Optimization of iron oxide nanoparticle detection using ultrashort echo time pulse sequences: comparison of T1, T2*, and synergistic T1- T2* contrast mechanisms.

Iron oxide nanoparticles (IONPs) are used in various MRI applications as negative contrast agents. A major challenge is to distinguish regions of signal void due to IONPs from those due to low signal tissues or susceptibility artifacts. To overcome this limitation, several positive contrast strategies have been proposed. Relying on IONP T(1) shortening effects to generate positive contrast is a particularly appealing strategy because it should provide additional specificity when associated with the usual negative contrast from effective transverse relaxation time (T(2)*) effects. In this article, ultrashort echo time imaging is shown to be a powerful technique which can take full advantage of both contrast mechanisms. Methods of comparing T(1) and T(2)* contrast efficiency are described and general rules that allow optimizing IONP detection sensitivity are derived. Contrary to conventional wisdom, optimizing T(1) contrast is often a good strategy for imaging IONPs. Under certain conditions, subtraction of a later echo signal from the ultrashort echo time signal not only improves IONP specificity by providing long T(2)* background suppression but also increases detection sensitivity, as it enables a synergistic combination of usually antagonist T(1) and T(2)* contrasts. In vitro experiments support our theory, and a molecular imaging application is demonstrated using tumor-targeted IONPs in vivo.