Extended Multi-Flip-Angle approach: a 3D B1unit+ mapping method for inhomogeneous fields

Introduction For successful RF coil development and improvement, coil characterization plays an essential role. A quantitative map of the magnetic field produced per unit current (B1unit+) provides information about the coil efficiency as well as its sensitivity distribution [1]. This allows one to predict SNR, homogeneity and sensitive volume. A mapping method with high accuracy over a large dynamic range is required, particularly for surface and array coils with inhomogeneous B1unit+ fields. Most techniques, such as the double-angle method (DA) [2], are based on only a few data points and rely on fixed assumptions about the corresponding signal. This leads to inaccurate results for small flip angles (FA), due to low SNR, as well as for signals which differ from the assumed FA dependency. The latter is the case for an effective B1+ field not orthogonal to the static magnetic field (B0), due to off-resonance effects, for example. The multi-flip-angle approach (MFA) [3] can overcome the SNR problem, but still relies on a fixed signal dependency, such as a sin(FA) course for a gradient echo (GE) image series. In this study we present an extension of the MFA method (ExMFA) with higher flexibility. As it is not based on a fixed assumption regarding the orientation of the effective B1+ field axis, it can handle complex signal courses as well as data with low SNR. To demonstrate its superiority over the commonly used DA method, especially in the case of inhomogeneous B1+ fields, a comparison between the two is given. Methods To map the B1unit+ field of a surface coil, a series of 80 3D GE images of a doped H20 phantom (T1/T2=250/200 ms) was acquired on a 9.4T BioSpec 94/20 system (Bruker BioSpin, Ettlingen, Germany) (matrix: 32x32x16; FOV: 8x8x6 cm, TE=5 ms, TR=1500 ms; rectangular RF pulse, τ =0.3 ms). For each image, the RF pulse amplitude was increased by a fixed increment. Power values were chosen such that a full period of signal oscillation (corresponding to a FA up to 360°) in the area of highest B1unit+ field was acquired. For analysis, the varying magnitude signal per voxel (S) was plotted over the coil current (I). The latter has been calculated from the pulse power measured in advance. B1unit+ is derived from the signal oscillation frequency by fitting the signal course with the following equation using Matlab: