What phenomenon describes the increase in average energy of an x-ray beam as it penetrates tissue?

The correct answer is beam hardening, which refers to the process where the average energy of the x-ray beam increases as it passes through tissue. This phenomenon occurs because lower-energy photons are absorbed more readily by the tissue compared to higher-energy photons. As the beam penetrates more tissue, the less penetrating (lower-energy) photons are selectively absorbed, resulting in a beam that is "hardened" or consists predominantly of higher-energy photons.

This concept is significant in computed tomography because it affects image quality and can lead to artifacts if not accounted for. Beam hardening is especially important in areas of high-density materials, such as bone, where the effect is pronounced. Understanding this phenomenon helps radiologic technologists and radiologists interpret CT images accurately and minimize potential distortions caused by varying tissue densities.