Biomedical Engineering Reference
In-Depth Information
2.3 Biomedical applications of emission tomography
depend on tracers
As already mentioned above, biomedical applications of emission tomog-
raphy use the injection of labeled molecules to trace a functional aspect or
molecular target and address specific scientific questions. In general, all of
these applications are grouped into \molecular imaging," which is dened
quite differently in the context of basic sciences vs. clinical diagnostics or nat-
ural sciences vs. medicine. In the literature, multiple definitions can be found;
for example, \Molecular imaging can be dened as the in vivo characterization
and measurement of biological processes at the cellular and molecular level...",
\...implies the convergence of multiple image-capture techniques, basic
cell/molecular biology, chemistry, medicine, pharmacology, medical physics,
biomathematics, and bioinformatics into a new imaging paradigm." [12, 29].
These definitions point to the important aspect of interdisciplinary team
work between biology, chemistry, mathematics, physics, computer sciences and
medicine to successfully develop and apply innovative molecular imaging tech-
nologies. One of these aspects, the generation and optimization of images from
emission raw data is within the focus of this topic. Signals that can be recon-
structed from emission data into image data stem from and depend on tracers,
and their chemical development and pharmaceutical validation is an important
step in molecular imaging. These tracers typically consist of two components:
(a) a drug (pharmacophor), which either follows a functional principle
and therefore traces it or targets specific molecular signatures (molecu-
lar targets). This molecule or drug compound of the tracer decides on
its biodistribution. For example, a tracer for measuring tissue perfusion
is typically based on a compound that is taken up by cells at a high first
pass rate and trapped in the cell. The resulting emitted imaging signal
therefore is quantitatively related to the degree of tissue perfusion. A
classical example for molecular imaging is the quantification of the den-
sity of a receptor on the cell surface. This approach is typically realized
by using receptor-ane molecules/drugs, such as a receptor antagonist,
which specifically bind to a receptor or receptor family.
(b) a flag (label), which is tightly bound to the drug component and is
emitting signals. This flag can consist of radioactive isotopes such as
positron emitters or gamma emitters allowing deep tissue penetration
with a fair resolution or a fluorescent probe having a limited application
for deep tissues but allowing micro- to nanoscopic resolution.
Upon injection of the drug-flag complex the fate of the drug can be followed
by its emission signals with crystal detectors, CCD cameras, etc., allowing the
non-invasive visualization and quantification of molecular targets or functional
parameters addressed by the drug (Figure 2.1).
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