X-ray contrast
Because most soft tissues in the body have an X-ray density (radiodensity/radio-opacity) that falls about in the middle of the black/white scale, an X-ray contrast medium (CM) is needed to bring out the detail. It is difficult or impossible with ordinary X-ray to differentiate between adjacent soft tissues/organs, or to distinguish diseased tissue, such as a tumour, from the surrounding healthy tissue from which the cancer may have arisen. A trained radiologist will be able to see the heart and perhaps the aorta on an unenhanced X-ray image. However, the injection of a CM will enable these tissues to be seen much more clearly and will in addition bring out details that were invisible without the contrast enhancement. The CM will also enable smaller blood vessels, as well as a number of other vital organs such as the kidneys, liver and spleen, to be more clearly differentiated from the surrounding tissues.
X-ray contrast agents usually contain iodine or sometimes barium. These two elements have a high atomic weight, and they are 'wrapped up' inside molecules to give them tissue-friendly characteristics. Owing to the high atomic weight of iodine or barium, the contrast medium will absorb more x-rays than the tissue by itself. Therefore, the organ containing the contrast agent, such as the kidney, will stand out clearly on the developed x-ray film and will appear whiter than surrounding tissues. The standard x-ray technique thus gives a silhouette 'in negative' of the interior of a body.
What we see in the image is not the tissue itself, because CM does not usually penetrate into individual cells. What we actually see is the CM, which gradually becomes evenly dispersed throughout the entire volume of circulating blood, filling even the smallest blood vessels including the capillaries.
Computer programmes today can amplify the small differences in grey intensity that exist. One can also take advantage of the fact that not all of the CM reaches its destination simultaneously. When CM is injected into an arm vein, the injected volume of CM is not immediately diluted by the blood but reaches the right heart chamber (ventricle) almost "in one piece". From the right ventricle, it is carried to the lungs, where it becomes a little more diluted before returning to the heart, this time into the left ventricle. Not until the CM is pumped out of the left ventricle of the heart through the aorta (main artery) does the dilution truly begin; the CM will now flow with the bloodstream to all parts of the body, from head to toe. After having circulated along with the blood through all body tissues, the CM returns to the right side of the heart and the cycle is repeated.
GE Healthcare Glossary