BACKGROUND: Numerous studies have shown that magnetic resonance imaging (MRI) can detect survival and migration of super paramagnetic iron oxide-labeled stem cells in models of focal cerebral infarction. OBJECTIVE: To observe distribution of bone marrow mesenchymal stem cells (BMSCs) in a rat model of global brain ischemia following cardiac arrest and resuscitation, and to investigate the feasibility of tracing iron oxide-labeled BMSCs using non-invasive MRI. DESIGN, TIME AND SETTING: The randomized, controlled, molecular imaging study was performed at the Linbaixin Medical Research Center, Second Affiliated Hospital, Sun Yat-sen University, and the Institute of Cardiopulmonary Cerebral Resuscitation, Sun Yat-sen University, China from October 2006 to February 2009.MATERIALS: A total of 40 clean, Sprague Dawley rats, aged 6 weeks and of either gender, were supplied by the Experimental Animal Center, Sun Yat-sen University, China, for isolation of BMSCs. Feridex (iron oxide), Gyroscan Inetra 1.5T MRI system, and cardiopulmonary resuscitation device were used in this study. METHODS: A total of 30 healthy, male Sprague Dawley rats, aged 6 months, were used to induce ventricular fibrillation using alternating current. After 8 minutes, the rats underwent 6-minute chest compression and mechanical ventilation, followed by electric defibrillation, to establish rat models of global brain ischemia due to cardiac arrest and resuscitation. A total of 24 successful models were randomly assigned to Feridex-labeled and non-labeled groups (n=12 for each group). At 2 hours after resuscitation, 5 x 10 6 Feddex-labeled BMSCs, with protamine sulfate as a carrier, and 5 × 10 6 non-labeled BMSCs were respectively transplanted into both groups of rats through the right carotid artery (cells were harvested in 1 mL phosphate buffered saline). MAIN OUTCOME MEASURES: Feridex-labeled BMSCs were observed by Prussian blue staining and electron microscopy. Signal intensity, celluar viability, and proliferative capacity of BMSCs were measured using MRI, Trypan blue test, and MTT assay, respectively. Distribution of transplanted cells was observed in rats utilizing MRI and Prussian blue staining prior to and 1, 3, 7, and 14 days after transplantation. RESULTS: Prussian blue staining displayed many blue granules in the Feridex-labeled BMSCs. High density of iron granules was observed in the cytoplasm under electron microscopy. According to MRI results, and compared with the non-labeled group, the signal intensity was decreased in the Feridex-labeled group (P＜0.05). The decrease was most significant in the 50 μg/mL Feridex-labeled group (P＜0.01). There were no significant differences in celluar viability and proliferation of BMSCs between the Feridex-labeled and non-labeled groups after 1 week (P＞0.05). Low-signal lesions were detected in the rat hippocampus and temporal cortex at 3 days after transplantation. The low-signal lesions were still detectable at 14 days, and positively stained cells were observed in the hippocampus and temporal cortex using Prussian blue staining. There were no significant differences in signal intensity in the non-labeled group. CONCLUSION: BMSC transplantation traversed the blood-brain barrier and distributed into vulnerable zones in a rat model of cardiac arrest-induced global brain ischemia. MRI provided a non-invasive method to in vivo dynamically and spatially trace Feridex-labeled BMSCs after transplantation.