Induced tissue displacement in magnetomotive ultrasound imaging - simulations and experiments

Magnetomotive ultrasound imaging is an emerging technique where superparamagnetic iron oxide nanoparticles can be used as an ultrasound contrast agent. A time-varying external magnetic field acts to move the particles lodged in tissue, and ultrasound is used to detect the resulting tissue movement. In phantom studies we have observed opposite phase motion next to regions containing nanoparticles. We hypothesize that this motion is caused by mechanical coupling from regions where nanoparticles are located. The present study compares experimental data to a numerical simulation with identical geometry as the experimental set-up. The magnetic force acting on particles was modeled as emanating from a coil with a cone shaped iron core, and applied as a body load in nanoparticle-laden regions. The simulation showed opposed motion in-between nanoparticle-laden phantom inserts, in a manner similar to the experimental situation. There is a slight mismatch in the extent of vertical movement, which we interpret as a result of the modeled slip condition tangentially to the surface, which in reality presumably is a combination of slip and stick due to friction.