An apparatus for determining at least one electromagnetic quantity characterizing an electromagnetic property of an object, in particular a human body, wherein said object contains magnetic particles, which apparatus comprises: - selection means comprising a selection field signal generator unit and selection field elements for generating a magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone having a low magnetic field strength and a second sub-zone having a higher magnetic field strength are formed in a field of view, - drive means comprising a drive field signal generator unit and drive field coils (136a, 136b, 136c) for changing the position in space of the two sub-zones (52, 54) in the field of view by means of a magnetic drive field so that the magnetization of the magnetic particles contained in said object changes locally, - receiving means comprising at least one signal receiving unit and at least one receiving coil for acquiring detection signals, which detection signals depend on the magnetization in the field of view, which magnetization is influenced by the change in the position in space of the first and second sub-zone (52, 54), - a reconstruction unit for reconstructing a particle distribution quantity characterizing a spatial distribution of the magnetic particles within at least a portion of the object depending on the detection signals, - a control unit for controlling the receiving unit for acquiring a first set of detection signals corresponding to a first drive field frequency and for acquiring a second set of detection signals corresponding to a second drive field frequency, wherein the first and the second drive field frequency differ from each other, and for controlling the reconstruction unit for reconstructing a first particle distribution quantity depending on the first set of detection signals and for reconstructing a second particle distribution quantity depending on the second set of detection signals, wherein the reconstruction unit contains a determination unit for determining the electromagnetic quantity depending on the first and the second particle distribution quantity. - selection means comprising a selection field signal generator unit and selection field elements for generating a magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone having a low magnetic field strength and a second sub-zone having a higher magnetic field strength are formed in a field of view, - drive means comprising a drive field signal generator unit and drive field coils (136a, 136b, 136c) for changing the position in space of the two sub-zones (52, 54) in the field of view by means of a magnetic drive field so that the magnetization of the magnetic particles contained in said object changes locally, - receiving means comprising at least one signal receiving unit and at least one receiving coil for acquiring detection signals, which detection signals depend on the magnetization in the field of view, which magnetization is influenced by the change in the position in space of the first and second sub-zone (52, 54), - a reconstruction unit for reconstructing a particle distribution quantity characterizing a spatial distribution of the magnetic particles within at least a portion of the object depending on the detection signals, - a control unit for controlling the receiving unit for acquiring a first set of detection signals corresponding to a first drive field frequency and for acquiring a second set of detection signals corresponding to a second drive field frequency, wherein the first and the second drive field frequency differ from each other, and for controlling the reconstruction unit for reconstructing a first particle distribution quantity depending on the first set of detection signals and for reconstructing a second particle distribution quantity depending on the second set of detection signals, wherein the reconstruction unit contains a determination unit for determining the electromagnetic quantity depending on the first and the second particle distribution quantity.