A medical apparatus, comprising: a flexible insertion tube having a distal end for insertion into a body cavity; first and second conduits (66, 68) contained within the flexible insertion tube and configured to deliver first and second fluids, respectively, to the distal end; and a terminal member fixed to the distal end of the insertion tube and comprising: a first balloon comprising one or more spray ports and coupled to the first conduit so that the first fluid inflates the first balloon and is delivered, via the one or more spray ports, to tissue in the body cavity; a second balloon contained within the first balloon and coupled to the second conduit so that the second fluid inflates the second balloon; and multiple splines comprising a flexible, resilient material and extending along a longitudinal axis of the terminal member, and configured to constrain the second balloon so that inflation of the second balloon creates lobes that form, along the longitudinal axis between the lobes, channels that direct the first fluid from the first conduit to the one or more spray ports, wherein the splines have cross-sections selected from a group consisting of rectangular and elliptical cross-sections, and wherein the flexible, resilient material is a shape-memory alloy. a flexible insertion tube having a distal end for insertion into a body cavity; first and second conduits (66, 68) contained within the flexible insertion tube and configured to deliver first and second fluids, respectively, to the distal end; and a terminal member fixed to the distal end of the insertion tube and comprising: a first balloon comprising one or more spray ports and coupled to the first conduit so that the first fluid inflates the first balloon and is delivered, via the one or more spray ports, to tissue in the body cavity; a second balloon contained within the first balloon and coupled to the second conduit so that the second fluid inflates the second balloon; and multiple splines comprising a flexible, resilient material and extending along a longitudinal axis of the terminal member, and configured to constrain the second balloon so that inflation of the second balloon creates lobes that form, along the longitudinal axis between the lobes, channels that direct the first fluid from the first conduit to the one or more spray ports, wherein the splines have cross-sections selected from a group consisting of rectangular and elliptical cross-sections, and wherein the flexible, resilient material is a shape-memory alloy. a first balloon comprising one or more spray ports and coupled to the first conduit so that the first fluid inflates the first balloon and is delivered, via the one or more spray ports, to tissue in the body cavity; a second balloon contained within the first balloon and coupled to the second conduit so that the second fluid inflates the second balloon; and multiple splines comprising a flexible, resilient material and extending along a longitudinal axis of the terminal member, and configured to constrain the second balloon so that inflation of the second balloon creates lobes that form, along the longitudinal axis between the lobes, channels that direct the first fluid from the first conduit to the one or more spray ports, wherein the splines have cross-sections selected from a group consisting of rectangular and elliptical cross-sections, and wherein the flexible, resilient material is a shape-memory alloy.