Abstract
Recently, pulsed magnetomotive ultrasound (pMMUS) imaging has been introduced to detect superparamagnetic iron oxide nanoparticles (SPIO) which is 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 rates. 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 SPIOs 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 SPIOs, which is highly correlated to tissue characteristics such as viscosity and elasticity, to an externally-applied magnetic pulse. In addition, a new pMMUS motion tracking algorithm based on ultrafast plane wave imaging is developed to reduce the effect of magnetic field inhomogeneity. In our experiments, ultrafast plane wave imaging with a 5 kHz frame rate was used to implement the pMMUS where the SPIO 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 SPIOs embedded. There was a monotonic increase in displacement with increased concentration of SPIOs. In addition, agarose phantoms with 0.5%, 1% and 1.5% agarose were used to mimic tissues with different elasticity. The dynamic responses of the excited SPIOs 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 SPIOs 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 SPIOs and the tissue viscosity and elasticity.