Attenuation of X-rays by matter ( 6 ) is due to physical interaction between photons and elementary particles. With the energies used in diagnostic radiology, and especially in CT scanning (70 keV), this interaction occurs between photons and electrons. Two types of interaction are involved : the Compton effect and the photoelectric effect. The Compton effect depends on the electronic density of the matter, which is related to the mass density of the tissues. Although chemical bonds between atoms do not interfere with absorption of X-rays by matter, the spatial configuration of the molecules affect the mass density and changes of this spatial configuration will modify the attenuation coefficients.
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