Assessing the limits of MPI tracer performance

Simulations of m'(H) (Fig 1B), which matched MPS (Fig 1C, D) and PSFs (Fig 1A), can be used to predict tracer behavior during imaging. Under applied field conditions commonly used in MPI (25 kHz, 20 mT/μ₀), our model predicts that hysteresis will appear in m(H) loops for particles larger than 20 nm diameter, when K_eff is 6000 [J m^-3], which is the average K_eff measured for our MPI tracer. Due to thermal fluctations, the model predicts a maximum coercive field that is less than predicted by the original Stoner-Wohlfarth model, which is given by 2K_eff/M_s, where M_S, is the saturation magnetization. For a fixed value of K_eff, the model predicts that coercive field increases with median particle size, however the FWHM of m'(H) remains relatively unchanged as the loop opens up with increasing particle size. In simulations, the FWHM of m'(H) decreased with K_eff. The minimum FWHM predicted by the model when K_eff is 6000 was 5 mT; this was consistent with the measured FWHM (5.5 mT) for 25 nm tracers.