  Solids and rigid tissues such as bone, ligaments, and tendons, typically
appear dark in magnetic resonance imaging (MRI), which is due to the extremely
short-lived proton nuclear magnetic resonance (NMR) signals. This short
lifetime is due to strong dipolar interactions between immobilized proton
spins, which render it challenging to detect these signals with sufficient
resolution and sensitivity. Here we show the possibility of exciting long-lived
signals in cortical bone tissue with a signature consistent with that of bound
water signals. Contrary to long-standing belief, it is further shown that
dipolar coupling networks are an integral requirement for the excitation of
these long-lived signals. The use of these signals could enhance the ability to
visualize rigid tissues and solid samples with high sensitivity, resolution,
and specificity via MRI.
