Figure 1.

(A) Handheld, portable frequency mixing-based readout device and (B) schematic cross-section of detection head composed of driving coil providing the low driving frequency (f₂), the excitation coil providing the high excitation frequency (f₁) and detection unit based on a detection coil detecting the resulting mixing frequency signal of magnetic particles (MPs; f₁ + 2f₂) together with the directly induced signal and the reference coil detecting only the directly induced signal. The finally resulting measuring signal does not contain the directly induced excitation due to the opposite winding direction of detection and reference coil.