ABSTRACT
Scattered photons are one of the main sources of image degradation in industrial X-ray computed tomography (CT) (see Section III, Chapter 32). The scattered radiation that reaches the detector represents an additional signal contribution that does not correspond to the classical model of straight line X-ray beam attenuation, described by the Beer–Lambert law. This means that the scattered radiation introduces high non-linearity in the CT scan. As a result, scattered radiation leads to a considerable blur in the measured X-ray images, which leads to a general loss of contrast in the radiographs as well as the reconstructed images. Since scattered photons add, by definition, spurious signals, their effects are generally similar to beam hardening artifacts. Moreover, the non-linearity introduced by scattered radiation adds to the image degrading effects beam hardening has on the measurement (Kasperl et al. 2003). This means scattered radiation enhances beam hardening artifacts such as cupping and streaking. Cupping artifacts can commonly be observed in large and uniform areas of an object. Instead of a homogeneous area, a reconstructed volume exhibits a non-uniform gray value distribution, where the attenuation values in the middle of the object are underestimated. Streaking artifacts mainly occur between high-contrast details in the object volume. They are caused mainly by photon starvation; however, beam hardening and scattered radiation add to the image degradation effect, leading to streak artifacts in the reconstructed images. Typically, these artifacts appear as dark streaks connecting the high-absorbing features with adjacent bright streaks.
