A device and a process for performing high temporal- and spatial-resolution MR imaging of the anatomy of a patient during intensity modulated radiation therapy (IMRT) to directly measure and control the highly conformal ionizing radiation dose delivered to the patient for the treatment of diseases caused by proliferative tissue disorders. This invention combines the technologies of open MRI, multileaf-collimator or compensating filter-based IMRT delivery, and cobalt teletherapy into a single co-registered and gantry mounted system.