Barium sulfate-labeled microcapsules for X-ray cell tracking (Xcaps) in peripheral arterial disease. (A) A bar graph of the average modified Thrombolysis In Myocardial Infarction ( TIMI) frame count, as a measure of collateral vessel development, in the MSC-Xcaps, empty microcapsules, unencapsulated MSCs, and sham injection treated animals demonstrating a significant improvement in distal filling only in the peripheral arterial disease (PAD) rabbits that received microencapsulated cells (*P < 0.001 empty microcapsules vs. MSC-Xcaps; P = NS naked MSCs vs. sham injections). B-G: Representative digital subtraction angiogram (DSA, red) obtained during peak contrast opacification performed at two weeks post injection of encapsulated MSCs-Xcaps (B) and empty microcapsules (C) with an overlay of microcapsules injections (green) obtained from mask image of DSA. The small collateral vessels are somewhat obscured by the Xcap radiopacity. However, the increased collateralization can be appreciated in the MSC-Xcap-treated animal DSA (D) relative to the Xcap-treated animal (E) Native mask digital radiographs demonstrating the location of the MSC-Xcaps (F) and empty Xcaps (G) in the same animals. There was no statistically significant difference in vessel diameter between treatment groups. Reprinted with permission from Kedziorek et al..