A therapeutic apparatus (900, 1000) comprising a high intensity focused ultrasound system (904) for heating a target zone (940, 1022). The therapeutic apparatus further comprises a magnetic resonance imaging system (902). The therapeutic apparatus further comprises a memory (952) containing machine executable instructions (980, 982, 984, 986, 988, 990) for execution by a processor (944). Execution of the instructions cause the processor to: generate (702, 802) heating commands (964) which cause the high intensity focused ultrasound system to sonicate the subject repeatedly acquire (704, 804) magnetic resonance data (954) during execution of the heating commands repeatedly calculate (706, 806) a spatially dependent parameter (970) and repeatedly modify (708, 808) the heating commands in accordance with the spatially dependent parameter such that within the target zone the spatially dependent parameter remains below a first predetermined threshold and above a second predetermined threshold.
