Ultrafast plane wave imaging based pulsed magnetomotive ultrasound

Recently, pulsed magnetomotive ultrasound (pMMUS) imaging has been introduced to detect magnetic nanoparticles (MNPs) which are not able to be visualized by conventional ultrasound. However, because of the used magnetic short pulse, the reported pMMUS only can use a single-element ultrasound transducer along with mechanical scanning to perform imaging, which significantly limits the imaging fame rate. To solve this problem, we propose an ultrafast plane wave imaging based pMMUS technique. The ultrafast frame rate of plane wave imaging is fast enough to track the magneto-motion of the excited MNPs during the period of the magnetic pulse being applied. Therefore, the proposed ultrafast plane wave pMMUS is capable of visualizing the dynamic response of the excited MNPs, which is highly correlated to tissue characteristics, to an externally-applied magnetic pulse. In our experiments, ultrafast plane wave imaging with a 5 kHz frame rate was used to implement the pMMUS where the MNP motion induced by an 8-ms magnetic pulse was tracked. The results showed that there were significant differences between the ultrafast plane wave pMMUS images of the phantoms with and without MNPs embedded. In addition, gelatin phantoms with 2%, 4% and 6% gelatin were used to mimic tissues with different elasticity. The dynamic responses of the excited MNPs in the three types of phantoms were distinguishable. Overall, it is demonstrated that the feasibility of our proposed ultrafast plane wave pMMUS imaging technique for the visualization of the magneto-motion and dynamic response of the MNPs under the excitation of a short magnetic pulse. More studies are required to further improve the magneto-motion tracking algorithm and explore the relationship between the dynamic response of the excited MNPs and the tissue viscosity and elasticity.