Lanthanide-based nanoparticles (Ln3+-based NPs) are promising candidates as magnetic resonance imaging (MRI) contrast agents. The present thesis aims to investigate the effect of the crystalline phase of Ln3+-based NPs on their MRI contrast performance. Understanding the phase-dependent MRI contrast behaviour of Ln3+-based NPs will provide insights into the development of brighter MRI contrast agents for future in vivo biomedical applications. A set of NaGdF4 NPs (6-8 nm) in cubic and hexagonal phases in the same size range was synthesized by employing a microwave-assisted approach, allowing the influence of host crystallinity on MRI T1 relaxivity to be investigated (chapter 4). The results showed that cubic NaGdF4 NPs exhibited superior performance as MRI T1 contrast agents than their hexagonal analogues, irrespective of the chosen surface modification, e.g. small citrate groups or longer chain poly(acrylic acid). NaDyF4 NPs (3 nm) were synthesized in both phases to assess whether phase-dependent MRI contrast behaviour consistently exists in other Ln3+-base NPs of the NaLnF4 family (chapter 5). Again, it was demonstrated that cubic NaDyF4 NPs had a better contrast performance as T2 contrast agents than the hexagonal NPs. Alternatively, cubic NaEuF4 NPs, exhibiting additional optical properties (e.g. red emission under UV excitation), were prepared as potential candidates for the preparation of chemical exchange saturation transfer (CEST) contrast agents (chapter 5). Chapter 6 introduces preliminary dispersion stability studies of cubic NaGdF4 NPs dispersed in different buffer solutions, the obtained hydrodynamic diameters indicated that NaGdF4 NPs possessed better dispersity in saline than that in PBS solution.