Tissue ablation device assembly and method for electrically isolating a pulmonary vein ostium from an atrial wall

A tissue ablation system for treating atrial arrhythmia by ablating a circumferential region of tissue at a location where a pulmonary vein extends from an atrium, comprising:

• a delivery member with a proximal end portion and a distal end portion having a longitudinal axis and a radial axis;
• a contact member located along the distal end portion and which is adjustable between a first condition and a second condition, wherein in the first condition the contact member is adapted to be delivered through a delivery sheath into the atrium, and wherein in the second condition the contact member comprises a circumferential wall that surrounds the longitudinal axis;
• and
• an ultrasound transducer assembly mounted onto the distal end portion of the delivery member and having a distally oriented face, wherein the ultrasound transducer cooperates with the contact member to emit a circumferential pattern of ultrasonic energy signal distally at an angle relative to the longitudinal axis and through the circumferential wall of the contact member and into a circumferential area normal to the circumferential wall without coupling an electrical current between the ablative energy source and the circumferential area, and wherein the ultrasound transducer and contact member are configured such that the circumferential area coincides with the circumferential region of tissue when the contact member is adjusted to the second condition at the location.