This invention relates to a method and device for calibrating the offset of an imaging system. The core idea of the invention is to place a reference object in the measurement chamber of the imaging system, measure the signals associated with the reference object at different points of time, calculate the merit function based on changes of the parameters representing the electromagnetic property of the reference object, and derive an optimal set of offset data that minimizes the value of the merit function for compensating the offset of the system in subsequent image reconstructions. In one embodiment, the invention uses a reference object comprising a non-conductive envelope and a cavity which can be filled with a conductive fluid and emptied, and in this way reduces the imaging interference caused by the reference object during monitoring.