Characterization of Soft Fibrous Materials by MRI of Ultrasound-Induced Shear Waves

本研究将开发一种新的实验技术来测量柔软、纤维材料的复杂性能，非侵入性和非破坏性。肌肉、肌腱和脑组织就是这类物质的例子，它们在人体和自然界中随处可见。人造纤维增强的柔软生物材料越来越多地用于工程应用。了解这些材料的机械性能是至关重要的，比如它们的刚度和储存和耗散能量的能力。在这个项目中，将使用磁共振成像和高强度聚焦超声的新组合来全面测量柔软纤维材料的机械性能，无论是在人体内还是在受控环境中。对社会的益处包括提高诊断损伤和纤维化疾病的能力，设计和评估人工组织，以及对创伤性脑损伤进行准确的计算机模拟。在劳动力发展方面，来自工程和成像的研究人员将共同构建、测试和演示新技术。研究生将发展一套复杂的研究技能，跨越这两个学科。利用暑期研究项目，以及暑期和周末研讨会，该项目将为来自不同背景的本科生和初高中学生提供工程和成像交叉的培训和研究经验。谐波超声诱导运动的磁共振成像将被发展和应用于精确测量软组织和纤维性生物材料的复杂、各向异性和非线性行为。首先，纤维性软质生物材料的线性、各向异性（横向各向同性和正交异性）、粘弹性模型的参数将从慢（纯横向）和快（准横向）剪切波中估计出来。剪切波相对于材料对称轴具有不同的传播和极化方向，将由聚焦超声诱导，并在人工排列的生物材料、肌肉和白质脑组织中通过MRI成像。通过将方向滤波平面波的速度拟合到解析表达式和模拟结果中来估计剪切模量和拉伸模量。其次，纤维性软材料的非线性行为将被表征。我们将探讨两种类型的非线性。(i)由强材料非线性引起的小应变状态的非线性，将通过量化波动的高谐波分量来测量。（ii）在大变形上叠加慢、快剪切波成像将是非线性、大应变行为的特征。这种新颖的方法有望以前所未有的分辨率，在这些日益重要的材料中提供各向异性和非线性的全面表征。

This research will develop a new experimental technique to measure the complex properties of soft, fibrous materials, non-invasively and non-destructively. Muscle, tendons, and brain tissue are examples of such materials, which are found throughout the human body and in nature. Artificial fiber-reinforced, soft biomaterials are increasingly being used in engineering applications. It is critical to know the mechanical properties of such materials, such as their stiffness and ability to store and dissipate energy. In this project, a novel combination of magnetic resonance imaging and high-intensity focused ultrasound will be used to comprehensively measure the mechanical properties of soft fibrous materials, either inside the human body, or in a controlled environment. Benefits to society include the improved ability to diagnose injury and fibrotic disease, to design and evaluate artificial tissues, and to perform accurate computer simulations of traumatic brain injury. In terms of workforce development, researchers from engineering and imaging will work together to build, test, and demonstrate the new technology. Graduate students will develop a set of sophisticated research skills that span these two disciplines. Leveraging a summer research program, as well as summer and weekend workshops, this project will provide training and research experience at the intersection of engineering and imaging to undergraduate students and middle/high school students from diverse backgrounds.MR imaging of harmonic ultrasound-induced motion will be developed and applied to accurately measure the complex, anisotropic, nonlinear behavior of soft tissue and fibrous biomaterials. First, parameters of linear, anisotropic (transversely isotropic and orthotropic), viscoelastic models of fibrous soft biomaterials will be estimated from slow (pure transverse) and fast (quasi-transverse) shear waves. Shear waves, with varying propagation and polarization directions relative to material symmetry axes, will be induced by focused ultrasound and imaged by MRI in artificially aligned biomaterials, in muscle, and in white matter brain tissue. Shear moduli and tensile moduli will be estimated by fitting speeds of directionally-filtered plane waves to analytical expressions and simulated results. Second, the nonlinear behavior of fibrous, soft materials will be characterized. Two types of nonlinearity will be explored. (i) Nonlinearity in the small-strain regime, arising from strong material nonlinearity, will be measured by quantifying the higher harmonic components of wave motion. (ii) Nonlinear, large-strain behavior will be characterized by imaging slow and fast shear waves superimposed on large deformations. This novel approach is expected to provide comprehensive characterization of anisotropy and nonlinearity, with unprecedented resolution, throughout these increasingly-important materials.