Imaging of Magnetic Nanoparticles Using Second-Harmonic Signals

Magnetic nanoparticle imaging (MPI) utilizes the non-linear magnetization response M for the detection of superparamagnetic iron oxide magnetic nanoparticles (MNPs). When an external ac magnetic field is applied to the MNPs, some harmonic responses such as second, third, and higher ones arise. We propose and demonstrate that the use of the second-harmonic response is the most effective technique to enhance the signal. The advantage of using the second-harmonic response is that the response can be measured even in a small ac field. A 2-D MPI system using this technique was realized and its performance was evaluated. The MPI system has no mechanical parts but instead magnetically scans an MNP phantom by changing the dc bias field in two directions. The cosine component of the signal is detected by a lock-in-amplifier and differentiated in one direction. The image of the phantom is then reconstructed using the d(cosθ)/dz data. As a result, the position of the phantom may be identified down to the iron content of 9 μg.