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Acoustically Activated Nanoparticle Agents for Molecular Imaging

T Shorrock, JC Bamber

Source of funding: EPSRC

We have reported previously that, with the aim of eventually overcoming the limitation that conventional ultrasound contrast microbubbles are restricted to the vasculature, we have conducted preliminary studies of a combined contrast agent and imaging concept, based on a transiently vaporisable perfluorocarbon emulsion with a droplet-size small enough (< 150 nm) to leave blood vessels. 

The method relies on two synchronised acoustic waves, a relatively short activating/manipulating low frequency wave, and a high-frequency wave that detects a transient imaging signature. In the presence of a 200nm-droplet perfluoropentane (boiling point 28 deg C) emulsion bright signals were observed on 20 MHz images at the focus of a low frequency wave, suggesting that exceptionally short duration droplet cavitation events may be induced and that a synchronised two-wave approach may have potential for imaging them. 

Recent work has concentrated on the theory underlying second-wave observation of extremely fast-moving structures such as the vaporisation-bubble wall, and an acoustic relativistic correction has been applied to derive equations for predicting how the bubble wall would appear to move when observed acoustically rather than optically. Differences between the predictions of traditional equations of motion for the bubble wall, such as the Keller-Miksis equation, and the acoustically observed motion predicted by the new equation, were found to be large at high-velocity phases of the motion (Figure 14), leading to significant differences in predicted acoustic scattering cross-sections for the two models.

Fig.14.  The time dependence of the wall velocity of a 2 µm diameter bubble excited by a 0.5 MHz acoustic wave at peak pressures 100 kPa, 300 kPa and 500 kPa, as predicted by calculation using the Keller-Miksis model (black - labelled optically measured) and a new acoustically-measured-Keller-Miksis model (red - labelled acoustically measured).