Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields.
PURPOSE
High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B(1)(+), and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult.


METHOD
We propose a two-step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the B(1)(+) map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this B(1)(+) remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B(1)(+), a simple pulse sequence design using time-varying SEMs can achieve a homogenous flip-angle distribution efficiently.


RESULTS
We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B(1)(+) inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple-pulse design, a similar average flip-angle distribution with less RF power, and/or a more homogeneous flip-angle distribution.


CONCLUSION
Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the B(1)(+) inhomogeneity in high-field MRI experiments.
['Hsu|Yi-Cheng|YC|', 'Chern|I-Liang|IL|', 'Zhao|Wei|W|', 'Gagoski|Borjan|B|', 'Witzel|Thomas|T|', 'Lin|Fa-Hsuan|FH|']
D000465:Algorithms D016477:Artifacts D001921:Brain D006801:Humans D007089:Image Enhancement D007090:Image Interpretation, Computer-Assisted D016247:Information Storage and Retrieval D060526:Magnetic Fields D008279:Magnetic Resonance Imaging D019047:Phantoms, Imaging D011846:Radio Waves D015203:Reproducibility of Results D012680:Sensitivity and Specificity
7T RF inhomogeneity SAR fast imaging nonlinear gradient