Low-frequency ultrasound contrast enhancement behavior of a new nano-system for dual-mode imaging

In recent years, intensive studies have been conducted on the development of nanosized contrast agents (CAs) for multimodal molecular imaging. In this context, we have demonstrated the possibility of realizing a shell of magnetic nanoparticles (NPs) on acoustically-visible silica nanospheres (SiNSs), in order to obtain new dual-mode CAs detectable through both ultrasound (US) and magnetic resonance imaging (MRI). In the present work we performed the first experimental investigation of the low-frequency acoustical behavior of one of these novel CAs, made of SiNSs of different diameters (range 160-660 nm) coated by FePt-iron oxide NPs. Magnetically-coated SiNSs were insonified with low-frequency US pulses (2.5, 3.5 and 4.5 MHz) and the results were compared with those of pure SiNSs. Magnetic shell addition was shown to produce only slight variations in acoustical response of SiNSs and the detection ability of each considered US frequency was quantified as a function of NP size.