Narrowband isotropic magnetic particle imaging with FFL-parallel excitation

Magnetic Particle Imaging (MPI) is an emerging medical imaging technique for high-sensitivity and quantitative visualization of tracer distributions. Narrowband MPI achieves high signal-to-noise ratio (SNR) by selectively detecting a few harmonics. However, in its conventional implementations, the point spread function (PSF) inevitably exhibits negative lobes, which introduce anisotropic resolution and destructive signal superposition. Approach. To address this, we propose a Narrowband Isotropic MPI system, utilizing the Field-Free-Line (FFL)-parallel excitation (FPE) scanning strategy to eliminate negative lobes. FPE-MPI employs single-channel acquisition with coaxial excitation and receive, applies an excitation field parallel to the FFL while maintaining a constant collinear offset field, and encodes harmonic responses on the projection plane orthogonal to the FFL. Main results. Through simulations and phantom experiments on our in-house scanner, we demonstrate that FPE-MPI eliminates the adverse effects of negative lobes, achieves isotropic imaging, and improves image quality. Compared with conventional narrowband MPI with FFL-orthogonal excitation, FPE-MPI preserves higher SNR in the acquired signals and yields robust reconstructions at low tracer concentrations, with only a modest trade-off in spatial resolution. Significance. FPE-MPI facilitates robust imaging at low tracer doses and provides a feasible scanning strategy for the development of future large field-of-view (FOV) narrowband MPI systems.