Biomedical Engineering Reference
In-Depth Information
2.2 Functional and molecular imaging by emission
tomography enables high sensitivity and
spatial resolution
Functional and molecular imaging aims at the visualization of molecu-
lar processes in complex biological systems ranging from synthetic molecular
structures, from cells to organs to individuals. Innovative functional imaging
technologies, namely the imaging of molecular processes on a single moleculu-
lar level, uniquely enable the exploration of chemical, physiological and patho-
physiological processes such as receptor expression and function, catalytic (en-
zymatic) activities and cell-cell interactions in biosystems.
The specific strength of imaging lies in the ability to resolve dynamic pro-
cesses in time and space in systems as small as ion transporters to large com-
plex organisms such as patients suffering from a specific disease. The knowl-
edge gained by imaging primarily supports the understanding of molecular
interactions. This is important for basic research of biological functions but
also, and most importantly, for future developments in medicine.
The diagnostic imaging of functional and molecular processes in vivo needs
a high sensitivity of the imaging technology plus a reasonable spatial and tem-
poral resolution tailored to the diagnostic question. Although the repertoire of
clinical imaging techniques covers a whole range from morphological to func-
tional to molecular imaging modalities, only the scintigraphic technologies,
single photon emission tomography (SPECT) and positron emission tomogra-
phy (PET), and optical imaging methods, such as bioluminescence and near-
infrared fluorescence imaging, are based on emission tomography and have an
exclusively high sensitivity.
As detailed in the previous chapter, scintigraphic techniques are preferable
for deep tissue imaging since gamma rays can travel long distances through tis-
sues without significant interferences. Therefore, SPECT and PET are mainly
used for functional and molecular imaging in clinical algorithms. \Miniatur-
ized" dedicated SPECT and PET systems have paved the way for applica-
tion of scintigraphic approaches in preclinical research [20]. Nowadays, optical
imaging using genetically encoded bioluminescence or injectable fluorescent
dyes is used extensively in preclinical research. However, fluorescence imaging
is just starting to be used for clinical imaging of structures on or close to the
body surface or in a catheter-based setup [15]. Both emission-based techniques
provide a very high molecular sensitivity in the nano- to picomolar range in
vivo, enabling the quantification of receptors, enzymes, transmitters in organ-
isms while having adequate spatial and temporal resolution. This exquisitely
high sensitivity is crucial, since low \tracer" amounts of substances, which do
not have a pharmacological effect and therefore do not influence the biosys-
tems one is looking at, can be detected.
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