Beyond Axial Symmetry: Tracking Vectorial Motion Enhances Nanoparticles Mapping With Magnetomotive Ultrasound

Magnetomotive ultrasound is an imaging technique that maps the distribution of magnetic nanoparticles within tissues by detecting motion induced by a time-varying magnetic field, overcoming the inherent limitations of ultrasound in nanoparticle detection. However, a major challenge in this technique is reducing the halo effect, which is the induced motion observed in tissues surrounding nanoparticles even when those regions are nanoparticle-free. This effect remains unresolved in current experimental methods, as they primarily rely on axial displacements due to the high resolution of ultrasound transducers in that direction. This study introduces a vectorial motion approach to enhance the detection of magnetic nanoparticles in magnetomotive ultrasound. Experiments were conducted on tissue-mimicking phantom, ex-vivo bovine liver, and a murine model, all labeled with iron oxide nanoparticles. A mathematical framework was established to describe the relationship between the magnetic properties of nanoparticles, the applied magnetic field, and the resulting tissue strains and displacements. Additionally, the Lucas-Kanade method is introduced as a reliable tool for analyzing the magnetically induced displacement field. Results demonstrate that vector-valued displacement analysis improves the interpretation of magnetomotive ultrasound data. However, displacement data alone proved insufficient for accurately resolving nanoparticle distribution. By incorporating external magnetic field information, the halo effect was effectively reduced, decreasing overestimation by 38% in phantom, 41% in ex-vivo tissues, and 58% in preliminary in-vivo experiment. Furthermore, this approach reduces dependence on axial symmetry between the excitation coil and transducer, offering greater flexibility in experimental setups, which is particularly beneficial for in-vivo applications.