Photon Interactions and Attenuation

Three Fates of a Photon

Attenuation is the reduction in x-ray beam intensity as the beam passes through the patient. Each photon in the primary beam is absorbed, scattered, or transmitted straight through without interacting. Absorption removes the photon and deposits its energy in the patient as dose. Scatter sends the photon in a new direction, where it can fog the image or reach anyone in the room. The photons that exit the patient, transmitted and scattered, make up the remnant (exit) beam that forms the image on the digital detector. Transmitted photons produce the darkest areas of the image, while areas where many photons were absorbed appear bright.