Journal of the Mechanical Behavior of Biomedical Materials ( IF 3.3 ) Pub Date : 2022-12-28 , DOI: 10.1016/j.jmbbm.2022.105638 Lauriane Jugé 1 , Patrick Foley 2 , Alice Hatt 2 , Jade Yeung 2 , Lynne E Bilston 1
|
Introduction
Knowledge of the nonlinear viscoelastic properties of the liver is important, but the complex tissue behavior outside the linear viscoelastic regime has impeded their characterization, particularly in vivo. Combining static compression with magnetic resonance (MR) elastography has the potential to be a useful imaging method for assessing large deformation mechanical properties of soft tissues in vivo. However, this remains to be verified. Therefore this study aims first to determine whether MR elastography can measure the nonlinear mechanical properties of ex vivo bovine liver tissue under varying levels of uniform and focal preloads (up to 30%), and second to compare MR elastography-derived complex shear modulus with standard rheological measurements.
Method
Nine fresh bovine livers were collected from a local abattoir, and experiments were conducted within 12hr of death. Two cubic samples (∼10 × 10 × 10 cm3) were dissected from each liver and imaged using MR elastography (60 Hz) under 4 levels of uniform and focal preload (1, 10, 20, and 30% of sample width) to investigate the relationship between MR elastography-derived complex shear modulus (G∗) and the maximum principal Right Cauchy Green Strain (C11). Three tissue samples from each of the same 9 livers underwent oscillatory rheometry under the same 4 preloads (1, 10, 20, and 30% strain). MR elastography-derived complex shear modulus (G∗) from the uniform preload was validated against rheometry by fitting the frequency dependence of G∗ with a power-law and extrapolating rheometry-derived G∗ to 60 Hz.
Results
MR elastography-derived G∗ increased with increasing compressive large deformation strain, and followed a power-law curve (G∗ = 1.73 × C11−0.38, R2 = 0.96). Similarly, rheometry-derived G∗ at 1 Hz, increasing from 0.66 ± 1.03 kPa (1% strain) to 1.84 ± 1.65 kPa (30% strain, RM one-way ANOVA, P < 0.001), and the frequency dependence of G∗ followed a power-law with the exponent decreasing from 0.13 to 0.06 with increasing preload. MR elastography-derived G∗ was 1.4–3.1 times higher than the extrapolated rheometry-derived G∗ at 60 Hz, but the strain dependence was consistent between rheometry and MR elastography measurements.
Conclusions
This study demonstrates that MR elastography can detect changes in ex vivo bovine liver complex shear modulus due to either uniform or focal preload and therefore can be a useful technique to characterize nonlinear viscoelastic properties of soft tissue, provided that strains applied to the tissue can be quantified. Although MR elastography could reliably characterize the strain dependence of the ex vivo bovine liver, MR elastography overestimated the complex shear modulus of the tissue compared to rheological measurements, particularly at lower preload (<10%). That is likely to be important in clinical hepatic MR elastography diagnosis studies if preload is not carefully considered. A limitation is the absence of overlapping frequency between rheometry and MR elastography for formal validation.
中文翻译：
离体牛肝非线性粘弹性特性：MR 弹性成像和流变学测量
介绍
了解肝脏的非线性粘弹性特性很重要，但线性粘弹性范围之外的复杂组织行为阻碍了它们的表征，尤其是在体内。将静态压缩与磁共振 (MR) 弹性成像相结合，有可能成为一种有用的成像方法，用于评估体内软组织的大变形机械特性。不过，这还有待考证。因此，本研究的目的首先是确定 MR 弹性成像是否可以在不同水平的均匀和焦点预紧力（高达 30%）下测量离体牛肝组织的非线性力学特性，其次是将 MR 弹性成像衍生的复合剪切模量与标准进行比较流变测量。
方法
从当地一家屠宰场采集了 9 块新鲜牛肝，并在死后 12 小时内进行了实验。从每个肝脏切下两个立方体样本 (~10 × 10 × 10 cm 3 )，并使用 MR 弹性成像 (60 Hz) 在 4 个水平的均匀和局部预载（样本宽度的 1%、10%、20% 和 30%）下成像研究 MR 弹性成像衍生的复数剪切模量 (G*) 与最大主右柯西格林应变 (C 11). 来自相同 9 个肝脏中每个肝脏的三个组织样本在相同的 4 个预加载（1%、10%、20% 和 30% 应变）下进行振荡流变测量。通过将 G* 的频率依赖性与幂律拟合并将流变测量得出的 G* 外推到 60 Hz，通过 MR 弹性成像法从均匀预紧力导出的复剪切模量 (G*) 针对流变测量进行了验证。
结果
MR 弹性成像得出的 G* 随着压缩大变形应变的增加而增加，并遵循幂律曲线 (G* = 1.73 × C 11 −0.38 , R 2 = 0.96)。同样，流变测量得出的 G* 在 1 Hz 时从 0.66 ± 1.03 kPa（1% 应变）增加到 1.84 ± 1.65 kPa（30% 应变，RM 单向方差分析，P < 0.001），以及 G* 的频率依赖性遵循幂律，随着预紧力的增加，指数从 0.13 下降到 0.06。MR 弹性成像衍生的 G* 在 60 Hz 时比外推的流变测量衍生的 G* 高 1.4-3.1 倍，但应变依赖性在流变测量和 MR 弹性成像测量之间是一致的。
结论
这项研究表明，MR 弹性成像可以检测由于均匀或局部预紧引起的离体牛肝复合剪切模量的变化，因此可以成为表征软组织非线性粘弹性特性的有用技术，前提是可以量化施加到组织的应变. 尽管 MR 弹性成像可以可靠地表征离体牛肝脏的应变依赖性，但与流变学测量相比，MR 弹性成像高估了组织的复数剪切模量，尤其是在较低的预紧力 (<10%) 下。如果不仔细考虑前负荷，这在临床肝脏 MR 弹性成像诊断研究中可能很重要。一个限制是流变测量和 MR 弹性成像之间没有用于正式验证的重叠频率。
京公网安备 11010802027423号