1. An arrangement ( 10 ) for detecting and/or locating a magnetic material ( 100 ) in a region of action ( 300 ), which arrangement comprises: selection means ( 210 ) for generating a magnetic selection field ( 211 ) having a pattern in space of its magnetic field strength such that a first sub-zone ( 301 ) having a low magnetic field strength and a second sub-zone ( 302 ) having a higher magnetic field strength are formed in the region of action ( 300 ), drive means ( 220 ) for changing the position in space of the two sub-zones ( 301 , 302 ) in the region of action ( 300 ) by means of a magnetic drive field ( 221 ) so that the magnetization of the magnetic material ( 100 ) changes locally, receiving means ( 230 ) for acquiring detection signals, which detection signals depend on the magnetization in the region of action ( 300 ), which magnetization is influenced by the change in the position in space of the first and second sub-zone ( 301 , 302 ), storage means ( 75 ) for storing a subset of system function data of the arrangement's system function, said system function comprising a system function data set describing the relation between spatial position of the magnetic material and the system response for said arrangement and the trajectory along which said first sub-zone ( 301 ) is moved for the acquisition of said system function data, and reconstruction means ( 74 ) for reconstructing the spatial distribution of the magnetic material ( 100 ) in the region of action ( 300 ) from the detection signals and the stored subset of the system function data, using additional knowledge about the structure of the system function.