Measurement and evaluation of local tissues elastic modulus using MR elastography

Magnetic resonance elastography (MRE) is a phase-contrast-based method that can visualize propagating strain waves in materials. The quantitative values of shear modulus can be calculated by estimating the local wavelength of the wave pattern. Low frequency mechanical motion must be used for soft tissue-like materials, because strain waves rapidly attenuate at higher frequency. Therefore, it is difficult to estimate local wavelength with high spatial resolution especially from noisy MRE. In the MRE sequence, motion-sensitizing gradient (MSG) are synchronized with the mechanical cyclic motion. MRE with multiple phase offsets can be generated with increasing delays between the MSG and mechanical excitation. In this paper, we describe a method of measuring local wavelength with high spatial resolution using MRE with multiple phase offsets. We also describe MRE sequences and a vibration system that can drive in the gantry of a magnetic resonance imaging system. To confirm the reliability of this method, a computer simulation and phantom study were performed. The shear modulus measured with various elastic objects was well consistent with the value obtained by MRE and the mechanical method. The shear moduli of various in-vitro and in-vivo tissues were also analyzed by this method. (author)