Local heating of metallic objects from switching magnetic gradients in MRI

Abstract

Previously it has been shown that heat induced in metallic objects due to fast switching magnetic gradients can pose a possible danger not only for patients undergoing MRI procedures, but also to medical staff present. In this study, we investigate magnetic flux changes and the effects of local heating in metallic elements by eddy currents due to fast switching magnetic gradients at several specific positions with respect to the isocenter of the gradient coils. Experiments were performed in a 3 T preclinical scanner with a 30 cm bore. To probe the induced electro motive force (EMF), which results in induced eddy currents, small pickup coils were used at various locations in the scanner. To investigate heating, metallic cylinders (12.5 mm diameter, 12.6 mm height) were prepared with a pinhole to accommodate miniature fiber optic temperature sensors. A gradient echo axial imaging sequence was applied for 4 minutes after achieving thermal equilibrium with the magnet. Three different materials were used in this study: copper, aluminum, and Ti-6AL-4V, a titanium alloy commonly used in orthopedic implants. In these experiments, a direct dependence on position and gradient strength on the induced EMF was observed. As expected, the increase of temperature depended directly on distance and material composition. Heating can be a danger during imaging of patients with conductive implants that are placed away from the isocenter. Thus, our findings are vital to patient safety and comfort during MRI procedures.