A system for use with a radiotherapy system comprising: a scanner (4) for producing images of at least one object within an imaging volume, a source of therapeutic radiation (6) adapted to deliver a beam of radiation into the or each object, a Treatment Planning System (TPS) for controlling the source so as to deliver radiation in accordance with a predetermined plan for treating a patient and with images of the or each object from the scanner of the imaging volume, and one or more devices (28) within and/or adjacent the volume, wherein the or each device (28) comprises at least one marker (30) which is/are adapted to be visible to the scanner and accurately to identify the device (28) and its position in relation to the volume, and at least one MR marker which is/are adapted to be visible to an MR scanner, and wherein the system further comprises a database containing data including the marker(s) associated with the device and at least its geometric and density characteristics. The invention encompasses the database, and methods of populating it with device-specific data.
