Bottom-up study of the MRI positive contrast created by the Off-Resonance Saturation sequence

Superparamagnetic iron oxide nanoparticles (SPM particles) are used in MRI to highlight regions such as tumors through negative contrast. Unfortunately, sources as air bubbles or tissues interfaces also lead to negative contrast, which complicates the images interpretation. New MRI sequences creating positive contrast in the particles surrounding, as the Off-Resonance Saturation sequence (ORS), have thus been developed. However, a theoretical study of the ORS sequence is still lacking, which hampers the optimization of this sequence. For this reason, this works provides a self-consistent analytical expression able to predict the dependence of the contrast on the sequence parameters and the SPM particles properties. This expression was validated by numerical simulations and experiments on agarose gel phantom on a 11.7T scanner system. It provides a fundamental understanding of the mechanisms leading to the contrast, allowing the sequence optimization for in vivo applications. The influences of the SPM particles relaxivities and concentration on the contrast, the echo time and the saturation pulse parameters were investigated. The best contrast was achieved with SPM particles possessing the smallest transverse relaxivity, an optimal particles concentration and for low echo times.