{HEAD ~ Active groups} are {TAIL ~ any} of various pharmacologically or therapeutically active moieties, including moieties useful for magnetic resonance contrast enhancement 
{ HEAD ~ Active groups } are { TAIL ~ any } of various pharmacologically or therapeutically active moieties , including moieties useful for magnetic resonance contrast enhancement
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH