1. A method for determining a spatial distribution of a magnitude of a radio frequency transmission B1 field in a magnetic resonance imaging apparatus by performing an MRI experiment in which a B1-sensitive complex image of a sample is obtained, with a real part and an imaginary part, wherein a phase distribution within the B1-sensitive complex image depends on the spatial distribution of the magnitude of the B1 field, the phase being determined by an arc tangent function, the method comprising the steps of: a) applying one or more adiabatic RF pulses with a frequency sweep ending off-resonance for establishing a dependency of the phase distribution within the B1-sensitive complex image on the spatial distribution of the B1 field; b) obtaining a reference complex image of the sample for eliminating any superimposed phase distribution components of the B1-sensitive complex image which are independent of the B1 field, the reference complex image of the sample having a same superimposed phase distribution components; c) generating a phase difference image of the B1-sensitive complex image and the reference complex image; and d) calculating a B1 magnitude image from the phase difference image of step c).