A system for delivering ultrasound energy to an internal anatomical target, the system comprising: an ultrasound transducer comprising a plurality of transducer elements collectively operable as a phased array; a plurality of driver circuits, each of the driver circuits being connected to at least one of the transducer elements ; a plurality of phase circuits ; a switch matrix selectably coupling the driver circuits with the phase circuits ; and a controller configured for (i) receiving as input at least one of a target average intensity level to be applied to the target, or an energy level to be applied to the target, (ii) identifying a plurality of sets of the transducer elements , each of the sets corresponding to a plurality of the transducer elements for focusing and/or shaping, 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 intensity level and/or energy level at the target, wherein the controller , via the switch matrix , operates each of the transducer element sets in accordance with a pulse-width modulation pattern having a duty cycle determined based at least in part on the ultrasound energy generated from the set of transducer elements and the beam energy levels detected at the target so as to achieve and maintain the target average intensity level and/or energy level at the target, the duty cycle having an off time smaller than an estimated time constant of the target tissue during sequential operation of the transducer-element sets . an ultrasound transducer comprising a plurality of transducer elements collectively operable as a phased array; a plurality of driver circuits, each of the driver circuits being connected to at least one of the transducer elements ; a plurality of phase circuits ; a switch matrix selectably coupling the driver circuits with the phase circuits ; and a controller configured for (i) receiving as input at least one of a target average intensity level to be applied to the target, or an energy level to be applied to the target, (ii) identifying a plurality of sets of the transducer elements , each of the sets corresponding to a plurality of the transducer elements for focusing and/or shaping, 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 intensity level and/or energy level at the target, wherein the controller , via the switch matrix , operates each of the transducer element sets in accordance with a pulse-width modulation pattern having a duty cycle determined based at least in part on the ultrasound energy generated from the set of transducer elements and the beam energy levels detected at the target so as to achieve and maintain the target average intensity level and/or energy level at the target, the duty cycle having an off time smaller than an estimated time constant of the target tissue during sequential operation of the transducer-element sets . <EOS>