Biomedical Engineering Reference
In-Depth Information
theranostics. A number of molecular imaging techniques have been developed to
visualize such biological processes at the cellular or molecular level. Among these
techniques are positron emission spectroscopy (PET) (Massoud and Gambhir
2003
), magnetic resonance imaging (MRI) (Weissleder et al.
2000
), and optical
imaging using bioluminescence or fluorescence (Moore et al.
1948
, Weissleder and
Ntziachristos
2003
), all of which have their pros and cons. For example, PET is a
highly sensitive technique but has low spatial resolution and requires the use of
radioactive isotopes. MRI offers better spatial resolution than does PET but has
lower sensitivity despite the fact that iron oxide or gadolinium derivatives, which
are expected to increase the sensitivity of the technique, are used as contrast agents.
Optical imaging techniques are relatively simple and inexpensive, but the pene-
tration depth and spatial resolution that can be achieved are very low because of the
Rayleigh scattering of optical waves by cells.
In this review, a novel molecular imaging technique based on the use of terahertz
(THz) electromagnetic waves and nanoparticle probes (Oh et al.
2009
) is introduced.
The basics of THz technology, including the generation and detection of THz waves,
are explained in
Sect. 2
. In addition, the THz characteristics of water are reviewed
because they form the basis of TMI. In
Sect. 3
, the principle of TMI is presented.
An example of nanoparticle delivery imaging of a cancerous tumor and the organs
in a mouse is shown in
Sect. 4
. A brief summary is presented in
Sect. 5
.
2
Review of Terahertz Technology
THz waves, ranging from 0.1 to 10 THz, occupy the electromagnetic spectrum
between microwave and infrared bands. This region is scientifically rich because of
the rotational and vibrational energies of the materials that lie in that frequency
range (see Fig.
1
), but it has gained importance only in recent times because of the
difficulty involved in the generation and detection of THz signals. In this section,
THz technology is reviewed with respect to generation and detection techniques. In
addition, the THz characteristics of water are discussed because they form the basis
of the THz molecular imaging technique.
2.1
Generation and Detection of Terahertz Waves
The generation of electromagnetic waves in the THz frequency range is difficult
because transport-type devices such as transistors cannot move electrons fast
enough to generate THz signals, and transit-type devices such as lasers do not have
natural gain media with a small energy separation of a only a few meV. However,
two decades ago when femtosecond lasers were developed, researchers worked to
combine optical pulses with semiconductors and thus generate THz electromagnetic
pulses. When 800-nm laser pulses from a mode-locked Ti:sapphire laser (Spence
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