Medical imaging: seeing inside the human body by Alvin Kwan
Increasing contrast
While it is easy to image dense tissues like bone by X-ray, air and fluid-filled cavities are not easy to visualise without the use of additional agents to enhance contrast. Clinical radiographers use radio-opaque substances such as barium sulphate to examine the gastrointestinal tract. Barium has a high atomic mass of 56 and is very good at absorbing electromagnetic radiation such as X-rays. The patient swallows the barium suspension, sometimes called a barium meal, to allow the visualisation of the lumen of the intestine. Intravenous administration of another radio-opaque element, iodine, is used to enhance contrast for imaging blood vessels. The image produced, known as an angiogram, is an important diagnostic tool for monitoring narrowing (stenosis) or dilation (aneurysm) or arteries. Since iodine is excreted naturally via the urinary system, iodine is also used in intravenous urography, a procedure employed to visualise the kidneys and the urinary tract.