An apparatus and method are provided for measuring flow velocity of a multi-phase fluid mixture. The proposed apparatus includes a radiation device, a detection device, and an analysis device. The radiation device generates a beam of photons to irradiate the mixture spatially over a section of flow of the mixture. The detection device is spatially configured to receive photons emanating from the section of flow of the mixture at different intervals of time. The detection device provides an image of a spatial distribution of the received photons for each the interval of time. The analysis device determines flow velocity of one or more phases of the mixture based on a temporal sequence of the images of the spatial distributions of the received photons.