A patient swallows a barium meal before an X-ray examination of their digestive system. Explain why barium is used and how it allows the digestive system to be imaged, including why it would not normally be visible on an X-ray without the barium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Barium sulfate is a substance that is given to patients before certain X-ray examinations. It passes through the digestive system without being absorbed into the body.
Model answer (5 marks)
Barium is used because it is a heavy, dense substance that absorbs X‑rays strongly. The digestive system is made of soft tissue, which absorbs X‑rays only weakly and therefore appears almost the same colour as the surrounding tissues on an X‑ray film. When a patient swallows a barium meal, the barium coats the lining of the stomach, small and large intestine and fills the lumen. Where the barium is present, the X‑rays are absorbed and fewer photons reach the detector, producing a bright (white) shadow or contrast on the image. This contrast outlines the shape of the digestive tract and any abnormalities can be seen.
Examiner tips
- Mention that barium is heavy and dense, so it absorbs X‑rays strongly; contrast the weak absorption of soft tissue; explain that the absorbed X‑rays give a white shadow on the film; link this to visualising the digestive system’s outline and abnormalities.
Common mistakes
- Confusing barium’s role as a contrast agent with a therapeutic drug; forgetting that soft tissue absorbs X‑rays weakly; not explaining that the white shadow is due to fewer X‑rays reaching the detector.
Mark scheme (5 marks)
- X-rays are absorbed (or transmitted) differently by different materials / X-rays are absorbed more by denser materials
- Soft tissue / the digestive system absorbs X-rays weakly / by a similar amount to surrounding tissue, so it does not show up on the X-ray image
- Barium absorbs X-rays strongly / more than soft tissue
- Where barium is present, fewer X-rays reach the detector / film, producing a (bright/white) shadow or contrast on the image
- This allows the shape / outline / any abnormalities of the digestive system to be seen / imaged
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →