Full Nuclear Magnetic Relaxation Dispersion of Superparamagnetic Nanoparticles with a Combination of Fast-Field-Cycling and Sample Shuttling Relaxometry

The optimization of superparamagnetic iron oxide (SPIOs), the most successful nanoparticle 'NP' contrast agent for in vivo imaging to date for MRI, requires the extensive characterization of water relaxation over a broad range of magnetic fields, especially at imaging fields. Nuclear Magnetic Relaxation Dispersion (NMRD) profiles, which represent the relaxation rate as a function of the magnetic field, provide important information to understand and quantify the mechanisms governing relaxation, but also constitute an original characterization tool of the particles providing, through an appropriate fitting, their radius and saturation magnetization1. Such profiles are usually obtained with the use of a fast-field-cycling relaxometer, which allows for the measurement of relaxation from the mT range up to about 1 T. However, MRI is performed at 1.5 T and above, a range that can only be accessed with sample shuttling relaxometry. The aim of this work is to study the field dependence of the water proton longitudinal relaxation rate (NMRD) of Ultrasmall-SuperParamagnetic Iron Oxide contrast agents 'USPIOs' (6 nm metallic core diameter) with a combination of fast-field-cycling relaxometer and sample shuttling relaxometry, also to illustrate the utility of using this shuttle device to access relaxation properties above 1.5 T.