This invention provides a system and method that improves the sensitivity and localization capabilities of Magnetic Particle Imaging (MPI) by using combinations of time-varying and static magnetic fields. Combinations of magnetic fields can be used to distribute the signals coming from the magnetic particles among the harmonics and other frequencies in specific ways to improve sensitivity and to provide localization information to speed up or improve the signal-to-noise ratio (SNR) of imaging and/or eliminate the need for saturation fields currently used in MPI. In various embodiments, coils can be provided to extend the sub-saturation region in which nanoparticles reside; to provide a static field offset to bring nanoparticles nearer to saturation; to introduce even and odd harmonics that can be observed; and/or to introduce combinations of frequencies for more-defined observation of signals from nanoparticles. Further embodiments provide for reading of the signal produced by cyclically saturated magnetic nanoparticles in a sample so as to provide a measurement of the temperature of those nanoparticles. The spectral distribution of the signal generated provides estimates of the temperature of the nanoparticles. Related factors may also be estimated—binding energies of the nanoparticles, phase changes, bound fraction of the particles or stiffness of the materials in which the nanoparticles are imbedded.