Apparatus for applying ultrasonic energy to tissue within the body of a living subject comprising: (a) an ultrasonic emitter having an emitting surface generally in the form of a surface of revolution about a central axis; (b) an inflatable lens, said lens having a refractive surface generally in the form of a surface of revolution about said central axis when said lens is in an inflated condition, said lens surrounding said emitter so that at least some ultrasonic energy emitted at said emitting surface will be directed through said refractive surface of said lens into the tissue of the subject, the apparatus further comprising an elongated probe, said probe having a proximal end and a distal end adapted for insertion into the body of a living subject, said emitter and said inflatable lens being mounted to said probe adjacent the distal end thereto, and further, wherein: i) the emitter includes an active piezo-electric element in the form of a cylindrical tube having an exterior electrode surface and an interior electrode surface; ii) an internal structure comprising an outer support tube with a pair of outwardly-projecting shoulders (1730, 1732) at its ends, each shoulder having arcuate surfaces connected by flats, the arcuate surfaces supporting the piezo-electric element concentric with the support tube and spaced from it by a passageway open at its ends via gaps in the shoulders at the flats; iii) whereby cooling liquid, advanced to the emitter along the length of the probe, can advance through the passageway to cool the emitter and then return along the length of the probe; iv) the said cooling liquid also flowing within the inflatable lens whereby the transmitter element is liquid-cooled on both its exterior and interior surfaces. the apparatus further comprising an elongated probe, said probe having a proximal end and a distal end adapted for insertion into the body of a living subject, said emitter and said inflatable lens being mounted to said probe adjacent the distal end thereto, and further, wherein: i) the emitter includes an active piezo-electric element in the form of a cylindrical tube having an exterior electrode surface and an interior electrode surface; ii) an internal structure comprising an outer support tube with a pair of outwardly-projecting shoulders (1730, 1732) at its ends, each shoulder having arcuate surfaces connected by flats, the arcuate surfaces supporting the piezo-electric element concentric with the support tube and spaced from it by a passageway open at its ends via gaps in the shoulders at the flats; iii) whereby cooling liquid, advanced to the emitter along the length of the probe, can advance through the passageway to cool the emitter and then return along the length of the probe; iv) the said cooling liquid also flowing within the inflatable lens whereby the transmitter element is liquid-cooled on both its exterior and interior surfaces. i) the emitter includes an active piezo-electric element in the form of a cylindrical tube having an exterior electrode surface and an interior electrode surface; ii) an internal structure comprising an outer support tube with a pair of outwardly-projecting shoulders (1730, 1732) at its ends, each shoulder having arcuate surfaces connected by flats, the arcuate surfaces supporting the piezo-electric element concentric with the support tube and spaced from it by a passageway open at its ends via gaps in the shoulders at the flats; iii) whereby cooling liquid, advanced to the emitter along the length of the probe, can advance through the passageway to cool the emitter and then return along the length of the probe; iv) the said cooling liquid also flowing within the inflatable lens whereby the transmitter element is liquid-cooled on both its exterior and interior surfaces.