Assessment of RF-Induced Heating of Stainless-Steel Sternal Wires at 7 T MRI.

PURPOSE: To investigate RF-induced heating of stainless-steel sternal closures at 7 T proton MRI for simple loop and figure-of-8 configurations.

METHODS

Simple loop and figure-of-8 sternal closure designs were implemented on plexiglass rods and immersed in a hydroxyethyl-cellulose phantom. Temperature was monitored by four fiber-optic sensors during RF exposure at average power levels of 3 W (conventional limit), 6 W, and 25 W for 10 min. Electromagnetic simulations were used to calculate specific absorption rate distributions, which served as input for thermal simulations. Simulated and measured temperature increases were compared at the probe locations.

RESULTS

At 3 W, the maximum measured temperature increase was 0.12°C for the simple loop and 0.29°C for the figure-of-8 configuration. At 6 W, the corresponding increases were 0.38°C and 0.50°C, respectively, and at 25 W they reached 1.30°C and 2.26°C. The simple loop configuration minimally affected the simulated B1 + and SAR distributions, whereas the figure-of-8 configuration produced greater field perturbations and elevated local SAR, particularly between closely spaced wire segments. Thermal simulations reproduced the measured heating trends but consistently overestimated the measured temperature increase.

CONCLUSION

RF-induced heating remained small under the investigated 7 T exposure conditions, with maximum measured temperature increases below 0.3°C for both closure configurations at the nominal operating power of 3 W. The figure-of-8 configuration produced greater electromagnetic field perturbations and heating than the simple loop configuration, indicating a more favorable RF behavior for simple loop closures.