Hounsfield units quantify X-ray attenuation; which statement best describes typical HU values for various tissues?

Hounsfield units quantify X-ray attenuation; which statement best describes typical HU values for various tissues?

• A. Air is approximately 0 HU; water approximately 100 HU; fat positive; soft tissue around 100 HU; bone around 500 HU.
• B. Air ≈ -1000 HU; water 0 HU; fat negative; soft tissue ~ 30-60 HU; bone >1000 HU.
• C. All tissues have identical HU values.
• D. Hounsfield units are used to measure a patient's blood pressure.

Answer: (B)

Hounsfield units are a scale of X-ray attenuation on CT, relative to water. On this scale, air has the lowest attenuation (around -1000 HU), water is the reference at 0 HU, fat attenuates less than water and sits on the negative side, soft tissues like muscle and organs attenuate a bit more than water (roughly 30–60 HU), and bone is very dense and attenuates strongly (usually >1000 HU). The statement that matches these ranges best is: air about -1000 HU, water 0 HU, fat negative, soft tissue around 30–60 HU, and bone greater than 1000 HU. The other options misstate these values or concepts (for example, air is not 0, fat isn’t positive, soft tissue isn’t around 100 HU, and HU isn’t used to measure blood pressure).

Hounsfield units are a scale of X-ray attenuation on CT, relative to water. On this scale, air has the lowest attenuation (around -1000 HU), water is the reference at 0 HU, fat attenuates less than water and sits on the negative side, soft tissues like muscle and organs attenuate a bit more than water (roughly 30–60 HU), and bone is very dense and attenuates strongly (usually >1000 HU). The statement that matches these ranges best is: air about -1000 HU, water 0 HU, fat negative, soft tissue around 30–60 HU, and bone greater than 1000 HU. The other options misstate these values or concepts (for example, air is not 0, fat isn’t positive, soft tissue isn’t around 100 HU, and HU isn’t used to measure blood pressure).