A high intensity focused ultrasound system (300, 400) comprising: an ultrasound window (316, 416), an ultrasound transducer (306, 406) with multiple transducer elements (312) for focusing ultrasound energy into a focal volume (320, 420) within a subject, wherein the ultrasound transducer is adapted for directing ultrasound through the ultrasound window into the subject an ultrasound transducer power supply (552) for adjusting the phase of electrical power supplied to each of the multiple transducer elements a mechanical positioning system (310, 410) the ultrasound transducer a processor (560) and a memory (564, 566) containing machine executable instructions (578, 580, 582, 584, 586). Execution of the instructions causes the processor to perform the steps of: moving (100, 208) the focal volume closer to the ultrasound transducer by controlling the ultrasound transducer power supply, and moving (102, 200) the ultrasound transducer closer to the ultrasound window by controlling the mechanical positioning system.
