As X-rays pass through the patient, they are attenuated; they are both absorbed and scattered within the patient. The amount of attenuation is dictated by the type of tissues present and by the X-ray beam energy. The X-rays that emerge from the patient arrive at the image receptor, where they are detected and recorded.

Image formation depends on a differential attenuation between tissues. The extent of this differential determines the amount of contrast between different tissues in the image formed. The amount of attenuation depends on the radiation energy and on three tissue characteristics: atomic number, density, and electrons per gram. Density

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