Abstract
Metal dental products lack precautionary statements regarding MR compatibility due to an exemption in the labelling obligation. Hence, it is difficult for radiologists to decide whether to remove fixed metal objects in patients prior to MRI. Asolution could be the direct determination of the magnetic permeability (µr) as adecisive material-related predictor of artifact formation and other interactions. Thus, the applicability of an industrially used measurement device as ascreening instrument and the relevance of the manufacturer's application restrictions in vitro and in vivo were tested. Precision and trueness were tested using self-made test objects with different dimensions and different permeability. To clarify whether the measurement results are affected by the remanence (BR) induced in the objects, 28brackets of different materials were exposed to aweak and astrong external magnetic field and the magnetic flux density before and after these exposures was compared. The clinical test was performed on avolunteer with an orthodontic appliance experimentally composed of brackets with different levels of magnetic permeability (µr). Validity and intra- and interrater reliability were calculated using two rater groups consisting of four dentists and four medical-technical radiology assistants (MTRA), respectively. With coefficients of variation below 0.14%, precision was excellent regardless of object surface and size. Trueness was high on objects with µr ≤ 1.002, and decreased with increasing µr, for which size-dependent correction factors were calculated. Intra- and interrater reliability and validity were excellent and independent of professional intraoral manipulation experience. The permeability measurement allows for avalid and reliable determination of the magnetizability of intraoral metal objects.When used as ascreening tool to detect nonartifact-causingobjects, no correction factor needs to be calculated.For the first time, it offers radiologists adecision support for the selective removal of only the highly permeable components of the multiband apparatus.
Highlights
Patients with medical implants can only be legally examined in an MRI if these products can be unmistakably identified and are labeled by the manufacturer as “MR safe” or “MR conditional” including the conditions to be met [1]
Forces acting on correctly anchored brackets and wires are harmless, which is about a factor of 1000 below the force required for debonding [13]
The 14 different bracket types in Table 2 can be divided into two groups: The brackets listed under numbers 1–8 consist of various materials: titanium, CoCrMo alloy, ceramic or metastable austenitic steels
Summary
Patients with medical implants can only be legally examined in an MRI if these products can be unmistakably identified and are labeled by the manufacturer as “MR safe” or “MR conditional” including the conditions to be met [1]. Radiologists lack the simplest source of information for assessing the risks emanated by these metal objects in MRI. There are more reliable data on the potential heating and force effects: brackets heat up by less than 1 °C, while wires were found to increase by a maximum of 3 °C [11, 20, 22, 26]. Forces acting on correctly anchored brackets and wires are harmless, which is about a factor of 1000 below the force required for debonding [13]
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