B1 mapping of liquid NMR transversal RF coils: Analysis of heterogeneity and nonlinearity

Abstract

AbstractB1 fields generated by transversal RF coils in a high‐resolution NMR probe were studied using a nutation experiment combined with gradient echo 1D‐profiling. Signal amplitude, B1 strength, and relaxation time were mapped as a function of longitudinal axial position. Numerical Bloch simulations in a rotating frame could reproduce the overall features of the experimentally observed nutation profiles. The 3D rotating frame trajectories of magnetization during pulsing are presented in spherical coordinates. Nutating magnetization (i.e., sinusoidal oscillation) was detectable even outside the RF coil, although the signal amplitude was low and the spin relaxation during pulsing was fast. As the axial position got closer to the edges of the RF coil, low frequency components began overlapping on the nutating component. At the edge regions, the low frequency magnetizations dominated and the oscillating component almost disappeared, resulting in a cycloidal 3D trajectory of magnetization. Inside the RF coil, the low frequency component increased the frequency of oscillation as the axial position got closer to the center. Only a narrow central region of the RF coil gave a near linear response against pulse width, wherein the highest B1 field strength and very small relaxation were observed. A significant nonlinearity between pulse duration and flip angle was observed in the edge portion of the RF coil and at the outside. The heterogeneity and nonlinearity led to an inaccurate 90° pulse width when the pulse duration at null points (e.g., 180° or 360°) was simply divided by 2 or 4. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 41B: 1–12, 2012