Example of PET and MR coregistered images. MR image (a), PET image (b), and registered PET-MR image (c) of the heart of a rat injected with FDG- and SPIO-labeled stem cells directly in the myocardium, showing the high concentration of stem cells in the myocardium. The dark regions (indicated by arrows) in the MRI image are the SPIO-labeled stem cells, whereas the bright regions (indicated by arrows) in the PET image are FDG-labeled stem cells. Images were registered using MIPAV software using landmark registration algorithm. The PET-MR registered image was used to accurately define the region of interest for quantifying the ID% of the stem cell retention.