Improved Backward Mode Pulsed Magnetomotive Ultrasound via Pre-magnetization of Superparamagnetic Iron Oxide Nanoparticles

Magnetomotive ultrasound (MMUS), capable of imaging in vivo magnetic nanoparticle distribution, has shown its potential in extravascular ultrasound molecular imaging, magnetic nano-drug delivery monitoring, and sentinel lymph node identification. To date, most of the research teams in the world mainly implemented forward mode MMUS where the imaging object has to lie in between an ultrasound probe and an electromagnet, which is not clinically translatable. However, in our previously proposed backward mode MMUS, the MMUS displacement map was relatively noisy because of the backward-mode setup. Learning from the principle of magnetic particle imaging, we propose a new strategy to improve the backward-mode MMUS via pre-magnetization of superparamagnetic iron oxide nanoparticles (SPIONs). The new MMUS probe consists of an electromagnet for magnetic excitation and a static magnet for pre-magnetization of SPIONs. The pre-magnetization is to magnetize the SPIONs with a preset magnetic field in advance so that according to the nonlinear characteristics of the SPION magnetization curve, SPIONs can have higher magnetic susceptibility of SPIONs during magnetic excitation; thus increasing the magnetomotive force applied to SPIONs. Therefore, the displacement resulting from the magnetomotion of the SPIONs can be increased, which in turn improves the MMUS detectability of the SPIONs in backward mode. Overall, it has been demonstrated that the effectiveness of the proposed strategy in the improvement of the backward mode MMUS.