Backward-mode ultrafast pulsed magnetomotive ultrasound

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Recently, magnetomotive ultrasound (MMUS) imaging has been introduced to detect magnetic nanoparticles (MNPs) which are not able to be visualized by conventional B-mode ultrasound. However, to date, only forward-mode MMUS where the imaging object is placed in between an ultrasound probe and an electromagnet is implemented in the literature. Such a MMUS mode is not suitable for clinical use. To facilitate clinical translation of MMUS, we propose a backward-mode ultrafast pulsed MMUS system. Firstly, we developed a MMUS probe integrating a 7.5-MHz ultrasound array transducer and an electromagnet which was able to apply cyclic pulsed magnetic excitation and 0.4-Tesla maximum magnetic field with the custom-made magnetic pulser. Secondly, cyclic pulsed magnetic excitation and ultrahigh frame-rate (up to 2.5 kHz) speckle tracking enabled matched filtering for identification of the unique cyclic magneto-motion from the excited MNPs. With the matched filtering, tissue motion artifacts can potentially be suppressed. Finally, using a projection window on the result of matched filtering was capable of rejecting the artifact caused by the intrinsic vibration of the working electromagnet in the MMUS probe. With the improved contrast, the distribution of the embedded magnetic nanoparticles was clearly imaged in phantom experiments. Overall, we demonstrate the capability of the proposed backward-mode ultrafast pulsed MMUS system, which is more clinically translatable.