Barium in a suitable form is administered to patients before an X-ray examination of the stomach, because

Barium is administered as barium sulfate because it is a positive (radiopaque) contrast medium that increases X‑ray attenuation, producing clear contrast between the stomach lumen and surrounding tissues. The BaSO4 particles provide radiodensity (radio‑opacity) determined by their concentration and physical density used in GI fluoroscopy/radiography, so coated mucosa appears bright on X‑ray images [1]. Barium’s effectiveness arises from its high atomic number and K‑edge characteristics, which increase absorption of diagnostic X‑rays and delineate anatomical detail. Barium sulfate is used specifically for gastrointestinal studies as an inert, insoluble coating agent (not for “cleaning” the stomach), so the visualization is due to X‑ray absorption, not colour [2].

Sources

1. [1] https://www.appliedradiology.org/courses/4386%2FPDF%2FBracco_07-21_barium_CE.pdf
2. [2] https://radiopaedia.org/articles/barium-sulfate-contrast-medium?lang=us