Magnetic resonance elastography: in vivo measurements of elasticity for human tissue

Elasticity is an important physical property of material. In the clinical practice, elasticity is used for physical examination in several ways, such as palpation or percussion. Differences in elasticity can help facilitate the diagnosis of tumors and their extent. Elasticity is an essential property in the diagnosis of liver cirrhosis, or soft degeneration in tissue necrosis. In addition, information of tissue elasticity is utilized in virtual reality systems such as telepalpation and computer assisted surgery. It was difficult to obtain such properties in vivo by using conventional measurement methods. To overcome this problem, magnetic resonance elastography (MRE) has been developed that provides noninvasive in vivo measurements of elasticity for human tissue. We summarize this MRE method in this paper. When an object is oscillated from the surface in a known frequency, acoustic strain waves propagate into the material and one can calculate the physical constants of a material elasticity by the wave velocity. In MRE measurements, a cyclic micromotion caused by the acoustic strain waves is obtained as an MR image that is synchronized to the oscillation. By measuring the local wavelength of the strain waves, we can obtain the elasticity constants. Several examples of MRE image including in vivo measurements are provided as well as several methods to estimate the local wavelength from MRE images are described.