Negative contrast media cannot be used for which structure?

• A. Heart
• B. Spinal cord
• C. Brain
• D. Liver

Answer: (B)

Negative contrast media (like air or CO2) are used to outline hollow spaces by filling them with gas, which makes those areas appear darker on radiographs. This approach can help visualize structures that can be safely filled with gas and separated from surrounding tissues. However, introducing gas into the spinal canal to image the spinal cord carries real risk: the gas can distort or compress neural tissue and cause serious complications, and modern practice prefers other methods (such as positive contrast or MRI) for spinal imaging. The brain also poses safety concerns with intrathecal gas, as pneumocephalus can occur, so gas is not used to image the brain. For these reasons, negative contrast media are not suitable for imaging the spinal cord, making it the structure that cannot be examined with this approach.