Ferumoxytol is a 'smart' magnetic resonance contrast agent consisting of ultrasmall superparamagnetic particles of iron oxide (USPIOs) that are taken up by macrophages. Concentration of USPIOs within macrophages changes the tissue relaxation properties and this can be detected by magnetic resonance imaging using R2* maps. In a recent early proof-of-concept study, we demonstrated that macrophages could be detected with USPIOs in the penumbra and infarct zone of the myocardium in patients with a recent myocardial infarction. The investigators therefore wish to establish whether USPIOs can be used to identify cellular inflammation within the myocardium in three distinct clinical conditions that currently lack a non-invasive diagnostic imaging test: cardiac allograft rejection, viral myocarditis and cardiac sarcoidosis. If successful, this would not only provide a useful diagnostic test but would also be a method of monitoring disease progression or response to therapy.