A primer on contrast agents for magnetic resonance imaging of post-procedural and follow-up imaging of islet cell transplant

beta-islet cell transplantation is a promising proposed treatment for type 1 diabetes mellitus (T1DM) which can provide more physiological glucose control when compared to exogenous insulin therapy and reverse secondary consequences of diabetes. Unfortunately, beta-islet cell transplantation is not a permanent solution for patients with T1DM as 85-90% of beta-islet recipients return to exogenous insulin treatment within 5 years of the transplant. Despite its lack of success, beta-islet cell transplantation still has potential for the treatment of T1DM and significant work has been done to non-invasively quantify beta-islet cell concentrations in transplantation to better understand the mechanism of beta-islet cell rejection. While several imaging modalities have been utilized, magnetic resonance imaging (MRI) remains at the forefront of beta-islet cell imaging due to its high resolution and lack of radiation exposure to patients. Several MRI contrast agents have been explored for the imaging of beta-islet cells including superparamagnetic iron oxide nanoparticles (SPIONs), gadolinium-based contrast agents (GBCA), manganese (II), fluorine-19, and theranostic agents, with SPIONs being the most extensively studied. In this review, we provide an overview of the history, recent progress, and challenges of MRI in islet transplantation with a focus on the molecular imaging agents used.