Tracking neuroprogenitor cells (NPCs) that are used to target tumors, infarction or inflammation, is paramount for cell-based therapy. We employed ultrasound imaging that can detect a single microbubble because it can distinguish its unique signal from those of surrounding tissues. NPCs efficiently internalized positively charged microbubbles allowing a clinical ultrasound system to detect a single cell at 7 MHz. When injected intravenously, labeled NPCs traversed the lungs to be imaged in the left ventricle and the liver where they accumulated. Internalized microbubbles were not only less sensitive to destruction by ultrasound, but remained visible in vivo for days as compared to minutes when given free. The extended longevity provides ample time to allow cells to reach their intended target. We were also able to transfect NPCs in vitro when microbubbles were preloaded with GFP plasmid only when cells were insonated. Transfection efficiency and cell viability were both greater than 90%.
Iron oxide nanoparticles (IONPs) are widely used as MR contrast agents because of their strong magnetic properties and broad range of applications. The contrast induced by IONPs typically depends on concentration, water accessibility, particle size, and heterogeneity of IONP distribution within the microenvironment. Although the latter could be a tool to assess local physiological effects at the molecular level, it renders IONP quantification from relaxation measurements challenging. This study aims to quantify IONP concentration using susceptibility measurements. In addition, further analysis of relaxation data is proposed to extract quantitative information about the IONP spatial distribution.
Mesenchymal stem cells were labeled with IONPs and the IONP concentration measured by mass spectroscopy. MR relaxation parameters (T1, T2, T2*) as well as magnetic susceptibility of cylindrical samples containing serial dilutions of mixtures of free and cell-internalized IONPs were measured and correlated with IONP concentration.
Unlike relaxation data, magnetic susceptibility was independent of whether IONPs were free or internalized, making it an excellent candidate for IONP quantification. Using IONP concentration derived from mass spectroscopy and measured relaxation times, free and internalized IONP fractions were accurately calculated.
Magnetic susceptibility was shown to be a robust technique to measure IONP concentration in this preliminary study. Novel imaging based susceptibility mapping techniques could prove to be valuable tools to quantify IONP concentration directly by MRI, for samples of arbitrarily shape. Combined with relaxation time mapping techniques, especially T2 and T2*, this could be an efficient way to measure both IONP concentration and the internalized IONP fraction in vivo using MRI, to gain insight into tissue function and molecular imaging paradigms.
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