Apparatus comprising an ultrasound ablation system (10), which comprises a reflection-facilitation element (12), configured to be placed at an extramyocardial site of a subject, and to provide an extramyocardial reflective region; and an ultrasound tool (20), which comprises at least one ultrasound transducer (40) configured to be positioned within a heart chamber of the subject. The ultrasound transducer (40) is configured to ablate myocardial tissue by applying ultrasound energy to the myocardial tissue such that at least a portion of the transmitted energy is reflected by the reflective region onto the myocardial tissue. Other embodiments are also described.