Rationale and Objectives. Concurrent visualization of differential targets in cellular and molecular imaging is valuable for resolving processes spatially and temporally, as in monitoring different cell subtypes. The purpose of this study was to demonstrate concurrent, dual (positive and negative) contrast visualization on magnetic resonance imaging (MRI) of two colocalized cell populations labeled with Gadolinium “Gd” oxide and iron “Fe” oxide nanoparticles. Materials and Methods. Human aortic endothelial cells (EC) and smooth muscle cells (SMC) were labeled with various concentrations of Gd oxide and Fe oxide, respectively. MRI on single- or mixed-cell samples was performed at 7 tesla. Proper cell phenotype expressions, cell uptake of contrast agents, and the effect of labeling on cell viability and proliferation were also determined. Results. Both contrast agents were efficiently taken up by cells, with viability and proliferation largely unaffected. On MRI, the positive contrast associated with Gd oxide-labeled EC and negative contrast associated with Fe oxide-labeled SMC discriminated the presence of each cell type, whether it existed alone or colocalized in a mixed-cell sample. Conclusion. It is feasible to use Gd oxide and Fe oxide for dual contrast and concurrent discrimination of two colocalized cell populations on MRI at 7 tesla. 1. Introduction Contrast agents play a critical role in cellular and molecular magnetic resonance imaging (MRI), as they enable sensitive and clear visualization of cellular and physiological processes. The different contrast agents available for labeling cells share in common their ability to enhance signal contrast and have facilitated cell-tracking studies in stem cell transplantation, neurodegenerative disorders and stroke, atherosclerosis, and cancer research [1–4]. Superparamagnetic iron oxide nanoparticles are by far the most common, owing to their strong negative -weighted contrast, and have been widely used for tracking different cell types in various organs, including lymphocytes, progenitor cells, and embryonic cells [5–9]. Less common are paramagnetic gadolinium (Gd-) based agents, which exploit positive -weighted contrast changes. These have been used in their chelated form for tracking stem cells in animal models of hemorrhagic transformation [10] and angiogenesis [11] and, more recently, in a nanoparticle formulation for improved cell uptake and retention [12]. However, in virtually all of the reported cellular MRI applications, only one cell population could be labeled and tracked. The ability to track
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