Vectorial magnetomotive ultrasound for mapping the viscoelastic properties of soft media.

Magnetic nanoparticles (MNPs) are attractive for biomedical applications due to their biocompatibility, controllable magnetic response, and multifunctionality in both diagnostic and therapeutic contexts. Magnetomotive ultrasound (MMUS) exploits these properties to detect MNPs-induced displacements, enabling nanoparticle mapping and quantitative assessment of tissue mechanics. However, conventional MMUS methods are limited by single-axis or quasi-static measurements, which fail to fully capture the multidirectional and dynamic behavior of soft tissues. In this study, we introduce a pixel-wise vectorial approach for the dynamic assessment of soft media, validated through experiments on tissue-mimicking phantoms and finite element simulations. By integrating lateral displacement data with axial measurements, the proposed method expanded viscoelastic map coverage from roughly 40% to nearly the full width of the lateral dimension within the transducer's field of view, enabling fully spatially resolved viscoelastic imaging. The proposed technique also quantifies viscoelastic variations, capturing differences of up to 50% in both stiffness and viscosity between the tested samples, thus providing an enhanced MMUS framework capable of assessing treatment efficacy and distinguishing the mechanical properties of regions containing MNPs from surrounding tissue.