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FIGURE 19.9 (See color insert.)
Correlation of MRM and iron staining of MPIO in a POD 94 allograft: Image from MRM shows
the discrete and circular spots of hypointensity (A). These dark spots of hypointensity are due
to the presence of MPIO particles, which was confirmed by the matching histological Perl's
Prussian blue sections for iron (B, ×40 magnification) that correspond to the same area as the
boxed region in MRM image (A). (C) The expansion of the boxed region in (B) (×200 magnifi-
cation). (Reprinted from Figure 3 of Ye, Q., Y.L. Wu, L.M. Foley, T.K. Hitchens, D.F. Eytan, H.
Shirwan, and C. Ho. 2008. Circulation 118, no. 2: 149-56. With permission.)
MPIO particles, they are stable once ingested by allowing macrophages, for
long-term longitudinal imaging studies following a single administration of
MPIO (Ye et al . 2008).
Because of the enormous amount of superparamagnetic iron packed into
one MPIO particle, signal attenuation across a distance larger than one imag-
ing pixel can be observed. Thus, single MPIO-labeled cells can be detected
with in vivo MRI even with an imaging resolution larger than one cell. This
enables in vivo imaging of sparse cell infiltration. In the early inflammation
stage, very sparse macrophage infiltration in both allograft and isograft trans-
plants can be detected in vivo with MPIO labeling ( Figure  19.10 ). Similarly,
sparse infiltration of macrophages in the brain after traumatic brain injury
can be imaged in vivo by MPIO labeling (Foley et al . 2009). Each individual
hypointensity represents a single macrophage, and the number of mac-
rophages infiltrated can be quantified by directly counting of the number of
hypointense or dark spots.
19.6 Beyond Mere Localization: Combining
Cellular and Functional MRI
In addition to tracking cells in vivo , MRI, with good soft tissue contrast and
penetration, has long been used for in vivo functional assessment. Renal per-
fusion (Sun et al . 2003; Wang et al . 1998; Yang et al . 2001), diffusion-weighted
MRI for renal function (Yang et al . 2004), cerebral perfusion with arterial spin
labeling (Foley et al . 2005, 2008), and cardiac regional wall motion with MRI
tagging and strain analysis (Wu et al . 2009) can be evaluated simultaneously
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