Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. Obstetric sonography is commonly used during pregnancy and is widely recognized by the public. In physics, the term "ultrasound" applies to all sound waves with a frequency above the audible range of human hearing, about 20,000 Hz. The frequencies used in diagnostic ultrasound are typically between 2 and 18 MHz.