ion, movement of patients during the scanning process or presence of metals on the body, problems with the scanner or during the reconstruction process (Barrett & Keat, 2004). The various types of artifacts and the methods used for reducing their occurrence are discussed below.
The most common physics-based artifact is beam hardening which is caused due to differential absorption of low and high energy photons of an x-ray beam as it passes through an object. Such beam hardening phenomena can give rise to cupping and streaking or dark band artifacts. Cupping artifacts arise when the x-rays passing through the middle portion of an object become hardened which causes a reduction in its attenuation rate resulting in an intense beam reaching the detector. Streaking is another common phenomenon where streaks or dark bands appear between two dense objects again due to the hardening effect. This is usually visualized in scans taken in bony regions (Barrett & Keat, 2004). The methods used to reduce artifacts due to beam hardening include filtration of low energy particles, calibration correction, and use of appropriate software algorithms to correct the hardening (Barrett & Keat, 2004; Huang, n.d; Petit et al, 2010).
Presence of a heterogeneous tissue mix can result in a CT number that is an attenuation average of all tissue types which can in turn result in a partial volume artifact as bands or streaks. Presence of off-axis objects in the path of the x-ray beam can result in the appearance of shading artifacts in the scan image. Such artifacts can be avoided using thinner sections and image noise can be limited by combining thinner sections to form a thicker section (Barrett & Keat, 2004; Huang, n.d).
This effect occurs in parts of the body where attenuation of the x-ray beam is greatest such as the shoulders and the hip. This results in low number of photons reaching the detector which causes noisy projections that are in turn magnified during the reconstruction