The promising applications of core-shell nanoparticles in the biological and medical field have been well investigated in recent years. One remaining challenge is the characterization of the structure of the hydrated polymer shell. Here we use small-angle X-ray scattering (SAXS) to investigate iron oxide core-poly(ethylene glycol) brush shell nanoparticles with extremely high polymer grafting density. It is shown that the shell density profile can be described by a scaling model that takes into account the locally very high grafting density near the core. A good fit to a constant density region followed by a star-polymer-like, monotonously decaying density profile is shown, which could help explain the unique colloidal properties of such densely grafted core-shell nanoparticles. SAXS experiments probing the thermally induced dehydration of the shell and the response to dilution confirmed that the observed features are associated with the brush and not attributed to structure factors from particle aggregates. We thereby demonstrate that the structure of monodisperse core-shell nanoparticles with dense solvated shells can be well studied with SAXS and that different density models can be distinguished from each other.