Carbon nanotubes (CNTs) strongly associate with the halide salts of gadolinium, chromium, erbium, and manganese, metals of which have utility as MRI contrast agents. A number of these CNT:salt complexes demonstrate increased T1-relaxivity in aqueous solution, a requirement of MRI contrast enhancement in accessible tissues. The CNT:salt complexes evaluated in this study show drastic enhancement in T1-relaxivity compared to the commercially used contrast agents and demonstrate little to no acute toxicity in cell culture, results of which suggest their potential use in MR angiography.