In an MRI method and apparatus a BLOCH-SIEGERT method is used to establish a scaling factor of an excitation magnetic field that excites nuclei in a subject. A method involving two flip angles is executed to obtain a spatial distribution of longitudinal relaxation times of the excited nuclei so as to reconstruct a longitudinal relaxation time spatial distribution image, wherein the scaling factor is used to correct the two flip angles. This MRI method and apparatus cannot only obtain the spatial distribution of an excitation magnetic field accurately, but also can greatly reduce scan time, while ensuring a similar image resolution and image signal-to-noise ratio.