In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.