Figure Caption: B: A microscopic view of stem cells inside X-ray visible bubbles, which protect the cells from rejection by the immune system. C-E: Using a reporter gene, one can determine whether stem cells in the bubbles are alive immediately after delivery (C). Only the X-ray-visible stem cells can be targeted under X-ray angiography at 24 hours (D) and 48 hours (E) after injection to show that the cells are still alive. (Kraitchman, Kedziorek, Fu, Walczak, Bulte, Wacker, et al.)