Abstract: An apparatus for measurement of a flow velocity of a multi-phase fluid mixture includes a radiation device configured to generate a sequence of pulses of photons for irradiating a section of the flow of the mixture. The photons are emitted at at least a first energy level and a second energy level. The apparatus includes a detection device configured to detect photons that transmitted the section of the flow at different time intervals so as to generate a first image of the spatial distribution of detected photons of the first energy level and a second image of the spatial distribution of detected photons of the second energy level. The apparatus includes an analysis device configured to determine the flow velocity of one or more phases of the mixture based on a temporal sequence of the first image and the second image of the spatial distributions.