*4.5. Oxygen-enhanced Magnetic Resonance Imaging*

Oxygen-enhanced (OE) MRI is an alternative in vivo technique to quantify and map changes, distributions, and the extent of oxygen concentrations in tumors. In OE-MRI, the longitudinal relaxation rate is used to evaluate changes in the level of molecular oxygen dissolved in blood plasma or interstitial tissue. Therefore, the measured variations are proportional to the variation of oxygen concentration in the tissue. This method allows the noninvasive identification, quantification, and mapping of tumor hypoxia with MRI in vivo, making the technique suitable for rapid clinical translation [131,133]. Indeed, tumor hypoxia and oxygen dynamics have been correlated to aggressiveness and therapeutic resistance in many tumors [135].
