Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1+ uniformity

Abstract

Purpose: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B1+ uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. Methods: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. Results: Two-port RF shimming, compared to circular polarization mode, can increase the B1+ uniformity on average by +32%. Worst-case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively. Conclusion: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986–997, 2016. © 2015 Wiley Periodicals, Inc.