This invention addresses the clinical problem of how to optimize biological cell based therapies, such as stem cell therapy. Currently, cell therapies administered by intravenous, intra-arterial, and/or direct tissue injection are limited by the lack of clinically available imaging methods to detect the in vivo fate of the administered cells. There are many efforts underway to develop imaging strategies for stem cells in vivo, including radionuclide and MRI-based approaches. However, these approaches are limited by potential safety issues (e.g. radioactive exposure of stem cells, toxicity of iron particles used for MRI) and difficulty in serial tracking due to complex instrumentation and/or the requirement for repetitive radiation exposure.