![]() When the incident x-ray energy is closer to the k-edge of the atom it encounters, photoelectric absorption is more likely to occur.ĭifferences in photoelectric absorption across different x-ray energies is harnessed in dual-energy CT, in which patients are scanned with two different x-ray spectra. Iodine has a particular advantage as a contrast agent because the k-shell binding energy (k-edge) is 33.2 keV, similar to the average energy of x-rays used in diagnostic radiography 1. As iodine has a high atomic number, 53, compared to most tissues in the body, the administration of iodinated material produces image contrast due to differential photoelectric absorption. Both these interactions depend upon physical density, but the latter also depends upon atomic number of the matter. The ability to distinguish between tissues of different x-ray attenuation (image contrast) depends upon two types of interactions between photons and matter: Compton scattering and photoelectric absorption.
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