Université Paris Diderot, Sorbonne Paris Cité, Clichy, France.
NMR Biomed. 2013 Oct;26(10):1326-35. doi: 10.1002/nbm.2958. Epub 2013 May 28.
In MR elastography (MRE), periodic tissue motion is phase encoded using motion-encoding gradients synchronized to an externally applied periodic mechanical excitation. Conventional methods result in extended scan time for quality phase images, thus limiting the broad application of MRE in the clinic. For practical scan times, researchers have been relying on one-dimensional or two-dimensional motion-encoding, low-phase sampling and a limited number of slices, and artifact-prone, single-shot, echo planar imaging (EPI) readout. Here, we introduce a rapid multislice pulse sequence capable of three-dimensional motion encoding that is also suitable for simultaneously encoding motion with multiple frequency components. This sequence is based on a gradient-recalled echo (GRE) sequence and exploits the principles of fractional encoding. This GRE MRE pulse sequence was validated as capable of acquiring full three-dimensional motion encoding of isotropic voxels in a large volume within less than a minute. This sequence is suitable for monofrequency and multifrequency MRE experiments. In homogeneous paraffin phantoms, the eXpresso sequence yielded similar storage modulus values as those obtained with conventional methods, although with markedly reduced variances (7.11 ± 0.26 kPa for GRE MRE versus 7.16 ± 1.33 kPa for the conventional spin-echo EPI sequence). The GRE MRE sequence obtained better phase-to-noise ratios than the equivalent spin-echo EPI sequence (matched for identical acquisition time) in both paraffin phantoms and in vivo data in the liver (59.62 ± 11.89 versus 27.86 ± 3.81, 61.49 ± 14.16 versus 24.78 ± 2.48 and 58.23 ± 10.39 versus 23.48 ± 2.91 in the X, Y and Z components, respectively, in the case of liver experiments). Phase-to-noise ratios were similar between GRE MRE used in monofrequency or multifrequency experiments (75.39 ± 14.93 versus 86.13 ± 18.25 at 28 Hz, 71.52 ± 24.74 versus 86.96 ± 30.53 at 56 Hz and 95.60 ± 36.96 versus 61.35 ± 26.25 at 84Hz, respectively).
在磁共振弹性成像（MRE）中，周期性组织运动通过与外部施加的周期性机械激励同步的运动编码梯度进行相位编码。传统方法会导致高质量相位图像的扫描时间延长，从而限制了 MRE 在临床中的广泛应用。为了实现实际的扫描时间，研究人员一直依赖于一维或二维运动编码、低相位采样和有限数量的切片，以及容易产生伪影的单次回波平面成像（EPI）读出。在这里，我们介绍了一种快速的多维运动编码脉冲序列，该序列不仅适用于同时对多个频率分量进行运动编码，而且还适用于同时对多个频率分量进行运动编码。该序列基于梯度回波（GRE）序列，并利用分数编码的原理。该 GRE MRE 脉冲序列已被验证能够在不到一分钟的时间内对大体积的各向同性体素进行全三维运动编码。该序列适用于单频和多频 MRE 实验。在均匀的石蜡体模中，eXpresso 序列产生的存储模量值与传统方法相似，尽管方差明显减小（GRE MRE 为 7.11±0.26kPa，传统自旋回波 EPI 序列为 7.16±1.33kPa）。在石蜡体模和肝脏的体内数据中，GRE MRE 序列获得的相位-噪声比优于等效的自旋回波 EPI 序列（在相同的采集时间下匹配）（X、Y 和 Z 方向的分别为 59.62±11.89 与 27.86±3.81、61.49±14.16 与 24.78±2.48 和 58.23±10.39 与 23.48±2.91）。在单频或多频实验中，GRE MRE 的相位-噪声比相似（28Hz 时为 75.39±14.93 与 86.13±18.25，56Hz 时为 71.52±24.74 与 86.96±30.53，84Hz 时为 95.60±36.96 与 61.35±26.25）。
