Magnetomotive ultrasound imaging using the nonlinear magnetization of nanoparticles

In magnetomotive ultrasound imaging, magnetic nanoparticle labeled tissue is moved by a magnetically induced force. This force is commonly modeled as linear with respect to the applied magnetic field strength. However, it is known that iron oxide nanoparticle suspensions exhibit a superparamagnetic, i.e., nonlinear, magnetization characteristic. In this contribution, a magnetic field configuration different from conventional setups was used to excite harmonics in the tissue displacement due to the nonlinear magnetization characteristic. These harmonics could be used for a localized magnetic excitation. For a proof of concept, displacements of a gelatin embedded nanoparticle bolus were measured with a vibrometer. Harmonics in the displacement were quantified and compared to a theoretical model. Deviations between the model and measurements exist that were explained by a misalignment of the probed bolus in the magnetic field and the instrumentation used.