125has the advantage of higher resolution and no radiation but comes with issues of low
sensitivity while nuclear imaging provides higher sensitivity with a disadvantage of
radiation exposure (Table 8.2). Other techniques that have been introduced includ-
ing genetic labeling with reporter genes that can be traced with imaging probes
allowing for repeatable tracking of cellular and subcellular function over a longer
period of time.
8.4.1 Direct Labeling of Cells Using Magnetic Resonance
Agents
MRI has become a fast and comprehensive technique for the assessment of cardiac
volumes, function, and mass in HF that is accurate but also highly reproducible [ 35 ].
Tracking transplanted stem cells using this technique with refined contrast agents
offers biologic insight into homing and engraftment. Contrast agents including
micron-scale particles like iron oxide and iron fluorophore particle (IFP) ensure maxi-
mum signal with minimum labeling. These agents thus provide detection of single
cells at a resolution that can be achieved in vivo while the cells retain biologic activity
with preservation of colony-forming ability and differentiation capacity [ 36 , 37 ].
Similar results can be achieved by nanoparticles of iron oxide for non-toxic labeling
of hematopoietic bone marrow-derived and mesenchymal stem cell populations with-
out affecting their transdifferentiation capacity [ 38 ]. Direct delivery can be coupled
with cell labeling in cardiac stem cell transplantation during endomyocardial injec-
tions. These labeled transplanted cells could be imaged shortly after delivery with a
high degree of spatial resolution using MRI [ 39 ]. The lowest detectable number of
cells is around 10^5 with use of conventional MRI scanners without any sequence mod-
ification. This can be lowered using high-field magnets such that single cells contain-
ing a single iron particle can be detected and tracked [ 40 ]. The disadvantage for MRI
include inability of imaging signal to link with viability. Also, there is a possible risk
of accumulation of magnetic resonance agents after cell death into surrounding cells
causing incorrect assessment of cell trafficking. Newer direct labeling techniques like
Table 8.2 MRI versus nuclear techniques for in vivo imaging of labeled transplanted cells
Method Advantages Disadvantages
MRI No radiation with high
resolution.Low sensitivity.
Possible non-reflection of viable cells.
Nuclear (direct
labeling)High sensitivity with high
translational capacity.Radiation exposure to individual and
therapeutic cells.
Decay of radioactivity, not reflection of
viable cells.
Nuclear (reporter
genes )High biologic specificity. Limitations due to weak signal and potential
adverse effect of gene modification.8 Cardiac Imaging and Stem Cell Transplantation