## 6 Metal Artifacts and Stratagems for Their Reduction
### Mri  
Metal material in the oral cavity can generate two kinds of artifacts on MRI images. Susceptibility artifacts are given by the interface between dental materials and adjacent tissues that create a variation in the magnetic field strength, causing magnetic distortions and signal loss (Figure 16). Non-susceptibility artifacts are given by eddy currents induced by alternations of gradients and radiofrequency fields [56]. Susceptibility artifacts are responsible for almost all oral artifacts since magnetic field inhomogeneities are due to ferromagnetic ingredients contained in dental materials [57]. The high magnetic susceptibility within the magnetic field not only determines the signal loss but could also accumulate the signal intensity in the region of interest (\"pile-up artifact\"), resulting in an abnormally high signal intensity (Figure 21).  
The artifact severity will vary depending on the shape, position, orientation, number of objects in the image, and type/parameters of sequence used [57]. Different materials have different influences on the formation of the artifacts. Their magnitude is greater for titanium dental implants than zirconia ones [56]. Such artifacts can be reduced by some expedients:  
* Spin-echo sequences with modified parameters (i.e., echo time, matrix, bandwidth, slice thickness, and echo train); high bandwidth per voxel, 3D spatial encoding, high-resolution matrix, and multi-echo spin-echo sequences or turbo/fast spin-echo sequences. Spin-echo sequences are less sensitive to susceptibility effects thanks to a 180\({}^{\circ}\) radiofrequency pulse that refocuses the spins diminishing the phase shifts in the voxel and therefore the field inhomogeneity [58];
* Gradient echo sequence with a very short echo time. These sequences are metal-sensitive and the signal loss is due to the intravoxel dephasing. Shortening the echo time and decreasing the voxel size can diminish the intravoxel dephasing and therefore the artifacts [58];  
Figure 21: MRI “pile-up” artifact. The high magnetic susceptibility within the magnetic field could accumulate the signal intensity in the region of interest resulting in an abnormally high signal (“pile-up artifact”, indicated by white arrows). “Pile-up artifact” mainly occurs on the periphery of the area produced by magnetic distortions and signal loss given by the interface between dental metal materials and adjacent tissues (*).  
* Optimised metal artifact-free sequences with a specific intensity and size of susceptibility artifacts, including the multi-acquisition variable-resonance image combination (MAVRIC) and slice encoding for metal artifact correction (SEMAC) techniques. SEMAC uses a slice selection gradient for excitation and a view-angle tilting compensation gradient for readout. MAVRIC and SEMAC show significantly smaller artifact extent than fast spin-echo imaging [59].  
Beyond artifacts, we should always remember that undesirable effects caused by the interaction of oral metal materials with the magnetic field also include the magnetically induced displacement and radiofrequency heating. The magnet can attract ferromagnetic objects, but the retention force of the dental cement (50-150 N) is strong enough to prevent the displacement of the prosthesis. However, position and fixing of the prosthesis must be inspected before and after every MRI examination because of the possibility of degeneracy cement phenomena [60]. The radiofrequency heating phenomenon causes a negligible rise in temperature, negligible even using a 3T magnet (<2 \({}^{\circ}\)C), that does not injure the prosthesis-neighbouring tissues and especially the richly vascularised periodontal ligament [54]. For further information on the use, safety, and diagnostic value of MRI in the presence of different metal materials and devices, please refer to the guidelines drawn up by [PERSON] et al. [60].