization, the transverse relaxation time depends on T2. While normally the echo time TE tends to be much smaller than the transverse relaxation time T2 (i.e. TE<<T2), at which point-TE/T2 approaches 0,near 1, the magnetic resonance signal strength S is directly and positively correlated with T2/T1, and the larger the T2/T1 is, the smaller the T1/T2 is, the larger the magnetic resonance signal strength S is, and the higher the signal-to-noise ratio is.
In this embodiment, it is known through calculation of the magnetic resonance signal intensity that the line scanning magnetic resonance imaging method of the present application can obtain a high-intensity magnetic resonance signal, the magnetic resonance signal intensity is directly related to the T2 contrast, and the signal intensity is avoided from the following Imaging contrast decreases due to magnetic field inhomogeneity during contrast transition to T2 contrast.
In one example embodiment, as shown in fig. 7, a TrueFISP (full steady state equilibrium precession sequence) based line scan magnetic resonance imaging method is provided for magnetic resonance imaging of 4 bottles of agarose gel molds (phantoms) of different concentrations.
Step 1: before line scanning magnetic resonance imaging of a full steady-state equilibrium precession sequence TrueFISP, a T2-map sequence is used for scanning T2 values of 4 agarose gel water modes with different concentrations in a magnetic resonance imaging system, T2 contrast values of 4 water modes are obtained through calculation, and the T2 contrast values of the 4 water modes are respectively 170, 220, 670 and 1130. Wherein, the T2-map sequence is a conventional sequence for calculating the contrast value of T2 by adopting a roll-over fitting method.
Step 2: setting optimal parameters of a full steady-state equilibrium precession sequence TrueFISP, wherein the echo time TE of a gradient echo is 1.93ms, the repeated excitation period time TR is 3.86ms, the resolution of a magnetic resonance image is 256×180, the FOV (field of view) is 256×180, the flip angle alpha=28° of an excitation pulse is 1180hz/pixel.
Step 3: and scanning the agarose gel molds with different concentrations of 4 bottles based on gradient echoes generated by the full steady-state equilibrium precession sequence TrueFISP. In the scanning process, the phase encoding is sequentially carried out on the repetition period of the gradient echo based on the phase gradient pulse in the phase encoding direction of the full steady-state balanced precession sequence, a group of phase encoding line elements corresponding to different moments in each period are acquired along the period repetition direction of the gradient echo, and the phase encoding line elements are sequentially and transversely filled into the phase encoding lines of the K space of each frame based on the frame number of the K space of the full steady-state balanced precession sequence, so that the magnetic resonance signals of the agarose gel water modes with different concentrations of 4 bottles are obtained.
And generating refocusing pulses with the same power as the phase gradient pulse and opposite polarity in the phase encoding direction of the full steady-state equilibrium precession sequence at the end of the gradient echo of each period, carrying out phase focusing on the residual transverse magnetization intensity, generating a residual magnetization signal, and carrying out signal enhancement on the gradient echo of the next period.
Step 4: based on the acquired magnetic resonance signals of the agarose gel molds with different concentrations of 4 bottles, acquiring corresponding magnetic resonance data to perform discrete Fourier transform, and reconstructing to obtain magnetic resonance images of the agarose gel molds with different concentrations of 4 bottles.
The process of steps 1 to 4 is shown in fig. 7.
In addition, in order to obviously show that the magnetic resonance image obtained by the method has high signal-to-noise ratio and high imaging contrast, the contrast test is carried out on the same 4 bottles of agarose gel molds with different concentrations by combining a small flip angle gradient echo sequence FLASH with line scanning magnetic resonance imaging in the prior art, and the method specifically comprises the following steps:
step 5: calculating and setting optimal parameters of a small flip angle gradient echo sequence FLASH, including: the echo time TE of the gradient echo is 2ms, the repetitive excitation cycle time TR is 4ms, the magnetic resonance image resolution is 256×180, the imaging field of view FOV is 256×180, the flip angle α=15° of the excitation pulse, and the bandwidth is 1200hz/pixel.
Step 6: based on the small flip angle gradient echo sequence FLASH combined with line scanning magnetic resonance imaging, magnetic resonance signals of 4 bottles of agarose gel molds with different concentrations are obtained, corresponding magnetic resonance data are obtained to carry out discrete Fourier transform, and the magnetic resonance images of the 4 bottles of agarose gel molds with different concentrations are obtained through reconstruction.
And (3) comparing the signal to noise ratio of the magnetic resonance image of the agarose gel water model obtained in the step (4) with the magnetic resonance image of the agarose gel water model obtained in the step (6). The calculating method is the ratio of the effective area average signal to the square of the background noise average signal, and the obtained calculating result is as follows: the imaging signal-to-noise ratio of the magnetic resonance image of the agarose gel water model obtained in the step 4 is 110, and the imaging signal-to-noise ratio of the magnetic resonance image of the agarose gel water model obtained in the step 6 is 40. According to the calculation result, the magnetic resonance image obtained by adopting the line scanning magnetic resonance imaging method based on the TrueFISP (full steady-state equilibrium precession sequence) has higher signal-to-noise ratio.
And (3) comparing the magnetic resonance image of the agarose gel water mold obtained in the step (4) with the T2 contrast of the magnetic resonance image of the agarose gel water mold obtained in the step (6). The imaging signals of agarose gel water modes with different concentrations are required to be analyzed, and the signal differences of magnetic resonance signals generated by different sequences on the agarose gel water modes with different concentrations are compared to obtain the response sensitivity of the imaging sequences to the T2 contrast ratio. As shown in fig. 8, the lower broken line is the T2 response of the magnetic resonance signal of the small flip angle gradient echo sequence FLASH combined with line scan magnetic resonance imaging in the prior art, and the abscissa indicates that the actual 4 water mode T2 contrast values calculated in step 1 include 170, 220, 670, 1130. The upper fold is the T2 response of the magnetic resonance signals of the line scan magnetic resonance imaging of the full steady state balanced precession sequence TrueFISP in this embodiment. Obviously, the imaging signal change of the T2 contrast ratio of agarose gel molds with different concentrations is larger by adopting the method of the embodiment, which shows that the method of the embodiment has better capability of detecting the T2 signal change compared with the prior method.
It should be understood that, although the steps in the flowcharts related to the embodiments described above are sequentially shown as indicated by arrows, these steps are not necessarily sequentially performed in the order indicated by the arrows. The steps are not strictly limited to the order of execution unless explicitly recited herein, and the steps may be executed in other orders. Moreover, at least some of the steps in the flowcharts described in the above embodiments may include a plurality of steps or a plurality of stages, which are not necessarily performed at the same time, but may be performed at different times, and the order of the steps or stages is not necessarily performed sequentially, but may be performed alternately or alternately with at least some of the other steps or stages.
In one embodiment, a computer device is provided, which may be a terminal, and the internal structure thereof may be as shown in fig. 9. The computer device includes a processor, a memory, an input/output interface, a communication interface, a display unit, and an input means. The processor, the memory and the input/output interface are connected through a system bus, and the communication interface, the display unit and the input device are connected to the system bus through the input/output interface. Wherein the processor of the computer device is configured to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and computer programs in the non-volatile storage media. The input/output interface of the computer device is used to exchange information between the processor and the external device. The communication interface of the computer device is used for carrying out wired or wireless communication with an external terminal, and the wireless mode can be realized through WIFI, a mobile cellular network, NFC (near field communication) or other technologies. The computer program is executed by a processor to implement a line scan magnetic resonance imaging method based on a full steady state equilibrium precession. The display unit of the computer device is used for forming a visual picture, and can be a display screen, a projection device or a virtual reality imaging device. The display screen can be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer equipment can be a touch layer covered on the display screen, can also be a key, a track ball or a touch pad arranged on the shell of the computer equipment, and can also be an external keyboard, a touch pad or a mouse and the like.
It will be appreciated by those skilled in the art that the structure shown in fig. 9 is merely a block diagram of a portion of the structure associated with the present application and is not limiting of the computer device to which the present application applies, and that a particular computer device may include more or fewer components than shown, or may combine some of the components, or have a different arrangement of components.
In one embodiment, a computer device is provided that includes a memory having a computer program stored therein and a processor that implements the foregoing embodiments when the computer program is executed.
In one embodiment, a computer readable storage medium is provided having a computer program stored thereon, which when executed by a processor, performs the steps of:
scanning an object to be scanned based on the acquired gradient echo, wherein the gradient echo is generated by a full steady-state equilibrium precession magnetic resonance imaging sequence;
in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different times in each period along the period repetition direction of the gradient echo, and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame in sequence based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, carrying out phase focusing on the residual transverse magnetization based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period;
A magnetic resonance image of the object to be scanned is generated based on the magnetic resonance signals.
In one embodiment, the computer program when executed by the processor further performs the steps of:
sequentially performing phase encoding on the repetition period of the gradient echo based on the phase gradient pulse of the magnetic resonance imaging sequence in the phase encoding direction; and acquiring a group of phase encoding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo.
In one embodiment, the computer program when executed by the processor further performs the steps of:
generating refocusing pulses of the residual transverse magnetization in the phase encoding direction of the magnetic resonance imaging sequence at the end of the gradient echo of each cycle, the refocusing pulses being equal in power and opposite in polarity to the phase gradient pulses of the sequence; and based on the refocusing pulse, carrying out phase-focusing on the residual transverse magnetization to generate the residual magnetization signal.
In one embodiment, the computer program when executed by the processor further performs the steps of:
acquiring magnetic resonance data of the object to be scanned based on the magnetic resonance signals; generating a data matrix of the magnetic resonance image based on the magnetic resonance data; reconstructing the magnetic resonance image using a discrete fourier transform based on the data matrix.
In one embodiment, the computer program when executed by the processor further performs the steps of:
determining first information of a K space of the magnetic resonance imaging sequence based on the data matrix, wherein the first information comprises a dimensional length and a dimensional complex unit root of the K space; and generating second information of an image domain corresponding to the data matrix by using the discrete Fourier transform based on the first information, and reconstructing the magnetic resonance image.
In one embodiment, the computer program when executed by the processor further performs the steps of:
and generating a gradient echo of the magnetic resonance imaging sequence by utilizing a dephasing gradient pulse and a dephasing gradient pulse in the frequency coding direction of the magnetic resonance imaging sequence based on the excitation pulse of the magnetic resonance imaging sequence, wherein the dephasing gradient pulse and the dephasing gradient pulse have the same width, the same amplitude and opposite polarities.
In one embodiment, the computer program when executed by the processor further performs the steps of:
based on the magnetic resonance signals, a corresponding signal strength is calculated and based on the signal strength, an imaging contrast of the magnetic resonance image is determined.
Those skilled in the art will appreciate that implementing all or part of the above described methods may be accomplished by way of a computer program stored on a non-transitory computer readable storage medium, which when executed, may comprise the steps of the embodiments of the methods described above. Any reference to memory, database, or other medium used in the various embodiments provided herein may include at least one of non-volatile and volatile memory. The nonvolatile Memory may include Read-Only Memory (ROM), magnetic tape, floppy disk, flash Memory, optical Memory, high density embedded nonvolatile Memory, resistive random access Memory (ReRAM), magnetic random access Memory (Magnetoresistive Random Access Memory, MRAM), ferroelectric Memory (Ferroelectric Random Access Memory, FRAM), phase change Memory (Phase Change Memory, PCM), graphene Memory, and the like. Volatile memory can include random access memory (Random Access Memory, RAM) or external cache memory, and the like. By way of illustration, and not limitation, RAM can be in the form of a variety of forms, such as Static Random access memory (Static Random access memory AccessMemory, SRAM) or dynamic Random access memory (Dynamic Random Access Memory, DRAM), and the like. The databases referred to in the various embodiments provided herein may include at least one of relational databases and non-relational databases. The non-relational database may include, but is not limited to, a blockchain-based distributed database, and the like. The processors referred to in the embodiments provided herein may be general purpose processors, central processing units, graphics processors, digital signal processors, programmable logic units, quantum computing-based data processing logic units, etc., without being limited thereto.
The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The above examples only represent a few embodiments of the present application, which are described in more detail and are not to be construed as limiting the scope of the present application. It should be noted that it would be apparent to those skilled in the art that various modifications and improvements could be made without departing from the spirit of the present application, which would be within the scope of the present application. Accordingly, the scope of protection of the present application shall be subject to the appended claims.
Claims (10)
1. A line scan magnetic resonance imaging method based on full steady state balanced precession, the method comprising:
scanning an object to be scanned based on the acquired gradient echo, wherein the gradient echo is generated by a full steady-state equilibrium precession magnetic resonance imaging sequence;
in the scanning process, sequentially carrying out phase coding on the repetition period of the gradient echo, collecting a group of phase coding line elements corresponding to different times in each period along the period repetition direction of the gradient echo, and transversely filling the phase coding line elements into the phase coding lines of the K space of each frame in sequence based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, carrying out phase focusing on the residual transverse magnetization based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period;
A magnetic resonance image of the object to be scanned is generated based on the magnetic resonance signals.
2. The line scan magnetic resonance imaging method based on the steady-state equilibrium precession of claim 1, wherein the sequentially phase encoding the repetition period of the gradient echo, and acquiring a set of phase encoded line elements corresponding to different moments in each period along the period repetition direction of the gradient echo comprises:
sequentially performing phase encoding on the repetition period of the gradient echo based on the phase gradient pulse of the magnetic resonance imaging sequence in the phase encoding direction;
and acquiring a group of phase encoding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo.
3. The line scan magnetic resonance imaging method based on the steady-state equilibrium precession of claim 1, wherein the generating a residual magnetization signal at the end of the gradient echo for each period based on a refocusing pulse of the residual transverse magnetization of the gradient echo comprises:
generating refocusing pulses of the residual transverse magnetization in the phase encoding direction of the magnetic resonance imaging sequence at the end of the gradient echo of each cycle, the refocusing pulses being equal in power and opposite in polarity to the phase gradient pulses of the sequence;
And based on the refocusing pulse, carrying out phase-focusing on the residual transverse magnetization to generate the residual magnetization signal.
4. The line scan magnetic resonance imaging method based on the steady-state balanced precession of claim 1, wherein the generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals comprises:
acquiring magnetic resonance data of the object to be scanned based on the magnetic resonance signals;
generating a data matrix of the magnetic resonance image based on the magnetic resonance data;
reconstructing the magnetic resonance image using a discrete fourier transform based on the data matrix.
5. The method of line scan magnetic resonance imaging based on full steady state balanced precession of claim 4, wherein the reconstructing the magnetic resonance image using a discrete fourier transform based on the data matrix comprises:
determining first information of a K space of the magnetic resonance imaging sequence based on the data matrix, wherein the first information comprises a dimensional length and a dimensional complex unit root of the K space;
and generating second information of an image domain corresponding to the data matrix by using the discrete Fourier transform based on the first information, and reconstructing the magnetic resonance image.
6. The line scan magnetic resonance imaging method based on the steady-state equilibrium precession of claim 1, wherein the acquiring gradient echoes, prior to scanning the object to be scanned, comprises:
and generating a gradient echo of the magnetic resonance imaging sequence by utilizing a dephasing gradient pulse and a dephasing gradient pulse in the frequency coding direction of the magnetic resonance imaging sequence based on the excitation pulse of the magnetic resonance imaging sequence, wherein the dephasing gradient pulse and the dephasing gradient pulse have the same width, the same amplitude and opposite polarities.
7. The line scan magnetic resonance imaging method based on the steady-state equilibrium precession of claim 1, further comprising, after generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals:
based on the magnetic resonance signals, a corresponding signal strength is calculated and based on the signal strength, an imaging contrast of the magnetic resonance image is determined.
8. A line scan magnetic resonance imaging apparatus based on a full steady state balanced precession, the apparatus comprising:
the gradient echo module is used for scanning the object to be scanned based on the acquired gradient echo, and the gradient echo is generated by a full steady-state equilibrium precession magnetic resonance imaging sequence;
The signal acquisition module is used for sequentially carrying out phase coding on the repetition period of the gradient echo in the scanning process, acquiring a group of phase coding line elements corresponding to different moments in each period along the period repetition direction of the gradient echo, and sequentially transversely filling the phase coding line elements into the phase coding lines of the K space of each frame based on the frame number of the K space of the magnetic resonance imaging sequence to obtain a magnetic resonance signal of an object to be scanned, wherein when the gradient echo of each period is finished, the residual transverse magnetization is subjected to phase focusing based on refocusing pulses of the residual transverse magnetization of the gradient echo to generate a residual magnetization signal, and the residual magnetization signal is used for carrying out signal enhancement on the gradient echo of the next period;
a magnetic resonance imaging module for generating a magnetic resonance image of the object to be scanned based on the magnetic resonance signals.
9. A magnetic resonance imaging system, characterized in that the system comprises scanning means for acquiring a magnetic resonance imaging sequence and a line scanning magnetic resonance imaging device based on a steady-state equilibrium precession as claimed in claim 8.
10. A computer readable storage medium, on which a computer program is stored, characterized in that the computer program, when being executed by a processor, implements the steps of the method of any one of claims 1 to 7.
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