1. An apparatus for detecting magnetic particles in a field of view, which apparatus comprises: selection and focus field coils disposed adjacent the field of view; a detection coil configured to measure background signals form the field of view and detect detection signals from the field of view; and a computer configured to: control the selection field coil to generate a magnetic selection field having a pattern in space of its magnetic field strength such that a first sub-zone has a low magnetic field strength where the magnetization of the magnetic particles is not saturated and a second sub-zone having a higher magnetic field strength where the magnetization of the magnetic particles is saturated, the first and second sub-zones being formed in the field of view, control the drive field coils to change a position in space of the two sub-zones in the field of view with a magnetic drive field so that the magnetization of the magnetic material changes locally, receive the background signals measured by the detection coil, receive the detection signals acquired by the detection coil, the detection signals depending 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-zones, wherein a spectrum of said detection signals includes a plurality of frequency components, and selecting one or more of said frequency components and/or weighting the frequency components by use of a frequency component specific signal quality factor obtained from background signal measurements, and reconstructing an image depicting the particles in the field of view using only selected and/or weighted frequency components.