Extending imaging range in magnetomotive ultrasound with tailored magnetic nanoparticles

In magnetomotive ultrasound (MMUS), magnetic nanoparticles (MNPs) are used as a contrast agent. A time-varying magnetic field displaces the MNPs which in turn move their closest surrounding - a motion detected with ultrasound. A limiting factor for MMUS is the magnetic force magnitude decay with depth, which restricts the imaging area. Several attempts to extend the imaging depth have been suggested, such as magnetic field generator configuration or choice of MNP core materials. In this study we present a new approach by modifying the design of the contrast agent using a commercially available nanoparticle with a nano-flower shaped iron oxide core (synomag®-D, micromod, Germany). In order to evaluate the performance of this particle, a conventional particle (perimag®, micromod, Germany), was used for comparison. Using an electromagnet with a cone shaped iron core as a field generator, the induced MMUS displacement of the two MNP varieties where analyzed when altering the magnetic field strength and magnetic field frequency. Further, by using a more clinically relevant magnetic field setup, the depth where the synomag MNPs could produce significant MMUS displacement, was evaluated. The synomag MNPs produced a 1.8 times larger displacement than the perimag MNPs. At 52mm, MMUS displacement was still detectable using synomag MNPs. This indicates that the particle design plays an important role in MMUS imaging. Moreover, this study also confirms that MMUS imaging at clinically relevant depths is possible.