Page 64 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Chapter 3
with the free suspensions, SPIOc particles exhibited dramatically increased relaxivity when incorporated into cells: r2* = 412 mM-1 s-1 for the cell dilution series and r2*= 758 mM-1 s-1 for the cell division series (Fig. 3).
Figure 4 shows the R2' relaxation rates for the different samples. Free SPIO and SPIOc dilutions exhibited zero r2' relaxivity. SPIOc particles incorporated in cells and MPIO particles yielded nonzero r2' relaxivity. The cell dilution and the MPIO dilution series resulted in very close R2' values when Fe concentrations were in the same range (0.1–0.3 mM, see fitted linear curves, Fig. 4). The estimated values of the iron content per particle for the MPIOs and for the labeled cells in cell dilution series were 1.1 and 1.4 pg respectively (based on optical emission spectrometry for labeled cells and manufacturer data for MPIO particles). Figure 5(a) depicts the relative changes in the R2, R2* and R2' values, where the density of labeled cells was kept constant with increasing concentration of extracellular SPIO complex. For the coexistence of extracellular and intracellular SPIO, the relative change in R2* value with increasing amount of extracellular SPIO was negligible. Although the total iron content was almost doubled at a fixed number of labeled cells, R2* remained constant within error. In contrast, R2 value showed sensitivity to total iron content (relative relaxivity was 0.92). R2' was less sensitive to the change in the extracellular SPIO concentration (relative relaxivity was - 0.29); it decreased with increasing extracellular iron content. Figure 5(b) shows the R2 relaxation rate dependence on iron concentration for three different compartmentalization models: dilution series of free SPIO complexes; dilution series of cells labeled with SPIO; and cell division series with SPIO labeled cells. The r2 relaxivity was enhanced for free SPIO complexes (r2 = 329 mM-1 s-1) compared with incorporated SPIO complexes (r2 = 169 mM-1 s-1 for cell division and r2 = 148 mM-1 s-1 for cell dilution).
Distribution of intracellular SPIO complexes
The R2* and R2' values at the same iron concentrations (also the r2* and r2'values) were different for the cell dilution and for the cell division series (see Figs 3 and 4.). Although the cell types, the labeling particles and the cell densities were identical for these series, the relaxivities were clearly different (r2*, 758 vs 412 mM-1 s-1; r2', 586 vs 314 mM-1 s-1).
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