Which of the following statements is NOT true regarding the calculation of CT numbers?

The statement regarding the calculation of CT numbers that is not true is that it is valid for any type of physical tissue. CT numbers, also known as Hounsfield units, are calculated based on the attenuation properties of different tissues relative to water and air. While CT numbers can provide useful information about various types of tissues, their accuracy and validity can vary significantly depending on the specific tissue type and its unique physical characteristics.

For instance, CT imaging utilizes a specific calibration based on the known attenuation values of water and air, which means that the relationship between CT numbers and the actual physical properties of tissues may not hold universally across all types of tissue. Some tissues may exhibit similar attenuation properties, leading to overlapping CT number ranges that can complicate diagnosis or evaluation. Therefore, while CT numbers can provide valuable diagnostic information, they are not universally valid for all tissues without considering the specific context and properties of each tissue type.

In contrast, the other statements are true: CT numbers do not provide absolute values, air is indeed used as a reference point, and calibration is essential for ensuring accurate measurements in CT imaging.