Combined MRE and SPAMM tagged MRI for the analysis of large strain soft tissue mechanical properties

Introduction: Clinical conditions affecting soft tissue often present with altered mechanical properties (e.g. tumours, fibrosis and/or malignancy). Hence, the MRI based non-invasive analysis of soft tissue mechanical properties has been the focus of many fields of research. Soft tissue mechanical properties are nonlinear with respect to strain, strain-rate and frequency and, due to strain stiffening of soft tissue, elasticity differences (e.g. between malignant and healthy tissue) may become more pronounced at higher strains. Analysis of non-linearity with respect to strain provides added tissue contrast capabilities and is therefore of interest to the current study. However this requires measurement of mechanical properties across a range of strain levels. Unlike ultrasound based methods which heavily rely on experimental boundary conditions, the current study aims at developing methods independent of these. Mechanical property measurement at macroscopic strain levels, such as those based on SPAtial Modulation of the Magnetization (SPAMM) tagged MRI [1], require complex inverse (e.g. finite element) analysis and detailed knowledge of the boundary (e.g. loading) conditions. MR Elastography (MRE) uses imaging of the propagation of low-frequency acoustic waves within soft tissue and allows for the estimation of shear elasticity and viscosity properties independent of boundary conditions, and has been shown to be beneficial in the study of liver fibrosis and breast lesions [2]. However, this technique has mainly been limited to microscopic strain levels. Using indentation of a silicone gel soft tissue phantom the current study presents the combination of a fast SPAMM tagged MRI sequence and MRE for the analysis of the large strain and non-linear mechanical behavior of soft tissue.