Some embodiments of the invention relate to a system for treating tissue internal to a body, such as heart tissue. For example, the system may be used to ablate tissue as a treatment for atrial fibrillation. In certain embodiments, the system is capable of causing scar tissue to form in ostial areas of the atrium rather than inside the pulmonary vein. In such embodiments, the system may include a tissue treatment member that is operable to form an annular area of ablated tissue along the outer portion of the ostium in an area known as the antrum.