Effect of coil geometry on sensitivity distribution in field-free line magnetic particle imaging

Magnetic particle imaging (MPI) with a field-free line (FFL) enables high-sensitivity visualization based on the nonlinear magnetization response of superparamagnetic nanoparticles. However, the superposition of the selection and drive fields modifies the local effective field and causes spatial variations in the system function. This paper presents a comparative analysis of two coil geometries: coaxial configuration (Co-Config.) and orthogonal configuration (Ortho-Config.), in which the drive-field direction is aligned with the FFL and perpendicular, respectively. Numerical simulations and experimental measurements showed that Co-Config. suppressed the polarity inversion and maintained a more uniform system function to enable stable single-harmonic detection. Conversely, Ortho-Config. produced sharper spatial variation together with polarity inversion in the system function. In the present third-harmonic reconstruction experiment, these sign-changing features appeared as localized reductions in reconstructed signal amplitude. These results show that the two coil geometries provide distinct native system-function characteristics, which should be considered when selecting the acquisition and reconstruction strategy for FFL-MPI systems.