We report the nanoscale loading and confinement of aquated Gd³⁺ _n-ion clusters within ultra-short single-walled carbon nanotubes (US-tubes); these Gd³⁺ _n@US-tube species are linear superparamagnetic molecular magnets with Magnetic Resonance Imaging (MRI) efficacies 40 to 90 times larger than any Gd³⁺-based contrast agent (CA) in current clinical use.