What it means
CT measures how strongly each tiny part of the body blocks X-rays. Hounsfield units turn that into a number. Water is set at 0 and air at minus 1000. Fat reads well below 0, usually around minus 100. Most organs and muscles sit a few dozen units above 0. Fresh blood reads a little higher, and calcium and bone higher still, up to a thousand or more.
When it matters
The number helps tell apart things that can look alike. A smooth spot close to 0 is usually fluid, such as a simple cyst. A reading around minus 100 means fat, which often points to something harmless such as a lipoma, a fatty lump. An adrenal nodule of 10 HU or less before contrast is very likely a benign adenoma, a growth that is not cancer. Very small spots give less reliable readings, so they may be rechecked later.
Questions to ask your doctor
- What Hounsfield number did my finding measure, and what does it suggest?
- Was the measurement taken before or after contrast?
- Is the spot large enough for the number to be reliable?
