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FIGURE 10.18
In vivo and ex vivo MRI of a mouse brain from an animal injected with SPIO-labeled mac-
rophages. (A) In vivo MRI showing three signal voids (100 × 100 µm 2 in plane, 200-µm slice
thickness). (B) Ex vivo MRI at the same resolution and slice location as (A) showing the same
three signal voids. (C), (D) High-resolution ex vivo MRI (100 × 100 µm 2 in plane, 100-µm slice
thickness). The three signal voids in (A) and (B) now appear in two different MR slices. cc,
corpus callosum; ac, anterior commissure; CPu, caudate putamen. (From Heyn, C., J.A. Ronald,
L.T. Mackenzie, I.C. Macdonald, A.F. Chambers, B.K. Rutt, and P.J. Foster. 2006. Magn Reson Med
55, no. 1: 23-29.)
macrophages embedded in gelatin, allowing for optical validation of
single-cell detection (Dodd et al. 1999; Foster-Gareau et al. 2003). In a com-
mon hepatocyte transplantation model, single cells labeled with 1.63-µm
MPIO particles were detected at 7 T in the livers of mice following injection
into the spleens. Careful validation using confocal microscopy confirmed
that single SPIO-labeled macrophages (~50 pg Fe/cell) were detected at 1.5
T in the brains of mice following intracardiac injection (Figure 10.18) (Heyn
et al. 2006a).
10.10 Limitations of Iron Oxide-Based Cellular MRI
There are certain limitations to the use of iron oxide-based reagents that must
be taken into account when analyzing MRI image data. Contrast agents are
diluted or lost when cells divide or die; as such, the time frame for imaging
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