A medical contrast medium is any substance introduced into a patient's body during a radiological study that allows different body tissues to show up as a different image. Some of the most common are:
Iodine - Iodine based materials dissolve readily in fluids and are generally used to outline blood vessels in X-ray technology, such as a CT Scan. It is commonly used in an angiogram, among other procedures. Iodine in a fluid makes it look dark on an x-ray, allowing the vessels to stand out in contrast.
Barium Sulphate - Barium is usually used for investigation of the digestive tract as it is generally insoluble in water and stays in the tract until expelled. Barium shows up as bright white in an x-ray as opposed to the darker soft tissues of the digestive tract.
Gadolinium - Gadolinium is used in MRI scans as it reacts quickly with water and enhances the ability of the MRI to detect such water molecules, providing more detail.
Microbubbles - Microbubbles are used in some ultrasound procedures. They are made up of nitrogen or another non-reactive gas. As these bubbles rapidly reflect sound waves, they show up more easily.
Contrast materials are not without risks (as was made clear in The Softer Side). Patients occasionally have allergic reactions to the material. They generally have to be avoided in patients with poor kidney function. They can also interact with other medications.