Field-Orientation Effects in Amplitude-Modulation Magnetic Particle Imaging for High-Resolution Navigation

Amplitude-modulation magnetic particle imaging (AM-MPI) has strong potential for real-time navigation of magnetic nanoparticles because it provides tracer-specific spatial feedback using a narrowband acquisition scheme with relatively low excitation power and simplified signal detection. However, improving spatial resolution by increasing the selection-field gradient becomes increasingly difficult as scanner dimensions increase. This study investigates whether field orientation can be used to improve and control the spatial resolution of three-dimensional AM-MPI with field-free-point (FFP) and field-free-line (FFL) encoding. Four scan-receive configurations were analyzed using a matrix point-spread-function (PSF) model, followed by two-point phantom simulations at equal physical and FWHM-normalized source separations. With a common y-directed scan, FFP-y produced a single-peaked collinear h_yy response with FWHM values of 1.29 mm along y and 5.89 mm along x and z. FFL-y retained the same y-direction FWHM while reducing the z-direction FWHM to 2.94 mm. FFP-x selected the transverse h_xy component, producing a central null and a multi-lobe response, whereas FFL-x was a null channel for the adopted field geometry. At equal physical spacing, the first tested separation satisfying the adopted Rayleigh-type criterion (M ≥ 0.26) was 3 mm along y for both FFP-y and FFL-y, 5 mm along z for FFL-y, and 7 mm along x and z for FFP-y. FFL-y also provided better z-direction separability than FFP-y. After normalization by the corresponding directional FWHM, the single-peaked responses showed similar two-point separability, indicating that the differences observed at equal physical spacing were mainly associated with directional PSF width. These results show that field orientation affects both the topology and directional resolution of the AM-MPI response and can be considered together with selection-field design when optimizing AM-MPI systems for nanoparticle navigation.