Narrowband Magnetic Particle Imaging Based on Multiharmonic System Matrix

Magnetic particle imaging (MPI) is an emerging medical imaging technique for quantitatvely visualizing the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIONs), which has shown great potential in biomedicine. In this study, we propose an approach of narrowband MPI based on multi-harmonic system matrix. A high-frequency (f₀ = 101 kHz) ac magnetic field in x-direction is used to excite SPIONs to generate spectra while a low-frequency (f_y = 33 Hz) ac magnetic field in y-direction is used to scan the field free point (FFP). The model for the forward problem is deduced based on multi-harmonic system matrix. Afterwards, the sideband harmonics of the third harmonic 3f0 are measured for imaging. With 24 sideband harmonics for MPI, the bandwidth is about 24 × 33 Hz = 792 Hz. An MPI scanner with a main gradient of 1.12 Tm^-1μ0^-1 is built for experiments. Phantom experiments are performed to evaluate the spatial resolution and limit of detection (LOD). Experimental results demonstrate that the proposed narrowband MPI approach based on multi-harmonic system matrix allows a spatial resolution of 1.5 mm and 1 mm for a gradient field of 0.56 Tm^-1μ0^-1 and 1.12 Tm^-1μ0^-1 in x- and y-directions, respectively. The MPI scanner achieves an LOD of 45 ng Fe (5 μg (Fe)/mL) using synomag®-D SPIONs at 8 mT.