The DC magnetic field of an installed MRI system can exceed the limit values in the surrounding area. This can endanger implant wearers. To avoid this, a shield made of ferromagnetic material is usually installed. The field strengths around the MRI scanner, in particular the course of the 5 Gauss field line, are determined using a magnetic field measuring system. This must not be measured outside the RF cabin or must be marked by suitable measures. This allows the effectiveness of the shielding to be verified and the safety for implant wearers in freely accessible areas to be determined.