Iron Oxide nano particles can generate contrast in two different ways depending on the size of the particle and it's composition. Most Iron oxide nano particles are on the order of 15-30 nm. This size regime can generate a substantial T1 relaxation effect due to the Outer-sphere mechanism where water rapidly exchanges through the steep field gradient of the particles. Larger nano-particles do not show this effect as greatly due to the geometrical change in the ratio of volume to surface area. Both sets of particles exhibit a T2* (pronounced T2 star) relaxation effect which can be many times larger than the particle itself. The T2* effect is best visualized in gradient echo images and can be seen most prominently there. The T2* effect increases with magnetic field so the stronger the magnet for the MRI the greater the effect.
There are many preclinical and clinical procedures that can use iron oxide NPs. Attached are two reviews that are publicly available.