Magnetic resonance elastography in nonlinear viscoelastic materials under load |
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Authors: | Capilnasiu Adela Hadjicharalambous Myrianthi Fovargue Daniel Patel Dharmesh Holub Ondrej Bilston Lynne Screen Hazel Sinkus Ralph Nordsletten David |
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Institution: | 1.Division of Biomedical Engineering and Imaging Sciences, King’s College London, London, UK ;3.Department of Biomedical Engineering and Cardiac Surgery, University of Michigan, Ann Arbor, USA ;4.Institute of Bioengineering, Queen Mary University of London, London, UK ;5.Prince of Wales Clinical School, University of New South Wales, Sydney, Australia ;6.Neuroscience Research Australia, Sydney, Australia ;7.Inserm U1148, LVTS, University Paris Diderot, University Paris 13, 75018, Paris, France ; |
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Abstract: | Characterisation of soft tissue mechanical properties is a topic of increasing interest in translational and clinical research. Magnetic resonance elastography (MRE) has been used in this context to assess the mechanical properties of tissues in vivo noninvasively. Typically, these analyses rely on linear viscoelastic wave equations to assess material properties from measured wave dynamics. However, deformations that occur in some tissues (e.g. liver during respiration, heart during the cardiac cycle, or external compression during a breast exam) can yield loading bias, complicating the interpretation of tissue stiffness from MRE measurements. In this paper, it is shown how combined knowledge of a material’s rheology and loading state can be used to eliminate loading bias and enable interpretation of intrinsic (unloaded) stiffness properties. Equations are derived utilising perturbation theory and Cauchy’s equations of motion to demonstrate the impact of loading state on periodic steady-state wave behaviour in nonlinear viscoelastic materials. These equations demonstrate how loading bias yields apparent material stiffening, softening and anisotropy. MRE sensitivity to deformation is demonstrated in an experimental phantom, showing a loading bias of up to twofold. From an unbiased stiffness of \(4910.4 \pm 635.8\) Pa in unloaded state, the biased stiffness increases to 9767.5 \(\pm \,\)1949.9 Pa under a load of \(\approx \) 34% uniaxial compression. Integrating knowledge of phantom loading and rheology into a novel MRE reconstruction, it is shown that it is possible to characterise intrinsic material characteristics, eliminating the loading bias from MRE data. The framework introduced and demonstrated in phantoms illustrates a pathway that can be translated and applied to MRE in complex deforming tissues. This would contribute to a better assessment of material properties in soft tissues employing elastography. |
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