Fig. 1.

Noninvasive MRI for simultaneously tracking and determining oxygenation of biomaterial implants. (A and B) Tracking capsules with contrast-based (Fe₃O₄) vs. label-based (¹⁹F) MRI. Alginate capsules loaded with iron oxide nanoparticles appear as local hypointensities (“cold spots,” highlighted with red arrows in A) in the anatomic background and cannot be easily distinguished from other hypointense structures of the IP space. Alginate capsules loaded with a fluorinated compound appear as hyperintensities (“hot spots” in B in null background). (C) Fluorine MRI is an imaging technique capable of distinguishing between structures of high and low oxygenation. The image is an MRI-derived oxygen map of two different groups of 1.5-mm diameter fluorinated alginate capsules. The “coldest” group has normal oxygen content and the “hottest” group has had its oxygen displaced by flushing nitrogen gas for several minutes. (D–G) The imaging flow in measuring oxygen content in alginate capsules using MRI. When the MRI coil is switched to fluorine mode, a scan that visualizes the presence of ¹⁹F nuclei can be acquired. (D) For PFC-loaded alginate capsules implanted in the IP space of mice, the scan will visualize only the capsules and their relative position. (E) To produce oxygen maps with MRI, multiple such scans are acquired while varying for each scan a parameter called inversion time, TI. (F) For each pixel in the set of images a plot of pixel intensity vs. TI value is created and the T1 relaxation times are calculated. (G) Using calibration curves (1/T₁ vs. pO₂, shown above the arrow) we are able to extract the pO₂ value corresponding to each T1 value of each pixel. (H) Combining anatomy and pO₂ maps. (H, Left) When the MRI coil is switched to hydrogen mode, an optional anatomic scan of the animal can be acquired that probes ¹H nuclei, thus providing information about the tissue in the IP space. (H, Center and Right) The anatomic scan can be combined with the pO₂ maps previously created (Center) to create an overlaid image that shows the capsule pO₂ and their position within the IP space (Right). Movie S1 shows a 3D rendering example of a combined anatomy and fluorine MRI image.