An ultrasound scan is performed using a machine that consists of a processing unit, a display screen, and a transducer which the sonographer or physician holds and places on the patient's skin over the area to be examined. Before placing the transducer, a special water-based gel is applied to the skin to facilitate the transmission of sound waves and prevent air gaps between the transducer and the skin. The transducer sends short pulses of sound waves into the body, and when these waves encounter boundaries between different tissues (such as between fluid and solid tissue), some are reflected back as echoes. The transducer captures these returning echoes and sends them to the computer, which interprets them and converts them into images displayed on the screen in real-time. The transducer can capture multiple cross-sectional images from different angles. There are specialized types of ultrasound such as Doppler ultrasound to evaluate blood flow in vessels, and 3D/4D ultrasound to create three-dimensional or moving images (especially in pregnancy). Ultrasound does not use ionizing radiation and is considered safe for pregnant women and children.