An intravascular, ultrasound-based ablation system comprising: a catheter having a proximal end and a distal end , at least one fluid lumen , a guidewire lumen and an electrical conductor lumen, the at least one fluid lumen, guidewire lumen and the electrical conductor lumen being eccentric relative to a central axis of the catheter ; a balloon positioned at a distal end of the catheter , an interior of the balloon being in fluid communication with the at least one fluid lumen of the catheter , wherein the balloon is configured to inflate when fluid is delivered into the interior through the at least one fluid lumen of the catheter ; a tip extending distally from a distal end of the balloon ; an ultrasound transducer positioned within the balloon and having a central axis, the ultrasound transducer comprising a cylindrical tube with inner and outer surfaces, each of the inner and outer surfaces comprising an electrode, wherein the ultrasound transducer defines an internal space adjacent the inner surface , the internal space being in fluid communication with the interior of the balloon so that, when in use, fluid entering the balloon passes along both the inner and outer surfaces to transfer heat away from the ultrasound transducer ; a backing member positioned concentrically within the internal space of the ultrasound transducer , the backing member having a central lumen extending from a proximal end of the backing member to a distal end of the backing member ; and further comprising a centering assembly located between the distal end of the catheter and the ultrasound transducer , the centering assembly having two or more wings that extend radially outwardly from a central hub, a central opening that communicates with the guidewire lumen , and a central axis aligned with the central axis of the ultrasound transducer . a catheter having a proximal end and a distal end , at least one fluid lumen , a guidewire lumen and an electrical conductor lumen, the at least one fluid lumen, guidewire lumen and the electrical conductor lumen being eccentric relative to a central axis of the catheter ; a balloon positioned at a distal end of the catheter , an interior of the balloon being in fluid communication with the at least one fluid lumen of the catheter , wherein the balloon is configured to inflate when fluid is delivered into the interior through the at least one fluid lumen of the catheter ; a tip extending distally from a distal end of the balloon ; an ultrasound transducer positioned within the balloon and having a central axis, the ultrasound transducer comprising a cylindrical tube with inner and outer surfaces, each of the inner and outer surfaces comprising an electrode, wherein the ultrasound transducer defines an internal space adjacent the inner surface , the internal space being in fluid communication with the interior of the balloon so that, when in use, fluid entering the balloon passes along both the inner and outer surfaces to transfer heat away from the ultrasound transducer ; a backing member positioned concentrically within the internal space of the ultrasound transducer , the backing member having a central lumen extending from a proximal end of the backing member to a distal end of the backing member ; and further comprising a centering assembly located between the distal end of the catheter and the ultrasound transducer , the centering assembly having two or more wings that extend radially outwardly from a central hub, a central opening that communicates with the guidewire lumen , and a central axis aligned with the central axis of the ultrasound transducer . <EOS>