There are several common misconceptions about pulmonary oxygen enhanced MRI. Most importantly, the relaxivity of molecular oxygen is, like other contrast agents, macromolecule dependent and not static. Moreover, the signal from the lung results from both oxygenated and non-oxygenated blood, where the oxygen enhancement effect will work to both shorten and elongate blood T1. The resultant relaxation enhancement primarily reflect blood partitioning. Finally, oxygen enhanced MRI has been shown to correlate with diffusing capacity in the lung, but this is only in severe disease. Rather, pulmonary shunt is the main determinant of arterial oxygenation and will affect the relaxation enhancement.