Infographics / X-ray Physics

K-absorption edges & contrast media

A curve of photoelectric mass attenuation coefficient (log scale, vertical) against photon energy in keV (horizontal, roughly 10 to 80 keV). The curve falls steeply as attenuation is proportional to Z cubed over E cubed. Just below the iodine K-shell binding energy of about 33 keV the attenuation is lower because the K-shell cannot be ionised; at 33 keV there is a sharp upward jump, the K-edge, after which the curve continues to fall. A highlighted band shows that diagnostic beam energies are chosen to sit just above the K-edge to maximise attenuation, which is why iodine and barium make good contrast media. A note lists how the K-edge rises with atomic number: iodine 33, barium 37, gadolinium 50, tungsten 70 keV.

Mass attenuation coefficient versus photon energy, showing the iodine K-edge A curve of photoelectric mass attenuation coefficient (log scale, vertical) against photon energy in keV (horizontal, roughly 10 to 80 keV). The curve falls steeply as attenuation is proportional to Z cubed over E cubed. Just below the iodine K-shell binding energy of about 33 keV the attenuation is lower because the K-shell cannot be ionised; at 33 keV there is a sharp upward jump, the K-edge, after which the curve continues to fall. A highlighted band shows that diagnostic beam energies are chosen to sit just above the K-edge to maximise attenuation, which is why iodine and barium make good contrast media. A note lists how the K-edge rises with atomic number: iodine 33, barium 37, gadolinium 50, tungsten 70 keV. Photoelectric mass attenuation versus photon energy (the K-edge) mass attenuation coefficient (log scale) photon energy (keV) 10 20 30 40 50 60 70 80 iodine K-edge ≈ 33 keV photoelectric absorption jumps up sharply (not a fall) photoelectric absorption ∝ Z³ / E³ attenuation falls between edges below the edge the K-shell cannot be ionised → attenuation is lower ← L-edges lie at only a few keV diagnostic beam energies sit just above the K-edge → maximum attenuation (why contrast media are chosen) K-edge energy rises with atomic number: iodine ≈ 33 · barium ≈ 37 · gadolinium ≈ 50 · tungsten ≈ 70 keV
Practise X-ray PhysicsDownload SVG

More in X-ray Physics

Practice the Physics paper

FRCRX is a Part 1 Physics question bank: true/false items with explanations, timed mocks and topic analytics.

Start question practiceSee plans