## 6 Metal Artifacts and Stratagems for Their Reduction
### Msct  
Metal artifacts are given by two main phenomena on MSCT examinations that take the name of beam hardening and photon starvation (Figure 18) based on the different atomic numbers of dental materials [52].  
Figure 17: Water distension of oral vestibule. The presence of water in the oral vestibule enables a better delineation and distinction of the buccal and gingival mucosa, allowing a better analysis of possible abnormalities.  
Beam hardening takes place when the X-ray beam hits metal materials with low atomic numbers (e.g., titanium \(Z=22\)). The metal absorbs radiation with lower energy, thus making the emerging beam \"harder\" since it is rich in high energy photons. Hypodense streaks gradually reducing from the area of interest towards the periphery is the typical aspect on MSCT images. On the contrary, when the X-ray beam hits metal materials with high thickness or atomic number (e.g., the amalgam: silver Z = 47 and mercury \(Z=80\)), a very high attenuation takes place and a \"photon starvation\" occurs, that is the condition where there is not a sufficient number of photons reaching the detector. The appearance of this artifact on MSCT images is that of alternated hypo/hyperdense streaks opening as a fan from the metallic area [52]. By comparing MSCT with CBCT, dental metal artifacts are significantly lower on CBCT images for three main reasons including low tube current, limited dynamic range of the detector, and high number of reconstructions around the metal object enabled by the cone/pyramidal-shaped beam [52] (Figure 19).  
Figure 18: Dental metal artifacts on MSCT examinations. **(A,B)** Beam hardening artifacts occurring when the X-ray beam hits a metal with a low atomic number. **(A)** MSCT count view shows two titanium implants located on the anterior lower jaw (arrow). **(B)** Axial image characterised by hypodense streaks gradually reducing from the area of interest towards the periphery. **(C,D)** Photon starvation artifacts occurring when the X-ray beam hits a metal with a high thickness or high atomic number. **(C)** MSCT count view in which many silver amalgam crowns and multiple dental restoration therapies are found. **(D)** Axial image shows the typical artifacts with alternated hypo/hyperdense streaks opening as a fan from the metallic area.  
Many \"stratagems\" have been employed over time to reduce metal artifacts on oral MSCT examinations including the examples described below with proven clinical efficacy.
* Gantry tilt scanning. Patients have to be positioned with tooth occlusal surfaces perpendicular to the table; the gantry tilt angle used is \(\geq\)20 degrees. In the post-processing phase the oblique images obtained are reconstructed to transverse images with information on the table position [53];
* Open mouth dynamic manoeuvre. See the above-mentioned paragraph and Figure 14);
* Metal artifact reduction algorithms. They replace the metal trace region with neighbouring data (also known as iterative reconstruction techniques) (Figure 20) [54].  
Figure 19: Dental metal artifacts. MSCT vs. CBCT. (**A**) Bone kernel–MSCT axial view of the lower jaw. (**B**) CBCT of the lower jaw carried out at the same level on the same patient. The artifacts from metal material are significantly less evident on CBCT images than MSCT ones.  
These three different strategies can also be used in combination to improve the dental artifact reduction. For instance, [PERSON] et al. [55] used the gantry tilt combined with a metal artifact reduction algorithm and lateral position of the patient on the table.