Quantitative imaging of iron oxide laden stem cells

When you inject stem cells for tissue repair, how many make it to the target and do they stay there?  To answer these questions, an Emory team is filling the cells with superparamagnetic iron oxide (SPIO) nanoparticles, which are the strongest known contrast agent for MR images.  A typical dose of cells is 10^5 to 10^6 cells, which can carry up to 30 picograms of SPIO iron each, making a total of up to 30 micrograms of iron.  In preliminary work, they injected 3 micrograms of nanoparticles (not in cells) into a leg of a dead rat.  Fig 1. 

Figure 3

The injection destroyed the proton signal locally making the region in the top pink sphere black.  No measurements can be made there.  However, the pink region itself gives a strong signal that changes in phase because of the SPIO.  Knowing the magnetic strength of the SPIO, they estimated without further calibration, that the black region contained 2.91micrograms of iron.  The pink sphere at 7 o’clock contains a black region too, but evidently not because of SPIO.  The same estimating procedure applied there gave minus 0.14 microgram.  The estimate for the 3rd sphere was minus 0.09 micrograms. 

Figure 2 is similar to fig 1, but the nanoparticles were loaded into human neural progenitor cells before microinjection into the spinal cord of a living pig (inside red circle).  The SPIO make their location known by the black spots.  Quantiatitve assessment eliminates black spots from other causes. 

The two methods shown by the figures are being combined for quantitative imaging in vivo.