19F-lanthanide complexes with increased sensitivity for 19F-MRI: optimization of the MR acquisition.

Fluorine-19 magnetic resonance methods offer advantages for molecular or cellular imaging in vivo due to the absence of radioactivity, lack of naturally occurring background signal, and the ability to easily combine measurements with anatomical MRI. Previous studies have shown that (19) F-MRI sensitivity is limited to millimolar concentrations by slow longitudinal relaxation. In this study, a new class of macrocyclic fluorinated lanthanide complexes is investigated where relaxation rates are significantly shortened by proximity of the fluorine group to a paramagnetic lanthanide ion located within the same molecule. Longitudinal and transverse relaxation rates are field dependent and in the range 50-150 s(-1) and 70-200 s(-1), respectively, at 7 T. Relaxation rates in these complexes are a function of the molecular structure and are independent of concentration at biologically relevant levels, so can be used as criteria to optimize imaging acquisition. Phantom experiments at 7 T indicate a lower limit for detection by imaging of 20 μM.