磁共振弹性成像的初步实验研究

摘要

Objective: To study magnetic resonance elastography (MRE) technique. Methods: An external force actuator was developed, the imaging pulse sequence of MRE was designed,and tissue simulating phantoms were constructed. The actuator controlled by the pulse sepuence produced shear waves at low frequency on the surface of the phantoms. A modified gradient echo sequence was developed with motion sensitizing gradient (MSG)imposed along X,Y or Z direction.Cyclic displacement within the medium 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 propatating shear waves within the medium were directly imaged. By adjusting the phase offsets,the dynamic propagation of shear waves in a wave cycle was obtained.The phase images were processed to aquire quantitative elasticity image using local frequency estimation(LFE)method. The experiments were implemented with 1.0% and 1.5% tissue simulating agarose gel. Shear waves at frequency of 150 Hz,200 Hz,250 Hz,and 300 Hz were applied. Results: The phase images of MRE directly imaged the propagating shear waves within the phantoms.The wavelength of shear waves varied with the change of exciting frequency and stiffness of the phantoms. The wavelength of shear waves was exactly proportional to the frequency and stiffness of the phantom. The contrast of elasticity in agarose gel with two concentrations was clearly demonstrated on elasticity images.Conclusion: The phase images of MRE can directly visualize the propagation of shear waves in the medium. The elasticity image of MRE can quantitatively image the elastic modulus of the medium%目的:研究磁共振弹性成像(MRE)技术.方法:研制外部激发装置,设计成像脉冲序列,制作模拟人体软组织的体模.激发装置由序列控制,于体模表面产生低频率剪切波.脉冲序列采用梯度回波序列,在x、y或z轴上施加运动敏感梯度(MSG).剪切波导致的介质内的周期性移位可使接收信号产生周期性相位位移,从测得的相位位移就能计算出每个体素的移位值,直接显示介质内剪切波的传播.通过调整相位偏置,获得一个完整周期内剪切波的动态传播图像.相位图经局部频率估算法(LFE)处理后计算出量化的弹性模量图.实验采用浓度为1.0%和1.5%不同弹性的琼脂凝胶体模,激发频率分别采用150Hz、200Hz、250Hz和300Hz.结果:MRE的相位图显示了剪切波在体模内的传播,剪切波的波长随激发频率和体模弹性变化.波长与激发频率呈反比,与体模弹性呈正比.剪切波的波长在不同激发频率和不同浓度体模之间呈严格的比例关系.计算出的弹性模量图清楚显示了两种浓度介质的弹性对比.结论:MRE的相位图可显示剪切波在介质内的传播,弹性图可量化和显示介质的弹性模量.