This disclosure is directed to a catheter having a lasso electrode assembly with a movable spine. The lasso electrode assembly may have an array of sensing electrodes and may be configured to engage the ostium of the vessel of a patient. The movable spine may have an ablation electrode and may travel along track around the circumference of the lasso electrode assembly. By adjusting the position of the movable spine, tissue may be ablated to form lesions around a circumference of the vessel.
