Which tissue absorbs the most x-rays?

• A. Air
• B. Fat
• C. Water
• D. Bone

Answer: (D)

Differential attenuation based on tissue density and atomic number is what drives why bone absorbs the most x-rays. X-ray photons are attenuated as they pass through the body, and the amount of attenuation depends on how dense the tissue is and how strongly its atoms interact with the photons. Bone has a high density and contains calcium with a higher effective atomic number, which increases the likelihood of interactions, especially the photoelectric effect that dominates diagnostic energies. This combination makes bone absorb more photons than soft tissues or air, so it appears the most radiopaque on radiographs. Air has very low density and hardly attenuates photons, so it looks black; soft tissues like fat and water attenuate less than bone, so they appear less bright.