System and apparatus for combined magnetic resonance imaging with magnetic spectroscopy of brownian motion and/or magnetic nanoparticle imaging

A system for measuring responses of magnetic nanoparticles has static magnetic bias field along first axis, and AC driving coils providing AC magnetic field along second axis perpendicular to the first axis, both fields passing through an imaging zone. Sensing coils are oriented to sense fields parallel to the first axis, but not parallel to the second axis. A processor determines responses of nanoparticles in the imaging zone to the AC field. Another system has DC bias on first axis and AC driving coils providing an AC magnetic field along a second axes, magnetic gradient oriented along the first and/or second axis, and the second axis is rotated mechanically or electronically. The signal processor provides a voxel-based model of magnetic nanoparticle distribution in imaging zone. In some embodiments, the static magnet is a main magnet of a magnetic resonance imaging system.