Department of Radiology, Charité Universitätsmedizin Berlin, Berlin, Germany.
Department of Mathematics and Statistics, University of Strathclyde, Glasgow, Scotland, United Kingdom.
Med Image Anal. 2018 May;46:180-188. doi: 10.1016/j.media.2018.03.003. Epub 2018 Mar 17.
A new viscoelastic wave inversion method for MRE, called Heterogeneous Multifrequency Direct Inversion (HMDI), was developed which accommodates heterogeneous elasticity within a direct inversion (DI) by incorporating first-order gradients and combining results from a narrow band of multiple frequencies. The method is compared with a Helmholtz-type DI, Multifrequency Dual Elasto-Visco inversion (MDEV), both on ground-truth Finite Element Method simulations at varied noise levels and a prospective in vivo brain cohort of 48 subjects ages 18-65. In simulated data, MDEV recovered background material within 5% and HMDI within 1% of prescribed up to SNR of 20 dB. In vivo HMDI and MDEV were then combined with segmentation from SPM to create a fully automated "brain palpation" exam for both whole brain (WB), and brain white matter (WM), measuring two parameters, the complex modulus magnitude |G*| , which measures tissue "stiffness", and the slope of |G*| values across frequencies, a measure of viscous dispersion. |G*| values for MDEV and HMDI were comparable to the literature (for a 3-frequency set centered at 50 Hz, WB means were 2.17 and 2.15 kPa respectively, and WM means were 2.47 and 2.49 kPa respectively). Both methods showed moderate correlation to age in both WB and WM, for both |G*| and |G*| slope, with Pearson's r ≥ 0.4 in the most sensitive frequency sets. In comparison to MDEV, HMDI showed better preservation of recovered target shapes, more noise-robustness, and stabler recovery values in regions with rapid property change, however summary statistics for both methods were quite similar. By eliminating homogeneity assumptions within a fast, fully automatic, regularization-free direct inversion, HMDI appears to be a worthwhile addition to the MRE image reconstruction repertoire. In addition to supporting the literature showing decrease in brain viscoelasticity with age, our work supports a wide range of inter-individual variation in brain MRE results.
一种新的用于 MRE 的粘弹性波反演方法，称为非均匀多频直接反演（Heterogeneous Multifrequency Direct Inversion，HMDI），通过在直接反演（DI）中包含一阶梯度并结合多个窄频带的结果，来适应弹性的非均匀性。该方法在不同噪声水平的有限元方法模拟的地面真实数据和 48 名 18-65 岁的前瞻性体内脑队列中，与亥姆霍兹型 DI、多频双弹性粘弹性反演（Multifrequency Dual Elasto-Visco inversion，MDEV）进行了比较。在模拟数据中，MDEV 在 SNR 为 20dB 时，恢复背景材料的误差在 5%以内，HMDI 在 1%以内。体内 HMDI 和 MDEV 随后与 SPM 的分割相结合，为整个大脑（Whole Brain，WB）和脑白质（White Matter，WM）创建了完全自动化的“脑触诊”检查，测量两个参数，即复模量幅度 |G*|，它衡量组织的“硬度”，以及频率之间 |G*| 值的斜率，衡量粘性分散。MDEV 和 HMDI 的 |G*| 值与文献值相当（对于以 50Hz 为中心的三频设置，WB 的平均值分别为 2.17kPa 和 2.15kPa，WM 的平均值分别为 2.47kPa 和 2.49kPa）。两种方法在 WB 和 WM 中均与年龄呈中度相关，对于 |G*| 和 |G*| 斜率，在最敏感的频率设置中，皮尔逊 r 值≥0.4。与 MDEV 相比，HMDI 显示出更好的目标形状恢复能力，对噪声的鲁棒性更强，在属性变化迅速的区域具有更稳定的恢复值，但是两种方法的汇总统计数据非常相似。通过在快速、全自动、无正则化的直接反演中消除均匀性假设，HMDI 似乎是 MRE 图像重建中一种有价值的补充。除了支持文献中显示的随着年龄增长脑粘弹性降低的结果外，我们的工作还支持脑 MRE 结果的个体间差异广泛。
