A system for delivering ultrasound energy to an internal anatomical target includes an ultrasound transducer having multiple transducer elements collectively operable as a phased array multiple driver circuits, each being connected to at least one of the transducer elements multiple phase circuits a switch matrix selectably coupling the driver circuits to the phase circuits and a controller configured for (i) receiving as input a target average intensity level and/or an energy level energy to be applied to the target and/or a temperature level in target, (ii) identifying multiple sets of the transducer elements, each of the sets corresponding to multiple transducer elements for shaping and/or focusing, as a phased array, ultrasound energy at the target across tissue intervening between the target and the ultrasound transducer, and (iii) sequentially operating the transducer-element sets to apply and maintain the target average energy level at the target. In various embodiments, the controller operates each of the transducer element sets in accordance with a pulse-width modulation pattern having a duty cycle selected to achieve the target average intensity level, energy level, and/or temperature level at the target in accordance with a time constant of the target tissue.
