Contrast Media

Soft tissues such as the stomach, intestines and blood vessels have attenuation coefficients very close to those of the tissue around them. On an ordinary X-ray image they show very poor contrast and are hard to see.A contrast medium is a substance containing atoms of high proton number Z that is put into the organ being studied. It has a large attenuation coefficient, so it absorbs X-rays strongly and the organ stands out clearly against its surroundings.It works because the chance of the photoelectric effect, the main attenuation mechanism at diagnostic energies, rises very steeply with proton number (roughly as Z³).• Barium (Z = 56): the patient swallows a barium sulfate meal. It coats the lining of the digestive tract, showing its outline and any blockages or ulcers. Barium sulfate is insoluble, so it is not absorbed into the body.
• Iodine (Z = 53): an iodine-based liquid is injected into the blood. It shows blood vessels (for example blocked arteries) and organs such as the kidneys.Worked example: compare the photoelectric absorption per atom of iodine with oxygen (Z = 8), a main element of soft tissue, using the Z³ rule.(53 / 8)³ = 6.6³ ≈ 290Each iodine atom absorbs roughly 290 times more strongly than an oxygen atom, which explains the sharp contrast in the image.