Abstract: The invention provides method of electron spin resonance enhanced magnetic resonance imaging of a subject, for example a human, animal or inanimate body, in which said subject is exposed to radiation of a frequency selected to stimulate an esr transition in a paramagnetic species in said subject whereby to effect dynamic nuclear polarization, wherein said exposure to radiation is arranged such that within the region of said subject so exposed the capacity of said radiation to stimulate a said transition is relatively enhanced in at least one volume (a region of interest (ROI)), i.e. a method in which one or more regions-of-interest within the subject under study are selected by making it possible for an enhancement agent to act more effectively in that region than outside. The method for example also allows flow processes within a body to be monitored to provide the operator with images or data sets indicative of the spatial and/or temporal characteristics of flow from the ROI.