Preliminary Study Of Artifacts From Dental Metal Alloys In Magnetic Resonance Imaging

During the last decades magnetic resonance images (MRI) has become a new medical imaging technique based on X-rays and CT with the development of the computer system. The advantages of MRI include its ability to provide sectional images of anatomic regions in any arbitrary plane, non-invasive, non-ionizing and its excellent soft-tissue contrast resolution. MRI is also used for diagnostics, treatment planning and therapy process control including neurodegenerative diseases, orthopedic surgery, craniomaxillofacial area. Metal objects are frequently located in the orofacial area. They are mostly inlays, crowns, fixed bridges and fixed dental splints. They are usually made of high-noble and noble alloys (including: gold, platinum, palladium, silver) as well as base metal alloys (cobalt, chromium, nickel). In MRI, degradation of image quality and its impact on image interpretation in the vicinity of metallic material are due to a combination of signal voids, image distortion, and signal inhomogeneity. Artifacts in MR images do not faithfully represent the tissue components being studied. Reducing and avoiding the effect of the metal artifacts is the problem to be solved. Recently, the development of MRI system is to the direction of lower field strength (0.3T) and the higher field strength (3.0T), and it can be anticipated that metal artifacts will be more extensive than at the more commonly used field strength of 1.5 T. Clinical use of magnetic field strengths of 3.0 Tesla has been approved in many countries. Understanding the extent of the metal artifacts and the relation with the MRI pulse sequences have great instruction on the practise of the 3.0T MRI system. Objective: To evaluate artifact sizes at 3.0 T compared to at 1.5 T caused by three kinds of metallic dental materials, and to evaluate the influence of different magnetic resonance imaging (MRI) sequences on artifacts.Methods: metal-ceramic crown samples of various dental materials( Ni-Cr alloy, Co-Cr alloy and Gold alloy ) were placed in a cylindrical plastic vessel filled with agarose gel. Images were performed by means of 1.5 T and 3.0 T MRI apparatus with five different sequences.The pulse sequences are fast spin echo T₁WI and T₂WI (FSE T₁WI, FSE T₂WI)sequences, fluid attenuated inversion recovery (FLAIR) sequence, 3D TOF of gradient echo (GRE) sequence and echo planar imaging DWI (EPI DWI) sequence, of which are all the common head sequences in clinical practise. A linear region of interest were defined across the centre of the sample in the transverse direction, and the susceptibility artifact diametre was calculated and compared quantitatively. The data were analyzed with factorial experiment's analysis of variance using SPSS 12.0. factorial design was used to test three factors (metal materials, magnetic field strengths and sequences).Results: Susceptibility artifact developed around all metal materials. The artifact diametres caused by Co-Cr alloy and Ni-Cr alloy were big and increased substantially at 3.0 T, compared to at 1.5 T. Gold alloy's artifact was very small and not influenced by magnetic field strengths. Different pulse sequences had different susceptibility artifacts. At 1.5T, FSE sequence produced the least artifacts, FLAIR sequence produced the second, 3D TOF sequence produced the third and DWI sequence produced the greatest artifact. At 3.0T, 3D TOF sequence produced the similar size of the artifacts with that of 1.5T. Analysis of variance showed that metal materials, magnetic field strengths and sequences were independently significant factors (P<0.05). Interactions between the three factors were not statistically significant (P >0.05).Conclusions:1. The metal artifacts on MRI depends on the composition of the metal alloy, with Gold alloy creating significantly less artifact than Co-Cr alloy and Ni-Cr alloy. Selection of specific metal materials could minimize the metal artifacts in MRI. 2. Sequences were important factors that affected the size of metal artifacts. FSE sequence produced the least artifacts and EPI sequence produced the greatest. The patient with the metal restoration should select FSE sequence, avoid GRE sequence and EPI sequence is strictly prohibited.3. Usually higher field strength results in greater imaging artifacts, so the lower field strength MR scanner is considered, but the increment of artifacts are only 10% at 3.0T compared to at 1.5T, while 3.0T MRI system has great advantages, it is eligible on the patient with metal restoration.