The invention provides a method of magnetic resonance investigation of a sample, said method comprising: (i) reacting para-hydrogen enriched hydrogen with a hydrogenatable MR imaging agent precursor containing a non-hydrogen non-zero nuclear spin nucleus to produce a hydrogenated MR imaging agent; (ii) administering said hydrogenated MR imaging agent to said sample; (iii) exposing said sample to radiation of a frequency selected to excite nuclear spin transitions of said non-zero nuclear spin nucleus in said hydrogenated MR imaging agent; (v) detecting magnetic resonance signals of said non-zero nuclear spin nucleus from said sample; and (vi) optionally, generating an image or biological functional data or dynamic flow data from said detected signals.