An ultrasonic emitter comprising: (a) a tubular piezoelectric element having interior and exterior surfaces, said element being operative to generate ultrasonic vibrations in response to an applied signal; and (b) an internal structure comprising an outer support tube, wherein the internal structure is spaced apart from said element so as to define a space disposed within and immediately adjacent said element, said space being open to an exterior of the element; wherein (c) the ultrasonic emitter is positionable within an interior of a balloon, and the ultrasonic emitter is constructed and arranged such that, in use, the interior surface of the element can be cooled by a liquid within the space, such that cooling via heat transfer with the liquid occurs both at the interior and exterior surfaces of the element, when the liquid is circulated through the balloon; characterized in that: (i) the internal structure further comprises an inner support tube and a fluid sealed in a gap between the support tubes; and (ii) the liquid in said space, said element and said internal structure cooperatively form a resonant unit, said resonant unit including a highly -reflective interface, said internal structure defining said highly-reflective interface, wherein the highly-reflective interface is formed at the inner surface of the outer support tube of said internal structure. (a) a tubular piezoelectric element having interior and exterior surfaces, said element being operative to generate ultrasonic vibrations in response to an applied signal; and (b) an internal structure comprising an outer support tube, wherein the internal structure is spaced apart from said element so as to define a space disposed within and immediately adjacent said element, said space being open to an exterior of the element; wherein (c) the ultrasonic emitter is positionable within an interior of a balloon, and the ultrasonic emitter is constructed and arranged such that, in use, the interior surface of the element can be cooled by a liquid within the space, such that cooling via heat transfer with the liquid occurs both at the interior and exterior surfaces of the element, when the liquid is circulated through the balloon; characterized in that: (i) the internal structure further comprises an inner support tube and a fluid sealed in a gap between the support tubes; and (ii) the liquid in said space, said element and said internal structure cooperatively form a resonant unit, said resonant unit including a highly -reflective interface, said internal structure defining said highly-reflective interface, wherein the highly-reflective interface is formed at the inner surface of the outer support tube of said internal structure. (i) the internal structure further comprises an inner support tube and a fluid sealed in a gap between the support tubes; and (ii) the liquid in said space, said element and said internal structure cooperatively form a resonant unit, said resonant unit including a highly -reflective interface, said internal structure defining said highly-reflective interface, wherein the highly-reflective interface is formed at the inner surface of the outer support tube of said internal structure.