Page 70 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis
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Chapter 3
relaxivity values close to the relaxivity of samples with labeled cells. When the highly aggregated extracellular iron coexist with the labeled cells in the imaging voxels, the R2' based cell quantification method fails.
Cell death
Large clusters of EC SPIO may also be produced during cell death. When dead cells do not disintegrate entirely, or the EC matrix prevents even distribution of released labeling particles (e.g. in the presence of a collagen matrix), particles of high iron content may be present in the EC space. Consequently R2' will not indicate changes in cell viability (30).
In the case of the aforementioned simplistic scenario (where dead cells spread their contents to EC evenly), though, theoretically, one can find an indication of cell death: the R2' value will decrease (owing to decreasing concentration of labeled cells, see Fig. 4, and owing to increasing concentration of extracellular iron, see Fig. 5a). On the other hand, R2 will increase if SPIO is transferred from the intracellular to the extracellular space (12). Figure 5(b) shows that the relaxivity for free SPIO is higher than that for SPIO incorporated in cells. Furthermore, R2 is sensitive to solute concentration, and the IC space is usually more concentrated. R2 will increase if cell content is transferred from the IC to the EC space (31–33). Therefore, the decrease in the R2' value and the simultaneous increase in the R2 value may well indicate on- going cell death in the labeled cell region.
Cell migration/mixing
Translocation of cells potentially leads to a mix of labeled and unlabeled cells. R2' and R2* prove to be reliable measures. Both show a linear dependence on iron concentration. If it is assumed that there is no other source for iron deposits resulting from other sources, e.g. from red blood cells, it is possible to quantify the number of labeled cells. However, a careful calibration is necessary. As Fig. 6(a) indicates, the variations in iron content per cell can lead to different relaxivities. The iron concentration cannot be determined unambiguously by relaxometry. However, with a priori knowledge of the iron content per cell the cell density can be derived from the R2' relaxation rate (see Fig. 6b).
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