A method is provided that includes inserting a catheter into an atrium of a subject, and deploying at least one loop from a wall of the catheter, such that a distal end of the loop is distal to a distal end of the catheter. The method further includes inserting the loop partially into an appendage of the atrium, and, while inserting the loop, atraumatically contacting a portion of a perimeter of an opening of the appendage with a portion of the loop. Thereafter, the loop is further inserted into the appendage. Thereafter, a size of the opening of the appendage is measured using the loop. Other applications are also described.