A method for measuring flow rate of at least one fluid in a multi-phase fluid comprises: providing a magnetic resonance module through which the fluid phases flow and at least one pre-polarization module of variable effective length upstream of the magnetic resonance module; and conducting, a measurement by: i) setting the pre-polarization module to have a first effective length, ii) applying a RF pulse sequence to the fluid, in the magnetic resonance module, iii) determining the intensity of a pre-determined number of spin echoes produced by the RF pulse sequence, iv) determining a line approximating the attenuation of the intensity during the RF sequence, v) determining slope and y-intercept of the line, vi) determining the ratio of the slope and intercept, vii) applying a calibration between the slope:intercept ratio and multi-phase flow rate so as to determine the flow velocity of the fluid in the multi-phase fluid.