An intravascular, ultrasound-based ablation system comprising: a catheter comprising at least one fluid lumen; 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, 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 electrode surface, the internal space being in fluid communication with the interior cavity 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; and wherein the cylindrical tube of the ultrasound transducer comprises a proximal end and a distal end, a backing member extending from the catheter to the tip and connecting the catheter with the tip, the backing member positioned within the internal space of the ultrasound transducer; at least one electrical cable configured to supply electrical power to the ultrasound transducer, wherein the at least one electrical cable comprises a first conductor and a second conductor; and a stand-off assembly located within an interior and along or near the proximal end of the cylindrical tube of the ultrasound transducer, the stand-off assembly being electrically conductive and in contact with, at least intermittently, the electrode along the inner surface of the cylindrical tube of the ultrasound transducer; and wherein the second conductor is connected to the stand-off assembly so that the second conductor is electrically coupled to the electrode along the inner surface of the cylindrical tube, characterized in that the proximal end of the cylindrical tube comprises a stepped portion, wherein a portion of the outer diameter formed by the outer surface of the cylindrical tube is smaller than a portion of the outer diameter of the cylindrical tube located distal to the stepped portion; and in that the first conductor is connected to an exterior of the cylindrical tube along the stepped portion. a catheter comprising at least one fluid lumen; 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, 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 electrode surface, the internal space being in fluid communication with the interior cavity 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; and wherein the cylindrical tube of the ultrasound transducer comprises a proximal end and a distal end, a backing member extending from the catheter to the tip and connecting the catheter with the tip, the backing member positioned within the internal space of the ultrasound transducer; at least one electrical cable configured to supply electrical power to the ultrasound transducer, wherein the at least one electrical cable comprises a first conductor and a second conductor; and a stand-off assembly located within an interior and along or near the proximal end of the cylindrical tube of the ultrasound transducer, the stand-off assembly being electrically conductive and in contact with, at least intermittently, the electrode along the inner surface of the cylindrical tube of the ultrasound transducer; and wherein the second conductor is connected to the stand-off assembly so that the second conductor is electrically coupled to the electrode along the inner surface of the cylindrical tube,