A device and a process for performing high temporal- and spatial-resolution MRimaging of the anatomy of a patient during intensity modulated radiationtherapy (IMRT) to directly measure and control the highly conformal ionizingradiation dose delivered to the patient for the treatment of diseases causedby proliferative tissue disorders. This invention combines the technologies ofopen MRI, multileaf-collimator or compensating filter-based IMRT delivery, andcobalt teletherapy into a single co-registered and gantry mounted system.