Department of Radiology, Dartmouth-Hitchcock Medical Center, One Medical Center Drive, Lebanon, NH 03756, USA.
Phys Med Biol. 2010 Nov 21;55(22):6801-15. doi: 10.1088/0031-9155/55/22/013. Epub 2010 Oct 28.
The mechanical model commonly used in magnetic resonance elastography (MRE) is linear elasticity. However, soft tissue may exhibit frequency- and direction-dependent (FDD) shear moduli in response to an induced excitation causing a purely linear elastic model to provide an inaccurate image reconstruction of its mechanical properties. The goal of this study was to characterize the effects of reconstructing FDD data using a linear elastic inversion (LEI) algorithm. Linear and FDD phantoms were manufactured and LEI images were obtained from time-harmonic MRE acquisitions with variations in frequency and driving signal amplitude. LEI responses to artificially imposed uniform phase shifts in the displacement data from both purely linear elastic and FDD phantoms were also evaluated. Of the variety of FDD phantoms considered, LEI appeared to tolerate viscoelastic data-model mismatch better than deviations caused by poroelastic and anisotropic mechanical properties in terms of visual image contrast. However, the estimated shear modulus values were substantially incorrect relative to independent mechanical measurements even in the successful viscoelastic cases and the variations in mean values with changes in experimental conditions associated with uniform phase shifts, driving signal frequency and amplitude were unpredictable. Overall, use of LEI to reconstruct data acquired in phantoms with FDD material properties provided biased results under the best conditions and significant artifacts in the worst cases. These findings suggest that the success with which LEI is applied to MRE data in tissue will depend on the underlying mechanical characteristics of the tissues and/or organs systems of clinical interest.
磁共振弹性成像（MRE）中常用的力学模型为线性弹性模型。然而，软组织可能表现出频率和方向相关（FDD）的剪切模量，以响应引起的激励，导致纯线性弹性模型对其力学特性提供不准确的图像重建。本研究的目的是描述使用线性弹性反演（LEI）算法对 FDD 数据进行重建的影响。制作了线性和 FDD 体模，并从具有频率和驱动信号幅度变化的时谐 MRE 采集获得 LEI 图像。还评估了 LEI 对从纯线性弹性和 FDD 体模的位移数据中人为施加的均匀相位偏移的响应。在所考虑的各种 FDD 体模中，LEI 似乎比由粘弹性数据模型不匹配引起的偏差更能容忍粘弹性数据模型不匹配，就视觉图像对比度而言，其偏差由多孔弹性和各向异性力学特性引起。然而，与独立的力学测量相比，即使在成功的粘弹性情况下，估计的剪切模量值也存在很大的误差，并且与均匀相位偏移、驱动信号频率和幅度相关的实验条件变化引起的平均值变化是不可预测的。总体而言，在具有 FDD 材料特性的体模中使用 LEI 对数据进行重建会在最佳条件下提供有偏差的结果，并在最坏情况下产生严重的伪影。这些发现表明，LEI 成功应用于组织中的 MRE 数据将取决于临床感兴趣的组织和/或器官系统的潜在力学特性。
