21nanoparticle called Gado CELLTrack™ to locate, track and quantify implanted
cells in in vivo using MRI, and thus far this is the only commercially available
nanoparticle designed for cell tracking in vivo (only in use for research, no clinical
applications at this stage) [ 21 ].
In MRI-assisted cell tracking, contrast is achieved through disturbances of the
local magnetic field experienced by surrounding hydrogen nuclei. Thus quantifica-
tion of the number of cells in vivo may be challenging. Furthermore, cell labeling
does not necessarily indicate viability of the labelled cell, for example a macrophage
can engulf a particle from a dead stem cell and lead to non-specific labeling.
Conversely, the contrast may be reduced due to division of stem cells in vivo.
Generated contrast can also easily be confounded with other contrast sources such as
bleeding or blood vessels [ 22 ]. Nevertheless, MRI is being explored as one of the
main imaging modalities in nanotechnology-assisted stem cell tracking, and the MRI
applicability will be discussed in association with each presented nanoparticle.
2.3.3 Photoacoustic Imaging: Photoacoustic Microscopy
and Photoacoustic Tomography
In photoacoustic imaging (PAI), a photoacoustic wave is generated by thermal
expansion of tissue after absorption of a short laser pulse [ 16 ]. By converting laser
into ultrasound emission, PAI combines rich optical contrast, high ultrasonic spatial
resolution (100 μm), and deep penetration depth (up to 2 cm) in a single modality,
and is becoming an alternative method to fluorescent, MRI, and radioactive imaging
for stem cell in vivo tracking [ 23 – 27 ]. It can be combined with ultrasound imaging,
and thus has great potential for stem cell therapy and tissue engineering due to non-
invasiveness, safety, selectivity, and ability to provide long-term monitoring
(Fig. 2.2). PAI has been used successfully in imaging and detecting single nanopar-
ticles in superficial tissues [ 28 – 30 ]. Plasmonic nanoparticles, including gold
Fig. 2.1 Photon microscopy. Reprinted with permission from Zagorovsky et al. [ 20 ]
2 Nanotechnology-Based Stem Cell Applications and Imaging