Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, United States of America. Author to whom any correspondence should be addressed.
Phys Med Biol. 2019 Mar 27;64(7):075006. doi: 10.1088/1361-6560/ab0a7d.
Intrinsic actuation MR elastography (IA-MRE) exploits natural pulsations of the brain as a motion source to estimate mechanical property maps. The low frequency motion of IA-MRE introduces new considerations for inversion algorithms relative to traditional external actuation MRE. Specifically, inertial forces become very small, which leaves low frequency viscoelastic inversions with a non-unique scalar multiplier. Biphasic poroelastic inversions include additional fluid-solid interaction forces to balance the elastic forces, which avoids the non-uniqueness. Analyzing the convergence behavior from different starting values using 1 Hz simulated data, IA-MRE data from a gelatin phantom and in vivo brain IA-MRE data reveal that higher frequency (50 Hz) viscoelastic inversion reaches the correct, unique solution regardless of initial property estimate; whereas, low frequency viscoelastic inversion recovers relative values of shear modulus. In the presence of measurement noise, the non-unique scalar multiplier is determined by the softest material reaching the prescribed lower bound on shear modulus. Poroelastic inversion produces a unique solution at both 50 Hz and 1 Hz; however, hydraulic conductivity must be known or accurately estimated in order to recover quantitatively accurate shear modulus maps at low frequency.
本征激励磁共振弹性成像（IA-MRE）利用大脑的自然脉动作为运动源来估计力学属性图。IA-MRE 的低频运动为反转算法引入了与传统外部激励 MRE 不同的新考虑因素。具体而言，惯性力变得非常小，这使得低频黏弹性反转具有非唯一标量乘数。双相多孔弹性反转包括额外的流固相互作用力来平衡弹性力，从而避免了非唯一性。使用 1 Hz 模拟数据从不同的初始值分析收敛行为，从明胶模型和活体大脑 IA-MRE 数据中得到的 IA-MRE 数据表明，无论初始属性估计如何，更高频率（50 Hz）黏弹性反转都能达到正确的、唯一的解；然而，低频黏弹性反转则恢复了剪切模量的相对值。在存在测量噪声的情况下，非唯一标量乘数由达到剪切模量规定下限的最软材料决定。多孔弹性反转在 50 Hz 和 1 Hz 时都产生了唯一的解；然而，为了在低频时恢复定量准确的剪切模量图，必须知道或准确估计水力传导率。
