A<unk>  (or<unk>  medium) is a substance used to increase the<unk>  of structures or fluids within the body in medical imaging.<unk> s absorb or alter external electromagnetism or ultrasound, which is different from radiopharmaceuticals, which emit radiation themselves. In x-rays,<unk> s enhance the radiodensity in a target tissue or structure. In MRIs,<unk> s shorten (or in some instances increase) the relaxation times of nuclei within body tissues in order to alter the<unk>  in the image.<unk> s are commonly used to improve the visibility of blood vessels and the gastrointestinal tract. Several types of<unk>  are in use in medical imaging and they<unk> n roughly be classified based on the imaging modalities where they are used. Most common<unk> s work based on X-ray attenuation and magnetic resonance signal enhancement.
Contrast agent
Radiocontrast agent
Iodinated contrast
Steroid
Antibiotic
Broad-spectrum antibiotic
Radioactive tracer
Thickened fluids