Fluorescent and magnetic resonance (MR) images revealing the biodistribution of B cells in mice. (a) Fluorescent image of a mouse that has been injected with B cells that have been labeled with ultrasmall superparamagnetic iron oxide (USPIO) and the fluorescent dye CellVue® NIR815. The fluorescent image clearly shows the localization of B cells within the spleen. (b) MR images of the spleen (white arrow) before and after the injection of the USPIO-loaded B cells. The migration of USPIO-loaded B cells to the spleen results in a loss in signal intensity on T2-weighted images.