WO2023035930A1 - 一种减少图像失真的磁共振平面回波成像方法 - Google Patents

一种减少图像失真的磁共振平面回波成像方法 Download PDF

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WO2023035930A1
WO2023035930A1 PCT/CN2022/114111 CN2022114111W WO2023035930A1 WO 2023035930 A1 WO2023035930 A1 WO 2023035930A1 CN 2022114111 W CN2022114111 W CN 2022114111W WO 2023035930 A1 WO2023035930 A1 WO 2023035930A1
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echo
gradient
chain
gradient echo
magnetic resonance
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French (fr)
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张志勇
陈浩
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上海交通大学
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    • 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
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities

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  • the present application relates to the field of magnetic resonance imaging (MRI, Magnetic Resonance Imaging), in particular to a magnetic resonance planar echo imaging method for reducing image distortion.
  • MRI Magnetic Resonance Imaging
  • Magnetic resonance imaging Magnetic Resonance Imaging, MRI
  • Magnetic Resonance Imaging can detect the clear anatomical structure of living tissue without damage, reflect the images of organic lesions, and provide physiological information to meet different diagnostic needs. It is one of the most commonly used medical detection methods today.
  • Magnetic resonance echo planar imaging (Echo Planar Imaging, EPI) only needs one radio frequency excitation to complete the scanning and acquisition of the entire two-dimensional k-space data, and can obtain a complete two-dimensional image within tens of milliseconds, which is the fastest imaging speed at present.
  • EPI breaks through the limitation of scanning speed on the application of MRI technology, and further expands the application range of MRI technology. EPI has been widely used in magnetic resonance imaging scans such as diffusion imaging, perfusion imaging and functional imaging, and is an important tool for clinical disease diagnosis and scientific research.
  • EPI is extremely sensitive to the inhomogeneity of the magnetic field and chemical shift artifacts.
  • image distortion and signal loss are prone to occur, especially at the junction of air and tissue, such as the forehead and eye sockets in brain scanning.
  • non-Cartesian sampling trajectories such as helical sampling can also effectively shorten the sampling time of MRI data
  • these non-Cartesian sampling methods are also sensitive to the imperfection of magnetic resonance gradient performance, and the stability needs further study.
  • some pre-scans to obtain correction information or multiple scans can correct or reduce image distortion caused by inhomogeneous magnetic fields, but it will reduce scanning efficiency and face possible inconsistencies between multiple scans.
  • the existing single-scan EPI realizes continuous and rapid encoding sampling of k-space through continuous gradient echo sampling and phase encoding blip gradient, so that the data filling of the entire k-space can be completed in one excitation scan, and then a two-dimensional image can be obtained.
  • the gradient echo sampling method cannot refocus the evolution phase generated by the inhomogeneity of the magnetic field like the spin echo, and as the spatial encoding progresses, the phase difference gradually accumulates, resulting in distortion and distortion of the final image.
  • a single-scan EPI sequence in the prior art is shown in FIG. 1 .
  • the existing multi-excitation scanning EPI method divides the k-space into multiple segments for phase encoding, which can shorten the time interval of the phase encoding dimension, reduce the accumulation of phase difference caused by the inhomogeneity of the magnetic field, and effectively reduce the distortion of the EPI image and improve the resolution.
  • the k-space is divided into multiple segments and needs to be combined with data from multiple excitation scans. It cannot be filled in a single excitation scan, and it is easily disturbed by the motion of the detection part, resulting in motion artifacts.
  • the motion artifact problem of multi-scan EPI can be improved by relying on technologies such as PROPELLER, the way of multiple excitation scanning makes the scanning time longer and the scanning efficiency is reduced. It is not applicable in some scenes that require high temporal resolution, which also limits Improvement and further application of EPI image quality.
  • the k-space filling trajectory in the prior art is shown in FIG. 2 .
  • the present application provides a magnetic resonance echo planar imaging method that reduces image distortion, based on a first radio frequency pulse with a flip angle of ⁇ , a second radio frequency pulse with a flip angle of ⁇ , and a spin echo
  • the module and the first gradient echo chain and the second gradient echo chain for collecting planar echo signals are characterized in that they specifically include the following steps:
  • Step 1 using the first radio frequency pulse to excite the layer, exciting the first part of the magnetization vector to the transverse plane, and keeping the second part of the magnetization vector on the longitudinal plane;
  • Step 2 After a delay, use the second radio frequency pulse to excite the layer, and excite the second part of the magnetization vector to the transverse plane;
  • Step 3 applying the spin echo module
  • Step 4 using gradient pulses to refocus the first part of the magnetization vector, and detecting the signal of the first part of the magnetization vector in the transverse direction in the first gradient echo chain to obtain a first signal;
  • Step 5 using gradient pulses to refocus the second part of the magnetization vector, and detecting the signal of the lateral second part of the magnetization vector in the second gradient echo chain to obtain a second signal;
  • Step 6 Alternately store the first signal and the second signal along the phase encoding direction, recombine them into k-space, and obtain a final image.
  • the flip angle of the first radio frequency pulse is 47°; the flip angle of the second radio frequency pulse is 122°.
  • the length of the delay is the time interval between the centers of the first gradient echo chain and the second gradient echo chain.
  • step 3 applying the spin echo module including 180° radio frequency pulses to obtain a T 2 weighted image with reduced distortion.
  • Step 3 a diffusion weighted gradient is applied on both sides of the 180° radio frequency pulse, so as to obtain a diffusion weighted image with reduced distortion.
  • step 3 the spin echo module is not applied to obtain a T 2 * weighted image with reduced distortion.
  • the momentum of the phase encoding gradient is selected as 2/( ⁇ H *FoV PE ), where ⁇ H is the gyromagnetic ratio of the hydrogen nucleus, and FoV PE is the imaging field of view in the phase encoding direction size.
  • the momentum of the phase encoding gradient is selected as 4/( ⁇ H *FoV PE ), where ⁇ H is the gyromagnetic ratio of the hydrogen nucleus, and FoV PE is the imaging field of view in the phase encoding direction size.
  • the navigation echo signals are respectively collected before the first gradient echo chain and the second gradient echo chain, and the amplitude difference of the two navigation echo signals is used to correct the The amplitude difference between the signals of the first gradient echo chain and the second gradient echo chain, the navigation echo is a gradient echo without phase encoding.
  • step 6 the signals of the first gradient echo chain and the second gradient echo chain are reconstructed using the MUSSELS method to obtain two pieces of image data, and the two pieces of image data are averaged to obtain the final image.
  • step 4 and step 5 the first gradient echo chain and the second gradient echo chain adopt phase encoding gradients of different polarities, and the first gradient echo chain and the The signals of the second gradient echo chain are reconstructed into images respectively, and the magnetic field distribution is estimated through the two images, and then image distortion is corrected.
  • step 5 the momentum of the phase encoding gradient is selected as 3/( ⁇ H *FoV PE ), where ⁇ H is the gyromagnetic ratio of the hydrogen nucleus, and FoV PE is the imaging field of view in the phase encoding direction.
  • the technical solution provided by the present application significantly improves the imaging precision and effectively reduces image distortion and distortion through the echo-planar imaging method based on multi-radio frequency excitation.
  • the technical solution of the present application still belongs to a single scan, so compared with the multi-scanning method in the prior art, the imaging efficiency of the technical solution of the present application is significantly improved.
  • FIG. 1 is a schematic diagram of a single-scan EPI sequence in the prior art
  • Fig. 2 is a schematic diagram of k-space filling trajectory in the prior art
  • Fig. 3 is a pulse sequence schematic diagram of an embodiment of the present application.
  • Fig. 4 is a schematic diagram of a k-space filling trajectory according to an embodiment of the present application.
  • Fig. 5 is the 9th layer human brain image adopting an embodiment of the present application.
  • Fig. 6 is the 9th layer human brain image adopting prior art
  • Fig. 7 is the 15th layer human brain image adopting an embodiment of the present application.
  • Fig. 8 is a layer 15 human brain image using the prior art.
  • the transverse magnetization vectors obtained by flipping the longitudinal magnetization vectors by ⁇ and ⁇ are respectively refocused in two different echo chains by gradient pulses.
  • the navigation echo signals are respectively collected before the two gradient echo chains, and the amplitude difference of the two gradient echo chain signals is corrected by the amplitude difference of the two navigation echo signals.
  • the momentum of the phase encoding blip gradient can be selected as 2/( ⁇ H *FoV PE ) or 4/( ⁇ H *FoV PE ).
  • ⁇ H is the gyromagnetic ratio of the hydrogen nucleus
  • FoV PE is the imaging field of view in the phase encoding direction.
  • phase encoding blip gradients of different polarities are respectively used in the first gradient echo chain and the second gradient echo chain, and the signals of the first gradient echo chain and the second gradient echo chain are respectively reconstructed into images,
  • the magnetic field distribution is estimated from two images to correct image distortion.
  • the signal is excited using three pulses and acquired using three gradient echo chains to reconstruct the resulting image.
  • this embodiment is a magnetic resonance echo-planar imaging method that reduces image distortion, based on a first radio frequency pulse with a flip angle of ⁇ , a second radio frequency pulse with a flip angle of ⁇ , a spin echo module and The first gradient echo chain and the second gradient echo chain are used to acquire echo planar signals.
  • the flip angle of the first radio frequency pulse is 47°; the flip angle of the second radio frequency pulse is 122° and specifically includes the following steps:
  • Step 1 using the first radio frequency pulse to excite the layer, exciting the first part of the magnetization vector to the transverse plane, and keeping the second part of the magnetization vector on the longitudinal plane;
  • Step 2 After a delay, use the second radio frequency pulse to excite the layer, and excite the second part of the magnetization vector to the transverse plane; the length of the delay is the time interval between the center of the first gradient echo chain and the second gradient echo chain;
  • Step 3 applying a spin echo module; wherein, applying a spin echo module including a 180° radio frequency pulse to obtain a T2 weighted image with reduced distortion; applying a diffusion weighted gradient on both sides of the 180° radio frequency pulse to obtain a T2 weighted image with reduced distortion Diffusion-weighted image; no spin-echo module applied to obtain T2 *-weighted image with reduced distortion.
  • Step 4 using the gradient pulse to refocus the first part of the magnetization vector, and detecting the signal of the first part of the transverse magnetization vector in the first gradient echo chain to obtain the first signal;
  • Step 5 using the gradient pulse to refocus the second part of the magnetization vector, and detecting the signal of the second part of the transverse magnetization vector in the second gradient echo chain to obtain the second signal;
  • the momentum of the phase encoding gradient can be selected as 2/( ⁇ H *FoV PE ) or 4/( ⁇ H *FoV PE ), where ⁇ H is the gyromagnetic ratio of the hydrogen nucleus, FoV PE is the size of the imaging field of view in the phase encoding direction.
  • ⁇ H is the gyromagnetic ratio of the hydrogen nucleus
  • FoV PE is the size of the imaging field of view in the phase encoding direction.
  • three pulses are used to excite signals and three gradient echo chains are used to acquire signals, at this time, the momentum of the phase encoding gradient can be selected as 3/( ⁇ H *FoV PE ).
  • Step 6 Alternately store the first signal and the second signal along the phase encoding direction, recombine them into k-space, and obtain the final image.
  • the navigation echo signals are respectively collected before the first gradient echo chain and the second gradient echo chain, and the first gradient echo chain and the second gradient echo chain are corrected by the amplitude difference of the two navigation echo signals.
  • the amplitude difference of the signal, the navigation echo is the gradient echo without phase encoding.
  • the signals of the first gradient echo chain and the second gradient echo chain are reconstructed by the MUSSELS method to obtain two pieces of image data, and the two pieces of image data are averaged to obtain a final image.
  • the echo planar imaging sequence based on multi-radio frequency excitation includes a first radio frequency pulse with a flip angle of ⁇ , a second radio frequency pulse with a flip angle of ⁇ , a spin echo module containing 180° radio frequency pulses and two gradient echo chains (the first gradient echo chain and the second gradient echo chain) for collecting echo plane signals,
  • RF corresponds to radio frequency pulses
  • G RO Corresponding to the gradient pulse applied in the readout direction
  • GPE corresponds to the gradient pulse applied in the phase encoding direction
  • Gss corresponds to the gradient pulse applied in the layer selection direction.
  • the k-space filling trajectory corresponding to the echo-planar imaging sequence based on multi-radio frequency excitation provided in this embodiment, k x corresponds to the readout direction k-space, ky corresponds to the phase encoding direction k-space, two gradient echoes
  • the signals collected by the wave chain are filled into the k-space according to the alternate combination shown in Fig. 4 .
  • the single-plane echo imaging sequence of the prior art and the echo-planar imaging sequence according to the embodiment of the present application were performed on a healthy volunteer on a 3T magnetic resonance imaging system equipped with a 32-channel head array receiving coil Control measurements with head scans.
  • Two sequences were used in the test: an echo-planar imaging sequence based on the prior art and an echo-planar imaging sequence according to this embodiment.
  • the imaging field of view is 240 ⁇ 240mm 2
  • the excitation layer thickness is 5mm
  • the TR is 4000ms
  • the TE is 92ms
  • the echo interval is 0.57ms
  • the in-plane resolution is 2.5 ⁇ 2.5mm 2
  • the number of collection layers is 19.
  • the signal is excited with an RF pulse with a flip angle of 90°, then a spin-echo module containing a 180° RF pulse is applied, and a gradient echo chain is used to acquire the signal, the gradient echo The chain length is 96.
  • a radio frequency pulse with a flip angle of 47° and a radio frequency pulse with a flip angle of 122° are used to excite the signal, and then a spin echo module containing a 180° radio frequency pulse is applied, using two
  • the signal is collected by gradient echo chains, each gradient echo chain has a length of 48, and the phase encoding gradient amplitude is twice that of the echo planar sequence based on the prior art.
  • the MUSSELS method is used to reconstruct the signals acquired by two gradient echo chains to obtain two pieces of image data, and the two pieces of image data are averaged to obtain a final image.
  • Figures 5 to 8 are the experimental results of volunteers using the above two sequences.
  • Fig. 5 adopts the 9th layer image of the echo-planar imaging sequence according to the present embodiment
  • Fig. 7 adopts the 15th layer image of the echo-planar imaging sequence according to the present embodiment
  • Fig. 6 adopts the image based on the prior art
  • Fig. 8 is the image of the 15th layer of the echo-planar imaging sequence based on the prior art. Comparing Fig. 5 with Fig. 6, the shape distortion of the forehead part in Fig. 5 is greatly reduced (the position indicated by the arrow). Comparing Figure 7 with Figure 8, it is obvious that the shape distortion of the eye in Figure 7 is greatly reduced (the position indicated by the arrow b), and the signal accumulation is also significantly improved (the position indicated by the arrow a).

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Abstract

一种减少图像失真的磁共振平面回波成像方法。使用两个射频脉冲α和β在不同时刻分别将纵向磁化矢量翻转到横向平面，得到可观测的横向磁化矢量。通过梯度脉冲在两个不同的回波链中分别重聚出通过α和β翻转纵向磁化矢量得到的横向磁化矢量。在两个梯度回波链之前分别采集导航回波信号，通过两个导航回波信号的幅值差异校正两个梯度回波链信号的幅值差异。使用低秩矩阵重建方法矫正两个梯度回波链采集的信号，得到两幅图像数据，对两幅图像数据取平均值，得到最终图像。相比于现有技术中的单次扫描方法，显著提高了成像精度，有效减少图像的畸变和失真。同时，与现有技术中的多次扫描方法相比，成像效率显著提升。

Description

一种减少图像失真的磁共振平面回波成像方法 技术领域
本申请涉及磁共振成像(MRI,Magnetic Resonance Imaging)领域，尤其涉及一种减少图像失真的磁共振平面回波成像方法。
背景技术
磁共振成像技术(Magnetic Resonance Imaging,MRI)能够无损伤地检测活体内组织清晰的解剖构造、反映器质性病变的图像，提供满足不同诊断需求的生理学信息，是现今最常用的医学检测手段之一。磁共振平面回波成像(Echo Planar Imaging,EPI)仅需要一次射频激发就能完成整个二维k空间数据的扫描采集，在几十毫秒内就能获得完整的二维图像，是目前成像速度最快的MRI技术之一。EPI突破了扫描速度对MRI技术应用的限制，进一步拓展了MRI技术的应用范围。EPI已经被广泛应用于扩散成像、灌注成像及功能成像等磁共振影像扫描中，是临床疾病诊断和科学研究的重要工具。
但EPI对磁场的不均匀性以及化学位移伪影都极为敏感，在非均匀磁场中容易出现图像畸变和信号丢失，尤其是在空气和组织交界等部位，例如大脑扫描中的前额、眼眶等部。尽管螺旋采样等非笛卡尔采样轨迹也能有效缩短MRI数据的采样时间，然而这些非笛卡尔采样方式对磁共振梯度性能的不完善性也很敏感，稳定性还需要进一步研究。另一方面，一些通过预扫描获取校正信息或者多次扫描可以校正或减少磁场不均匀引起的图像失真，但会降低扫描效率并面临多次扫描间可能的不一致问题。
在现有技术中，多采用单次扫描EPI或多次激发扫描EPI方法。
现有的单次扫描EPI通过连续梯度回波采样和相位编码blip梯度，实现k空间的连续快速编码采样，从而在一次激发扫描中就能完成整个k空间的数据填充，进而得到二维图像。然而梯度回波的采样方式无法像自旋回波那样重聚磁场不均匀产生的演化相位，并且随着空间编码的进行，相位差逐步累积，导致最终的图像出现扭曲和失真。现有技术中的单次扫描EPI序列如图1所示。
现有的多次激发扫描EPI方法将k空间分多段进行相位编码，能够缩短相位编码维的时间间隔，降低了磁场不均匀引起的相位差的累积，可以有效减少EPI图像失真并提高分辨率，但k空间被分成多段需要多次激发扫描的数据组合而成，无法在单次激发扫描中完成填充，容易受到检测部位运动的干扰而出现运动伪影。虽然依靠PROPELLER等技术可以改善多扫描EPI的运动伪影问题，但是多次激发扫描的方式使得扫描时间变长，扫描效率降低，在一些对时间分辨率要求较高的场景无法适用， 这也限制了EPI图像质量的提升和进一步应用。现有技术中的k空间填充轨迹如图2所示。
因此，本领域的技术人员致力于开发一种减少图像失真的磁共振平面回波成像方法，不仅能够缩短空间编码的时间，降低磁场不均匀相位的累积，减少图像的畸变失真，而且不增加扫描次数或扫描时间，保证扫描效率。
发明内容
为实现上述目的，本申请提供了一种减少图像失真的磁共振平面回波成像方法，基于一个翻转角为α的第一射频脉冲、一个翻转角为β的第二射频脉冲、一个自旋回波模块和用于采集平面回波信号的第一梯度回波链、第二梯度回波链，其特征在于，具体包括以下步骤：
步骤一、使用所述第一射频脉冲激发层面，将第一部分磁化矢量激发到横向平面，将第二部分磁化矢量保持在纵向平面；
步骤二、经过一段延迟，使用所述第二射频脉冲激发层面，将所述第二部分磁化矢量激发到横向平面；
步骤三、施加所述自旋回波模块；
步骤四、使用梯度脉冲重聚出所述第一部分磁化矢量，在所述第一梯度回波链中检测横向的所述第一部分磁化矢量的信号，得到第一信号；
步骤五、使用梯度脉冲重聚出所述第二部分磁化矢量，在所述第二梯度回波链中检测横向的所述第二部分磁化矢量的信号，得到第二信号；
步骤六、将所述第一信号和所述第二信号沿相位编码方向交替存放，重组成k空间，进而得到最终图像。
可选地，所述第一射频脉冲的翻转角为47°；所述第二射频脉冲的翻转角为122°。
可选地，步骤二中，所述延迟的长度为所述第一梯度回波链与所述第二梯度回波链中心的时间间隔。
可选地，步骤三中，施加包含180°射频脉冲的所述自旋回波模块，以获得失真减少的T 2加权图像。
可选地，步骤三中，在所述180°射频脉冲两侧施加扩散加权梯度，以获得失真减少的扩散加权图像。
可选地，步骤三中，不施加所述自旋回波模块，以获得失真减少的T 2*加权图像。
可选地，在步骤四、步骤五中，选取相位编码梯度的动量为2/(γ H*FoV PE)，其中γ H为氢原子核的旋磁比，FoV PE为所述相位编码方向成像视野大小。
可选地，在步骤四、步骤五中，选取相位编码梯度的动量为4/(γ H*FoV PE)，其中γ H为氢原子核的旋磁比，FoV PE为所述相位编码方向成像视野大小。
可选地，步骤六中，在所述第一梯度回波链和所述第二梯度回波链之前分别采集 导航回波信号，通过两个所述导航回波信号的幅值差异校正所述第一梯度回波链和所述第二梯度回波链信号的幅值差异，所述导航回波为未进行相位编码的梯度回波。
可选地，步骤六中，所述第一梯度回波链和所述第二梯度回波链的信号采用MUSSELS方法重建，得到两幅图像数据，对所述两幅图像数据取平均值，得到所述最终图像。
可选地，步骤四、步骤五中，所述第一梯度回波链和所述第二梯度回波链采用不同极性的相位编码梯度，并将所述第一梯度回波链和所述第二梯度回波链的信号分别重建成图像，通过两幅图像估算磁场分布，进而校正图像畸变。
可选地，还包括第三射频脉冲以及第三梯度回波链，采用所述第一梯度回波链、所述第二梯度回波链、所述第三梯度回波链采集信号，在步骤四、步骤五中，选取相位编码梯度的动量为3/(γ H*FoV PE)，其中γ H为氢原子核的旋磁比，FoV PE为所述相位编码方向成像视野大小。
相比于现有技术中的单次扫描方法，本申请提供的技术方案通过基于多射频激发的平面回波成像方法，显著提高了成像精度，有效减少图像的畸变和失真。同时，本申请的技术方案仍然属于单次扫描，因此与现有技术中的多次扫描方法相比，本申请的技术方案的成像效率显著提升。
以下将结合附图对本申请的构思、具体结构及产生的技术效果作进一步说明，以充分地了解本申请的目的、特征和效果。
附图说明
图1是现有技术中单次扫描EPI序列示意图；
图2是现有技术中k空间填充轨迹示意图；
图3是本申请的一个实施例的脉冲序列示意图；
图4是本申请的一个实施例的k空间填充轨迹示意图；
图5是采用本申请的一个实施例的第9层人脑图像；
图6是采用现有技术的第9层人脑图像；
图7是采用本申请的一个实施例的第15层人脑图像；
图8是采用现有技术的第15层人脑图像。
具体实施方式
以下参考说明书附图介绍本申请的多个优选实施例，使其技术内容更加清楚和便于理解。本申请可以通过许多不同形式的实施例来得以体现，本申请的保护范围并非仅限于文中提到的实施例。
本申请的总体设计思路如下：
1、使用两个射频脉冲α和β在不同时刻分别将纵向磁化矢量翻转到横向平面，得到可观测的横向磁化矢量。
2、通过梯度脉冲在两个不同的回波链中分别重聚出通过α和β翻转纵向磁化矢量得到的横向磁化矢量。
3、在两个梯度回波链之前分别采集导航回波信号，通过两个导航回波信号的幅值差异校正两个梯度回波链信号的幅值差异。
4、使用MUSSELS方法重建两个梯度回波链采集的信号，得到两幅图像数据，对两幅图像数据取平均值，得到最终图像。
其中，相位编码blip梯度的动量可以选取为2/(γ H*FoV PE)或4/(γ H*FoV PE)。其中γ H为氢原子核的旋磁比，FoV PE为所述相位编码方向成像视野大小。从而进一步降低图像失真，得到更高分辨率图像。
其中，在第一梯度回波链和第二梯度回波链中分别使用不同极性的相位编码blip梯度，并将第一梯度回波链和第二梯度回波链的信号分别重建成图像，通过两幅图像估算磁场分布，进而校正图像畸变。
使用三个脉冲激发信号，并使用三个梯度回波链采集信号，重建得到图像。
其中，具体地，施加包含180°射频脉冲的自旋回波模块，以获得失真减少的T 2加权图像；在180°射频脉冲两侧施加扩散加权梯度，以获得失真减少的扩散加权图像；不施加自旋回波模块，以获得失真减少的T 2*加权图像。
具体地，本实施例是一种减少图像失真的磁共振平面回波成像方法，基于一个翻转角为α的第一射频脉冲、一个翻转角为β的第二射频脉冲、一个自旋回波模块和用于采集平面回波信号的第一梯度回波链、第二梯度回波链。其中，第一射频脉冲的翻转角为47°；第二射频脉冲的翻转角为122°具体包括以下步骤：
步骤一、使用第一射频脉冲激发层面，将第一部分磁化矢量激发到横向平面，将第二部分磁化矢量保持在纵向平面；
步骤二、经过一段延迟，使用第二射频脉冲激发层面，将第二部分磁化矢量激发到横向平面；延迟的长度为第一梯度回波链与第二梯度回波链中心的时间间隔；
步骤三、施加自旋回波模块；其中，施加包含180°射频脉冲的自旋回波模块，以获得失真减少的T 2加权图像；在180°射频脉冲两侧施加扩散加权梯度，以获得失真减少的扩散加权图像；不施加自旋回波模块，以获得失真减少的T 2*加权图像。
步骤四、使用梯度脉冲重聚出第一部分磁化矢量，在第一梯度回波链中检测横向的第一部分磁化矢量的信号，得到第一信号；
步骤五、使用梯度脉冲重聚出第二部分磁化矢量，在第二梯度回波链中检测横向的第二部分磁化矢量的信号，得到第二信号；
其中，在步骤四与步骤五中，相位编码梯度的动量可以选取为2/(γ H*FoV PE)或4/(γ H*FoV PE)，其中γ H为氢原子核的旋磁比，FoV PE为相位编码方向成像视野大小。 在一些实施例中，采用三个脉冲激发信号，并使用三个梯度回波链采集信号，此时相位编码梯度的动量可以选取为3/(γ H*FoV PE)。
步骤六、将第一信号和第二信号沿相位编码方向交替存放，重组成k空间，进而得到最终图像。具体地，在第一梯度回波链和第二梯度回波链之前分别采集导航回波信号，通过两个导航回波信号的幅值差异校正第一梯度回波链和第二梯度回波链信号的幅值差异，导航回波为未进行相位编码的梯度回波。同时，第一梯度回波链和第二梯度回波链的信号采用MUSSELS方法重建，得到两幅图像数据，对两幅图像数据取平均值，得到最终图像。
如图3所示为本实施例提供的基于多射频激发的平面回波成像序列，该平面回波成像序列包括一个翻转角为α的第一射频脉冲、一个翻转角为β的第二射频脉冲、一个包含180°射频脉冲的自旋回波模块以及两个用于采集平面回波信号的梯度回波链(第一梯度回波链和第二梯度回波链)，RF对应射频脉冲，G RO对应读出方向所施加梯度脉冲，G PE对应相位编码方向所施加梯度脉冲，Gss对应层选方向所施加梯度脉冲。如图4所示为本实施例提供的基于多射频激发的平面回波成像序列对应的k空间填充轨迹，k x对应读出方向k空间，k y对应相位编码方向k空间，两条梯度回波链采集的信号按照图4中交替组合的方式填充到k空间中。
将现有技术的单平面回波成像序列和根据本申请的实施例的平面回波成像序列，在配有32个通道头部阵列接收线圈的3T磁共振成像系统上对一位健康志愿者进行了头部扫描的对照测量。测试中采用了两个序列：基于现有技术的平面回波成像序列和根据本实施例的平面回波成像序列。两个平面回波成像序列的对照实验中，成像视野为240×240mm 2，激发层厚为5mm，TR为4000ms，TE为92ms，回波间隔为0.57ms，平面内分辨率为2.5×2.5mm 2，采集层数为19。对于基于现有技术的平面回波成像序列，使用一个翻转角为90°的射频脉冲激发信号，然后施加包含180°射频脉冲的自旋回波模块，使用一条梯度回波链采集信号，梯度回波链长度为96。对于根据本实施例的平面回波成像序列，使用一个翻转角为47°的射频脉冲和一个翻转角为122°的射频脉冲激发信号，然后施加包含180°射频脉冲的自旋回波模块，使用两条梯度回波链采集信号，每条梯度回波链长度为48，相位编码梯度幅度为基于现有技术的平面回波序列的两倍。对于根据本实施例的平面回波成像序列，使用MUSSELS方法重建两个梯度回波链采集的信号，得到两幅图像数据，对两幅图像数据取平均值，得到最终图像。
图5～图8是采用了上述两种序列的志愿者实验结果。其中图5是采用根据本实施例的平面回波成像序列的第9层图像，图7是采用根据本实施例的平面回波成像序列的第15层图像，图6是采用基于现有技术的平面回波成像序列的第9层图像，图8是采用基于现有技术的平面回波成像序列的第15层图像。图5与图6相比，图5中前额部分的形状畸变大为减少(箭头所指位置)。图7与图8相比，很明显图7中眼部的形状畸变大为减少(箭头b所指位置)，信号堆积情况也显著改善(箭头a所指位置)。
以上详细描述了本申请的较佳具体实施例。应当理解，本领域的普通技术无需创造性劳动就可以根据本申请的构思作出诸多修改和变化。因此，凡本技术领域中技术人员依本申请的构思在现有技术的基础上通过逻辑分析、推理或者有限的实验可以得到的技术方案，皆应在由权利要求书所确定的保护范围内。

Claims (10)

  1. 一种减少图像失真的磁共振平面回波成像方法，基于一个翻转角为α的第一射频脉冲、一个翻转角为β的第二射频脉冲、一个自旋回波模块和用于采集平面回波信号的第一梯度回波链、第二梯度回波链，其特征在于，具体包括以下步骤：
    步骤一、使用所述第一射频脉冲激发层面，将第一部分磁化矢量激发到横向平面，将第二部分磁化矢量保持在纵向平面；
    步骤二、经过一段延迟，使用所述第二射频脉冲激发层面，将所述第二部分磁化矢量激发到横向平面；
    步骤三、施加所述自旋回波模块；
    步骤四、使用梯度脉冲重聚出所述第一部分磁化矢量，在所述第一梯度回波链中检测横向的所述第一部分磁化矢量的信号，得到第一信号；
    步骤五、使用梯度脉冲重聚出所述第二部分磁化矢量，在所述第二梯度回波链中检测横向的所述第二部分磁化矢量的信号，得到第二信号；
    步骤六、将所述第一信号和所述第二信号沿相位编码方向交替存放，重组成k空间，进而得到最终图像。
  2. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，所述第一射频脉冲的翻转角为47°；所述第二射频脉冲的翻转角为122°。
  3. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤二中，所述延迟的长度为所述第一梯度回波链与所述第二梯度回波链中心的时间间隔。
  4. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤三中，施加包含180°射频脉冲的所述自旋回波模块，以获得失真减少的T 2加权图像。
  5. 如权利要求4所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤三中，在所述180°射频脉冲两侧施加扩散加权梯度，以获得失真减少的扩散加权图像。
  6. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤三中，不施加所述自旋回波模块，以获得失真减少的T 2*加权图像。
  7. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤六中，在所述第一梯度回波链和所述第二梯度回波链之前分别采集导航回波信号，通过两个所述导航回波信号的幅值差异校正所述第一梯度回波链和所述第二梯度回波链信号的幅值差异，所述导航回波为未进行相位编码的梯度回波。
  8. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于， 步骤六中，所述第一梯度回波链和所述第二梯度回波链的信号采用MUSSELS方法重建，得到两幅图像数据，对所述两幅图像数据取平均值，得到所述最终图像。
  9. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，步骤四、步骤五中，所述第一梯度回波链和所述第二梯度回波链采用不同极性的相位编码梯度，并将所述第一梯度回波链和所述第二梯度回波链的信号分别重建成图像，通过两幅图像估算磁场分布，进而校正图像畸变。
  10. 如权利要求1所述的减少图像失真的磁共振平面回波成像方法，其特征在于，还包括第三射频脉冲以及第三梯度回波链，采用所述第一梯度回波链、所述第二梯度回波链、所述第三梯度回波链采集信号，在步骤四、步骤五中，选取相位编码梯度的动量为3/(γ H*FoV PE)，其中γ H为氢原子核的旋磁比，FoV PE为所述相位编码方向成像视野大小。
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