Figure 2. Quantitative $P_{{\text{O}}_2}$ imaging with spin lattice relaxation EPR.

A, chemical structure of oxygen spin probe OX063-d₂₄, which is infused into the mouse via tail vein. B, in the EPR spin-lattice relaxation signal, the relaxation rate of the spin probe is higher for high $P_{{\text{O}}_2}$ and lower for low $P_{{\text{O}}_2}$. C, tomographic reconstruction shows a 3D quantitative distribution of $P_{{\text{O}}_2}$ in the tumour in isotropic voxels of 0.67 × 0.67 × 0.67 mm³, where $P_{{\text{O}}_2}$ voxels ≤ 10 Torr show hypoxia (blue).