A catheter for delivering ultrasonic energy and therapeutic compounds to a treatment site within a patient's vasculature comprises a tubular body. The tubular body has a proximal region and a distal region opposite the proximal region. The catheter further comprises a plurality of fluid delivery lumens formed into the tubular body. The catheter further comprises an inner core configured for insertion into the tubular body. The inner core comprises an elongate electrical conductor having a plurality of flattened regions, each flattened region having a first flat side and a second flat side opposite the first flat side. The inner core further comprises a plurality of ultrasound radiating members mounted in pairs to the flattened regions of the elongate electrical conductor. A first ultrasound radiating member is mounted to the first flat side of the elongate electrical conductor, and a second ultrasound radiating member is mounted to the second flat side of the elongate electrical conductor. The catheter further comprises control electronics configured to apply a driving signal to at least two, but fewer than all, of the ultrasound radiating members.