WO2021196865A1 - 一种振荡梯度准备的3d梯度自旋回波成像方法及设备 - Google Patents

一种振荡梯度准备的3d梯度自旋回波成像方法及设备 Download PDF

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WO2021196865A1
WO2021196865A1 PCT/CN2021/074841 CN2021074841W WO2021196865A1 WO 2021196865 A1 WO2021196865 A1 WO 2021196865A1 CN 2021074841 W CN2021074841 W CN 2021074841W WO 2021196865 A1 WO2021196865 A1 WO 2021196865A1
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gradient
module
diffusion
sequence
epi
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French (fr)
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吴丹
徐义程
李浩天
张祎
孙毅
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浙江大学
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Priority to US17/952,341 priority patent/US20230125982A1/en

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    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
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    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/563—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution of moving material, e.g. flow contrast angiography
    • G01R33/56341—Diffusion imaging
    • G—PHYSICS
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    • G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
    • G01R33/4824—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory
    • G01R33/4826—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory in three dimensions
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
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    • G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
    • G01R33/4833—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices
    • G01R33/4835—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices of multiple slices
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    • G01—MEASURING; TESTING
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    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5611—Parallel magnetic resonance imaging, e.g. sensitivity encoding [SENSE], simultaneous acquisition of spatial harmonics [SMASH], unaliasing by Fourier encoding of the overlaps using the temporal dimension [UNFOLD], k-t-broad-use linear acquisition speed-up technique [k-t-BLAST], k-t-SENSE
    • G—PHYSICS
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    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G01R33/5616—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using gradient refocusing, e.g. EPI
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G01R33/5618—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using both RF and gradient refocusing, e.g. GRASE
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
    • G01R33/5619—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences by temporal sharing of data, e.g. keyhole, block regional interpolation scheme for k-Space [BRISK]

Definitions

  • This application relates to the field of magnetic resonance technology, in particular to the field of diffusion magnetic resonance imaging.
  • Diffusion MRI can detect the microstructure of the tissue based on the restricted diffusion of water molecules in the biological tissue under the microstructure environment.
  • dMRI Diffusion MRI
  • the signal strength depends not only on the direction of diffusion and the value of b, but also on the diffusion time and relaxation time.
  • the diffusion time determines the sensitivity of dMRI to microstructural features at different spatial scales.
  • the acquisition technology based on the oscillatory gradient dMRI may achieve a shorter diffusion time ( ⁇ 5ms), which is more conducive to the short-term measurement of diffusion parameters.
  • the oscillating code gradient is composed of multiple periods of cosine or trapezoidal cosine waveforms, and has been widely used in animal imaging systems to study the time-dependent dMRI of animals and water models.
  • the main challenge for the conversion of this technology to clinical applications is the need for a strong oscillation gradient. Since the b value of the OG waveform is inversely proportional to the cube of its oscillation frequency, even if a high-end clinical scanner with a maximum gradient of 80mT/m is used, the highest oscillation frequency is limited to between 40-60Hz, and the corresponding b value is about 300- 500s/mm 2 . This results in a limited short diffusion time that can be achieved on a clinical scanner and low signal contrast-to-noise ratio.
  • the conventional OG-dMRI sequence still has other problems that will reduce the image quality and hinder the clinical conversion of OG-dMRI.
  • a long oscillation gradient with multiple cycles is usually required to achieve a reasonable b value on a clinical scanner. Therefore, a longer echo time (TE) is required for image acquisition, resulting in a low signal-to-noise ratio (SNR) The problem.
  • TE echo time
  • SNR signal-to-noise ratio
  • 2D multislice acquisition since strong oscillation gradients need to be repeatedly applied to each slice, it will result in a higher duty cycle and system heat generation.
  • TRs repetition times
  • the present invention proposes a 3D gradient spin echo sequence (OGprep-GRASE) prepared by an oscillating gradient to solve the above-mentioned two OG-dMRI sequence problems and improve dMRI imaging performance.
  • OGprep-GRASE 3D gradient spin echo sequence
  • the present invention provides a 3D gradient spin echo imaging method prepared by oscillation gradient, which includes:
  • S1 Add a global saturation module at the beginning of the sequence to destroy the previous residual transverse magnetization; then wait for a post-saturation delay (PSD) time to restore the longitudinal magnetization vector;
  • PSD post-saturation delay
  • MUSE Multiplex Sensitivity Code Reconstruction
  • each step can further provide the following preferred implementation manners. It should be noted that the technical features in each preferred manner can be combined with each other without conflict. Of course, these preferred manners can also be implemented in other manners that can achieve the same technical effect, and do not constitute a limitation.
  • the method for establishing the global saturation module in step S1 is as follows:
  • the post-saturation delay is the time interval between the saturation module and the diffusion coding module.
  • the method for establishing the diffusion preparation module in step S2 is as follows:
  • the method for establishing the fat saturation module in step S3 is as follows:
  • the method for establishing the gradient spin echo sequence module in step S4 is as follows:
  • 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; set the number of spin echoes to N SE and the number of gradient echoes The number is N EPI ; the segment reading can be performed along the EPI direction, while the center encoding is enforced along the TSE direction; the stabilizer gradient is applied along the Z axis before and after each EPI reading.
  • EPI echo planar imaging
  • TSE turbo spin echo
  • the method for establishing the multiplex sensitivity coding reconstruction module in step S5 is as follows:
  • the conventional Sensitivity Encoding (SENSE) technique is used to estimate the phase changes between multiple EPI segments caused by motion, and then the amplitude signals of aliasing voxels are jointly calculated in all interleaved EPIs at the same time.
  • SENSE Sensitivity Encoding
  • the present invention provides a magnetic resonance imaging device, which includes a magnetic resonance scanner and a control unit, the control unit stores an imaging sequence, and when the imaging sequence is executed, it is used to implement the first The 3D gradient spin echo imaging method of oscillating gradient preparation according to any one of the aspects.
  • the present invention has the following characteristics: a 3D oscillating gradient preparation gradient spin echo sequence is proposed, which combines the oscillating gradient diffusion coding sequence and the gradient spin echo sequence for the first time.
  • the 3D OGprep-GRASE sequence not only shortens the image scanning time but also improves the signal-to-noise ratio.
  • Figure 1 is a timing diagram of the 3D OGprep-GRASE sequence.
  • Figure 2 is a comparison of the results before and after the MUSE reconstruction using the OGprep-GRASE and PGprep-GRASE sequences that were stimulated twice.
  • Figure 3 is the comparison result of scanning time and signal-to-noise ratio of OG-dMRI using 2D EPI and 3D GRASE readout sequences.
  • Figure 4 is the comparison result of OG-dMRI using 2D EPI and 3D GRASE readout sequences to quantify DTI indicators: apparent diffusion coefficient map, anisotropy score map, and diffusion direction coded color map.
  • Figure 5 shows the comparison results of the diffusion time-dependent comparison of DTI quantitative indicators obtained by OG-dMRI using 2D EPI and 3D GRASE readout sequences respectively.
  • Fig. 1 shows the 3D gradient spin echo sequence (3D OGprep-GRASE) of the oscillation gradient preparation provided in a preferred implementation of the present invention, which is a new type of oscillating gradient field dispersion magnetic resonance Three-dimensional imaging sequence.
  • 3D gradient spin echo sequence (3D OGprep-GRASE) of the oscillation gradient preparation provided in a preferred implementation of the present invention, which is a new type of oscillating gradient field dispersion magnetic resonance Three-dimensional imaging sequence.
  • the following detailed description of the specific method of the sequence includes the following steps:
  • Step 1 Add a global saturation module at the beginning of the sequence to destroy the previous residual transverse magnetization.
  • the specific method of the global saturation module is: at the beginning of the sequence sequence, first apply the B1 radio frequency pulse; each time the radio frequency pulse is applied, gradients are applied in the three directions of X, Y, and Z at the same time, and the amplitudes of the three gradients are completely equal. After applying 3 radio frequency pulses, it can be considered that the transverse magnetization remaining in the previous signal acquisition has been completely eliminated. Then wait for a period of time called post-saturation delay (PSD) to wait for the recovery of the longitudinal magnetization vector.
  • PSD post-saturation delay
  • the time interval between the saturation module and the diffusion coding module is recorded as PSD.
  • Step 2 Immediately after the PSD, the diffusion preparation module is used to embed a pair of trapezoidal cosine oscillation gradients or pulse gradients into the 90° x -180° y -90° -x RF pulse to realize the separation of diffusion coding and signal acquisition.
  • the specific method of the diffusion preparation module is as follows: firstly, launch a non-slice-selective hard pulse excitation with a flip angle of 90° along the X axis; then, apply a trapezoidal cosine oscillation gradient along the direction of setting the dispersion, and the magnitude of the gradient and the setting
  • the b value of is related; then, due to the insensitivity of the adiabatic tangent pulse to the B1 field, the adiabatic tangent pulse excitation with a flip angle of 180° along the Y-axis direction will refocus the divergent transverse magnetization vector; then, apply the same as before Apply a trapezoidal cosine oscillation gradient with exactly the same amplitude, shape, duration, etc.; after that, apply a stabilizer gradient (G stb ) along the Z axis to reduce signal modulation related to phase error, and set the gradient amplitude to The same amplitude as the crusher in the GRASE readout module.
  • G stb stabilizer
  • a non-chip-selective hard pulse excitation with a flip angle of 90° is emitted along the -X axis to convert the refocused transverse magnetization vector into a longitudinal magnetization vector.
  • the echo time in the diffusion preparation module is denoted as TE1.
  • Step 3 Add a fat saturation module after the diffusion preparation module to suppress the fat signal.
  • the specific method of the fat saturation module is: apply a frequency-selective Gaussian pulse with a flip angle of 110°, flip the cos(110°) part of the fat longitudinal magnetization vector, wait for T1*ln(2) time, and restore the When the magnetization vector is 0, the imaging model of step 4 is applied, and the gradient in the Z-axis direction is simultaneously applied to completely eliminate the transverse magnetization vector of fat.
  • Step 4 After the fat saturation module, use the gradient spin echo sequence module to collect signals in 3D K space.
  • the specific method of the gradient spin echo sequence module is to implement 3D GRASE readout by performing echo planar imaging (EPI) encoding in the Y direction and turbo spin echo (TSE) encoding in the Z direction.
  • EPI echo planar imaging
  • TSE turbo spin echo
  • Segmented reading can be performed along the EPI direction, where the segmented reading is the entire or segmented acquisition of N EPI along the EPI direction, and the center encoding is enforced along the TSE direction, that is, the EPI reading after the first focus pulse
  • the way of taking the signal fills the k-space center in the kz direction.
  • the stabilizer gradient is applied along the Z axis at the same time as the breaker gradient beside the refocusing pulse before and after each EPI readout.
  • the echo time in the GRASE module is denoted as TE2, which is equal to the echo interval (ESP) for subsequent signal acquisition in TSE mode. Therefore, the total echo time of the acquired signal is TE1+TE2.
  • the phase encoding in the EPI direction is divided into N segments for signal acquisition, it is necessary to repeat steps 1 to 4 for each segment of the signal acquisition.
  • the N EPI of the segment acquisition method is reduced.
  • the output time is short and the signal attenuation is small, which can improve the image resolution and reduce the deformation effect of EPI, but the acquisition time becomes N times the original, and it is more sensitive to motion, and the motion error between each segment needs to be corrected. But it greatly improves the signal-to-noise ratio of the image.
  • Step 5 Use the multiplex sensitivity coding (MUSE) reconstruction module to correct the phase error that exists between multiple excitations. That is to say, if the segmented signal is used for acquisition along the EPI direction in step 4, MUSE reconstruction is required to correct the motion error between the signals collected in each segment.
  • MUSE reconstruction the conventional Sensitivity Encoding (SENSE) technology is first used to estimate the phase changes between multiple EPI segments caused by motion, and then the amplitude signals of aliased voxels are jointly calculated in all interleaved EPIs at the same time.
  • SENSE Sensitivity Encoding
  • the MUSE method greatly improves the matrix inversion conditions, so images with higher signal-to-noise ratio can be obtained.
  • the 3D OGprep-GRASE sequence in the present invention not only shortens the scanning time of the image but also improves the signal-to-noise ratio.
  • the present invention also carries out the following steps.
  • Step 6 After processing steps 1 to 5, post-processing is performed to obtain the quantitative indicators of DTI: Apparent Diffusion Coefficient (ADC), Anisotropy Score (FA), and Diffusion Direction Encoded Color Image (DEC).
  • ADC Apparent Diffusion Coefficient
  • FA Anisotropy Score
  • DEC Diffusion Direction Encoded Color Image
  • the post-processing method is as follows: take out the acquired images with different b values with diffusion gradient applied along the three directions of X, Y, and Z, and calculate the diffusion value Dxx of each pixel of the image in the three directions of X, Y, and Z respectively. , Dyy, Dzz, the ADC value can be obtained by taking the average of the three; the general formula for calculating the diffusion value D in any one direction is:
  • S1 and S2 are the image pixel values under two different b values in the current calculation direction.
  • D values in 6 directions are obtained according to the calculation formula of diffusion (the subscript of D indicates the direction), and the D values in 6 directions can be constructed into a matrix D:
  • the FA value of each voxel in the image is obtained by the following formula:
  • the eigenvector corresponding to the largest eigenvalue ⁇ among the three eigenvalues is regarded as the main direction of diffusion, and the diffusion direction coding color map is drawn according to the difference of the three-dimensional space direction.
  • the gradient spin echo sequence method prepared by the above 3D oscillation gradient was tested in 7 healthy young male volunteers.
  • the specific parameters here are described below:
  • the MRI scan uses a Siemens Prisma 3.0T scanner, all scans Both use 64-channel head coils at a maximum gradient of 80mT/m. Two sets of experiments were carried out in this example.
  • Figure 2 shows the images acquired using OG- and PG-prepared GRASE sequences before and after MUSE reconstruction. It can be seen from the figure that for the GRASE acquisition method with two excitations, the motion artifact is not obvious in the OG-dMRI image, but it is obvious in the PG-dMRI image, and MUSE can correct the phase caused by the motion to a large extent. error.
  • Figure 3 shows that in experiment 1, for the 10-level and 20-level acquisition protocols, compared with the 2D EPI sequence, the scanning speed of 3D OGprep-GRASE is increased by 1.34 and 1.38 times, respectively.
  • GRASE and EPI sequences show similar image signal-to-noise ratios, but for 3D GRASE sequences, when the number of layers increases to 20 layers, the image signal-to-noise ratio doubles, while the image signal-to-noise ratio of 2D EPI data The ratio remains the same.
  • Figure 4 shows the indicators of apparent dispersion system (ADC), anisotropy (FA) and direction-encoded color image (DEC) obtained by OG-dMRI using the readout data of 2D EPI and 3D GRASE respectively after diffusion tensor reconstruction. result. It can be seen from the image that noise points can be observed in the 2D EPI data of the 10, 20, and 30 level protocol. In 10 slices of 3D GRASE data, the noise is similar to EPI data, but as the number of slices increases to 20 and 30, the image gradually becomes clear. It shows that the 3D GRASE read-out sequence captures an image with a higher signal-to-noise ratio.
  • ADC apparent dispersion system
  • FA anisotropy
  • DEC direction-encoded color image
  • the 3D OGprep-GRASE sequence of the present invention effectively improves the imaging time and the signal-to-noise ratio, and contributes to the time-dependent The clinical transformation of diffusion MRI technology.
  • a magnetic resonance imaging device which includes a magnetic resonance scanner and a control unit, and the control unit stores an imaging sequence, that is, a 3D OGprep-GRASE sequence.
  • the imaging sequence is executed, it is used to realize the 3D gradient spin echo imaging method of the oscillation gradient preparation in the foregoing steps 1 to 5.
  • the hardware structure of the magnetic resonance scanner and the control unit here 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.
  • the control unit should contain the necessary data processing and image reconstruction software for imaging.
  • the 3D OGprep-GRASE sequence can be read by the control unit and used to control the magnetic resonance scanner. In other words, the 3D OGprep-GRASE sequence can be directly applied to the corresponding magnetic resonance imaging equipment.

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Abstract

一种振荡梯度准备的3D梯度自旋回波成像方法及设备。该成像方法包括以下步骤：首先，利用全局饱和模块中破坏先前残余的横向磁化；其次，通过扩散编码模块，将一对梯形余弦振荡梯度嵌入到90° x-180° y-90° -x射频脉冲中，将扩散编码与信号采集分开；然后，使用脂肪饱和模块对脂肪信号进行抑制；最后，采用梯度自旋回波的读出方式采集信号，并使用多路复用灵敏度编码重建校正了多次激发之间的相位误差。与在3T临床系统上使用的基于2D平面回波的振荡梯度弥散序列相比，3D振荡梯度准备的梯度自旋回波序列有效提高了成像时间和信噪比，有助于时间依赖性的弥散MRI技术的临床转化。

Description

一种振荡梯度准备的3D梯度自旋回波成像方法及设备 技术领域
本申请涉及磁共振技术领域，尤其涉及弥散磁共振成像领域。
背景技术
扩散MRI(dMRI)可以根据生物组织中的水分子在微结构环境下受限扩散的情况对组织的微结构进行探测。通过改变扩散编码方案，可以采集到多维的dMRI信号，该信号强度不仅取决于扩散的方向和b值，还取决于扩散时间和弛豫时间。具体来说，扩散时间决定了dMRI对在不同空间尺度下的微结构特征的敏感性。在不同的扩散时间测量dMRI信号，即可获取水分子扩散的时间依赖性，而时间依赖性曲线反映了重要的微结构特性，例如细胞大小，细胞内分数，表面积与体积比等。相比于采用传统脉冲梯度dMRI(PG-dMRI)序列，基于振荡梯度dMRI(OG-dMRI)的采集技术可能实现更短的扩散时间(<5ms)，更有利于扩散参数的短时测量。
振荡编码梯度由多个周期的余弦或梯形的余弦波形组成，目前已经被广泛用于在动物成像系统中对动物和水模的时间依赖性dMRI的研究。然而，该技术转化到临床应用面临的主要挑战是需要很强的振荡梯度。由于OG波形的b值与其振荡频率的立方成反比，所以即使使用最大梯度为80mT/m的高端临床扫描仪，最高振荡频率也被限制在40-60Hz之间，对应的b值约为300-500s/mm 2。这导致了临床扫描仪上可以实现的短扩散时间有限且信号的对比度-噪声比较低。同时，常规的OG-dMRI序列仍然存在其他的问题会降低图像质量并阻碍OG-dMRI的临床转换。一方面，通常需要具有多个周期的长振荡梯度才能在临床扫描仪上达到合理的b值，因此图像采集时需要采用较长的回波时间(TE)，进而导致信噪比(SNR)低的问题。另一方面，对于2D多层面(multislice)采集，由于需要对每个层面重复应用强振荡梯度，因此会导致较高的占空比和系统产热量。综上所述，与PG-dMRI相比，OG-dMRI需要使用较长的重复时间(TRs)以降低占空比，使得总采集时间变长。
发明内容
为了克服现有序列中存在的不足，本发明提出了一种振荡梯度准备的3D梯度自旋回波序列(OGprep-GRASE)，以解决上述提及的两个OG-dMRI序列的问题，提高dMRI成像性能。
为了达到上述目的，本发明采用以下技术方案予以实现：
第一方面，本发明提供了一种振荡梯度准备的3D梯度自旋回波成像方法，其包括：
S1：在序列开始的位置添加全局饱和模块，以破坏之前残余的横向磁化；然后等待一段饱和后延迟(PSD)时间，使纵向磁化矢量恢复；
S2：在饱和后延迟(PSD)之后添加扩散准备模块，将一对梯形余弦振荡梯度或脉冲梯度嵌入到90° x-180° y-90° -x射频脉冲中，以实现扩散编码与信号采集的分离；
S3：在扩散准备模块后添加脂肪饱和模块，抑制脂肪信号；
S4：在脂肪饱和模块后，使用梯度自旋回波序列模块对3D K空间进行信号采集；
S5：使用多路复用灵敏度编码重建(MUSE)模块校正多次激发之间存在的相位误差。
基于上述第一方面的方案，各步骤还可以进一步提供以下优选的实现方式。需要注意的是，各优选方式中的技术特征在没有冲突的情况下均可进行相互组合。当然这些优选方式也可以通过其他能够实现相同技术效果的方式实现，不构成限制。
作为优选，所述步骤S1中的全局饱和模块建立方法如下：
每施加一次射频脉冲，同时在X、Y、Z三个方向施加梯度，将上次信号采集时残留的横向磁化完全消除；施加3次射频脉冲后，等待一段饱和后延迟(PSD)时间，所述饱和后延迟(PSD)为饱和度模块和扩散编码模块之间的时间间隔。
作为优选，所述步骤S2中的扩散准备模块建立方法如下：
首先，沿X轴方向进行90°翻转角的非层选性硬脉冲激励；然后，沿着设置弥散的方向施加梯形余弦振荡梯度；接着，沿Y轴方向进行180°翻转角的非层选性绝热正切脉冲，然后施加完全相同的梯形余弦振荡梯度；之后沿Z轴方向 施加稳定器梯度(G stb)；最后，沿-X轴方向进行90°翻转角的非层选性方硬脉冲激励，将横向磁化矢量转化为纵向磁化矢量。
作为优选，所述步骤S3中的脂肪饱和模块建立方法如下：
施加110°翻转角的频率选择性的高斯脉冲，将脂肪纵向磁化矢量的cos(110°)部分进行翻转并等待T1*ln(2)时间后将其置0，并同时施加Z轴方向的梯度，将脂肪的横向磁化矢量完全消除，其中T1为脂肪的纵向弛豫时间。
作为优选，所述步骤S4中的梯度自旋回波序列模块建立方法如下：
通过在Y方向上进行回波平面成像(EPI)编码以及在Z方向上进行涡轮自旋回波(TSE)编码来实现3D GRASE读出；设定自旋回波个数为N SE，梯度回波个数为N EPI；沿EPI方向可执行分段读取，而沿TSE方向上强制执行中心编码；在每段EPI读出的前后沿Z轴方向施加稳定器梯度。
作为优选，所述步骤S5中的多路复用灵敏度编码重建模块建立方法如下：
首先使用常规的灵敏度编码(SENSE)技术来估计运动引起的多个EPI段之间的相位变化，然后同时在所有交错EPI中联合计算混叠体素的幅度信号。
第二方面，本发明提供了一种磁共振成像设备，其包括磁共振扫描器以及控制单元，所述控制单元中存储有成像序列，所述成像序列被执行时，用于实现如上述第一方面中任一方案所述的振荡梯度准备的3D梯度自旋回波成像方法。
相对于现有技术，本发明具有以下特点：提出了一种3D振荡梯度制备梯度自旋回波序列，首次将振荡梯度扩散编码序列与梯度自旋回波序列结合起来。与传统的临床扫描仪上OG-dMRI序列进行的2D-EPI采集相比,3D OGprep-GRASE序列不仅缩短了图像的扫描时间而且提高了信噪比。通过比较PG和OG-dMRI对成人大脑进行成像的结果，表明无论使用GRASE或者EPI读出序列，人脑的灰质和白质都表现出明显的扩散时间依赖性。
附图说明
图1是3D OGprep-GRASE序列的时序图。
图2是对分别使用两次激发的OGprep-GRASE和PGprep-GRASE序列进行MUSE重建的前后结果对比。
图3是对OG-dMRI分别使用2D EPI和3D GRASE读出序列的扫描时间和信噪比的对比结果。
图4是OG-dMRI分别使用2D EPI和3D GRASE读出序列对DTI量化指标：表观扩散系数图、各向异性分数图、以及扩散方向编码彩图的对比结果。
图5是OG-dMRI分别使用2D EPI和3D GRASE读出序列得到的DTI量化指标的扩散时间依赖性对比结果。
具体实施方式
如附图1下面基于本发明提出的方法结合实施例展示其具体的技术效果，以便本领域技术人员更好地理解本发明的实质。
参见图1所示，展示了本发明的一种较优实现方式中提供的振荡梯度准备的3D梯度自旋回波序列(3D OGprep-GRASE)，该序列是一种新型的振荡梯度场弥散磁共振三维成像序列。下面详细描述该序列具体方法包括以下步骤：
步骤一：在序列开始的位置添加全局饱和模块，以破坏之前残余的横向磁化。全局饱和模块的具体做法为：在序列时序开始时，首先施加B1射频脉冲；每施加一次射频脉冲，同时在X、Y、Z三个方向施加梯度，三个梯度的幅值完全相等。在施加3次射频脉冲后，可以认为已经将上次信号采集时残留的横向磁化完全消除。之后等待一段被称为饱和后延迟(PSD)的时间，等待纵向磁化矢量的恢复。本发明中，将饱和度模块和扩散编码模块之间的时间间隔记为PSD。
步骤二：在PSD之后紧接着扩散准备模块，将一对梯形余弦振荡梯度或脉冲梯度嵌入到90° x-180° y-90° -x射频脉冲中，以实现扩散编码与信号采集的分离。扩散准备模块的具体做法为：首先，沿X轴方向发射90°翻转角的非层选性的硬脉冲激励；然后，沿着设置弥散的方向施加梯形余弦振荡梯度，梯度的幅值大小与设置的b值有关；接着，由于绝热正切脉冲对B1场的不敏感性，沿Y轴方向进行180°翻转角的绝热正切脉冲激励，将散向的横向磁化矢量进行重聚；然后，施加与之前施加的幅值、形状、持续时间等完全相同的梯形余弦振荡梯度；之后，沿Z轴方向施加稳定器梯度(G stb)，以减轻与相位误差有关的信号调制，并将梯度幅值设置为与GRASE读出模块中的破碎机梯度(crusher)相同的幅度。最后，沿-X轴方向发射90°翻转角的非片选性的硬脉冲激励，将重聚的横向磁化矢量转化为纵向磁化矢量。扩散准备模块中的回波时间表示为TE1。
步骤三：在扩散准备模块后添加脂肪饱和模块，抑制脂肪信号。脂肪饱和模块的具体做法为：施加110°翻转角的频率选择性的高斯脉冲，将脂肪纵向磁化矢量的cos(110°)部分进行翻转，等待T1*ln(2)时间，当翻转后恢复的磁化矢量为 0时施加步骤四的成像模型，并同时施加Z轴方向的梯度，将脂肪的横向磁化矢量完全消除。
步骤四：在脂肪饱和模块后，使用梯度自旋回波序列模块对3D K空间进行信号采集。梯度自旋回波序列模块的具体做法为：通过在Y方向上进行回波平面成像(EPI)编码以及在Z方向上进行涡轮自旋回波(TSE)编码来实现3D GRASE读出。在EPI方向上的相位编码的数目被表示为N EPI，而在TSE方向上的相位编码的数目被表示为N SE，即自旋回波个数为N SE，梯度回波个数为N EPI。沿EPI方向可执行分段读取，此处分段读取是沿EPI方向对N EPI进行整段或分段采集，而沿TSE方向上强制执行中心编码，即第一次聚焦脉冲后的EPI读取信号的方式在kz方向填充k-space中心。稳定器梯度沿Z轴方向在每段EPI读出的前后与重聚焦脉冲旁边的破碎机梯度同时施加。GRASE模块中的回波时间表示为TE2，它等于之后以TSE方式进行信号采集的回波间隔(ESP)。因此，所获取信号的总回波时间为TE1+TE2。
当把EPI方向的相位编码分成N段进行信号采集时，对每一段的信号采集都需要重复一遍步骤一到步骤四，这样相对于常规采集方式，分段采集方式的N EPI减少，每次读出时间较短，信号衰减较小，可提高图像分辨率和减小EPI的变形效应，但采集时间变为原来的N倍，同时对运动较为敏感，需要矫正每段之间的运动误差。但是极大地提高了图像的信噪比。
步骤五：使用多路复用灵敏度编码(MUSE)重建模块校正多次激发之间存在的相位误差。也就是说，步骤四中沿EPI方向如果使用分段信号进行采集，则需要MUSE重建来矫正每段采集到的信号之间的运动误差。MUSE重建时，首先使用常规的灵敏度编码(SENSE)技术来估计运动引起的多个EPI段之间的相位变化，然后同时在所有交错EPI中联合计算混叠体素的幅度信号。与常规的SENSE方法相比，MUSE方法大大改善了矩阵求逆条件，因此可以得到更高信噪比的图像。
由此可见，通过上述步骤一到五，本发明中的3D OGprep-GRASE序列，不仅缩短了图像的扫描时间而且提高了信噪比。为了进一步展示该做法的效果，本发明还进行了下列步骤。
步骤六：经过步骤一到五处理后，再进行后处理得到DTI的量化指标：表观扩散系数(ADC)、各向异性分数(FA)、以及扩散方向编码彩图(DEC)。 其中后处理方法如下：将获取到的沿X、Y、Z三个方向施加弥散梯度的不同b值的图像取出，分别计算图像每个像素点在X、Y、Z三个方向的扩散值Dxx、Dyy、Dzz，三者取均值即可得到ADC值；其中任意一个方向的扩散值D的计算通式为：
D＝-ln(S2/S1)/(b2-b1)
式中S1和S2分别为当前计算方向中两个不同b值下的图像像素值。
另外，根据扩散的计算公式求得6个方向的D值(D的下标表示方向)，6个方向的D值可以构建成矩阵D：
Figure PCTCN2021074841-appb-000001
通过求取矩阵D的3个特征向量和3个特征值λ 1、λ 2、λ 3，再通过以下公式求得图像中每个体素的FA值：
Figure PCTCN2021074841-appb-000002
将3个特征值中最大特征值λ对应的特征向量当作扩散的主方向，根据三维空间方向的不同绘制了扩散方向编码彩图。
下面基于上述方法的步骤一到六，结合实施例对其技术效果进行展示，以便本领域技术人员更好地理解本发明的实质。
实施例
将上述的3D振荡梯度准备的梯度自旋回波序列方法在7名健康的年轻男性志愿者中进行了测试，下面介绍此处的具体参数：核磁共振扫描使用了西门子Prisma 3.0T扫描仪，所有扫描均使用64通道头部线圈在最大梯度为80mT/m上进行。本实施例中进行了两组实验。
实验一：为了比较3D OGprep-GRASE和2D OG-EPI序列的扫描时间和图像信噪比，以50Hz的振荡梯度，b＝500s/mm 2、12个方向，2次重复，FOV＝220×220mm，分辨率为2.75×2.75mm 2，切片厚度为3mm(3D GRASE为2.75×2.75×3mm 3)执行OG-dMRI扫描，采用以下方案：
(1)单次激发的GRASE读出序列，N EPI＝80，N SE＝10，turbo因子＝12， EPI因子＝63，层面过采样＝20％，部分傅立叶因子＝61％，带宽＝2,500Hz，TE1/TE2/TR＝124/33.6/3000ms，扫描时间＝1.5min；与之对应的2D-EPI读出序列，扫描了10个层面，部分傅立叶因子＝6/8，带宽＝1,690Hz，TE/TR＝158/4200ms(最小TR)，扫描时间＝2.0min。
(2)两次激发的GRASE读出序列，N EPI＝80，N SE＝20，turbo因子＝24，EPI因子＝27，TE1/TE2/TR＝124/22.9/3000ms，扫描时间＝2.9min，其他参数与单次激发的GRASE读出序列的参数一致；与之对应的2D-EPI读出序列，扫描了20个层面，部分傅里叶因子＝6/8，TE/TR＝147/8400ms(最小TR)，扫描时间＝4.0min，其他参数与单次激发对应的2D-EPI相同。
(3)三次激发的GRASE读出序列，N EPI＝80，N SE＝30，turbo因子＝36，EPI因子＝21，TE1/TE2/TR＝124/20.2/3000ms，扫描时间为4.3min，其他参数与单次激发的GRASE读出序列的参数一致；与之对应的2D-EPI读出序列，扫描了20个层面，部分傅里叶因子＝6/8，TE/TR＝144/12600ms(最小TR)，扫描时间＝6.0min，其他参数与单次激发对应的2D-EPI相同。
实验二：为了测试本发明中的序列对不同扩散时间下扩散系数测量的可靠性，以25Hz和50Hz的振荡频率，b＝600s/mm 2，6个方向，TE1/TE2/TR＝84/32/3000ms执行单次激发的OGprep-GRASE序列，矩阵尺寸＝80×80×10，成像分辨率＝2.75×2.75×5mm；PGprep-GRASE序列，其δ＝20ms，Δ＝30和60ms，其他参数与OGprep序列一致。作为比较，还以25Hz，50Hz的振荡梯度和δ＝20ms，Δ＝30/60ms的脉冲梯度以及其他与prep-GRASE扫描一致的参数采集了2D EPI数据。最后采集了一组受试者的两次激发PG-dMRI和OG-dMRI，以测试多路复用灵敏度编码(MUSE)重建算法对运动伪影校正的有效性。
附图2显示了MUSE重建前后使用OG-和PG-prepared GRASE序列采集得到的图像。从图中可以看出，对于两次激发的GRASE采集方式，运动伪影在OG-dMRI图像中不明显，但在PG-dMRI图像中明显，并且MUSE可以在很大程度上纠正运动引起的相位误差。
附图3显示了在实验1中对于10层面和20层面的采集协议，与2D EPI序列相比，3D OGprep-GRASE的扫描速度分别提高了1.34和1.38倍。对于10层面协议，GRASE和EPI序列显示出相似的图像信噪比，但是对于3D GRASE序 列，当层面数量增加到20层面时，图像信噪比增加了一倍，而2D EPI数据的图像信噪比保持不变。
附图4是OG-dMRI分别使用2D EPI和3D GRASE的读出数据经过弥散张量重建后得到的指标表观弥散系统(ADC)，各向异性(FA)和方向编码彩图(DEC)的结果。从图像中可以看到在10、20和30层面协议的2D EPI数据中可以观察到噪声点。在10层面的3D GRASE数据中，噪声类似于EPI数据，但是随着层面数量增加到20和30，图像逐渐变得清晰。说明3D GRASE读出序列采集得到了较高信噪比的图像。
在OG-dMRI(50Hz和25Hz)和PG-dMRI(30ms和60ms)的3D GRASE和2D EPI序列成像得到的图像中，分别求得在人脑的皮层下白质和深部灰质中的ADC数据。如附图5所示，来自两个序列的ADC值在白质和灰质区域均表现出显著的时间依赖性(p<0.0001)，并且通过双向ANOVA测试并未发现显著的序列差异，说明3D OGprep-GRASE序列可有效测量ADC的时间依赖信，结果与普通2D EPI序列一致。
由此可见，与在3T临床系统上使用的基于2D平面回波的振荡梯度弥散序列相比，本发明的3D OGprep-GRASE序列有效提高了成像时间和信噪比，有助于时间依赖性的弥散MRI技术的临床转化。
基于上述3D OGprep-GRASE序列，在其他实施例中还可以进一步提供一种磁共振成像设备，其包括磁共振扫描器以及控制单元，控制单元中存储有成像序列，即3D OGprep-GRASE序列。该成像序列被执行时，用于实现前述步骤一到五中的振荡梯度准备的3D梯度自旋回波成像方法。
需要注意的是，此处的磁共振扫描器和控制单元的硬件结构可采用常规磁共振成像系统实现。磁共振扫描器应当包括磁体部分和磁共振波谱仪部分，控制单元中应当含有实现成像必要的数据处理和图像重建软件，3D OGprep-GRASE序列可由控制单元读取并用于控制磁共振扫描器。也就是说，3D OGprep-GRASE序列可以直接应用至相应的磁共振成像设备中。
需要指出的是，以上所述的实施例只是本发明的一种较佳的方案，然其并非用以限制本发明。有关技术领域的普通技术人员，在不脱离本发明的精神和范围 的情况下，还可以做出各种变化和变型。因此凡采取等同替换或等效变换的方式所获得的技术方案，均落在本发明的保护范围内。

Claims (7)

  1. 一种振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，包括：
    S1：在序列开始的位置添加全局饱和模块，以破坏之前残余的横向磁化；然后等待一段饱和后延迟(PSD)时间，使纵向磁化矢量恢复；
    S2：在饱和后延迟(PSD)之后添加扩散准备模块，将一对梯形余弦振荡梯度或脉冲梯度嵌入到90° x-180° y-90° -x射频脉冲中，以实现扩散编码与信号采集的分离；
    S3：在扩散准备模块后添加脂肪饱和模块，抑制脂肪信号；
    S4：在脂肪饱和模块后，使用梯度自旋回波序列模块对3D K空间进行信号采集；
    S5：使用多路复用灵敏度编码重建模块校正多次激发之间存在的相位误差。
  2. 如权利要求1所述的振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，所述步骤S1中的全局饱和模块建立方法如下：
    每施加一次射频脉冲，同时在X、Y、Z三个方向施加梯度，将上次信号采集时残留的横向磁化完全消除；施加3次射频脉冲后，等待一段饱和后延迟(PSD)时间，所述饱和后延迟(PSD)为饱和度模块和扩散编码模块之间的时间间隔。
  3. 如权利要求1所述的振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，所述步骤S2中的扩散准备模块建立方法如下：
    首先，沿X轴方向进行90°翻转角的非层选性硬脉冲激励；然后，沿着设置弥散的方向施加梯形余弦振荡梯度；接着，沿Y轴方向进行180°翻转角的非层选性绝热正切脉冲，然后施加完全相同的梯形余弦振荡梯度；之后沿Z轴方向施加稳定器梯度(G stb)；最后，沿-X轴方向进行90°翻转角的非层选性方硬脉冲激励，将横向磁化矢量转化为纵向磁化矢量。
  4. 如权利要求1所述的振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，所述步骤S3中的脂肪饱和模块建立方法如下：
    施加110°翻转角的频率选择性的高斯脉冲，将脂肪纵向磁化矢量的cos(110°)部分进行翻转并等待T1*ln(2)时间后将其置0，并同时施加Z轴方向的梯度，将脂肪的横向磁化矢量完全消除，其中T1为脂肪的纵向弛豫时间。
  5. 如权利要求1所述的振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，所述步骤S4中的梯度自旋回波序列模块建立方法如下：
    通过在Y方向上进行回波平面成像(EPI)编码以及在Z方向上进行涡轮自旋回波(TSE)编码来实现3D GRASE读出；设定自旋回波个数为N SE，梯度回波个数为N EPI；沿EPI方向可执行分段读取，而沿TSE方向上强制执行中心编码；在每段EPI读出的前后沿Z轴方向施加稳定器梯度。
  6. 如权利要求1所述的振荡梯度准备的3D梯度自旋回波成像方法，其特征在于，所述步骤S5中的多路复用灵敏度编码重建模块建立方法如下：
    首先使用常规的灵敏度编码(SENSE)技术来估计运动引起的多个EPI段之间的相位变化，然后同时在所有交错EPI中联合计算混叠体素的幅度信号。
  7. 一种磁共振成像设备，其特征在于，包括磁共振扫描器以及控制单元，所述控制单元中存储有成像序列，所述成像序列被执行时，用于实现如权利要求1～6任意所述的振荡梯度准备的3D梯度自旋回波成像方法。
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