The invention provides for a medical apparatus (300, 500, 600) comprising a high intensity focused ultrasound system (302), a memory containing instructions, and a processor. Execution of the instructions causes the processor to receive (100, 202) surface data (342) descriptive of the location of a surface (322). The surface data defines the location of the heating volume. Execution of the instructions further cause the processor to determine (102, 204) a set of transducer switching commands (344) using the heating location data and an ultrasound transducer element model (352) and control (104, 206) the high intensity focused ultrasound system with the set of transducer switching commands to heat the heating volume. The set of transducer switching commands comprises an intensity level for the multiple ultrasound transducer elements either above or below a predetermined intensity for controlling a cross sectional shape of the heating volume to encompass the surface.
