Gadolinium-labeled quantum dots for molecular magnetic resonance imaging: R1 versus R2 mapping

Quantum dots labeled with paramagnetic gadolinium chelates can be applied as contrast agent for preclinical molecular MRI combined with fluorescence microscopy. Besides increasing the longitudinal relaxation rate, gadolinium-labeled quantum dots may increase the transverse relaxation rate, which might be related to their magnetic properties. Furthermore, molecular MRI experiments are primarily conducted at high magnetic fields, where longitudinal relaxation rate becomes less effective, and the use of transverse relaxation rate as a source of contrast may become attractive. Consequently, the optimal method of contrast enhancement using gadolinium-labeled quantum dots is a priori unknown. The objective of this study was to compare longitudinal relaxation rate- and transverse relaxation rate-based contrast enhancement, proton visibility, and changes thereof induced by gadolinium-labeled quantum dots targeted to the angiogenic vasculature of murine tumors, using in vivo longitudinal and transverse relaxation rate mapping. At a field strength of 7 T, longitudinal relaxation rate-based measures were superior to transverse relaxation rate-based measures in detecting both the level and spatial extent of contrast agent-induced relaxation rate changes.