(e) X-ray imaging with contrast media; barium and iodine

The idea

X-ray imaging relies on differential absorption: dense tissue such as bone absorbs more X-rays than soft tissue, so it casts a clearer shadow on the detector. Soft organs such as the gut or blood vessels absorb X-rays only weakly, so they produce little contrast. A contrast medium is a substance introduced into the body to increase absorption in the region of interest. Barium, usually swallowed as a suspension of barium sulfate, is used for the gastrointestinal tract because it is dense and effectively insoluble, so it coats the lining and outlines it. Iodine compounds, injected or infused into the bloodstream, are used for the urinary tract and blood vessels because iodine has a high atomic number and absorbs X-rays strongly. The medium must be safe, and its high attenuation relative to surrounding tissue is what makes the structure visible.

What earns the marks

• Contrast media increase the attenuation of X-rays in the region of interest so that soft tissue structures become visible.
• Barium is used for imaging the gastrointestinal tract, typically swallowed as a suspension of barium sulfate.
• Iodine compounds are used for imaging blood vessels and the urinary tract, usually introduced into the bloodstream.
• The medium must be safe for the patient, which is why barium is used as the insoluble sulfate rather than a soluble barium salt.

Common mistakes and how to put them right

• Thinking contrast media emit X-rays.

  Put it right: They do not; they absorb X-rays more strongly than surrounding tissue, increasing contrast.

• Believing barium is used for blood vessels.

  Put it right: Barium is used for the gastrointestinal tract, while iodine compounds are used for blood vessels and the urinary tract.

• Assuming any barium compound is safe.

  Put it right: Soluble barium salts are toxic, so barium sulfate, which is effectively insoluble, is used.

• Confusing contrast media with radioactive tracers.

  Put it right: Contrast media are not radioactive; they work by differential X-ray absorption.

Exam tips

• Link each medium to the body region it images: barium to the gut, iodine to blood vessels and the urinary tract.
• Explain contrast in terms of attenuation: the medium absorbs more X-rays than soft tissue, so the shadow it casts is clearer.
• If asked why barium sulfate is used, state that it is insoluble and therefore not absorbed as toxic barium ions.