Which of the following tissues have approx. the same density?

• A. Muscle and 2) Fat
• B. Muscle and 3) Water
• C. Fat and 3) Water
• D. 2) 3) all

Answer: (B)

Density on radiographs reflects how much X‑ray energy a tissue attenuates. Muscle and water have similar attenuation because both are relatively dense soft tissues with comparable composition, so they produce almost the same brightness on a plain X‑ray. Fat, on the other hand, attenuates X‑rays less and appears noticeably darker (less dense) than muscle or water, so it doesn’t match their density. That’s why the pair with approximately the same density is muscle and water. In CT terms, water is about 0 Hounsfield units and muscle is around +40 to +60 HU, which sits between fat (~-100 HU) and bone, but still closer to water than fat, reinforcing why muscle and water look alike.