Difference method for the measurement of the liver iron concentration by biomagnetic liver susceptometry: From the start position (left), the patient is lowered in an inhomogeneous magnetic field of 20–30 milliTesla by about 5 to 10 cm (right) to a field of 1 milliTesla. During this vertical scan, the water coupling bag follows the patient and the change of the magnetic flux as a function of the distance is registered in the detector coils and transformed by the SQUID electronics.