1354863 - Ischemic tissue pO2 mapping by 19F MR relaxometry in an in vivo murine model

Peripheral arterial disease (PAD) is characterized by varying degrees of hypoxia due to chronic progredient occlusion of peripheral arteries. While many techniques have been introduced to determine the extent of PAD, most approaches remain focused on the vascular compartment without assessing the homeostasis of the tissue affected by limited perfusion. Here, we report an in vivo approach for direct determination of deep tissue pO₂ in PAD with background-free, non-invasive fluorine magnetic resonance imaging (¹⁹F MRI) using physiologically inert perfluorocarbon nanoemulsions (PFCs). PFCs dissolve oxygen proportional to the ambient pO₂ which leads to a linear increase of the ¹⁹F relaxation rate R₁ due to the paramagnetic properties of oxygen¹. A key condition for the applicability of ¹⁹F relaxometry is the consideration of the temperature dependence of the ¹⁹F T₁ relaxation and a fast, artefact-free MRI acquisition technique.

Methods: ¹⁹F T₁ relaxation times were determined with a flow-sensitive alternating inversion recovery echo planar imaging sequence (FAIR-EPI) at 9.4 T using a 25 mm ¹⁹F quadrature resonator. Rescaling of the trajectories acquired in reference ¹H EPI scans allowed the elimination of artefacts along the phase encoding direction for the corresponding ¹⁹F measurements. A 20% perfluoro-15-crown-5 ether nanoemulsion² was used to record the relation between the relaxation rate R₁ = 1/T₁ to the surrounding pO₂ and the temperature in a three-dimensional surface-plot (Fig. A). ¹⁹F MR relaxometry was applied to assess tissue pO₂ in a murine hindlimb ischemia model (HLI)³, induced by occlusion of the femoral artery as depicted in figure B, left. For determination of the tissue pO₂, 100 µl PFCs were injected into the muscle of the upper legs just below the first site of ligation. FAIR-EPI sequence was utilized to quantify ¹⁹F T1 for the ischemic and control hindlimb one day after surgery.

Results:

Through the phantom experiments, a linear relationship was found between ¹⁹F relaxation rate R₁ to pO₂ and temperature. In the in vivo experiments, the sham hindlimb exhibited a pO₂ of 31.2±7.2 mmHg (n=9), the ischemic leg showed – as expected – a significantly decreased pO₂ of 13.0±6.9 mmHg (Fig. C).

Conclusion:

In conclusion, non-invasive ¹⁹F MR relaxometry using an optimized fast acquisition technique can reliably be used for the in vivo determination of spatially resolved pO₂ maps and the degree of tissue hypoxia in a PAD model. Beyond current perfusion techniques or superficial O₂ measurements, our approach allows assessment of deep tissue pO₂, providing a meaningful measure of the consequences of impaired perfusion that reflects the true mismatch between demand and supply of the affected tissue. Since similar PFCs have already been evaluated in clinical trials and human scanners can readily be equipped with ¹⁹F interfaces, our technology implies a high translational potential for diagnosis, personalized therapy as well as identification of new therapeutic targets in PAD.