1. A method of mapping a field of a RF Excitation pulse (B 1 +) in the presence of a static field (B 0 ) for magnetic resonance imaging (MRI), comprising the steps of: a) acquiring MRI signals in response to RF excitation pulse sequences alternately applied at two flip-angles, and b) for each RF excitation pulse sequence at one flip angle, applying a reset pulse sequence for returning excited nuclei spins to a pre-excitation state.