 Ultrasonography is based on the principle that sound can pass through human body tissues and is reflected by the tissue interfaces in the same way that light can reflect back on itself, from a mirror. Tissue in the body will offer varying degrees of resistance, known as acoustic impedance, to the path of the ultrasound beam. When there is a high impedance difference between two tissues, the interface between them will strongly reflect the sound. When the ultrasound beam meets air, or solid tissue such as bone, their impedance difference is so great that most of the acoustic energy is reflected making it impossible to see any underlying structures. The examiner will see just a shadow, instead of the image expected. Air will impede sound waves which is why a gel is used. The gel prevents air bubbles from forming between the probe, and the patient's skin and so helps the conduction of the sound waves from the transducer into the body. The watery medium also helps to conduct the sound waves. Liquids, including blood have a low impedance, which means that little energy will be reflected and no visualization possible. One of the important exceptions is that when the blood flow is very slow it can in fact be seen, in what is termed "spontaneous contrast".
