This thesis is centered on the Gadonanotubes (GNTs), an ultra-high-performance magnetic resonance imaging (MRI) contrast agent material discovered in our laboratories in 2005. The GNTs are a new paradigm in MRI contrast agent design with small clusters of Gd3+ ions within ultra-short carbon nanocapsules (ca. 50 nm) cut from full-length single-walled carbon nanotubes. Here, the factors underlying the performance efficacy of the GNTs have been investigated for the first time by variable-field (-50,000 Oe to 50,000 Oe at 2K) and variable-temperature (2K to RT at 100 Oe) magnetic susceptibility measurements using a Magnetic Property Measurement System (MPMS, based on a SQUID magnetometer). Additionally, experiments focused on the effects of hydroxylation of the GNTs’ exterior surface regarding water-solubility are examined. Finally, the use of the GNTs as potential replacements for traditional metal-chelating/sequestering agents is explored. More specifically, the internal Gd3+-ion clusters of the GNTs have been radiolabeled: (1) with 153Gd3+ ions to test Gd3+-ion stability to simulated biological challenge, (2) with 225Ac3+ ions to generate a new concept for a GNT-based agent for α-radiotherapy, and finally (3) with 64Cu2+ ions to produce the first bimodal MRI/PET (PET = positron emission tomography) imaging agent derived from the GNTs.