3D gradient spin echo diffusion imaging method and device based on 3D navigator

Abstract

The invention discloses a 3D gradient spin echo diffusion imaging method and device based on a 3D navigator. The method comprises the following steps: firstly, destroying previous residual transverse magnetization in a global saturation module; secondly, embedding a pair of trapezoidal cosine oscillation gradients or pulse gradients into 90-degree x-180-degree y-90-degree-x radio frequency pulses through a diffusion coding module, and separating diffusion coding from signal acquisition; then, a fat saturation module is used for inhibiting a fat signal; acquiring an image signal by adopting a 3D gradient spin echo reading mode; and finally, acquiring a 3D navigation echo signal in the spin echo at the tail to correct a phase error between multiple times of excitation. According to the invention, phase errors among multiple sections of acquisition can be effectively eliminated, and whole-brain high-resolution diffusion imaging is realized. Compared with 2D planar echo imaging, the method has the advantages that the signal-to-noise ratio of diffusion magnetic resonance imaging is increased, and higher-resolution whole-brain 3D diffusion magnetic resonance imaging is supported.

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance technology, in particular to the field of diffusion magnetic resonance imaging. Background Art

[0002] Diffusion MRI (dMRI) can detect tissue microstructure based on the characteristics of restricted diffusion of water molecules in the microenvironment of biological tissues. 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, while 2D multi-slice acquisition often has suboptimal slice resolution and low signal-to-noise ratio. Therefore, 3D acquisition is ideal for high-resolution dMRI, which can mitigate the loss of signal-to-noise ratio caused by diffusion-weighted attenuation and capture the microstructure of tissues in a continuous 3D space.

[0003] However, due to the lengthy phase encoding process of 3D imaging, whole-brain acquisition often requires segmented acquisition. In 3D-dMRI, additional eddy currents and motion introduce serious phase errors between signals in segmented acquisition, resulting in motion artifacts and distortion in the image, so the technology faces challenges in human clinical application. In segmented acquisition, adding navigator echoes is an effective way 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 and do not require or only require phase encoding in 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 urgently. Summary of the invention

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

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

[0006] In a first aspect, the present invention provides a 3D gradient spin echo diffusion imaging method based on a 3D navigator, comprising:

[0007] S1: The whole-brain signal is acquired by repeatedly executing the 3D gradient spin echo diffusion imaging sequence based on the 3D navigator. After each execution of the imaging sequence, a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal are obtained. All 3D GRASE readout signal segments constitute the whole-brain 3D K-space data.

[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 uses a 3D navigation echo signal acquired during the same imaging sequence to correct the phase error, and performing vacant row filling on each segment of the 3D GRASE readout signal after the phase error correction, and finally performing 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 implementations. It should be noted that the technical features in each preferred implementation can be combined with each other without conflict. Of course, these preferred implementations 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, a gradient with completely equal amplitude is applied in the X, Y, and Z directions of the 3D imaging space to eliminate the transverse magnetization. After the last B1 RF pulse is executed, wait until the longitudinal magnetization vector is restored according to the preset post-saturation delay time (PSD) before executing the next diffusion preparation module.

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

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

[0020] Preferably, in the imaging sequence, the process executed in the fat saturation module is as follows:

[0021] 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 set 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 pulse, gradients are synchronously applied in the X, Y, and Z axis 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 through three navigation echoes, and then 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and fast spin echo (TSE) encoding in the Z direction to obtain a 3D GRASE readout signal; in the 3D GRASE readout process, linear encoding is performed along the EPI direction, center encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. At the same time, a stabilizer gradient needs to be applied along the Z-axis direction 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.

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

[0025] N segments of spin echo signals at the center of the k-space in the Z direction are collected, EPI encoding is performed along the X and Y directions, and TSE encoding is performed in the Z direction, so as to obtain a segment of 3D navigation echo signal; before and after each segment of EPI readout, a stabilizer gradient needs to be applied along the Z axis direction, 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 general automatic calibration and parallel acquisition and reconstruction operation on the whole-brain 3D K-space data is as follows:

[0027] S21, extracting a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal obtained after each execution of the imaging sequence from the whole-brain 3D K-space data, using 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 using the GRAPPA technology to fill in the missing rows in the k-space for performing parallel accelerated imaging acquisition along the EPI direction;

[0028] S22, for each segment of the 3D GRASE readout signal after the missing lines are filled, the phase error between multiple excitations of the 3D GRASE readout signal is corrected using the corresponding 3D navigation echo signal, and then the GRAPPA technology is used to fill the missing lines of each segment of the 3D GRASE readout signal after the phase error correction, in which the parallel accelerated imaging acquisition is performed 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 by using the 3D navigation 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 three-dimensional image space of xyz to obtain the first phase signal φ of each voxel (x, y, z) of the 3D navigator echo signal 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 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);

[0033] 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.

[0034] In a second aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the computer program is used to control an external device to implement a 3D gradient spin echo diffusion imaging method based on a 3D navigator as described in any one of the schemes of the first aspect above.

[0035] In a third aspect, the present invention provides a magnetic resonance imaging device, which includes 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 a 3D navigator as described in any one of the schemes of the first aspect above.

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

[0037] The present invention proposes a 3D gradient spin echo diffusion imaging sequence (3DDW-GRASE) based on a 3D navigator, which combines diffusion gradient encoding, 3D gradient spin echo imaging, and a 3D navigator echo sequence for the first time. Based on this sequence, the whole brain signal can be divided into N segments for signal acquisition in sequence to obtain N segments of 3D GRASE readout signals and N segments of 3D navigator echo signals, 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, due to its higher resolution, can detect the diffusion time dependence of the gray matter cortex of the brain in t space. The 3D DW-GRASE sequence based on the 3D navigator effectively eliminates the phase error between multi-segment acquisitions and can achieve high-resolution diffusion imaging of the whole brain. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0039] Figure 2 yes 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 navigation 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 show the results without and with phase correction using 1D, 2D, and 3D navigator echoes.

[0042] Figure 5 The results are a comparison of the signal-to-noise ratios of b0 and b1000 images obtained on 7 subjects using 3D DW-GRASE and 2D EPI sequences in three regions of interest. DETAILED DESCRIPTION

[0043] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with 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 different from 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 each embodiment 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 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 functions of each module are as follows:

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

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

[0049] The navigation echo module is used to collect signals from the 3D navigator to obtain a 3D navigation echo signal.

[0050] In an embodiment of the present invention, the timing diagram of the 3D DW-GRASE sequence is as follows: Figure 1 As shown below. Figure 1 , each module in the imaging sequence is described in detail.

[0051] In the imaging sequence, the first module is the global saturation module, and the process executed in this module is as follows:

[0052] At the beginning of the imaging sequence, three B1 RF pulses are applied, and each time a B1 RF pulse is applied, a gradient with completely equal amplitude is applied in the X, Y, and Z directions of the 3D imaging space to eliminate the transverse magnetization. After the last B1 RF pulse is executed, wait until the longitudinal magnetization vector is restored according to the preset post-saturation delay time (PSD) before executing the next diffusion preparation module.

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

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

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

[0056] 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 set 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 pulse, gradients are synchronously applied in the X, Y, and Z axis directions to completely eliminate the transverse magnetization vector of the fat; where T1 is the longitudinal relaxation time of the fat.

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

[0058] First, signal acquisition is performed through three navigation echoes, and then 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and fast spin echo (TSE) encoding in the Z direction to obtain a 3D GRASE readout signal; in the 3D GRASE readout process, linear encoding is performed along the EPI direction, center encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. At the same time, a stabilizer gradient needs to be applied along the Z-axis direction 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.

[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 at the center of the k-space in the Z direction are collected, EPI encoding is performed along the X and Y directions, and TSE encoding is performed in the Z direction, so as to obtain a segment of 3D navigation echo signal; before and after each segment of EPI readout, a stabilizer gradient needs to be applied along the Z axis direction, 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 the 3D gradient spin echo diffusion imaging sequence based on the 3D navigator. After each execution of the imaging sequence, a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal are obtained. 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 uses a 3D navigation echo signal acquired during the same imaging sequence to correct the phase error, and performing vacant row filling on each segment of the 3D GRASE readout signal after the phase error correction, and finally performing 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 general automatic calibration 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 navigation echo signal obtained after each execution of the imaging sequence from the whole-brain 3D K-space data, use the signals collected at the three navigation 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 fill in 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 navigation echo signal, and then the GRAPPA technology is used to fill the missing rows of each segment of the 3D GRASE readout signal after the phase error correction, in which parallel accelerated imaging acquisition is performed along the TSE direction, to obtain 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 navigation 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 three-dimensional image space of xyz to obtain the first phase signal φ of each voxel (x, y, z) of the 3D navigator echo signal 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, using the above-mentioned 3D DW-GRASE sequence based on the 3D navigator, the phase error between multiple acquisitions can be effectively eliminated to 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, it 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 the above steps S1 and S2, the S1 step needs to collect whole-brain signals in segments, so it needs to rely on the cooperation of the magnetic resonance scanner, while the S2 step mainly processes the whole-brain signals collected in the S1 step, which relies on the computer program to implement. Therefore, in the above computer program, a part of the subroutine is used to control the magnetic resonance scanner to collect whole-brain signals in segments according to the S1 step, and another part of the subroutine is used to perform a general automatic calibration parallel acquisition and reconstruction operation on the whole-brain 3D K-space data according to the S2 step to obtain a reconstructed image.

[0076] In addition, in other embodiments, a magnetic resonance imaging device may be provided, which includes a magnetic resonance scanner and a control unit, wherein the control unit stores a computer program through the above-mentioned computer-readable storage medium, 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 as described in the above-mentioned steps S1 and S2.

[0077] The hardware structure of the magnetic resonance scanner and control unit can be implemented by a conventional magnetic resonance imaging system. The magnetic resonance scanner should include a magnet part and a magnetic resonance spectrometer part, and the control unit should contain a computer program for implementing the above steps S1 and S2, and should also have a control program necessary for implementing magnetic resonance imaging.

[0078] It should be noted that the above-mentioned computer-readable storage medium may include random access memory (Random Access Memory, RAM), and may also include non-volatile memory (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 (Central Processing Unit, CPU), a network processor (Network Processor, NP), etc.; it may also be a digital signal processor (Digital Signal Processing, DSP), an application-specific integrated circuit (Application Specific Integrated Circuit, ASIC), a field-programmable gate array (Field-Programmable Gate Array, FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components. Of course, the device should also have the necessary components to realize the operation of the program, such as power supply, communication bus, etc.

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

[0080] Example

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

[0082] Step 1: Continuously execute the 3D gradient spin echo diffusion imaging sequence (3D DW-GRASE) based on the 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 timing, three B1 RF pulses are applied first, and each time a RF pulse is applied, gradients need to be applied synchronously in the X, Y, and Z directions, and the amplitudes of the gradients in the three directions are exactly equal. After applying three B1 RF pulses, it can be considered that the transverse magnetization remaining from the last signal acquisition has been completely eliminated. After waiting for a period of time called post-saturation delay (PSD), the longitudinal magnetization vector is restored, and the next diffusion preparation module can be executed after PSD.

[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 direction as a radio frequency excitation pulse; then, a diffusion gradient is applied along a 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 ; Next, an adiabatic tangent pulse excitation with a flip angle of 180° is applied along the Y-axis direction as a radio frequency focusing pulse to focus the scattered transverse magnetization vector; then, a diffusion gradient with the same amplitude, shape, duration, and direction as previously applied is applied; then, a stabilizer gradient (G stb ) to reduce the signal modulation related to the phase error, and set the gradient amplitude to the same amplitude as the stabilizer gradient in the subsequent gradient spin echo module; finally, a non-layer-selective hard pulse excitation with a flip angle of 90° is emitted along the -X axis as the RF excitation pulse to convert the transverse magnetization vector into the longitudinal magnetization vector. The echo time in the diffusion preparation module is recorded 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] Apply three frequency-selective Gaussian pulses with a flip angle of 95° to flip the cos(95°) part of the longitudinal magnetization vector of the fat three times and wait for T1*ln(2) time before setting the fat signal to 0, so that most of the longitudinal magnetization vector of the fat is converted into a transverse magnetization vector. While applying the Gaussian pulse, apply gradients in the X, Y, and Z axis directions 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 signals in 3D K-space 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 fast 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; and a stabilizer gradient is applied along the Z-axis before and after each EPI readout moment.

[0092] First, the signal is collected by three navigator echoes (i.e., 3-navigator echo), and 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 to obtain a 3D GRASE readout signal (including 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 for N TSE Interval acceleration factor value (N a2 ) TSE / N a2 The EPI reading signal after the first focusing pulse is collected in k z The center of k-space is filled in the direction; at the same time, a stabilizer gradient is applied along the Z-axis before and after each EPI readout time, and the gradient amplitude of the stabilizer gradient applied here is the same as the amplitude of the stabilizer gradient applied in the aforementioned diffusion preparation module. The echo time in the gradient spin echo module is recorded as TE2, which is equal to the echo interval (ESP) for signal acquisition in the TSE mode later. Therefore, the total echo time of the acquired signal is TE1+TE2.

[0093] Step 1.5: embed a navigation echo module at the end of the sequence, which is executed after the gradient spin echo module, to collect 3D spin echo navigation signals for correcting the phase error between multiple excitations in the 3D K-space data collected in the gradient spin echo module. Figure 3 As shown, the processing flow in the navigation echo module is as follows:

[0094] Collect 4 segments of navigator echoes in the center of the k-space Z direction, set gradient encoding to perform EPI encoding in the X and Y directions, and TSE encoding in the Z direction, to achieve the acquisition of 3D navigator echoes and obtain a segment of 3D navigator echo signal. The navigator echo module uses the same echo interval (ESP) as the gradient spin echo module, and also needs to apply a stabilizer gradient (stablizer) along the Z axis before and after each EPI readout moment. The number of phase encodings and acceleration factor values ​​in the EPI direction are consistent with those of the gradient spin echo module, and are recorded as N and N respectively. EPI and N a1 .

[0095] Each time the imaging sequence is executed, the above steps 1.1 to 1.5 are repeated, and a 3D GRASE readout signal and a 3D navigation echo signal are obtained accordingly. By repeating steps 1.1 to 1.5 N times, the whole-brain signal can be divided into N segments for signal acquisition in sequence. All N segments of 3D GRASE readout signals constitute the whole-brain 3D K-space data. Corresponding to the whole-brain 3D K-space data are 3D navigator data composed of N segments of 3D navigation echo signals, which can be used for subsequent phase correction. The difference between this acquisition method and the conventional acquisition method is that N TSE The acquisition time is reduced to N times of the original time. At the same time, 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 the acquisition of the whole-brain 3D K-space data is completed, the general automatic calibration parallel acquisition and 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 invention uses N a1 >1 or N a2>1, so a general automatic calibration partial parallel acquisition and reconstruction is needed to fill in the k-space data of the uncollected rows. And because segmented signals are used for acquisition, 3D navigation echo reconstruction is needed to correct the motion error between the signals acquired in each segment. Therefore, in the general automatic calibration parallel acquisition and reconstruction module, the 3-navigator-echo technology is first used to estimate the phase change between the EPI odd and even rows, and then the phase is corrected on all the EPI odd and even row signals acquired for images and navigation. The GRAPPA technology is used to fill in the k-space data of the uncollected 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 general automatic calibration parallel acquisition and reconstruction module is as follows:

[0099] Step 2.1, extracting a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal obtained after each execution of the imaging sequence from the whole-brain 3D K-space data, and using the 3-navigator-echo technology, i.e., using the signals collected at the three navigator echos in the 3D GRASE readout signal, to estimate the phase change between the EPI odd and even rows, and then using the GRAPPA technology to complete the missing rows in the k-space caused by the GRAPPA parallel accelerated imaging in the EPI direction, that is, to complete the missing rows of the 3D GRASE readout signal acquired by the parallel imaging performed along the EPI direction, and obtain the 3D GRASE readout signal after the missing rows 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 navigation echo signal, and then the GRAPPA technology is used to complete the missing rows in the k-space caused by the GRAPPA parallel accelerated imaging in the TSE direction, that is, the missing rows in 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, are filled completely to obtain a complete 3D GRASE readout signal;

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

[0102] First, both the 3D GRASE readout signal and the 3D navigator echo signal are transformed from the 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 sth 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 of the 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 the k-space, thereby obtaining the phase-corrected s-th segment 3D GRASE readout signal.

[0106] The correction process for the sth segment of 3D GRASE readout signal is similar for the remaining segments, that is, s = 1, 2, ..., N. The above operation is repeated for each segment of the signal, and then the empty line 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 with conventional phase correction methods, 3D navigation echo is more sensitive to motion, which greatly solves the problem of image motion artifacts. Compared with conventional acquisition methods, the GRAPPA parallel acquisition method greatly improves the acquisition efficiency, not only shortens the acquisition time, but also reduces TE, so that images with higher signal-to-noise ratio can be obtained.

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

[0110] This example was tested on 7 healthy young male volunteers. The specific parameters are as follows: Siemens Prisma 3T scanner (maximum gradient 80mT / m, maximum switching rate 200mT / m) was used for magnetic resonance scanning, and a 64-channel head coil was used for all scans. Two groups of experiments were conducted in this example, and the 2D EPI sequence commonly used in the prior art was used as a comparison during the experiment.

[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 / 2500ms, only one acquisition, scanning time = 24min.

[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 signal was collected at a frequency of ≈5ms. All other parameters of the scans at all diffusion times remained the same: TR / TE 1 / TE 2=2600 / 84 / 46ms, N EPI = 67, 12 segments were collected along the TSE encoding direction. Average times = 2, b = 500s / mm 2 , 6 directions, b = 0, 4 averages, N a1 =2, N a2 = 2. The scanning time for each diffusion time image was 5 minutes. The other scanning parameters were the same as the 3D PG-GRASE sequence based on the 3D navigator in Experiment 1.

[0116] Figure 4 The b0 image (3Dnavigator_b0) and b1000 image (3D navigator_b1000) obtained by 3D navigator echo correction and the b0 image (nonnavigator_b0) and b1000 image (non navigator_b1000) obtained without navigator echo correction using 3D PG-GRASE are shown. In contrast, the motion artifacts of the b1000 image after 3D navigator echo correction are corrected, such as the residual artifacts along the z direction (white arrows in the figure) are 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 signal-to-noise ratio of the 3D PG-GRASE sequence is significantly higher than that of the 2D EPI sequence in WM, BG, and Thal of b0 and b1000 images.

[0118] It should be noted that the above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent conversion falls within the protection scope of the present invention.

Claims

1. A 3D gradient spin echo diffusion imaging method based on a 3D navigator, characterized in that: include: S1: The whole-brain signal is acquired by repeatedly executing the 3D gradient spin echo diffusion imaging sequence based on the 3D navigator. After each execution of the imaging sequence, a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal are obtained. 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, thereby obtaining 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 uses a 3D navigation echo signal acquired during the same imaging sequence to correct the phase error, and performing vacant row filling on each segment of the 3D GRASE readout signal after the phase error correction, and finally performing 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, a gradient with completely equal amplitude is applied in the X, Y, and Z directions of the 3D imaging space to eliminate the transverse magnetization. After the last B1 RF pulse is executed, wait until the longitudinal magnetization vector is restored according to the preset post-saturation delay time (PSD) before executing the next diffusion preparation module.

3. 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 diffusion preparation module is as follows: First, a radio frequency excitation pulse with a flip angle of 90° is applied along the X-axis direction; then, a diffusion gradient is applied along the preset diffusion direction, and the diffusion gradient is a trapezoidal cosine oscillation diffusion gradient or a pulse diffusion gradient; then, a radio frequency focusing pulse with a flip angle of 180° is applied along the Y-axis direction, and then the same diffusion gradient is applied again; then, a stabilizer gradient (G stb ); finally, a radio frequency excitation pulse with a flip angle of 90° is performed along the -X axis direction 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, characterized in that: In the imaging sequence, the execution process in the fat saturation module is as follows: 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 set 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 pulse, gradients are synchronously applied in the X, Y, and Z axis 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, characterized in that: In the imaging sequence, the execution process in the gradient spin echo sequence module is as follows: First, signal acquisition is performed through three navigation echoes, and then 3D GRASE readout is achieved by performing echo planar imaging (EPI) encoding in the Y direction and fast spin echo (TSE) encoding in the Z direction to obtain a 3D GRASE readout signal; in the 3D GRASE readout process, linear encoding is performed along the EPI direction, center encoding is performed along the TSE direction, and GRAPPA parallel accelerated imaging is performed in both the EPI and TSE directions. At the same time, a stabilizer gradient needs to be applied along the Z-axis direction 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.

6. 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 processing flow in the navigation echo module is as follows: N segments of spin echo signals at the center of the k-space in the Z direction are collected, EPI encoding is performed along the X and Y directions, and TSE encoding is performed in the Z direction, so as to obtain a segment of 3D navigation echo signal; before and after each segment of EPI readout, a stabilizer gradient needs to be applied along the Z axis direction, 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, characterized in that: In S2, the specific process of performing the general automatic calibration and parallel acquisition and reconstruction operation on the whole brain 3D K-space data is as follows: S21, extracting a segment of 3D GRASE readout signal and a segment of 3D navigator echo signal obtained after each execution of the imaging sequence from the whole-brain 3D K-space data, using the signals collected at three navigator echos in the 3D GRASE readout signal to estimate the phase change between the EPI odd and even rows, and then using the GRAPPA technology to fill in the missing rows in the k-space for performing parallel accelerated imaging acquisition along the EPI direction; S22, for each segment of the 3D GRASE readout signal after the missing lines are filled, the phase error between multiple excitations of the 3D GRASE readout signal is corrected using the corresponding 3D navigation echo signal, and then the GRAPPA technology is used to fill the missing lines of each segment of the 3D GRASE readout signal after the phase error correction, in which the parallel accelerated imaging acquisition is performed 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 by using the 3D navigation 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 three-dimensional image space of xyz to obtain the first phase signal φ of each voxel (x, y, z) of the 3D navigator echo signal in the image space. n,s (x, y, z) and the second phase signal ψ of the 3DGRASE readout signal for 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 the processor, is used to control an external device to implement the 3D gradient spin echo diffusion imaging method based on a 3D navigator as claimed in any one of claims 1 to 8.

10. A magnetic resonance imaging device, characterized in that: It 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 a 3D navigator as claimed in any one of claims 1 to 8.

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