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In 2013 we proposed the magneto-thermo-acoustic effect [Piao et al, Med. Phys. 2013], which refers to acoustic emission from magnetic nanoparticles (MNPs) when thermally mediated under an alternating magnetic field (AMF) applied in a pulsed or frequency-chirped mode. Several independent experimental studies have since validated magneto-thermo-acoustic effect in association with various modes of AMF application including pulsing and chirping of the intensity modulation [Feng et al, Appl Phys. Lett., 2013]. Later Kellnberger et al demonstrated that [Phys Rev Lett. 2016] a continuous-wave (CW) AMF applying to MNPs activated acoustic emission at ONLY the second-harmonic frequency of AMF. We predict that applying a static bias magnetic field to MNPs in the presence of a CW AMF produces acoustic emission at the fundamental frequency of AMF, in addition to the known second harmonic frequency of AMF. The mechanism of this dual-frequency acoustic emission is projected as the partial magnetization of MNPS by the bias field affecting the AMF mediation of MNPs. The coupling between the two magnetic fields in modulating the magnetic susceptibility of MNPs causes acoustic emission at both the fundamental and second harmonic frequencies of AMF. Interestingly, the intensity ratio of the acoustic emission at the two frequencies is determined uniquely by the intensity ratio of the two magnetic fields. This spectral intensity characteristic of the dual-frequency acoustic emission from MNPs can thus be made spatially unique by controlling the bias field over AMF. This potentiates spatial encoding or magnetically scanned imaging of MNPs towards theragnostic applications.
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