CT Basics

Abstract

Computed tomographic (CT) images are depictions of relative and not absolute tissue densities. These relative depictions are produced by measuring x-ray attenuation resulting from its travel through tissue. A tissue’s attenuation is expressed by its attenuation coefficient, μ. μ is an indicator of the likelihood that a photon will interact with an atom of a tissue rather than passing through it. The greater the density of a tissue, the greater its attenuation coefficient. However, specific tissues do not have constant attenuation coefficients. The attenuation coefficient of a tissue is also affected by the photon energy of an x-ray (KeV) and by the tissue thickness. Increasing photon energy and decreasing tissue thickness will reduce the attenuation coefficient of a specific tissue, thus allowing more photons to pass through to the detector (Fig. 2.1).