Elasticity Imaging and Sensing Using Targeted Motion: From Macro to Nano

The methods used to estimate tissue elasticity based on the motion of local inhomogeneities such as laser-induced gas microbubbles, magnetic nanoparticles and other targets under an externally applied force are reviewed. The theoretical bases of the motion of a target in a viscoelastic medium are described. Given various targets differing in size, these methods allow for elasticity measurements at different levels of spatial resolution. Short acoustic radiation force and magnetic field pulses were used to initiate the motion of various test objects (solid spheres, laser-induced microbubbles and magnetic nanoparticles) with sizes ranging from several millimeters to tens of nanometers. The induced motion was monitored by conventional pulse-echo ultrasound systems adapted for these measurements. The elasticity of the medium was evaluated based on a theoretical model describing the motion of a particle in a viscoelastic medium. Shear elasticity, evaluated using the developed approach, agrees well with independent measurements of mechanical properties of the medium. In this paper we review our recent theoretical and experimental results and discuss the applications of elasticity imaging and sensing using targeted motion, i.e., motion induced via the external forces acting on local inhomogeneities.