Pulsed magneto-motive ultrasound (pMMUS) imaging is a relatively new ultrasound-based molecular imaging technique capable of visualizing the spatial distribution of non-toxic magnetic nanoparticles in real time and at sufficient depth. In pulsed magneto-motive ultrasound imaging, an external high-strength pulsed magnetic field is applied to induce motion within the magnetically labelled tissue and ultrasound is used to detect the induced internal tissue displacement.

The magnetically induced displacement depends on several parameters including size, geometry and magnetic properties of the nanoparticles. Larger magnetic nanostructures with a large volumetric ratio of magnetic materials experience a stronger magnetic force and produce a larger displacement, which improves the sensitivity and signal-to-noise ratio (SNR) of pMMUS imaging. However, although small magnetic nanoparticles (generally below 10–20 nm) exhibit superparamagnetism, particles above a certain size called the "superparamagnetic limit" start to exhibit ferromagnetic properties. The strong attractive magnetic force between ferromagnetic particles often causes aggregation, making colloidal stabilization very challenging.

It is possible to stabilize ferromagnetic nanoparticles through a polymer coating that prevents their aggregation, however these polymer coated nanoparticles usually have a small volume fraction of magnetic materials, which can make them unsuitable for pMMUS imaging.

Nanoclusters in action

In this study, we investigated clusters of magnetic nanoparticles as a contrast agent for pMMUS imaging with higher sensitivity and contrast. Individual superparamagnetic particles and nanoclusters of different sizes were studied. We demonstrated that larger magnetic nanostructures increase the quality (sensitivity and signal-to-noise ratio) of pMMUS images while preserving their superparamagnetic properties.

The results suggest that imaging at larger depth and using lower concentrations of nanoparticles can be achieved by increasing the size of the magnetic nanostructures.

The authors presented their results in the journal Nanotechnology.