Predictability of magnetic susceptibility artifacts from metallic orthodontic appliances in magnetic resonance imaging.

Abstract

Orthodontic appliances are often prophylactically removed prior to MRI examinations, although they are sometimes left in situ (out of ignorance). Either way, there is a risk of adverse consequences for the patient, as removing the appliance may incur avoidable costs and extensive dental treatment, whereas leaving them in can cause artifacts that can significantly impair the diagnostic quality of MRI. The aim of this study was to measure the size of experimental artifacts created by orthodontic devices and to develop criteria using sound material-science research for making MRI more compatible, thereby supporting radiologists and orthodontists in their efforts. Sixteen orthodontic small-device and wire specimens made of different steel and titanium or CoCr alloys were placed in a chambered water-filled phantom for MRI. Each was subjected to spin-echo and gradient-echo sequences at 1.5 and 3 Tesla. We observed that artifact formation depends on the material properties (specimen size, crystalline structure, manufacture-related processing) and on the specifications of the MRI system used (main field strength, sequence type). Our results varied considerably according to the steel grades. Artifact radii ranged from 14 mm (spin echo at 1.5 Tesla) to 51 mm (gradient echo at 3 Tesla). No artifacts occurred at 1.5 Tesla around the titanium and CoCr specimens; the same observation was made with one of the steel grades. Artifact size cannot be predicted merely from the designation "steel". Nor did the crystalline structure of the baseline material from which a steel device had been produced have major implications for artifact size. Relevant, however, was the magnetic permeability (or susceptibility) of the final products, which is not disclosed by the manufacturers, and it cannot be measured on fixed intraoral appliances. Furthermore, the present investigation reveals that some steel devices can remain in situ without triggering adverse consequences.