A method for treating atrial fibrillation in a heart of a patient includes placing an ultrasonic catheter in a first chamber of the heart； acquiring two-dimensional ultrasonic images of a second chamber of the heart and at least a portion of surrounding structures of the second chamber using the ultrasonic catheter placed in the first chamber； reconstructing a three-dimensional ultrasonic image based on the two- dimensional ultrasonic images； displaying the reconstructed three-dimensional ultrasonic image； identifying at least one key landmark on the reconstructed three-dimensional ultrasonic image； marking the least one key landmark on the reconstructed three- dimensional ultrasonic image； penetrating the septum for accessing the second chamber of the heart while using the marked at least one key landmark for guidance； positioning a sheath through the penetrated septum and within the second chamber of the heart； inserting an ablation catheter through the sheath and into the second chamber of the heart； and ablating a portion of the second chamber of the heart using the ablation catheter while under observation with the ultrasound catheter located in the first chamber of the heart.
