DOI: 10.1002/adma. 201502784 the brain to exert their therapeutic effects. However, many animals are needed to monitor transplant dynamics of a graft over a long period of time due to restricted one specific time point. In this condition, for clinical use, it is critical to develop multimodal and noninvasive in vivo imaging approaches to track stem cells, monitor their homing, migration, and proliferation, assess therapeutic response, and confirm the transplantation parameters such as optimal time window and route for optimizing cell delivery of the treatment of stroke and maximizing the therapeutic benefit.

Recently, labeling stem cells with superparamagnetic iron oxide (SPIO) particles has been used to provide a noninvasive method for tracking the fate of the administered stem cells in vivo by magnetic resonance imaging (MRI),[14–17] but the contrast generated by SPIO-labeled cells is not specific due to ambiguous background from negative image contrast artifacts, eg, hemorrhage, air, and microvascular obstruction in the infarcted brain area. In contrast, photoacoustic imaging (PAI) can overcome the intrinsic limits of current imaging techniques by converting the absorbed light to an outgoing