Effect of nanoparticle size and magnetic field strength on the displacement signal in magnetomotive ultrasound imaging

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 tissue embedded particles, and ultrasound is used to detect the resulting tissue movement. In this experimental phantom study we observed a variation in the magnetomotive response in respect to physical size of the embedded superparamagnetic iron oxide nanoparticles. Given the same Fe concentration a weaker response, by a factor of 2, was detected with the larger nanoparticles. However, approximately seven times larger response remains, given the volume ratio between particles, implying a seven times larger response per binding event. We hypothesize that this can have bearing in molecular imaging.