中文摘要：

目的探讨MR弹性成像（MRE）在正常人脑的应用。方法研制用于人脑的激发装置，激发装置附加于头线圈上，并固定在志愿者头部，激发装置产生的低频率（f）振荡波经颅骨传至颅内，引起剪切波在脑组织内传播；设计成像脉冲序列，脉冲序列以梯度回波序列为基础，在X、Y或Z轴上施加运动敏感梯度（MSG）。剪切波导致的脑组织内周期性移位可使接收信号产生周期性相位位移，获得相位图像，从而显示脑组织内剪切波的传播。通过调整相位偏置，获得1个周期内剪切波的动态传播图像；相位图像经局部f估算法处理后获得局部剪切波的波长（γ），计算出剪切模量，获得弹性图像；激发f采用100、150和200Hz。结果MRE的相位图清楚显示了剪切波在脑组织内的传播，剪切波从脑表面向中心传播。剪切波γ随激发f而变化，γ与激发f呈反比。脑白质内剪切波γ大于脑灰质；弹性图像显示脑白质的剪切模量大于脑灰质的剪切模量。结论MRE的相位图可显示剪切波在脑组织内的传播，弹性图像显示了脑灰质与白质之间的弹性变化。

英文摘要：

Objective To study the application of magnetic resonance elastography （MRE） in the human brain. Methods An external force actuator was developed. The actuator was fixed to the head coil. During MRE scan, one side of the actuator was attached to the volunteers＇ head. Low frequency oscillation was produced by the actuator and generated shear waves propagating into brain tissue. The pulse sequence of MRE was designed. A modified gradient echo sequence was developed with motion sensitizing gradient （MSG） imposed along X, Y or Z direction. Cyclic displacement within brain tissue induced by shear waves caused a measurable phase shift in the received MR signal. From the measured phase shift, the displacement at each voxel could be calculated, and the shear waves within the brain were directly imaged. By adjusting the phase offset, the dynamic propagation of shear waves in a wave cycle was obtained. Phase images were processed with local frequency estimation （LFE） technique to obtain the elasticity images. Shear waves at 100 Hz, 150 Hz, and 200 Hz were applied. Results The phase images of MRE directly imaged the propagating shear waves within the brain. The direction of the propagation was from surface of the brain to the center. The wavelength of shear waves varied with the change of actuating frequency. The change of wavelength of shear waves in gray and white matter of the brain was identified. The wavelength of shear waves in gray matter was shorter than that in white matter. The elasticity image of the brain revealed that the shear modulus of the white matter was higher than that of gray matter. Conclusion The phase images of MRE can direcdy visualize the propagation of shear waves in the brain tissue. The elasticity image of the brain can demonstrate the change of elasticity between gray and white matter.