Power Scaling Methods for RF Excitation Fields in MRI Systems

In this paper, we analyze the homogeneity of the transmitted radiofrequency (RF) field by applying different power scaling methods ranging from low-field (12.8 MHz, 0.3 T) to high-field (298 MHz, 7.0 T) magnetic resonance imaging (MRI) systems. Homogeneity strongly depends on RF power scaling, especially in higher-field MRI. While no notable homogeneity problem is usually observed in lower-field MRI, strong center brightening occurs at a higher field, owing to shortened wavelength. In particular, the dependence of homogeneity on RF power scaling appears during the initial MRI system calibration. In this study, we evaluate the 10-g averaged specific absorption rate (SAR) to evaluate RF safety. This rate may suffer from severe phase interferences, especially in the case of higher-field MRI, due to the increased strength of the static magnetic field and the complex distribution of electrical properties in structures, such as the human head. In particular, the 10-g averaged SAR level is up to six times higher in higher-field MRI with entire-area RF power scaling than with center scaling. Overall, this study establishes the importance of accounting for MRI scan homogeneity and RF safety during MRI examinations.