In the vicinity of an MRI system, moving ferromagnetic masses (elevators, trucks, conveyor belts, etc.) that are magnetized by the earth's magnetic field can cause interference. Similarly, direct currents in overhead lines of trams, suburban trains or subway trains can also cause interference. In order to achieve optimum image quality in MR imaging, this interference must be measured, evaluated and eliminated by taking appropriate measures before a project is planned. Subsequent reduction of interference sources is difficult, time-consuming and cost-intensive.