In vivo imaging of graftversus-host disease and graftversus-leukemia - Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation - page 75

the laser is replaced with radio waves or microwaves. The main advantage
of these technologies is that they break through the barrier of optical imag-
ing which is one transport mean free path (TMFP), which is typically 1 mm
in tissue [118] . With linear scanning microwave-induced thermoacous-
tic tomography, however, it was possible to achieve tissue penetration of
1.2 cm in muscle and 9 cm in fat [119] .

Multiphoton in vivo flow cytometry

In multiphoton in vivo flow cytometry, cells of interest are fluorescently
labeled and injected i.v. into anesthetized mice. A small vessel of approxi-
mately 40-100 μm in diameter (typically a vessel in a mouse ear) is immo-
bilized and imaged under a microscope. The vessel is then excited by a two
photon femtosecond laser and fluorescence is detected through the micro-
scope. This information is then used to determine the frequency by which
the cell of interest passes through the microscope's objective. This technol-
ogy has already been applied to in vivo enumeration of circulating tumor
cells in several studies (reviewed in Tkaczyk et al. [120] ).

Conclusion

76

There is no doubt that in vivo imaging has provided novel insights into the
complex mechanisms involved in GVHD and GVL reactions. Optical meth-
ods such as bioluminescence and in vivo fluorescence have gained great
popularity due to their simplicity, convenience, ability to use multiple
reporters and relative inexpensiveness. However, these methods have lim-
ited tissue penetrating ability and spatial resolution and are not applicable
to the clinic. While MRI is best for resolution and PET/SPECT for sensitiv-
ity and tissue penetration, these both require expensive equipment. Thus
a combination of BLI or fluorescence, coupled with CT X-ray for anatomi-
cal location, might provide the best solution. Hopefully, newer technolo-
gies such as photoacoustic imaging will also find more widespread use and
become accessible to more researchers.

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