An ultrasound catheter system and a method for operating an ultrasonic catheter at a treatment site within a patient's vasculature or tissue are disclosed. The method of operating an ultrasonic catheter comprises advancing a catheter with at least one ultrasonic element to a treatment site in a patient's vascular system or tissue, driving the at least one ultrasonic element to generate ultrasonic energy, delivering a therapeutic compound to the treatment site through the catheter, and varying a power parameter and a physiological parameter of the ultrasonic element by ramping the power parameter and the physiological parameter between two set of minimum and maximum values.