Evaluation of harmonic signals from magnetic nanoparticles and application to magnetic nanoparticle imaging

We developed a magnetic nanoparticle (MNP) imaging system that uses the harmonic signals of MNPs on the basis of their nonlinear magnetization curves. Since the interference of the excitation field can be significantly decreased by measuring the harmonic signals, the detection sensitivity can be improved. First, we have characterized the nonlinear properties of MNPs in order to evaluate the harmonic signals. We clarified what proportion of MNPs contributes to the harmonic signal. It is important to choose a sample having a large portion of MNPs with large magnetic moment and short relaxation time for the harmonic signal-based detection. Then, we measured a contour map of the signal field from the MNPs with our simple detection system using second harmonic signal. We could detect 100 μg of MNPs located at z = 25 mm below the pickup coil with a high signal-to-noise ratio. Analyzing the contour map using a mathematical technique called singular value decomposition (SVD), we could clearly distinguish two MNP samples separated by ?x = 25 mm and located at z = 25 mm. This simple detection system will be useful for medical applications such as sentinel lymph node detection.