What is the CT number for air?

The correct answer is that the CT number for air is -1000. In computed tomography (CT), the CT number, also known as the Hounsfield unit (HU), is a quantitative measure of radiodensity. The scale is defined such that distilled water has a CT number of 0, and the CT number for air is set at -1000 to provide a reference point for the lowest radiodensity typically encountered in imaging.

Air is significantly less dense than water, thus it appears with a very low (negative) value on the Hounsfield scale, indicating it is highly radiolucent. This helps in differentiating structures during imaging because air contrasts sharply with soft tissues, which have positive Hounsfield units.

A Hounsfield unit of -1000 specifically indicates that the area in question is almost entirely devoid of matter, such as in the case of air-filled spaces in the body (like lungs). The other options either represent values for different materials or do not accurately characterize air's radiodensity in the context of CT imaging. For example, 0 is water, -100 might represent fat or other less dense tissues, and 20 would indicate a higher density than air.