Objectives: To investigate the potential influence of standard dental materials on dental MRI (dMRI) by estimating the magnetic susceptibility with the help of the MRI-based geometric distortion method and to classify the materials from the standpoint of dMRI. Methods: A series of standard dental materials was studied on a 1.5 T MRI system using spin echo and gradient echo pulse sequences and their magnetic susceptibility was estimated using the geometric method. Measurements on samples of dental materials were supported by in vivo examples obtained in dedicated dMRI procedures. Results: The tested materials showed a range of distortion degrees. The following materials were classified as fully compatible materials that can be present even in the tooth of interest: the resinbased sealer AH Plus ® (Dentsply, Maillefer, Germany), glass ionomer cement, gutta-percha, zirconium dioxide and composites from one of the tested manufacturers. Interestingly, composites provided by the other manufacturer caused relatively strong distortions and were therefore classified as compatible I, along with amalgam, gold alloy, gold-ceramic crowns, titanium alloy and NiTi orthodontic wires. Materials, the magnetic susceptibility of which differed from that of water by more than 200 ppm, were classified as non-compatible materials that should not be present in the patient's mouth for any dMRI applications. They included stainless steel orthodontic appliances and CoCr. Conclusions: A classification of the materials that complies with the standard grouping of materials according to their magnetic susceptibility was proposed and adopted for the purposes of dMRI. The proposed classification can serve as a guideline in future dMRI research.

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ABSTRACT:The MRI-based contrast-enhanced method of tooth surface digitization provides an alternative possibility for making a dental impression. It has recently been demonstrated that tooth surface digitization with an accuracy sufficient for production of dental restorations is possible using a clinical whole-body MRI scanner. However, for an in vivo application of the technique, the required high nominal resolution and signal-tonoise ratio have to be achieved within reasonable measurement times. Existing radio frequency (RF) volume and surface coils have not been able to fulfill this requirement. The objective of this work was to develop a dedicated RF receiver coil for contrast-enhanced dental MRI which provides a high sensitivity in the region of the whole dental arch and allows for application of the contrast media into the mouth of the patient. To achieve this goal, an intraoral dental coil design with a built-in contrast medium applicator was developed and implemented. The coil was shown to enable high-resolution contrast-enhanced dental MRI in vivo. This was demonstrated on four teeth prepared for dental restorations. The average accuracy obtained for all four teeth was below 100 lm. For two of the teeth, copings were modeled based on MRI data and produced from zirconium oxide using computer-aided design and manufacturing (CAD/CAM) technology.

The results presented demonstrate the feasibility of high-resolution dental MRI to three-dimensionally visualize and quantify carious lesions, including approximal and occult caries lesions, and measure the minimum distance to the dental pulp.