Development and Validation of Noninvasive Magnetic Resonance Relaxometry for the In Vivo Assessment of...

Techniques to monitor the oxygen partial pressure (pO₂) within implanted tissue-engineered grafts (TEGs) are critically necessary for TEG development, but current methods are invasive and inaccurate. In this study, we developed an accurate and noninvasive technique to monitor TEG pO₂ utilizing proton (¹H) or fluorine (¹⁹F) magnetic resonance spectroscopy (MRS) relaxometry. The value of the spin-lattice relaxation rate constant (R₁) of some biocompatible compounds is sensitive to dissolved oxygen (and temperature), while insensitive to other external factors. Through this physical mechanism, MRS can measure the pO₂ of implanted TEGs. We evaluated six potential MRS pO₂ probes and measured their oxygen and temperature sensitivities and their intrinsic R₁ values at 16.4 T. Acellular TEGs were constructed by emulsifying porcine plasma with perfluoro-15-crown-5-ether, injecting the emulsion into a macroencapsulation device, and cross-linking the plasma with a thrombin solution. A multiparametric calibration equation containing R₁, pO₂, and temperature was empirically generated from MRS data and validated with fiber optic (FO) probes in vitro. TEGs were then implanted in a dorsal subcutaneous pocket in a murine model and evaluated with MRS up to 29 days postimplantation. R₁ measurements from the TEGs were converted to pO₂ values using the established calibration equation and these in vivo pO₂ measurements were simultaneously validated with FO probes. Additionally, MRS was used to detect increased pO₂ within implanted TEGs that received supplemental oxygen delivery. Finally, based on a comparison of our MRS data with previously reported data, ultra-high-field (16.4 T) is shown to have an advantage for measuring hypoxia with ¹⁹F MRS. Results from this study show MRS relaxometry to be a precise, accurate, and noninvasive technique to monitor TEG pO₂ in vitro and in vivo.