A combination catheter includes an ultrasound transducer and RF ablation electrode. The ultrasound transducer transmits ultrasound signals into and receives echo signals from a vessel. The echo signals are processed and used to produce an image of the tissue surrounding the catheter. A driveshaft rotates the ultrasound transducer to obtain a 360° view of the vessel wall. At the distal end of the driveshaft is an electrode. An RF generator is electrically coupled to the driveshaft to deliver RF energy to the electrode at the distal end of the driveshaft to ablate occluding material in the vessel. The electrode may have a variety of tip shapes including concave, roughened, or expandable configurations, depending on the size of the vessel and composition of the occluding material to be ablated. Alternatively, the RF ablation energy may be delivered to a guidewire that is used to route the ultrasound catheter through a patient's vasculature. Finally, a steerable catheter can be used to further position the ultrasound catheter and ablation electrode in the vessel.