Before X-ray examination (coloured X-ray) of the stomach, patients are given suitable salt of barium because—

Barium sulfate is used as a radiopaque contrast medium for gastrointestinal X-ray examinations. Because barium has a high atomic number (56), it possesses high radiopacity, which allows it to absorb X-rays effectively. This absorption creates a stark contrast between the barium-filled structures and the surrounding soft tissues, which would otherwise appear faint or transparent on an X-ray. When ingested as a 'barium meal,' the salt coats the lining of the stomach and esophagus, making these organs appear bright or white on the radiographic image [1]. This allows radiologists to clearly visualize the structure and function of the digestive tract to identify ulcers, narrowing, or other abnormalities [1]. Unlike other barium salts, barium sulfate is insoluble in water and is not absorbed by the body, making it safe for medical use [3].

Sources

1. [1] https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/barium-tests
2. [3] https://medlineplus.gov/druginfo/meds/a606010.html