A tissue ablation apparatus includes a body having at least two lumens extending therethrough and an ultrasound ablation element positioned within a distal region of the body. The ultrasound ablation element includes an ultrasound transducer and a membrane. One of the lumens is arranged so as to deliver a fluid into a cavity defined between the transducer and the membrane. A first orifice is provided in the body proximal of the ultrasound ablation element and is open to the first lumen to define a first vacuum pathway. A distal vacuum chamber, opening to a second orifice in the body, is defined by the body distal of the ultrasound ablation element and is in communication with the first lumen to define a second vacuum pathway. At least one flow control apparatus is provided to regulate flow through one of the vacuum pathways independent of flow through the other vacuum pathway.