12
in combination with oxygen, magnesium and ATP results in a reddish light
(500–700 nm).
On the other hand, fluorescent techniques depend on incorporating an inor-
ganic substrate into the cells that have fluorescence properties that is they emit
light when excited by external light or wavelengths. These substrates can be
organic, like green fluorescence protein or small molecule polymethines or inor-
ganic semiconductor nanocrystals (quantum dots) [ 24 ].
Optical bioluminescence imaging has been successfully used in molecular
imaging of stem cells [ 2 , 25 ] (Fig. 1.3). However there are certain limitations of
this technique. Firstly, only visible light is generated by the luciferase (400–
700 nm wavelength). Also, the light is produced in small quantities requiring an
ultra- sensitive charge coupled device (CCD) camera to generate images. But
more importantly, optical imaging has limited tissue penetration (around 2 mm)
restricting the use of the technique to superficial tissues and small animals like
mice and rat [ 26 ]. The technique is also a planar imaging with no limited depth
perception and resolution (4–10 cm). There is also signal loss due to subsequent
cell divisions and possible stem cell uptake by macrophages after stem cell death
[ 27 ].- Radionuclide Imaging: Use of radionuclides for labeling and imaging of stem
cells also employs several strategies. Direct labeling techniques similar to
SPIOs can be used where the radiotracer is introduced into the stem cells prior
to transplantation. The radionuclides emit gamma radiation that is imaged using
a gamma camera and/or SPECT. Common radionuclides are Tc99m (Half life
6 h) and In111 (Half life 2.8 days). Some radionuclides (like F-18-Fluoro-
Fig. 1.3 Antibody-stained fluorescent images after RF ablation (magnification 20, zoom ×0.7)
show stem cell uptake at coagulation margin with fluorescent stem cells (arrows), coagulation area
(A) and the more peripheral hepatic parenchyma not subjected to substantial changes from RF
ablation heating (C). Focal blue areas of fluorescence represent 4′,6-diamidino-2-phenylindole
stain of nuclei as anatomic markers and are unrelated to stem cell labeling. (Courtesy, Nikolic B
et al. The effect of hepatic radiofrequency ablation on stem cell trafficking in the rat model. J Vasc
Interv Radiol. 2009 May;20(5):640–7)
T. Pa ndey