T1 reactivity can be used as a marker for myocardial perfusion reserve in the setting of ischemia or hypoperfusion. We hypothesize that ferumoxytol, as a pure intravascular agent with high r1 relaxivity, sensitizes T1 reactivity for assessment of myocardial perfusion. We selectively induced acute myocardial hypoperfusion in twelve healthy male Yorkshire swine. We then performed native and ferumoxytol-enhanced adenosine stress testing with the MOLLI sequence at 3.0T. Ferumoxytol increased absolute T1 reactivity in remote regions by 4.62-fold. The normalized difference in T1 was 4.5-fold greater in FE images compared to native T1.