Alteration of flux density of highly coercive SmCo magnets used in prosthodontics and epithetics in 1.5 T and 3 T MRI

The opposing field stability of highly coercive dental magnets in external magnetic fields of 1.5 and 3 T magnetic resonance imaging (MRI) was investigated. It was further assessed if remagnetizing can reverse the flux density in the magnets. Using an adjustable fixture, 20 SmCo magnets were exposed and 6 positions of prosthodontics and epithetics were simulated: P : in the lower jaw parallel to the main field B${}_0$, A: in the upper jaw antiparallel to B${}_0$ in a straight position, Ad: antiparallel, reclined by 45°, Av: antiparallel, inclined by 45°, G: glabellar region 90° to B${}_0$ and M: mastoid region 90° to B${}_0$. The effects of exposure in the exterior field directly at the opening for the parallel (P${}_{ex}$), antiparallel (A${}_{ex}$), glabellar (G${}_{ex}$) and mastoid (M${}_{ex}$) positions were also investigated. After each exposure the magnets were remagnetized. The flux density was determined as an equivalent of the adhesive force. With 1.5 T clinically relevant loss of flux density between 7% and 10% occurred only in the angled positions Ad and Av and the external position A${}_{ex}$. In the antiparallel positions A and A${}_{ex}$ the strong external field of 3 T caused very high losses of 72% and 33%, respectively. In the inclined and reclined antiparallel positions Ad and Av the magnets lost 96% of their flux density and were almost fully demagnetized. All of the magnets could be fully remagnetized regardless of the degree of damage. Highly coercive SmCo magnets can remain in situ during a 1.5 T MRI scan unless the resulting artifacts are diagnostically relevant. Exposure to the 3 T main field in antiparallel position may result in a complete loss of the adhesive force. In this case the magnets should be remagnetized by the manufacturer. Inclination or reclination of the head reinforces the effect of the main field.