A method for determining individual component flow-rates in a multi-phase fluid flowing in a pipe comprises the steps of: a) flowing the fluid through a magnetic resonance based apparatus comprising a magnetic resonance module and at least one pre-polarization module through which the fluid phases flow before entering the magnetic resonance module, wherein said pre-polarization module comprises a plurality of cylindrical segments that can be selectively combined so as to modify the effective length of the pre-polarization module, b) selectively using the prepolarization module to polarize a component in a volume of the fluid, c) applying a selective excitation to said fluid volume, d) using a signal resulting from said excitation to obtain a velocity measurement for said component, and e) repeating steps b)-d) until velocity measurements for a desired number of components have been obtained.