High-Field MRI Contrast Agents and their Synergy with Optical Imaging: the Evolution from Single Molecule Probes towards Nano-architectures

Contrast agents for magnetic resonance imaging have historically been based on paramagnetic metal complexes, particularly Gd³⁺ chelates, which tend to lose their contrast enhancement ability with increasing magnetic field strength. Emerging high-field MRI applications require the development of novel contrast agents that exhibit high relaxation enhancement as a function of magnetic field strength. Paramagnetic ions such as Dy³⁺, Tb³⁺ or Ho³⁺ incorporated into supramolecular or inorganic nano-architectures represent promising platforms for the development of high field MRI contrast agents. Furthermore, such platforms allow facile inclusion of multiple imaging modalities, therapeutic loading, and targeting vectors. This Minireview examines the application of contrast agents for high-field MRI, which range from single molecules to nanoparticles. Approaches to create multimodal agents by combining high-field MRI contrast properties with another imaging modality are also discussed.