T1 mapping is a promising technique to indirectly assess myocardial blood volume changes during vasodilator-induced stress, but native T1 reactivity (defined as the percent change in T1 between rest and vasodilator-induced stress) has a small amplitude and narrow dynamic range. Ferumoxytol, as an intravascular magnetic resonance imaging contrast agent has potential to enhance the amplitude and dynamic range of myocardial T1 reactivity. We aim to compare vasodilator-induced variations in native relative to ferumoxytol-enhanced (FE) myocardial T1 reactivity and to assess the FE T1 reactivity in normal, remote, and ischemic myocardium. We found that vasodilator stress testing with FE T1 reactivity shows early promise in swine models of inducible hypoperfusion and is feasible in patients with IHD.