A system or use with a radiotherapy system includes a scanner for producing images of at least one object within an imaging volume, a radiotherapy apparatus including a source of therapeutic radiation adapted to deliver a beam of radiation into the at least one object, a treatment planning system for controlling the source so as to deliver radiation in accordance with a predetermined plan for treating a patient and with the images of the at least one object, and at least one device within and/or adjacent to the imaging volume. The at least one device comprises at least one first marker which is visible to the scanner and at least one MR second marker which is visible to an MR scanner. The system further comprises a database containing data including the markers associated with the at least one device and at least its geometric and density characteristics.