Mass density The essential physics of medical imaging By Jerrold T. Bushberg 2/e p3.56 writes? "What do CT numbers correspond to physically in the patient? CT images are produced with a highly filtered, high-kV x-ray beam, with an average energy of about 75 keV. At this energy in muscle tissue, about 91% of x-ray interactions are Comptom scatter. For .fat and hone, Compton scattering interactions are 94% and 74%, respectively. Therefore, CT numbers and CT images derive their contrast mainly .from the physical propeies of tissue that influence Compton scatter. Density (g/cubic cm) is a very impoant discriminating propey of tissue, and the linear attenuation coefficient (p) tracks linearly with density. Other than physical density, the Compton scatter cross section depends on the electron density in tissue: Electron density = NZ/A where N is Avogadro's number (a constant) Z is the atomic number, A is the atomic mass The main constituents of soft tissue are hydrogen (Z = 1, A = 1), carbon (Z = 6, A = 12), nitrogen (Z = 7, A = 14), and oxygen (Z = 8, A = 16). Carbon, nitrogen, and oxygen all have the WA ratio of 0.5, so their electron densities are the same. Because the Z/A ratio .for hydrogen is 1.0, the relative abundance of hydrogen in a tissue has some influence on CT number. Hydrogenous tissue such as fat is well visualized on CT Neveheless, density (g/cubic cm) plays the dominant role in forming contrast in medical CT." "The difference in electron density of tissues depends upon physical density. Therefore, the soft tissue contrast achieved in an image is primarily determined by the relative physical densities of the structures, not by the relative differences in atomic numbers."- Neuroimaging By Robe A. Zimmerman, Wendell A. Gibby, Raymond F. Carmody P5 CT is a tomographic imaging technique that generates cross-sectional images in the axial plane. The basic concept of x-ray CT is simple. By looking at an object from multiple directions a three dimensional representation of that object can be formed. A thin beam of x-rays is passed through the body from multiple different projections. When an x-ray beam passes through the body, there is a logarithmic attenuation of its intensity over distance. A measurement of the intensity of the exiting beam is known as the linear attenuation coefficient when the attenuator thickness is 1 cm. The linear attenuation coefficient is related to the absorption of a given material both photoelectric interaction and Comptom scattering interactions. This, of course, depends upon the energy of the x-ray beam and the type of material being irradiated. CT images are maps of the relative linear attenuation coefficient of tissues. The relative attenuation coefficient is normally expressed in Hounsfield units (HU), also known as CT numbers (Named in honor of the inventor of CT, Geoffrey Hounsfield). The HU of material Xis HU = 1000 X (14-14)/1.1.,,., where x, is the attenuation coefficient of the material X, and ix,. is the attenuation coefficient of water. By definition, the HU value for water is always zero. HU value for air is -1000 ( minus 1000) HU value for coical bone is 1000