This invention provides a method of MR investigation of a sample, the method comprising: (i) placing in a uniform magnetic field a composition comprising an OMRI contrast agent and an MR imaging agent containing nuclei (MR imaging nuclei) capable of emitting magnetic resonance signals (e.g. the primary magnetic field B0) and capable of exhibiting a T1 relaxation time of 6 s or more (at 37° C. in D2O in a field of 7T); (ii) exposing the composition to a first radiation of a frequency selected to excite electron spin transitions in the OMRI contrast agent; (iii) optionally but preferably separating the whole, substantially the whole, or a portion of said OMRI contrast agent from said MR imaging agent; (iv) administering said MR imaging agent to said sample, (v) exposing the sample to a second radiation of a frequency selected to excite nuclear spin transitions; (vi) detecting magnetic resonance signals from the sample; and (vii) optionally, generating an image or dynamic flow data from the detected signals.