The AAPM/RSNA physics tutorial for residents. X-ray interactions.

The diagnostic information in a radiograph or fluoroscopic image is largely the result of the quantity of x rays that are not removed from the incident x-ray beam. The information content of the image is delivered by the percentage of noninteracting photons that are successfully recorded. There are four major x-ray interactions: Rayleigh (coherent) scattering. Compton scattering, photoelectric absorption, and pair production. The degree of attenuation and the predominant mechanisms involved in the interactions are influenced by the x-ray energy and tissue composition. In the diagnostic energy range, photoelectric absorption and Compton scattering are the predominant modes of attenuation. One of the challenges in diagnostic imaging is to optimize image acquisition by controlling x-ray attenuation to obtain the appropriate contrast between the tissues while minimizing patient dose and scattered radiation in the image. Imaging techniques such as use of contrast material and dedicated mammography equipment exploit the differences in these types of x-ray interactions to improve the quality and diagnostic utility of the examination. Rayleigh scattering and pair production are presented but do not occur to any significant degree in diagnostic radiography.