Biomedical Engineering Reference
In-Depth Information
Fig. 4 Temperature-dependent power absorptions and refractive indices of water (From Son
( 2009 ). Copyright © 2009 by American Institute of Physics. Reprinted by permission of American
Institute of Physics)
the molecule's motions are active in the THz frequency range (Son 2009 ). The
power absorption becomes greater as the temperature increases, and reaches a local
maximum around 6 THz due to the hydrogen-bond resonance (Zelsmann 1995 ).
THz measurement with a Debye model fit has shown that bulk water has two
relaxation dynamics with fast and slow times. The slow time of approximately 8 ps
is related to the loosening of hydrogen bonds, and the fast time of 170 fs arises from
the realignment of molecules under an external electric field (Oh et al. 2007 ).
3
Principle of Terahertz Molecular Imaging
THz molecular imaging (TMI) is achieved by monitoring the changes in water's
THz optical property such as absorption and index caused by temperature variation.
Cells and organs contain an abundant amount of water, and the water temperature
can be controlled by nanoparticle probes (nanoprobes) delivered to the cells and
organs. The gold nanorods (GNRs) shown by the transmission electron microscopy
(TEM) images in Fig. 5 (Oh et al. 2009 ) are an example of nanoprobes. The GNRs
have resonant features around 800 nm, as shown in Fig. 5d , and their resonant
wavelength can be engineered by adjusting the dimension. GNRs can be targeted
to specific cells or tumors by antibody conjugation, and they can be used to generate
heat in the targeted location by surface-plasmon resonance under infrared (IR) irra-
diation at the resonant wavelength. To simulate GNRs in cells, the GNRs were
immersed in water and the THz response was measured by the reflection-mode THz
imaging setup shown in Fig. 3 . The increased heat induced by the surface plasmon
elevated the temperature of the water, and THz reflectivity increased, as shown in
Fig. 6 . Similar results were obtained with an A431 skin cancer cell endocytosed
with GNRs. Fig. 7 shows the cancer cell images with and without GNRs. Figs 7b,
c are the THz images with and without IR irradiation, respectively. The cell with
GNRs subjected to IR irradiation was the only one to display higher THz reflectivity
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