Spatial resolution is a key metric for characterizing any imaging modality. Here we describe magnetic particle imaging (MPI) experiments and complementary magnetic particle spectroscopy (MPS) experiments that reveal and probe significant variations in MPI spatial resolution that are intrinsically coupled to the magnetization dynamics of superparamagnetic nanoparticles. In particular, employing a phase-sensitive MPI detection scheme, we observe a marked spatial sharpening effect as the phase of the demodulation reference is advanced. This increase in spatial resolution is accompanied by degradation of image contrast, to the point where reconstructed images effectively vanish when the demodulation phase is advanced by approximately 90 degrees relative to the phase that yields images with the highest signal-to-noise ratio (SNR). Relaxation- or phase-weighted magnetic particle images with apparent spatial resolutions of order a few hundred microns or better, and with SNRs of order 10 dB or more, are presented, and a qualitative framework for interpreting the role of phase as it pertains to spatial resolution in MPI is introduced.
