What is an example of a substance that has a CT number of around 20?

The CT number, or Hounsfield unit, is a measure of radiodensity used in computed tomography (CT) imaging to characterize different types of tissues based on their inherent density compared to water. A CT number of around 20 typically corresponds to substances that are less dense than water but denser than air.

Blood is indeed a fluid that has a CT number close to this range. It is primarily composed of water and has a density slightly greater than that of soft tissue because of its cellular components and proteins. The general CT number for blood can vary, but it often falls around this approximate value due to its composition.

Soft tissue generally has a higher CT number, typically in the range of about 30 to 40, while muscle, which is slightly denser than soft tissue, ranges around 40 to 50. Air, on the other hand, has a significantly negative CT number, usually around -1000, as it is much less dense than the surrounding tissues.

Thus, blood is the substance that accurately corresponds to a CT number of around 20, as it balances the characteristics of density found in both liquid and soft tissue forms.