Novel carbon nanohybrids based on unmodified metallofullerenes have been successfully fabricated for use as a new magnetic resonance imaging (MRI) contrast agent. The nanohybrids showed higher R₁ relaxivity and better brightening effect than Gd@C₈₂(OH)_X, in T₁-weighted MR images in vivo. This is a result of the proton relaxivity from the original gadofullerenes, which retained a perfect carbon cage structure and so might completely avoid the release of Gd³⁺ ions. A "secondary spin-electron transfer" relaxation mechanism was proposed to explain how the encaged Gd³⁺ ions of carbon nanohybrids interact with the surrounding water molecules. This approach opens new opportunities for developing highly efficient and low toxicity MRI contrast agents.