An apparatus for detecting magnetic particles in a field of view, 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 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 are formed in the field of view, - drive means comprising a drive field signal generator unit and drive field coils (124; 125, 126, 127) 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 material 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),- wherein said apparatus is adapted for performing background signal measurements interspersed with system function measurements and for deriving signal quality factors from said background signal measurements; and - reconstruction means for reconstructing an image of the field of view from said detection signals, wherein the spectrum in the temporal frequency domain of said detection signals includes a plurality of frequency components, wherein one or more of said frequency components are selected and/or weighted by use of said frequency component specific signal quality factor obtained from said background signal measurements and wherein only selected and/or weighted frequency components are used for reconstruction of the image. 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 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 are formed in the field of view, - drive means comprising a drive field signal generator unit and drive field coils (124; 125, 126, 127) 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 material 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),- wherein said apparatus is adapted for performing background signal measurements interspersed with system function measurements and for deriving signal quality factors from said background signal measurements; and - reconstruction means for reconstructing an image of the field of view from said detection signals, wherein the spectrum in the temporal frequency domain of said detection signals includes a plurality of frequency components, wherein one or more of said frequency components are selected and/or weighted by use of said frequency component specific signal quality factor obtained from said background signal measurements and wherein only selected and/or weighted frequency components are used for reconstruction of the image.