3D navigator-based 3D gradient spin echo diffusion imaging method and apparatus

Abstract

The application discloses a 3D gradient spin echo diffusion imaging method and device based on a 3D navigator. The method first destroys previous residual transverse magnetization in a global saturation module; secondly, a pair of trapezoidal cosine oscillation gradients or pulse gradients are embedded into 90° x -180° y -90° ‑x RF pulses to separate diffusion encoding from signal acquisition; then, a fat saturation module is used to suppress fat signals; then, a 3D gradient spin echo readout mode is used to acquire image signals; finally, 3D navigator echo signals are acquired in the last spin echo to correct phase errors between multiple excitations. The application can effectively eliminate phase errors between multi-section acquisitions and realize high-resolution diffusion imaging of the whole brain. Compared with 2D planar echo imaging, the application improves the signal-to-noise ratio of diffusion magnetic resonance imaging, thereby supporting higher-resolution whole brain 3D diffusion magnetic resonance imaging.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of magnetic resonance technology, in particular to the field of diffusion magnetic resonance imaging. BACKGROUND

[0002] Diffusion MRI (dMRI) can detect the microstructure of biological tissue according to the characteristics of the restricted diffusion of water molecules in the microenvironment of the tissue. 3D MRI pulse sequences have high signal-to-noise ratio (SNR) efficiency in high-resolution acquisition, especially when using 3D k-space encoding to define very thin slices. Two-dimensional multi-slice acquisition often has an undesirable slice resolution and a low signal-to-noise ratio. Therefore, 3D acquisition is an ideal choice for high-resolution dMRI, which can reduce the signal-to-noise ratio loss caused by diffusion weighting and capture the microstructure of the tissue in continuous 3D space.

[0003] However, due to the lengthy phase encoding process of 3D imaging, whole-brain acquisition often needs to be completed in segments. In 3D-dMRI, additional eddy currents and motion introduce severe phase errors between signals in each segment in the segmented acquisition, resulting in image motion artifacts and distortion, and therefore the technology faces challenges in clinical applications in humans. In segmented acquisition, adding navigator echoes is an effective method to correct the phase errors between signals in each segment. However, previous studies have been largely limited to 1D or 2D navigators, which track the phase of the signal along the readout direction, without or with phase encoding in only one direction. Therefore, for diffusion-weighted 3D gradient-spin echo sequences (DW-GRASE), how to track the phase changes of image signals in the entire 3D k-space and improve the accuracy of inter-segment phase error correction is a technical problem that needs to be solved at present. SUMMARY

[0004] In order to realize whole-brain 3D high-resolution diffusion imaging, the present application proposes a 3D gradient-spin echo diffusion imaging sequence (3D DW-GRASE) based on a 3D navigator to solve the problems of the dMRI sequence mentioned above and improve the clinical practicability of 3D diffusion imaging.

[0005] In order to achieve the above purpose, the present application adopts the following technical solutions:

[0006] In a first aspect, the present application provides a 3D gradient-spin echo diffusion imaging method based on a 3D navigator, which comprises:

[0007] S1: segmentally acquiring whole-brain signals by repeatedly executing a 3D gradient-spin echo diffusion imaging sequence based on a 3D navigator, obtaining a 3D GRASE readout signal segment and a 3D navigator echo signal segment after each execution of the imaging sequence, and constructing whole-brain 3D K-space data from all 3D GRASE readout signal segments;

[0008] The 3D gradient spin echo diffusion imaging sequence based on the 3D navigator is composed of a global saturation module, a diffusion preparation module, a fat saturation module, a gradient spin echo module and a 3D navigator echo module in sequence;

[0009] The global saturation module is used to destroy the residual transverse magnetization by applying a gradient to the entire 3D imaging space;

[0010] The diffusion preparation module is used to embed a pair of diffusion gradients into 90° x -180° y -90° -x In the radio frequency pulse, the diffusion coding and signal acquisition are separated; the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient;

[0011] The fat saturation module is used to suppress fat signals;

[0012] The gradient spin echo module is used to collect signals in 3D K-space, thereby obtaining a 3D GRASE readout signal;

[0013] The navigation echo module is used to collect signals from the 3D navigator, thereby obtaining a 3D navigation echo signal.

[0014] S2. Performing a universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data, wherein each segment of the 3D GRASE readout signal in the whole-brain 3D K-space data is phase-corrected using a 3D navigator echo signal acquired during the same imaging sequence, and performing missing-line filling on each segment of the 3D GRASE readout signal after phase error correction. Finally, performing a 3D Fourier transform on all the filled 3D GRASE readout signals to obtain a reconstructed image.

[0015] Based on this solution, each step can further provide the following preferred implementation methods. It should be noted that the technical features in each preferred method can be combined with each other without conflict. Of course, these preferred methods can also be implemented by other methods that can achieve the same technical effects, which does not constitute a limitation.

[0016] Preferably, in the imaging sequence, the process performed in the global saturation module is as follows:

[0017] At the beginning of the imaging sequence, three B1 RF pulses are applied, and each time a B1 RF pulse is applied, gradients of completely equal amplitude are applied in the X, Y, and Z directions of the 3D imaging space to eliminate transverse magnetization. After the last B1 RF pulse is executed, the longitudinal magnetization vector is restored according to the preset post-saturation delay (PSD) before the next diffusion preparation module is executed.

[0018] Preferably, in the imaging sequence, the process executed in the diffusion preparation module is as follows:

[0019] First, a radio frequency excitation pulse with a 90° flip angle is applied along the X-axis direction; then, a diffusion gradient is applied along the preset diffusion direction, wherein the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient; then, a radio frequency focusing pulse with a 180° flip angle is applied along the Y-axis direction, and then the same diffusion gradient is applied again; finally, a stabilizer gradient (G stb ); finally, a radio frequency excitation pulse with a 90° flip angle is performed along the -X axis to convert the transverse magnetization vector into a longitudinal magnetization vector.

[0020] Preferably, in the imaging sequence, the fat saturation module performs the following process:

[0021] Three frequency-selective Gaussian pulses with a flip angle of 95° are applied to flip the cos(95°) portion of the longitudinal magnetization vector of the fat three times and reset it to 0 after waiting for T1*ln(2) time, so that most of the longitudinal magnetization vector of the fat is converted into a transverse magnetization vector. While applying the Gaussian pulses, gradients are synchronously applied in the X, Y, and Z directions to completely eliminate the transverse magnetization vector of the fat; where T1 is the longitudinal relaxation time of the fat.

[0022] Preferably, in the imaging sequence, the execution process in the gradient spin echo sequence module is as follows:

[0023] First, signal acquisition is performed using three navigator echoes. Then, 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction, obtaining a segment of 3D GRASE readout signal. During the 3D GRASE readout process, linear encoding is performed along the EPI direction, central encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. Simultaneously, a stabilizer gradient is applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as that applied in the diffusion preparation module.

[0024] Preferably, in the imaging sequence, the processing flow in the navigator echo module is as follows:

[0025] N segments of spin echo signals are collected from the center of the k-space in the Z direction, and EPI encoding is performed in the X and Y directions, and TSE encoding is performed in the Z direction, thereby obtaining a segment of 3D navigator echo signal; a stabilizer gradient needs to be applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as the amplitude of the stabilizer gradient applied in the diffusion preparation module; N is an even number not less than 4.

[0026] Preferably, in S2, the specific process of performing the universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data is as follows:

[0027] S21. Extracting a 3D GRASE readout signal and a 3D navigator echo signal acquired after each imaging sequence from the whole-brain 3D k-space data, using signals acquired at three navigator echoes in the 3D GRASE readout signal to estimate phase changes between even and odd EPI lines, and then using GRAPPA technology to fill in the missing lines in k-space acquired by performing parallel accelerated imaging along the EPI direction;

[0028] S22. For each segment of the 3D GRASE readout signal after the missing rows are filled, correct the phase error between multiple excitations of the 3D GRASE readout signal using the corresponding 3D navigator echo signal, and then use the GRAPPA technique to completely fill the missing rows of each segment of the 3D GRASE readout signal after the phase error correction, which are acquired by performing parallel accelerated imaging along the TSE direction, to obtain a complete 3D GRASE readout signal.

[0029] S23. All complete 3D GRASE readout signal segments are reassembled into complete whole-brain 3D K-space data, and a reconstructed image is obtained through 3D Fourier transform.

[0030] Preferably, in S22, the specific method of correcting the phase error of the 3D GRASE readout signal using the 3D navigator echo signal is as follows:

[0031] First, the 3D GRASE readout signal and the 3D navigator echo signal are transformed from the k-space to the xyz three-dimensional image space to obtain the first phase signal φ of the 3D navigator echo signal at each voxel (x, y, z) in the image space. n,s (x, y, z) and the second phase signal ψ of the 3D GRASE readout signal at each voxel (x, y, z) in the image space i,s (x,y,z);

[0032] Then, the 3D GRASE readout signal is obtained by subtracting the second phase signal from the first phase signal. i,s (x,y,z)=ψ i,s (x,y,z)-φ n,s (x,y,z);

[0033] Finally, the three-dimensional image of the phase-corrected 3D GRASE readout signal is re-transformed to k-space, thereby obtaining the phase-corrected 3D GRASE readout signal.

[0034] In a second aspect, the present application provides a computer readable storage medium, wherein the storage medium stores a computer program, and when the computer program is executed by a processor, the computer program is used to control an external device to implement the 3D navigator-based 3D gradient spin echo diffusion imaging method according to any one of the above first aspect.

[0035] In a third aspect, the present application provides a magnetic resonance imaging device, comprising a magnetic resonance scanner and a control unit, wherein the control unit stores a computer program, and when the computer program is executed, the computer program is used to control the magnetic resonance scanner to implement the 3D navigator-based 3D gradient spin echo diffusion imaging method according to any one of the above first aspect.

[0036] Compared with the prior art, the present application has the following beneficial effects:

[0037] The present application proposes a 3D navigator-based 3D gradient spin echo diffusion imaging sequence (3DDW-GRASE), which first combines diffusion gradient encoding, 3D gradient spin echo imaging, and 3D navigator echo sequence. Based on the sequence, the whole brain signal can be divided into N segments for signal acquisition in turn, and N 3D GRASE readout signals and N 3D navigator echo signals are obtained, and the phase of the 3D GRASE readout signal can be corrected based on the 3D navigator echo signal. Compared with the traditional 2D-EPI acquisition used on a 3T clinical system, the 3D DW-GRASE sequence not only improves the signal-to-noise ratio of whole brain magnetic resonance diffusion imaging, but also has higher resolution and can detect the diffusion time dependence of the cerebral gray matter cortex in t space. The 3D navigator-based 3D DW-GRASE sequence effectively eliminates the phase error between multi-segment acquisition and can realize whole brain high-resolution diffusion imaging. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a timing diagram of the 3D navigator-based 3D DW-GRASE sequence.

[0039] Figure 2 is Figure 1A magnified timing diagram of the global saturation module, diffusion preparation module, fat saturation module, and gradient spin echo module in the 3D DW-GRASE sequence.

[0040] Figure 3 yes Figure 1 A magnified timing diagram of the 3D navigator echo module within the 3D DW-GRASE sequence.

[0041] Figure 4 The b0 and b1000 images were acquired using a 3D DW-GRASE sequence, and the results without and with 1D, 2D, and 3D navigator echoes were shown.

[0042] Figure 5 The results are a comparison of the signal-to-noise ratios of b0 and b1000 images acquired on seven subjects using 3D DW-GRASE and 2D EPI sequences, respectively, in three regions of interest. DETAILED DESCRIPTION

[0043] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined accordingly without conflicting with each other.

[0044] The present invention designs a 3D gradient spin echo diffusion imaging sequence (3DDW-GRASE) based on a 3D navigator. The 3D DW-GRASE sequence consists of a global saturation module, a diffusion preparation module, a fat saturation module, a gradient spin echo module, and a 3D navigator echo module. The functions of each module are as follows:

[0045] A global saturation module for destroying residual transverse magnetization by applying gradients across the entire 3D imaging volume;

[0046] Diffusion preparation module for embedding a pair of diffusion gradients at 90° x -180° y -90° -x In the radio frequency pulse, the diffusion coding and signal acquisition are separated; the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient;

[0047] The fat saturation module is used to suppress fat signals;

[0048] a gradient spin echo module for signal acquisition of 3D K-space, thereby obtaining a 3D GRASE readout signal;

[0049] a navigator echo module for signal acquisition of 3D navigator, thereby obtaining a 3D navigator echo signal.

[0050] In the embodiment of the present application, the timing diagram of the 3D DW-GRASE sequence is shown as follows. Figure 1 The modules in the imaging sequence will be described in detail below. Figure 1

[0051] In the imaging sequence, the first module is a global saturation module, and the flow performed in the module is as follows:

[0052] At the beginning of the imaging sequence, three B1 radio frequency pulses are applied, and at the same time, gradients with equal amplitude are applied in the X, Y and Z directions of the 3D imaging space, thereby eliminating the transverse magnetization; after the last B1 radio frequency pulse is performed, the longitudinal magnetization vector is recovered according to the preset post-saturation delay time (PSD), and the next diffusion preparation module is performed.

[0053] In the imaging sequence, the second module is a diffusion preparation module, and the flow performed in the module is as follows:

[0054] First, a radio frequency excitation pulse with a 90° flip angle is performed along the X-axis direction; then, a diffusion gradient is applied along a preset diffusion direction, the diffusion gradient being a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient; next, a radio frequency refocusing pulse with a 180° flip angle is performed along the Y-axis direction, and then the same diffusion gradient is applied again; then, a stabilizer gradient (G stb ) is applied along the Z-axis direction; finally, a radio frequency excitation pulse with a 90° flip angle is performed along the -X-axis direction, thereby converting the transverse magnetization vector into the longitudinal magnetization vector.

[0055] In the imaging sequence, the third module is a fat saturation module, and the flow performed in the module is as follows:

[0056] Three frequency-selective Gaussian pulses with a 95° flip angle are applied, thereby inverting the cos(95°) part of the longitudinal magnetization vector of fat three times and setting it to 0 after waiting for a T1*ln(2) time, so that most of the longitudinal magnetization vector of fat is converted into the transverse magnetization vector; at the same time that the Gaussian pulses are applied, gradients are applied synchronously in the X, Y and Z axis directions, thereby completely eliminating the transverse magnetization vector of fat; wherein T1 is the longitudinal relaxation time of fat.

[0057] In the imaging sequence, the fourth module is a gradient spin echo sequence module, and the flow performed in the module is as follows:​

[0058] First, signal acquisition is performed using three navigator echoes. Then, 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction, obtaining a segment of 3D GRASE readout signal. During the 3D GRASE readout process, linear encoding is performed along the EPI direction, central encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. Simultaneously, a stabilizer gradient is applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as that applied in the diffusion preparation module.

[0059] In the imaging sequence, the fifth module is the navigation echo module, and the process executed in this module is as follows:

[0060] N segments of spin echo signals are collected from the center of the k-space in the Z direction, and EPI encoding is performed in the X and Y directions, and TSE encoding is performed in the Z direction, thereby obtaining a segment of 3D navigator echo signal; a stabilizer gradient needs to be applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as the amplitude of the stabilizer gradient applied in the diffusion preparation module; N is an even number not less than 4.

[0061] The above-mentioned 3D gradient spin echo diffusion imaging sequence based on the 3D navigator can be built into a magnetic resonance imaging device, and then K-space data is acquired and reconstructed based on the imaging sequence to complete magnetic resonance imaging.

[0062] Therefore, in an embodiment of the present invention, based on the above-mentioned 3D gradient spin echo diffusion imaging sequence based on a 3D navigator, a 3D gradient spin echo diffusion imaging method based on a 3D navigator is provided, which includes the following steps S1 and S2.

[0063] S1: Whole-brain signals are acquired by repeatedly executing a 3D gradient spin echo diffusion imaging sequence based on a 3D navigator. Each execution of the imaging sequence corresponds to obtaining a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal. All 3D GRASE readout signal segments constitute the whole-brain 3D K-space data.

[0064] S2. Performing a universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data, wherein each segment of the 3D GRASE readout signal in the whole-brain 3D K-space data is phase-error corrected using a 3D navigator echo signal acquired during the same imaging sequence, and performing missing-line filling on each segment of the 3D GRASE readout signal after phase error correction. Finally, performing a 3D Fourier transform on all the filled 3D GRASE readout signals to obtain a reconstructed image.

[0065] In an embodiment of the present invention, in the above step S2, the specific process of performing the universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data is as follows:

[0066] S21. Extract a 3D GRASE readout signal and a 3D navigator echo signal obtained after each imaging sequence from the whole-brain 3D K-space data, use the signals collected at the three navigator echoes in the 3D GRASE readout signal to estimate the phase change between the EPI odd and even rows, and then use the GRAPPA technology to complete the missing rows in the k-space for parallel accelerated imaging acquisition along the EPI direction.

[0067] S22. For each segment of the 3D GRASE readout signal after the missing rows are filled, the phase error between multiple excitations of the 3D GRASE readout signal is corrected using the corresponding 3D navigator echo signal. Then, the GRAPPA technology is used to fill the missing rows of the parallel accelerated imaging acquisition along the TSE direction in each segment of the 3D GRASE readout signal after the phase error correction, thereby obtaining a complete 3D GRASE readout signal.

[0068] In an embodiment of the present invention, a specific method for correcting the phase error of a 3D GRASE readout signal using a 3D navigator echo signal is as follows:

[0069] First, the 3D GRASE readout signal and the 3D navigator echo signal are transformed from the k-space to the xyz three-dimensional image space to obtain the first phase signal φ of the 3D navigator echo signal at each voxel (x, y, z) in the image space. n,s (x, y, z) and the second phase signal ψ of the 3D GRASE readout signal at each voxel (x, y, z) in the image space i,s (x,y,z);

[0070] Then, the corrected phase ψ' of the 3D GRASE readout signal at each voxel (x, y, z) in the image space is obtained by subtracting the second phase signal from the first phase signal. i,s (x,y,z)=ψ i,s (x,y,z)-φn,s (x,y,z);

[0071] Finally, the three-dimensional image of the phase-corrected 3D GRASE readout signal is retransformed into k-space to obtain the phase-corrected 3D GRASE readout signal.

[0072] S23. All complete 3D GRASE readout signal segments are reassembled into complete whole-brain 3D K-space data, and reconstructed images are obtained through 3D Fourier transform, i.e., whole-brain high-resolution diffusion imaging.

[0073] Therefore, the above-mentioned 3D DW-GRASE sequence based on the 3D navigator can effectively eliminate the phase error between multiple acquisition segments and achieve high-resolution diffusion imaging of the whole brain.

[0074] In addition, in other embodiments, a computer-readable storage medium may be provided, on which a computer program is stored. When the computer program is executed by a processor, the computer program is used to control an external device to implement the 3D gradient spin echo diffusion imaging method based on a 3D navigator as described in steps S1 and S2 above.

[0075] It should be noted that in steps S1 and S2 above, step S1 requires segmented acquisition of whole-brain signals, thus relying on the cooperation of the MRI scanner. Step S2 primarily processes the whole-brain signals acquired in step S1, relying on a computer program for implementation. Therefore, within the aforementioned computer program, one subroutine controls the MRI scanner to acquire whole-brain signals segmented according to step S1, while another subroutine performs universal automatic calibration and parallel acquisition and reconstruction operations on the whole-brain 3D K-space data according to step S2 to obtain a reconstructed image.

[0076] In addition, in other embodiments, a magnetic resonance imaging device may be provided, comprising a magnetic resonance scanner and a control unit, wherein the control unit stores a computer program via the computer-readable storage medium. When the computer program is executed, the computer program is used to control the magnetic resonance scanner to implement the 3D gradient spin echo diffusion imaging method based on the 3D navigator as described in steps S1 and S2 above.

[0077] The hardware structure of the magnetic resonance scanner and control unit described above can be implemented using a conventional magnetic resonance imaging system. The magnetic resonance scanner should include a magnet and a magnetic resonance spectrometer. The control unit should contain a computer program that implements steps S1 and S2 above, as well as the control program necessary to implement magnetic resonance imaging.

[0078] It should be noted that the above-mentioned computer-readable storage medium may include random access memory (RAM) and may also include non-volatile memory (NVM), such as at least one disk storage. The above-mentioned processor may be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it may also be a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components. Of course, the device should also have the necessary components to implement program operation, such as power supply, communication bus, etc.

[0079] The following is a specific embodiment to illustrate the specific implementation and technical effects of the 3D gradient spin echo diffusion imaging method based on the 3D navigator described in S1 to S2 of the present invention.

[0080] Example

[0081] In this embodiment, a 3D gradient spin echo diffusion imaging method based on a 3D navigator includes the following steps:

[0082] Step 1: Continuously execute a 3D gradient spin echo diffusion imaging sequence (3D DW-GRASE) based on a 3D navigator to acquire whole-brain signals in segments.

[0083] Each time a 3D DW-GRASE sequence is executed, the global saturation module, diffusion preparation module, fat saturation module, gradient spin echo module, and 3D navigation echo module need to be executed in sequence. The specific process is as follows:

[0084] Step 1.1: Execute a global saturation module at the beginning of the imaging sequence, see Figure 2 As shown, the processing flow within the module is as follows:

[0085] At the beginning of the sequence, three B1 RF pulses are applied. Each RF pulse is accompanied by simultaneous gradients in the X, Y, and Z directions, with equal amplitudes. After these three B1 RF pulses, any residual transverse magnetization from the previous signal acquisition is considered completely eliminated. A period known as the post-saturation delay (PSD) is then allowed for the longitudinal magnetization to recover. Only after the PSD is reached can the next diffusion preparation module be executed.

[0086] Step 1.2: After the PSD of the global saturation module, the diffusion preparation module is executed. Figure 2 As shown, the processing flow within the module is as follows:

[0087] First, a non-layer-selective hard pulse excitation with a flip angle of 90° is applied along the X-axis as the RF excitation pulse; then, a diffusion gradient is applied along the preset diffusion direction (the diffusion direction can be selected according to actual needs. In this embodiment, gradients are applied along the X, Y, and Z directions). The diffusion gradient can be a trapezoidal cosine oscillation diffusion gradient with a certain frequency or a pulse diffusion gradient with a certain diffusion interval. The amplitude of the gradient depends on the preset b value. The diffusion time is recorded as t d ; Then, an adiabatic tangential pulse excitation with a flip angle of 180° is applied along the Y-axis as a radio frequency focusing pulse to make the scattered transverse magnetization vector converge; then, a diffusion gradient with the same amplitude, shape, duration and direction as the previous one is applied; after that, a stabilizer gradient (G stb ) to mitigate signal modulation associated with phase error, and the gradient amplitude is set to the same value as the stabilizer gradient in the subsequent gradient spin echo module. Finally, a non-slice-selective hard pulse with a 90° flip angle is emitted along the -X axis as the RF excitation pulse to convert the transverse magnetization vector to the longitudinal magnetization vector. The echo time in the diffusion preparation module is denoted as TE1.

[0088] Step 1.3: Execute the fat saturation module after the diffusion preparation module to suppress the fat signal. Figure 2 As shown, the processing flow within the module is as follows:

[0089] Three frequency-selective Gaussian pulses with a flip angle of 95° are applied to flip the cos(95°) part of the longitudinal magnetization vector of the fat three times and then the fat signal is set to 0 after waiting for T1*ln(2) time, so that most of the longitudinal magnetization vector of the fat is converted into a transverse magnetization vector. While applying the Gaussian pulse, gradients in the X, Y, and Z axis directions are applied to completely eliminate the transverse magnetization vector of the fat; where T1 is the longitudinal relaxation time of the fat.

[0090] Step 1.4: Execute the gradient spin echo module after the fat saturation module to acquire 3D K-space signals and obtain 3D K-space data. Figure 2 As shown, the processing flow in the module is as follows:

[0091] 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction. Linear encoding is performed along the EPI direction, and central encoding is performed along the TSE direction. GRAPPA parallel acceleration imaging is performed in both directions. Stabilizer gradients are applied along the Z axis before and after each EPI readout moment.

[0092] First, signal acquisition is performed using three navigator echoes (i.e., 3-navigator echo). 3D-GRASE readout is then achieved by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction, obtaining a 3D GRASE readout signal (which includes the signals of the three navigator echoes). In the above 3D GRASE readout process, linear encoding is performed along the EPI direction, center encoding is performed along the TSE direction, and the number of phase encodings in the EPI direction is recorded as N. EPI , and the number of phase encodings in the TSE direction is recorded as N TSE In addition, in the above 3D GRASE readout process, this embodiment performs 2×2 GRAPPA parallel accelerated imaging in both EPI and TSE directions, that is, parallel acquisition of N images along the EPI direction. EPI Interval acceleration factor value (N a1 ) EPI / N a1 Line number collection, along the TSE direction N TSE Interval acceleration factor value (N a2 ) TSE / N a2 The EPI signal is read after the first focusing pulse in k z The center of k-space is filled in the direction of the gradient spin echo. Stabilizer gradients are applied along the z-axis before and after each EPI readout. The amplitude of the stabilizer gradient applied here is the same as that applied in the diffusion preparation module. The echo time in the gradient spin echo module is denoted as TE2, which is equal to the echo interval (ESP) for subsequent TSE signal acquisition. Therefore, the total echo time of the acquired signal is TE1 + TE2.

[0093] Step 1.5, embedding a navigation echo module after the gradient spin echo module at the end of the sequence, collecting a 3D spin echo navigation signal for correcting the phase error between multiple excitations in the 3D K-space data collected in the gradient spin echo module. Referring to Figure 3 The processing flow in the navigation echo module is as follows:

[0094] Four segments of navigation echoes in the center of the k-space Z direction are collected by setting the gradient encoding to perform EPI encoding in the X and Y directions and TSE encoding in the Z direction, so as to realize 3D navigation echo collection and obtain a 3D navigation echo signal. The navigation echo module uses the same echo spacing (ESP) as the gradient spin echo module, and also needs to apply a stabilizer gradient in the Z axis direction at the leading and trailing edges of each EPI readout time. The number of phase encodings in the EPI direction and the acceleration factor value are consistent with those of the gradient spin echo module, and are denoted as N EPI and N a1 .

[0095] Each execution of the imaging sequence, i.e., repeating the above steps 1.1-1.5, corresponds to obtaining a 3D GRASE readout signal and a 3D navigation echo signal. Repeating steps 1.1-1.5 a total of N times can divide the whole brain signal into N segments for signal collection, and the whole brain 3D K-space data is composed of all N 3D GRASE readout signals. The 3D navigator data composed of N 3D navigation echo signals corresponding to the whole brain 3D K-space data can be used for subsequent phase correction. The difference between this acquisition method and the conventional acquisition method is that N TSE is reduced, the acquisition time is N times that of the original, and it is more sensitive to motion and needs to correct the motion error between each segment. However, this acquisition method greatly improves the signal-to-noise ratio of the image.

[0096] Step 2, after completing the acquisition of the whole brain 3D K-space data, a general automatic calibration parallel acquisition reconstruction module is executed to reconstruct the whole brain 3D K-space data to obtain a complete image.

[0097] It should be noted that, since the present application uses N a1 >1 or N a2>1, so universal automatic calibration is required to partially parallelize acquisition and reconstruction to fill in the k-space data of the unacquired rows. Moreover, since segmented signals are used for acquisition, 3D navigator echo reconstruction is required to correct the motion errors between the signals acquired in each segment. Therefore, in the universal automatic calibration parallel acquisition and reconstruction module, the 3-navigator-echo technology is first used to estimate the phase changes between the EPI odd and even rows, and then the phases of all acquired image and navigation EPI odd and even row signals are corrected. The GRAPPA technology is used to fill in the k-space data of the unacquired rows, and the k-space image data of each segment and the corresponding navigator data are reconstructed into a three-dimensional image space.

[0098] In this embodiment, the processing flow in the universal automatic calibration parallel acquisition and reconstruction module is as follows:

[0099] Step 2.1. Extracting a 3D GRASE readout signal and a 3D navigator echo signal from the whole-brain 3D k-space data after each imaging sequence, and using the 3-navigator-echo technique, i.e., using the signals collected at the three navigator echo locations in the 3D GRASE readout signal, to estimate the phase change between the EPI odd and even lines. Then, using the GRAPPA technique, complete the missing lines in k-space caused by GRAPPA parallel accelerated imaging along the EPI direction. That is, complete the missing lines in the 3D GRASE readout signal acquired by parallel imaging along the EPI direction, thereby obtaining a 3D GRASE readout signal after the missing lines are filled.

[0100] Step 2.2: For each segment of the 3D GRASE readout signal after the missing rows are filled, the phase error between multiple excitations of the 3D GRASE readout signal is corrected using the corresponding 3D navigator echo signal. GRAPPA is then used to complete the missing rows in k-space caused by GRAPPA parallel acceleration imaging in the TSE direction. That is, the missing rows in each segment of the 3D GRASE readout signal after phase error correction, acquired by parallel acceleration imaging along the TSE direction, are completed to obtain a complete 3D GRASE readout signal.

[0101] In this embodiment, the 3D navigator echo signal needs to be used to correct the phase error of the 3D GRASE readout signal acquired during the same imaging sequence. The process of achieving phase error correction is as follows:

[0102] First, both the 3D GRASE readout signal and the 3D navigator echo signal are transformed from k-space to the xyz three-dimensional image space. is the voxel (x, y, z) phase signal of the sth segment 3D navigation echo signal in the image space, ψi,s (x, y, z) is the phase signal of the s-th segment 3D GRASE readout signal at voxel (x, y, z) in the image space. The corrected phase ψ' of the 3D GRASE readout signal at voxel (x, y, z) in the image space is calculated according to formula (1): i,s (x,y,z):

[0103]

[0104] In the image space, after the corrected phase is calculated for each voxel, the phase correction of the three-dimensional image corresponding to the s-th segment 3DGRASE readout signal can be completed.

[0105] Then, the three-dimensional image of the phase-corrected s-th segment 3D GRASE readout signal is retransformed into k-space, thereby obtaining the phase-corrected s-th segment 3D GRASE readout signal.

[0106] The correction process for the sth segment of the 3D GRASE readout signal is similar for the remaining segments, i.e., s = 1, 2, ..., N. The above operation is repeated for each segment, and then the gap filling in the TSE direction is performed to obtain N segments of complete 3D GRASE readout signals.

[0107] Step 2.3: All N segments of complete 3D GRASE readout signals are reassembled into complete whole-brain 3D K-space data, and reconstructed whole-brain high-resolution diffusion imaging is obtained through 3D Fourier transform.

[0108] Compared to conventional phase correction methods, 3D navigation echo is more sensitive to motion, significantly reducing the problem of image motion artifacts. Compared to conventional acquisition methods, the GRAPPA parallel acquisition method significantly improves acquisition efficiency, shortening acquisition time and reducing TE, resulting in images with a higher signal-to-noise ratio.

[0109] In this embodiment, since the diffusion gradients in the diffusion preparation module of the above-mentioned 3D gradient spin echo diffusion imaging sequence based on the 3D navigator (3D DW-GRASE) include trapezoidal cosine oscillation diffusion gradient and pulsed diffusion gradient, the 3D DW-GRASE imaging sequence when the diffusion gradient in the diffusion preparation module is a trapezoidal cosine oscillation diffusion gradient is recorded as 3D OG-GRASE, and the 3D DW-GRASE imaging sequence when the diffusion gradient in the diffusion preparation module is a pulsed diffusion gradient is recorded as 3D PG-GRASE.

[0110] This example was tested on seven healthy young male volunteers. The specific parameters are described below: MRI scans were performed using a Siemens Prisma 3T scanner (maximum gradient 80 mT / m, maximum switching rate 200 mT / m), and all scans were performed using a 64-channel head coil. Two sets of experiments were conducted in this example, using a 2D EPI sequence commonly used in the prior art for comparison.

[0111] Experiment 1: To compare the signal-to-noise ratio of 3D PG-GRASE and 2D EPI sequences based on 3D navigator at the same acquisition time. 2 , 30 directions, FOV = 220 × 200 × 144 mm 3 mm, resolution 1.5×1.5×1.5mm 3 , slice direction oversampling = 20%, partial Fourier factor = 76%, N a1 =2 Perform a scan using the following scheme:

[0112] (1) GRASE readout of 18 excitations, N EPI =113,N TSE =4, navigation echo Z direction N TSE = 4. TE1 / TE2 / TR = 38.68 / 43.92 / 2500 ms, only one acquisition, scanning time = 24 min.

[0113] (2) 2D EPI readout of multi-slice scanning, TR / TE = 11250 / 83ms and N EPI =110, 4 repeated acquisitions were performed, and the scanning time was 24 min.

[0114] Experiment 2: Assessing the time dependence of diffusion in cortical gray matter.

[0115] The diffusion time dependence of the apparent diffusion coefficient (ADC) in the cortical gray matter area was evaluated using 3D OG-GRASE and 3D PG-GRASE sequences based on a 3D navigator. PG encoding (δ / △=15 / 25ms, 0Hz, Δ eff =20ms; δ / △=15 / 35ms,0Hz,Δ eff =30ms; δ / △=15 / 45ms,0Hz,Δ eff =40ms) and OG encoding (25Hz, 2 cycles, Δ eff ≈10ms; 50Hz, 4 cycles, Δ eff The other parameters of the scans at all diffusion times remained the same: TR / TE1 / TE2 = 2600 / 84 / 46 ms, N EPI= 67, 12 segments were collected along the TSE encoding direction. Average times = 2, b = 500s / mm 2 , 6 directions, b=0 and averaged 4 times, N a1 =2, N a2 = 2. The scanning time for each diffusion time image was 5 minutes. The remaining scanning parameters were the same as those of the 3D PG-GRASE sequence based on the 3D navigator in Experiment 1.

[0116] Figure 4 The following figure shows the b0 image (3Dnavigator_b0) and b1000 image (3D navigator_b1000) acquired using 3D PG-GRASE after 3D navigator echo correction, as well as the b0 image (nonnavigator_b0) and b1000 image (non navigator_b1000) acquired without navigator echo correction. In comparison, the b1000 image corrected for motion artifacts after 3D navigator echo correction, such as the residual artifact along the z direction (white arrow in the figure), is better removed.

[0117] Figure 5 The SNR of three regions (white matter (WM), basal ganglia (BG), and thalamus (Thal)) measured using 3D PG-GRASE and 2D EPI sequences at the same acquisition time is shown. The results show that the SNR of the 3D PG-GRASE sequence is significantly higher than that of the 2D EPI sequence in the WM, BG, and Thal at b0 and b1000 images.

[0118] It should be noted that the above-described embodiment is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent substitution or equivalent transformation falls within the scope of protection of the present invention.

Claims

1. A 3D gradient spin echo diffusion imaging method based on a 3D navigator, characterized in that: include: S1: Whole-brain signals are acquired by repeatedly executing a 3D gradient spin echo diffusion imaging sequence based on a 3D navigator. Each imaging sequence generates a corresponding segment of 3D GRASE readout signal and a segment of 3D navigator echo signal. All 3D GRASE readout signal segments constitute the whole-brain 3D K-space data. The 3D gradient spin echo diffusion imaging sequence based on the 3D navigator is composed of a global saturation module, a diffusion preparation module, a fat saturation module, a gradient spin echo module and a 3D navigator echo module in sequence; The global saturation module is used to destroy the residual transverse magnetization by applying a gradient to the entire 3D imaging space; The diffusion preparation module is used to embed a pair of diffusion gradients into 90° x -180° y -90° -x In the radio frequency pulse, the diffusion coding and signal acquisition are separated; the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient; The fat saturation module is used to suppress fat signals; The gradient spin echo module is used to collect signals in 3D K-space, thereby obtaining a 3D GRASE readout signal; The navigation echo module is used to collect signals from the 3D navigator to obtain a 3D navigation echo signal; S2. Performing a universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data, wherein each segment of the 3D GRASE readout signal in the whole-brain 3D K-space data is phase-corrected using a 3D navigator echo signal acquired during the same imaging sequence, and performing missing-line filling on each segment of the 3D GRASE readout signal after phase error correction. Finally, performing a 3D Fourier transform on all the filled 3D GRASE readout signals to obtain a reconstructed image.

2. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, characterized in that: In the imaging sequence, the process performed in the global saturation module is as follows: At the beginning of the imaging sequence, three B1 RF pulses are applied, and each time a B1 RF pulse is applied, gradients of completely equal amplitude are applied in the X, Y, and Z directions of the 3D imaging space to eliminate transverse magnetization. After the last B1 RF pulse is executed, the longitudinal magnetization vector is restored according to the preset post-saturation delay (PSD) before the next diffusion preparation module is executed.

3. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, wherein: In the imaging sequence, the process performed in the diffusion preparation module is as follows: First, a radio frequency excitation pulse with a 90° flip angle is applied along the X-axis direction; then, a diffusion gradient is applied along the preset diffusion direction, wherein the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient; then, a radio frequency focusing pulse with a 180° flip angle is applied along the Y-axis direction, and then the same diffusion gradient is applied again; finally, a stabilizer gradient (G stb ); finally, a radio frequency excitation pulse with a 90° flip angle is performed along the -X axis to convert the transverse magnetization vector into a longitudinal magnetization vector.

4. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, wherein: In the imaging sequence, the fat saturation module performs the following steps: Three frequency-selective Gaussian pulses with a flip angle of 95° are applied to flip the cos(95°) portion of the longitudinal magnetization vector of the fat three times and reset it to 0 after waiting for T1*ln(2) time, so that most of the longitudinal magnetization vector of the fat is converted into a transverse magnetization vector. While applying the Gaussian pulses, gradients are synchronously applied in the X, Y, and Z directions to completely eliminate the transverse magnetization vector of the fat; where T1 is the longitudinal relaxation time of the fat.

5. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, wherein: In the imaging sequence, the execution process in the gradient spin echo sequence module is as follows: First, signal acquisition is performed using three navigator echoes. Then, 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction, obtaining a segment of 3D GRASE readout signal. During the 3D GRASE readout process, linear encoding is performed along the EPI direction, central encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. Simultaneously, a stabilizer gradient is applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as that applied in the diffusion preparation module.

6. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, wherein: In the imaging sequence, the processing flow in the navigation echo module is as follows: N segments of spin echo signals are collected from the center of the k-space in the Z direction, and EPI encoding is performed in the X and Y directions, and TSE encoding is performed in the Z direction, thereby obtaining a segment of 3D navigator echo signal; a stabilizer gradient needs to be applied along the Z axis before and after each EPI readout moment, and the gradient amplitude of the stabilizer gradient is the same as the amplitude of the stabilizer gradient applied in the diffusion preparation module; N is an even number not less than 4.

7. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 1, wherein: In S2, the specific process of performing the universal automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data is as follows: S21. Extracting a 3D GRASE readout signal and a 3D navigator echo signal acquired after each imaging sequence from the whole-brain 3D k-space data, using signals acquired at three navigator echo locations in the 3D GRASE readout signal to estimate phase changes between even and odd EPI lines, and then using GRAPPA technology to fill in the missing lines in k-space acquired by performing parallel accelerated imaging along the EPI direction; S22. For each segment of the 3D GRASE readout signal after the missing rows are filled, correct the phase error between multiple excitations of the 3D GRASE readout signal using the corresponding 3D navigator echo signal, and then use the GRAPPA technique to completely fill the missing rows of each segment of the 3D GRASE readout signal after the phase error correction, which are acquired by performing parallel accelerated imaging along the TSE direction, to obtain a complete 3D GRASE readout signal. S23. All complete 3D GRASE readout signal segments are reassembled into complete whole-brain 3D K-space data, and a reconstructed image is obtained through 3D Fourier transform.

8. The 3D gradient spin echo diffusion imaging method based on a 3D navigator according to claim 7, characterized in that: In S22, the specific method of correcting the phase error of the 3D GRASE readout signal using the 3D navigator echo signal is as follows: First, the 3D GRASE readout signal and the 3D navigator echo signal are transformed from the k-space to the xyz three-dimensional image space to obtain the first phase signal φ of the 3D navigator echo signal at each voxel (x, y, z) in the image space. n,s (x, y, z) and the second phase signal ψ of the 3DGRASE readout signal at each voxel (x, y, z) in the image space i,s (x,y,z); Then, the corrected phase ψ' of the 3D GRASE readout signal at each voxel (x, y, z) in the image space is obtained by subtracting the second phase signal from the first phase signal. i,s (x,y,z)=ψ i,s (x,y,z)-φ n,s (x,y,z); Finally, the three-dimensional image of the phase-corrected 3D GRASE readout signal is retransformed into k-space to obtain the phase-corrected 3D GRASE readout signal.

9. A computer-readable storage medium, characterized in that The storage medium stores a computer program, which, when executed by a processor, is used to control an external device to implement the 3D gradient spin echo diffusion imaging method based on a 3D navigator according to any one of claims 1 to 8.

10. A magnetic resonance imaging device, characterized in that: The invention comprises a magnetic resonance scanner and a control unit, wherein a computer program is stored in the control unit, and when the computer program is executed, it is used to control the magnetic resonance scanner to implement the 3D gradient spin echo diffusion imaging method based on the 3D navigator according to any one of claims 1 to 8.

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