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During the last two decades, magnetic resonance imaging (MRI) has evolved into one of the most powerful techniques in medical diagnosis. Among the various medical diagnostic modalities, such as X-ray or ultra-sound imaging, MRI is the technique of choice especially because of its non-invasive character, the absence of ionizing radiations and its high spatial resolution. The principle of an MRI examination relies on the relaxation of water protons in the human body placed in a magnetic field after radiofrequency excitations. MRI is sensitive to the nature of the soft tissues, to the proton density and to the relaxation rate of protons to produce a contrast. The development of MRI is closely related to the successful use of paramagnetic contrast agents, essentially gadolinium(III) complexes. Unlike contrast agents used in other clinical imaging techniques, the MRI contrast agents are not themselves imaged but rather enhance the nuclear relaxation rate of water protons in their vicinity. The use of MRI contrast agents induces not only a better image contrast and a better delineating of diseased tissues but also a shortening of the examination time. Since the discovery of GdIII chelates as MRI contrast agents, a lot of effort has been made to increase their relaxivity, i.e. the gauge of efficiency of the contrast agent. In the work presented in this thesis, two groups of paramagnetic compounds have been studied: GdIII poly(amino carboxylates) and GdIII trapped in carbon cage compounds. For traditional monohydrated GdIII poly(amino carboxylate) complexes, the Solomon- Bloembergen-Morgan equations predict maximum proton relaxivities of 100 mM-1 s-1, instead of 4-5 mM-1 s-1 for commercial agents, when the three most important influencing factors, the rotation, the electron spin relaxation and the water exchange rate are simultaneously optimized. On the basis of structural considerations in the inner sphere of nine-coordinate, monohydrated GdIII poly(amino carboxylate) complexes, we succeeded in accelerating the water exchange by inducing steric compression around the water binding site. We modified the common DTPA5- ligand by replacing one ethylene bridge of the amine backbone by a propylene one (EPTPA5--based ligand), leading to a water exchange rate two orders of magnitude higher than that for the commercial [Gd(DTPA)(H2O)]2-. The replacement of two ethylene bridges (DPTPA5-) results in the elimination of the water molecule from the inner sphere. Although the thermodynamic stability of [Gd(EPTPA-bz-NO2)(H2O)]2- is reduced to a slight extent in comparison to [Gd(DTPA)(H2O)]2-, it is stable enough to be used in medical diagnosis. The next step to obtain high relaxivity contrast agents is the covalent coupling of GdIII EPTPA complexes to three generations (5, 7 and 9) of PAMAM dendrimers to ensure fast water exchange rate and slow rotation. These macromolecular GdIII complexes allow high relaxivities with maximum values at magnetic fields presently used in clinical applications thanks to slow molecular tumbling. The relaxivities show a strong and reversible pH dependency for all three dendrimer generations. This smart behaviour originates from the pHdependent rotational dynamics of the dendrimer skeleton. From the studies on the monomeric and dendrimeric GdIII EPTPA complexes, general considerations have been proposed about the crucial need of more rigid macromolecular assemblies to obtain the highest relaxivity values predicted by the Solomon-Bloembergen-Morgan equations. Water-soluble fullerenes and carbon nanotubes have shown a great potential for various biological and medical applications. Gadofullerenes have been proposed not only as high relaxivity contrast agents thanks to the high number of water molecules close to the paramagnetic centre and their slow tumbling arising from aggregation phenomena but also as smart pH-responsive drugs. Proton relaxivity was proposed as an ideal aggregation-state sensor for the disruption of the water-soluble Gd@C60(OH)x (x ≈ 27) and Gd@C60[C(COOH)2]10 aggregates upon salt addition. This phenomenon has important implications for biological or biomedical applications of fullerene-based materials, since real biological fluids present a rather high salt concentration. With the aim of gaining more insight into the paramagnetic relaxation mechanism induced by water-soluble gadofullerenes, an NMR analysis with the common techniques used for GdIII MRI contrast agents allowed a comparison between the aggregated and the disaggregated systems. In analogy to C60 buckyballs, ultra-short single-walled nanotubes have been loaded with Gd3+ aqua-ions clusters. These Gdn3+@US-tubes, which self-organise into bundles-like structures, exhibit relaxivities up to 90 times higher (depending on the magnetic field) than that of any GdIII-based MRI contrast agent in current clinical use.