Assessing the MR compatibility of dental retainer wires at 7 Tesla

Abstract

ii Thesis abstractThe radio frequent (RF) safety of dental retainer wires at 7 tesla has been investigated. Previous studies on metal implant wires at lower field strengths indicate possible SAR increase and directly related a temperature rise of the tissue around the wire. In the Netherlands up to 40% of the younger adults have such a dental retainer wire, which at the moment is listed as a contraindication for 7 T head scans.The radio frequent (RF) field transmitted by the MR excitation head coil is responsible for heating of the wire and its surroundings, of which the electric fields directly contribute to tissue heating. A range of retainer wire lengths of 0 to 70 mm have been tested in a gelled saline phantom in the 7T head coil. Temperature measurements on these wires followed the ASTM F2182-02a standard test method for measurement of radio frequency induced heating near passive implants. The temperature results after 15 minutes of excitation are compared to electromagnetic and temperature simulations. By confirming that phantom simulation and measurement results agree well and therefore are reliable we can make valid conclusions extending to a head model.Head model electromagnetic simulations provided results in which a retainer wire of common wire length did not create a 1 gram SAR higher than than the maximum 1g SAR present in the head without the wire. It could be argued that due to the fact that the wire is not creating the highest SAR, this wire can be rated MR compatible. However, the phantom study showed a 10 fold increase in the 1 gram SAR for a resonant wire. A long retainer wire in the head could become resonant and therefore unsafe, although depending on the surrounding tissues most of the common wire lengths are below resonance length. A solution to this problem has been investigated which resulted in an electrically non-conductive spacer around the wire that lowers the local SAR. The spacer, which is fully surrounding the wire, does not allow conduction currents, induced by the incident field of the MR head coil, to flow from the tissue towards the wire. An experimentally verified spacer with a layer thickness of 0.38 reduced the 1 gram SAR of a resonant wire by a factor of 8. The head model with a 3 mm spacing between tongue and wire had no local SAR increase compared to the model without wire, making the retainer wire with spacer MR compatible.Concluding, small retainer wires up to 32 mm of length are MR compatible for a head scan. Longer wires could be safe, depending on the tissues present around the wire. To be certain, the longer wires can be made safe when a non-conductive spacer is applied, covering the whole wire. A practical solution would be to use a silicone sports mouthguard. As a consequence of these outcomes, people with a retainer wire have the possibility of going into the 7 tesla MR scanner. iii PrefaceThe work for this thesis has been carried out at the site of the 7 tesla MR scanner of the C.J. Gorter center for high field MRI at the LUMC in Leiden. This exceptionally powe...

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