Method and apparatus for measuring an amount of superparamagnetic material in an object, the method including a) applying a magnetic field having a first component alternating with a first period to the object and a magnetic field strength lower than a magnetic field strength at which the superparamagnetic material is driven in saturation; b) measuring a first magnetic susceptibility of the object with a detection coil; c) applying a static second component to the magnetic field for a second period being equal or larger than the first period, the strength of the magnetic field during the second period is such that the superparamagnetic material is driven towards saturation; d) measuring a second magnetic susceptibility of the object with the detection coil during the application of the static second component; and e) determining the amount of superparamagnetic material from a difference between the measured first and second susceptibility of the object.