A method of treating an occlusion at a treatment site within a patient's vasculature comprises providing an ultrasonic catheter. The ultrasonic catheter has a distal region, a proximal region opposite the distal region, a fluid delivery lumen having at least one opening in the distal region, and an ultrasound radiating member positioned within the distal region. The method further comprises positioning the ultrasonic catheter at the treatment site, such that at least a portion of the distal region is within the occlusion. The ultrasonic catheter further comprises passing a therapeutic compound through the fluid delivery lumen such that the therapeutic compound is delivered to the treatment site. The ultrasonic catheter further comprises emitting ultrasonic energy from the ultrasound radiating member. The emitted ultrasonic energy has an amplitude that is periodically varied between a low amplitude and a high amplitude.