Ultrasound is a diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, their size, structure, and any pathological lesions with real time tomographic images. It is also used to visualize a fetus during routine and emergency prenatal care. Ultrasound scans are performed by medical health care professionals called sonographers. Obstetric sonography is commonly used during pregnancy. Ultrasound has been used to image the human body for at least 50 years. It is one of the most widely used diagnostic tools in modern medicine. The technology is relatively inexpensive and portable, especially when compared with modalities such as magnetic resonance imaging (MRI) and computer tomography (CT). As currently applied in the medical environment, ultrasound poses no known risks to the patient. Ultrasound is generally described as a "safe test" because it does not use ionizing radiation.