A system for determining the spatial distribution of magnetic particles in an examination area. Magnetic field generator generates a spatially inhomogeneous gradient magnetic field with at least one region with a low field strength in which the magnetization of the particles is in a state of non-saturation, whereas they are in a state of saturation in the remaining region. By an arrangement to shift the area with a low field strength within the examination area, a change in the magnetization of the magnetic particles is brought about which can be detected from outside by a detector. At least one of the magnetic field generator, the arrangement and the detector are arranged at least partially on a medical instrument.
