Enhanced pulsed magneto-motive ultrasound imaging using superparamagneticnanoclusters

Abstract

Recently, pulsed magneto-motive ultrasound (pMMUS) imaging augmented withultra-small magnetic nanoparticles has been introduced as a tool capable of imaging eventsat molecular and cellular levels. The sensitivity of a pMMUS system depends on severalparameters, including the size, geometry and magnetic properties of the nanoparticles.Under the same magnetic field, larger magnetic nanostructures experience a strongermagnetic force and produce larger displacement, thus improving the sensitivity andsignal-to-noise ratio (SNR) of pMMUS imaging. Unfortunately, large magnetic iron-oxidenanoparticles are typically ferromagnetic and thus are very difficult to stabilize againstcolloidal aggregation. In the current study we demonstrate improvement of pMMUS imagequality by using large size superparamagnetic nanoclusters characterized by strongmagnetization per particle. Water-soluble magnetic nanoclusters of two sizes (15 and 55 nmaverage size) were synthesized from 3 nm iron precursors in the presence of citratecapping ligand. The size distribution of synthesized nanoclusters and individualnanoparticles was characterized using dynamic light scattering (DLS) analysis andtransmission electron microscopy (TEM). Tissue mimicking phantoms containing singlenanoparticles and two sizes of nanoclusters were imaged using a custom-built pMMUSimaging system. While the magnetic properties of citrate-coated nanoclusters areidentical to those of superparamagnetic nanoparticles, the magneto-motive signaldetected from nanoclusters is larger, i.e. the same magnetic field produced largermagnetically induced displacement. Therefore, our study demonstrates that clusters ofsuperparamagnetic nanoparticles result in pMMUS images with higher contrast and SNR.