Ultra-short single-walled carbon nanotubes (US-tubes) have been previously shown to be efficient carriers of imaging agents. In particular, gadonanotubes (GNTs) synthesized by loading and nanoscale confinement of Gd3+ ions within US-tubes have been established as high-performance MRI contrast agents (CAs) with efficiencies 40 to 90 times greater than the current clinical CAs. Using nuclear magnetic resonance dispersion (NMRD) and electron spin resonance (ESR) techniques, this work discusses the origin of the magnetic and proton relaxation behavior in MRI of the GNTs and related structures at low magnetic fields. The likely causes for the observed paramagnetism for these materials are explored and their effect on water proton relaxation is discussed. In addition, Gd3+ chelates, which are currently approved for clinical MRI use, provide relaxivities (or contrast enhancement) well below their theoretical limit, and they also lack tissue specificity. In this dissertation, using vascularly injectable mesoporous silicon nanoparticles (SiMPs), general methods for increasing the efficiency of Gd3+-based MRI CAs are described. Two different strategies have been successfully tested where Gd3+ chelates are either geometrically confined within the pores of SiMPs or covalently attached to the surface of SiMPs. For both the approaches, SiMPs with different pore sizes have been used to generate a dominant role in the resulting relaxivity. The nanoconstructs designed using these approaches have been shown to produce relaxivities that are many-fold greater than the free CAs in solution. This enhancement is attributed to the optimization of the molecular parameters that govern relaxivity. Co-loading the pores with a Gd3+-based CA and a fluorescently-labeled antibody has shown the potential of SiMP nanoconstructs as multimodal agents. The strategies outlined in this dissertation are general and can be successfully applied to any imaging agent and porous nanosystem. In summary, this work highlights two key outcomes. First, it provides a better understanding of the magnetic and MRI behavior of the GNTs. Second, it demonstrates that geometrical confinement of CAs and covalent functionalization of nanoparticles are universal strategies for enhancing the performance of Gd3+-based CAs.