Final answer:
The attenuation of a specific tissue relative to water is known as the CT number, and it is affected by the density and thickness of the tissue as well as the x-ray energy level.
Step-by-step explanation:
The attenuation of a specific tissue relative to water is known as the CT number. The CT number, or Hounsfield unit, quantitatively represents the degree to which a tissue absorbs x-rays, based on a scale where water is assigned a value of zero. When x-ray energy increases, the Compton effect becomes more significant, affecting the attenuation of the x-rays as they scatter from outer electron shells, which in turn depends on the material's density and thickness. For clinical imaging, compounds with a high atomic number, like barium or iodine, can be introduced to the body to enhance contrast due to their increased attenuation properties.