20
2.3 Tracking of Nanoparticles In Vivo
The common techniques of imaging of stem cells in vivo have been covered else-
where in this book. In general, it can be said that the majority of the employed
contrast agents often present disadvantages like photo-bleaching over time, interfer-
ence derived from tissue autofluorescence, chemical and/or metabolic degradation
in vivo, and even low transfection efficiency in primary cells and thus are consid-
ered suboptimal for in vivo imaging [ 16 ]. To overcome this limitation, several engi-
neered nanoparticles with unique magnetic and/or optical properties have been
developed and employed in biomedicine, due to their capability to offer real-time
methods of tracking intracellular processes at a biomolecular level [ 17 – 19 ].
However, it is important to underline that almost no nanoparticles have been used
for stem cell tracking in human patients yet, and thus the bulk of research described
is experimental. As particles and technologies develop, so do imaging techniques.
We foresee that not only will nanoparticles and stem cell medicine evolve, but imag-
ing technology will also grow to accommodate modern molecular imaging into rou-
tine clinical modalities.
The modalities that can currently be utilized for tracking of nanoparticles and
conjugated stem cells in vivo are:
2.3.1 Fluorescent Imaging (Light/Confocal/Two-Photon
Microscopy)
Several of the nanoparticles can be detected using microscopic optical imaging
techniques. Optical imaging is often accessible, of low cost, of high spatial and
temporal sensitivity, but lacking in deep tissue penetration. One-, two- and three-
photon microscopy has been experimented in imaging for example tumors close to
the skin (Fig. 2.1). While currently the penetration depth has a maximum of ~3 mm,
scientists are hopeful that it is possible to achieve greater imaging depth by manipu-
lating the fluorescence processes and instrumental set-up [ 20 ]. The development of
super-resolved fluorescence microscopy by E. Betzig, W.E. Moerner and S. Hell
was awarded with the 2014 Nobel Prize.
2.3.2 Magnetic Resonance Imaging (MRI)
Although there are no concerns regarding penetration depth or invasiveness using
MRI, the resolution is often insufficient at a molecular and cellular scale, unless
contrast agents are employed [ 16 ]. Contrast agents usually have a very short life-
span in the body. However, when the contrast agent is incorporated into the cell as
a nanoparticle, it greatly enhances the lifespan and allows for long-term tracking
capability. A company called BioPAL has developed a gadolinium colloid
H.A. Jensen et al.