Abstract
Traditional diagnosis relies on identifying anatomical abnormality, which offers a stage for various anatomical imaging techniques, such as X-ray computed tomography (CT), ultrasonic imaging, and magnetic resonance imaging (MRI). The good capacity of providing anatomical details, especially for soft tissues, popularizes the clinical use of MRI. However, as the understanding of various diseases reaches the molecular level, it is gradually accepted that molecular anomaly often precedes anatomical abnormality. Therefore, molecular imaging, which is aimed at gathering various molecular information in organisms via imaging, starts to gain momentum. Unfortunately, traditional MRI is not capable of molecular imaging. As a result, there is an urgent demand for probes that enable MRI to "see" molecules. A promising design strategy for these probes is to elicit a signal change triggered by the presence of molecular targets, i.e. activation. Benefiting from the rapid development of nanotechnology, a number of nanoparticle-based activatable MRI probes have been developed for molecular imaging. This review summarizes recent advances of activatable MRI nanoprobes for imaging pathological characteristics of cancer, inflammation, and neurodegenerative diseases, with a focus on the design strategies and applications of these probes. In addition, the prospects and challenges of activatable MRI nanoprobes are also discussed.