The word "radiopaque" is the adjective form of the word radiopacity. It refers to something that does not allow x-rays and other forms of radiation to pass. A synonym of the word is radio-opaque and an antonym is radiolucent. Radiopacity is used as a measurement to determine the ability of electromagnetism to pass through different materials. Those materials that do not allow this electromagnetic radiation to pass through are known as "radiopaque". The concept is of considerable practical use in the field of modern medicine. The field of Radiologic imaging in medicine has seen major developments since the use of radiopaque dyes. These dyes are used to study the body by injecting them into the blood steam, cerebral spinal fluid or the intestinal tract and are so used to highlight X-ray or CT images.