Personalized diagnosis and treatment with allogenic or autologous cells are becoming a reality in the field of medicine (1, 2). Cytotoxic or engineered T cells are under clinical trial for the treatment of hematopoietic or other malignant diseases (3). Contrast agent–tagged macrophages are used as cellular probes to image the early inflammatory processes in macrophage-rich conditions such as inflammation, atherosclerosis, and acute cardiac graft rejection (1, 4). The roles of stem cells are under intensive investigation in therapeutic and regenerative medicine such as regenerating cardiomyocytes, neurons, bone, and cartilage (2, 5). Genetically modified cells are used to treat genetic disorders (6). With the promising results from these studies, a critical issue is how to monitor the temporal and spatial migration and homing of these cells, as well as the engraftment efficiency and functional capability of the transplanted cells in vivo (7-9). Histopathological techniques have only been used to obtain the information on the fate of implanted cells at the time of animal euthanization or via biopsy or surgery. To understand the temporal changes of cell location, viability, and functional status, cell imaging techniques have been introduced during the last few years. Cells of interest are labeled with reporter genes, fluorescent dyes, or other contrast agents that transform the tagged cells into cellular probes or imaging agents (10). There are three fundamentally different routes for labeling cells of interest (7, 8). One route is to label the cells through systemic contrast agent application, as seen in the systemic use of superparamagnetic iron oxides (SPIO), subsequent phagocytosis of the SPIO by macrophages, and accumulation in macrophage-rich lesions. The second route is to label the cells in situ by injecting contrast agents into the tissue area of interest to monitor target cell migration after phagocytosis. The more widely used route is to label the cells in vitro, which is achieved by in vitro incorporation of contrast agents or by transfecting one or more reporter genes into cells. Each labeling method has its own limitations.
In vivo optical cell imaging is a rapidly developing field in small animal imaging that depends on the use of reporter genes and fluorescent dyes (9). The Reporter gene–based approach is crucial for molecular imaging, but it strongly depends on the stable, persistent, and long-term expression of desired proteins. Long-term expression of the reporter genes may lead to host immune response and may carry on to the daughter cells in the proliferating population. The fluorescence-based approach is simple, cost-effective, and relatively sensitive, but issues of tissue-to-detector geometry, auto-fluorescence, and tissue absorption and scattering remain to be solved. Accurate quantification may only be possible when measurements are properly controlled and signals are normalized. Organic dyes are also less valuable for long-term cell tracking strategies. Nevertheless, the fluorescence-based approach is easily used for a variety of straightforward short-term labeling applications in cell imaging. In an attempt to noninvasively trace and monitor macrophages for better localization, visualization, and quantification of inflammation processes, Eisenblatter et al. developed a protocol for rapid and safe macrophage labeling with near-infrared fluorescent dye, and the investigators further tested the feasibility to image inflammation in a mouse granuloma inflammation model (1). Imaging results showed that the tagging of macrophages with the lipophilic tracer 1,1-dioctadecyl-3,3,3,3-tetramethylindotricarbocyanine iodide (DiR) allowed the noninvasive tracking of inflammatory cells for several days in vivo. DiR has an excitation spectrum of 750 nm and an emission spectrum of 782 nm.