Biology Reference
In-Depth Information
computational analysis based on the absorption and scattering properties
of the tissue allows determinations to be made of the point of origin of
the fluorescence signal and a 3D image to be reconstructed, even from tissue
several centimeters deep. However, FMT also necessitates longer image
acquisition times, and care must be taken in the interpretation of data, as
the reconstruction algorithms are not infallible. This technology is now of-
fered in commercial animal imaging systems and has allowed quantitation
and distribution of tumor-targeted fluorescent probes throughout the entire
depth of the animal. 58 FMT borrows much of its methodology from diffrac-
tion tomography and diffuse optical tomography in which a NIR light
source on the body surface delivers NIR range photons several centimeters
into the tissue (usually brain or breast) and the emitted photonic energy is
used to acquire chemical and anatomical information about the tissue. These
systems have been tested clinically for the detection of shallow tumors in
breast and other subdermal locations. 40,41 Newer detectors also allow the
extraction of information from the emitted fluorescence based on
properties other than intensity, such as FLI which yields information
about chemical processes taking place in the vicinity of the dye molecule
or probe. FLI is also desirable in some applications because the measured
signals are independent of the fluorophore concentration.
Last, the ability of a fluorescent signal to relay usable information about a
disease process will be largely determined by the targeting capability of the
probe, that is, the ability to be selectively taken up by the tissue of interest
and/or to target a specific biomolecule. The latter is no small feat for enzyme
targets, given the large number of proteins with similar or redundant func-
tions and associated enzymatic activities found in the body. For example,
roughly 600 proteases have been identified by bioinformatic analysis of
the mouse and human genome. 59 The delivery of fluorescent probes to tar-
get sites is also challenging, given the efficiency of the reticuloendothelial
system at sequestering and eliminating foreign molecules. However, as
we discuss in a later section, a number of innovative strategies have been
developed to circumvent these problems.
4. PRINCIPLES OF ENZYMATIC ACTIVITY SENSORS
BASED ON FLUORESCENCE CHANGES
As previously discussed, aberrant enzyme activity has been implicated
in a number of diseases, and therefore its detection in vivo could substantially
impact clinical outcomes and aid in research studies of the disease. Although
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