SPIO-enhanced MRI As a Nondestructive in Vivo Method to Assess Vascularization of 3D Degrapol® Scaffolds Planted on the Chorioallantoic Membrane of the Chick Embryo in Ovo

MRI has recently been presented as a nondestructive in vivo readout to report perfusion capacity in biomaterials planted on the CAM in the living chick embryo in ovo. Perfusion capacity was assessed through changes in T1 relaxation pre- and post-injection of a paramagnetic contrast agent, Gd-DOTA (Dotarem®). Hence local contrast agent concentration was dependent on perfusion, vascular permeability, and extravascular compartment size. In the present study we, therefore, explore intravascular SPIO particles of the FeraSpin® series to deliver a more direct measure of vascularization in a 3D polymer DegraPol® scaffold. Furthermore, we present contrast enhancement upon SPIOs of different particle size, namely FeraSpin® series XS, M, XXL and Endorem® for comparison, and hence different efficiency on T1 and T2, and study respective dose-effects. No signal change was observed within the egg yolk, consistent with the SPIO remaining in the vasculature. Consequently, T1 positive signal enhancement (reduction in T1) and T2 negative contrast (reduction in T2) were observed only in the vasculature and hence were restricted mainly to the surface of the CAM at the interface to the biomaterial. Furthermore, the effect upon T2 appears stronger than in T1 with all SPIOs investigated and at blood concentrations between 0.46 mM to 4.65 mM. Comparison of different concentrations shows larger T1 enhancement at the highest dose, as expected. Vessel structures in and around the scaffold as seen in MRI were corroborated by histology. Different particle sizes show reduced T1 effect with larger particles, yet the effect on T2 was less apparent. In sum, SPIO-enhanced MRI provides measures for vascularization nondestructively in biomaterials connected to the CAM, based on intravascular contrast enhancement in T1 and T2, in ovo in the living chick embryo. Small SPIOs provide the best efficiency for that purpose, and contrast enhancement is most prominent in T2.