Radio-Frequency Vector Magnetic Field Mapping in Magnetic Resonance Imaging

A method is presented to measure the radio-frequency (RF) vector magnetic field inside an object using magnetic resonance imaging (MRI). Conventional " \text{B}_{{1}} mapping" in MRI can measure the proton co-rotating component ( \text{B}_{{1}}^{+}{)} of the RF field produced by a transmit coil. Here we show that by repeating \text{B}_{{1}}^{+} mapping on the same object and coil at multiple (8) specific orientations with respect to the main magnet, the magnitudes and relative phases of all (x, y, z) Cartesian components of the RF field can be determined unambiguously. We demonstrate the method on a circularly polarized volume coil and a loop coil tuned at 123.25 MHz in a 3 Tesla MRI scanner, with liquid phantoms. The volume coil measurement showed the axial component of the RF field, which is normally unmeasurable in MRI, away from the center of the coil. The measured RF vector field maps of both coils compared favorably with numerical simulation, with volumetric normalized root-mean-square difference in the range of 7~20%. While the proposed method cannot be applied to human imaging at present, applications to phantoms and small animals could provide a useful experimental tool to validate RF simulation and verify certain assumptions in \text{B}_{{1}}^{+} map-based electrical properties tomography (EPT).