Attenuation physics rule DE imaging

November 1, 2007

X-ray matter interactions in the diagnostic imaging energy range are dominated by photoelectric and Compton effects. Both interactions increase in proportion to electron density, which is proportional to physical density. In conventional CT imaging at 120 kVp to 140 kVp, the Compton effect predominates, and hence image quality is primarily governed by physical density.

X-ray matter interactions in the diagnostic imaging energy range are dominated by photoelectric and Compton effects. Both interactions increase in proportion to electron density, which is proportional to physical density. In conventional CT imaging at 120 kVp to 140 kVp, the Compton effect predominates, and hence image quality is primarily governed by physical density.

At lower kilovoltages, the frequency of Compton interactions remains relatively constant, whereas the frequency of photoelectric interactions increases exponentially. While the frequency of photoelectric interactions is strongly dependent on the atomic number Z (approximately proportional to Z3) the Compton effect is independent of the atomic number. For certain substances, the presence of elemental k-edges will further increase the frequency of photoelectric interactions at energies at, or just above, the k-edge (e.g., Iodine at 33keV).

Ultimately, attenuation displayed within a CT voxel is determined by the sum of different x-ray matter interactions, dominated by photoelectric and Compton interactions. Therefore, different substances will demonstrate different CT Hounsfield unit values at different energies. If the substances imaged have sufficiently distinct atomic numbers, it should, in principle, be possible to differentiate these substances based on their known attenuation properties at two different energies.