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Positive contrast media are substances that brighten structures on radiographs by highly attenuating X-rays, making the area of interest appear white or very bright. In veterinary radiology, the classic positive agents are barium sulfate and iodinated contrast media (such as Omnipaque or Conray). These two types of agents are used to outline hollow organs and vessels because they stand out against surrounding tissues on X-ray images. Air, on the other hand, is a negative contrast agent—it appears dark on X-ray because it attenuates fewer X-rays than surrounding tissues. Gadolinium-based agents are primarily used to enhance MRI (and sometimes CT), not standard X-ray imaging, so they aren’t considered positive contrast media for radiographs. Therefore, substances that are positive contrast media include both barium and iodine-based agents.