To determine which of the given substances is a water-soluble contrast, we need to consider the properties of each option:
- Barium: Barium sulfate is commonly used as a contrast agent in X-ray imaging but is not water-soluble. It remains in the gastrointestinal tract, providing a contrast without being absorbed.
- Iodine: Iodine compounds, particularly iodinated contrast media, are widely used and are water-soluble. These compounds are easily excreted by the kidneys, making them suitable for various imaging techniques.
- Bromium: While brominated compounds can be used in certain contexts, they are not commonly recognized for their water solubility or use as standard contrast agents.
- Calcium: Calcium compounds, such as calcium carbonate, are not water-soluble and are not used as contrast agents in medical imaging.
Based on these properties, the correct answer is Iodine, as it is readily water-soluble and effectively used in medical imaging.