<unk>  agents (also simply contrast agents or contrast materials) are compounds used to improve the visibility of internal bodily structures in an X-ray image.
Contrast agents are also used in MRI (Magnetic Resonance Imaging). Although MRI is usually considered a branch of radiology, it is not based on X-rays. MRI contrast agents are usually gadolinium-based, and work not by being radioopaque, but rather by altering the magnetic properties of nearby hydrogen nuclei.
Radiocontrast
Contrast 
Contrast medium
Imaging agent
Fluid
Palpation
Colloid
Radionuclide