Magnetic resonance imaging contrast agents are currently designed by modifying their structural and physiochemical properties to improve relaxivity and to enhance image contrast. Here, we show a general method for increasing relaxivity by confining contrast agents inside the nanoporous structure of silicon particles. Magnevist, gadofullerenes and gadonanotubes were loaded inside the pores of quasi-hemispherical and discoidal particles. For all combinations of nanoconstructs, a boost in longitudinal proton relaxivity r1 was observed: Magnevist, r1 ≈ 14 mM−1 s−1/Gd3+ ion (~8.15 × 10+7 mM−1 s−1/construct); gadofullerenes, r1 ≈ 200 mM−1 s−1/Gd3+ ion (~7 × 10+9 mM−1 s−1/construct); gadonanotubes, r1 ≈ 150 mM−1 s−1/Gd3+ ion (~2 × 10+9 mM−1 s−1/construct). These relaxivity values are about 4 to 50 times larger than those of clinically available gadolinium-based agents (~4 mM−1 s−1/Gd3+ ion). The enhancement in contrast is attributed to the geometrical confinement of the agents, which influences the paramagnetic behaviour of the Gd3+ ions. Thus, nanoscale confinement offers a new and general strategy for enhancing the contrast of gadolinium-based contrast agents.