BRAINCAKEContrast 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).
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Key terms in this lesson
- photoelectric effect
- The complete absorption of a photon by an inner electron of an atom, which is ejected, transferring the photon's energy to the electron.
- attenuation
- A decrease in the intensity of a beam of radiation as it passes through matter, due to absorption or scattering.
More in Medical Imaging
- X-ray Tubes
- X-ray Attenuation Mechanisms
- Photoelectric Effect, Compton Scattering and Pair Production
- Attenuation of X-rays
- CAT Scanning
- Medical Tracers
- Technetium-99m and Fluorine-18
- The Gamma Camera
All 20 lessons in Medical Imaging · All OCR A-level Physics topics