Computerised tomography performance.

Generally, computerised tomography (CT) is an X-ray based non-destructive testing method that was developed for medical purposes in the seventies and introduced for industrial applications during the eighties. CT produces maps of the X-ray linear attenuation coefficient of an object’s interior; presented either as cross section images (two-dimensional CT) or as volume information (three-dimensional CT). The linear attenuation coefficient is the fraction of X-ray photons absorbed or scattered per unit length as the ray propagates through an object. The linear attenuation coefficient depends on the X-ray photon energy, and both object density and atomic composition. Most industrial CT systems are equipped with conventional X-ray tubes that produce X-ray photons with an energy distribution, that is, a spectrum. Consequently, the effective linear attenuation coefficient of an object, shown by the CT image, depends on the full energy spectrum, how it changes as it propagates through the object, and how it interacts with the detector. To emphasise contrasts in the final CT image, caused by density or compositional variations in the object investigated, the energy spectrum has to be chosen and shaped with filters in such a way that the differences in effective linear attenuation coefficient increase. However, it is empirically tedious to find optimal CT parameter settings for each individual imaging task, particularly for industrial CT, because of the wide range of object and defect combinations of interest.