Contrast enhancement; use of X-ray opaque material as illustrated by the…

The idea

Contrast in a radiograph depends on how strongly each tissue attenuates the X-ray beam, which depends on density and atomic number. Where neighbouring soft tissues attenuate almost equally, the shadow on the detector is nearly uniform and detail is lost. Contrast enhancement introduces a material with much higher attenuation than the surrounding tissue. Barium sulfate is a heavy, high-atomic-number compound that is chemically inert and not absorbed, so it can be swallowed as a suspension. It coats the oesophagus, stomach and intestine, and those coated surfaces attenuate the beam far more than soft tissue, producing a clearly darker shadow on the image. The patient is imaged while the barium moves, so the outline and motility of the gut can be studied. The same principle explains iodine-based contrast agents in the bloodstream and kidneys.

What earns the marks

• Contrast arises from differences in X-ray attenuation between adjacent regions, so similar attenuation gives poor visibility of detail.
• Attenuation increases with material density and with atomic number, so a high-Z material absorbs and scatters X-rays strongly.
• Barium sulfate is used because barium has a high atomic number, it is insoluble and chemically inert, and it is not absorbed into the body.
• The barium meal coats the lining of the gastrointestinal tract, so the coated surfaces attenuate the beam much more than soft tissue and cast a strong shadow.
• Imaging during swallowing and transit allows the outline, filling and movement of the oesophagus, stomach and intestine to be assessed.
• The technique is an example of the general principle of using an X-ray opaque contrast medium to enhance contrast in a chosen organ.

Common mistakes and how to put them right

• Thinking contrast enhancement increases the X-ray tube voltage or exposure.

  Put it right: It changes the attenuation of the subject by adding a high-attenuation material.

• Believing barium is chosen because it is radioactive or emits X-rays.

  Put it right: It is chosen because it strongly attenuates the transmitted beam and is biologically inert.

• Assuming the barium meal improves spatial resolution or magnification.

  Put it right: It improves contrast between the gut lumen and surrounding soft tissue, not the sharpness of the imaging system.

Exam tips

• Link every statement about contrast to attenuation and to the atomic number and density of the material involved.
• When describing the barium meal, state the route of administration, the structures coated and the effect on the recorded shadow.
• Use the barium meal as a named example of the general principle of X-ray opaque contrast media rather than treating it as an isolated fact.