A quantification of a particular element comprised in magnetic particles enclosed in a volume involves applying a first time-varying magnetic field to the volume, having a first magnitude and a first frequency and applying a second time varying magnetic field, not parallel with the first magnetic field for causing precession of the magnetized particles. The second magnetic field is an RF field having a second frequency equal to the Larmor-frequency of the particular element. Thereafter the resultant magnetization originating from the volume and modulated by the time-varying field is measured, and at least one frequency component of the resultant magnetization is determined. A power and/or voltage of the at least one frequency component is calculated and a quantity of the magnetic particles enclosed in the volume is determined based thereon.