26.1 INTRODUCTION

The object of x-ray computed tomography is to reproduce, in the form of digital data, the internal structure of three-dimensional bodies with as little distortion as possible. This is achieved by the mathematical reconstruction of a series of two-dimensional images produced by the projection of an x-ray beam through the body in question. In this respect the form and functioning of x-ray computed tomography systems are closely linked to the physics of the interaction of x-ray photons with the material body and to the principles that govern the mathematical process of image reconstruction. In depth these are complex matters, but the fundamentals can be simply explained to provide a sound basis for further study.1 ^,2 ^,3 ^,4

The relative values of absorption and transmission of adjacent elemental ray paths in an x-ray beam provide the contrast in a projected image. The associated interaction of the x-rays with the material body can be defined in terms of collision cross sections that describe the probability of particular events occurring along the ray path.5 Here, the principal effects, for x-ray energies below 1 MeV, are identified in Sec. 26.2 as the photoelectron event and the Compton scattering event.6 ^,7 These effects are combined to define a linear attenuation coefficient ?( x) that describes the variation of photon intensity ? along the...