Abstract
Techniques to monitor the oxygen partial pressure (pO(2)) 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 pO(2) utilizing proton ((1)H) or fluorine ((19)F) magnetic resonance spectroscopy (MRS) relaxometry. The value of the spin-lattice relaxation rate constant (R(1)) 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 pO(2) of implanted TEGs. We evaluated six potential MRS pO(2) probes and measured their oxygen and temperature sensitivities and their intrinsic R(1) 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 R(1), pO(2), 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. R(1) measurements from the TEGs were converted to pO(2) values using the established calibration equation and these in vivo pO(2) measurements were simultaneously validated with FO probes. Additionally, MRS was used to detect increased pO(2) 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 (19)F MRS. Results from this study show MRS relaxometry to be a precise, accurate, and noninvasive technique to monitor TEG pO(2) in vitro and in vivo.