Nanoparticles made of polymers containing fluorine have promising applications since magnetic resonance imaging using fluorinated contrast agents (19F MRI) enables enhanced resolution and selectivity due to the negligible fluorine background in living tissues. We prepared several types of fluorinated diblock copolymers for this purpose A critical parameter is the density of 19F atoms in the core of the micelles, which has to be relatively high to ensure sufficient imaging contrast. To evaluate this density, the radius of the core and the aggregation number Ng of the micelles needs to be known to determine the number of fluorinated blocks in the core. The degree of hydration of the core related to the density also influences magnetic relaxivity of the fluorine atoms in it. SANS will be used to determine Ng and the core radius of micelles using contrast matching of the shell.