Terahertz-wave water concentration and distribution measurement in thin biotissue based on a novel sample preparation.
ABSTRACT The measurement of water concentration and distribution in thin biotissues with terahertz (THz)-wave has been proposed. In this paper, a novel sample preparation approach was introduced to effectively preserve tissue freshness at room temperature. Excellent stability of this method was demonstrated by measuring the transmittance spectroscopy and imaging many times within a certain time. Moreover, the reliability of water volume concentration measurement with THz-wave was evaluated. Measurement results using THz-wave were in good agreement with volume concentration measurement results based on other quantitative methods. The results suggest that water concentration and distribution measurement in thin biotissues using THz-wave will be a potential modality for medical and biological diagnosis.
Terahertz-wave water concentration and distribution measurement in thin biotissue based on
a novel sample preparation
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PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol. 56 (2011) 4517–4527
Terahertz-wave water concentration and distribution
measurement in thin biotissue based on a novel sample
Y Y Wang, T Notake, M Tang, K Nawata, H Ito and H Minamide
RIKEN ASI, 519-1399, Aramaki, Aoba, Sendai 980-0845, Japan
Received 19 January 2011, in final form 21 April 2011
Published 30 June 2011
Online at stacks.iop.org/PMB/56/4517
The measurement of water concentration and distribution in thin biotissues
with terahertz (THz)-wave has been proposed. In this paper, a novel sample
preparation approach was introduced to effectively preserve tissue freshness
at room temperature. Excellent stability of this method was demonstrated by
measuring the transmittance spectroscopy and imaging many times within
a certain time.Moreover, the reliability of water volume concentration
measurement with THz-wave was evaluated.
THz-wave were in good agreement with volume concentration measurement
results based on other quantitative methods. The results suggest that water
concentration and distribution measurement in thin biotissues using THz-wave
will be a potential modality for medical and biological diagnosis.
Measurement results using
(Some figures in this article are in colour only in the electronic version)
In the last decade, terahertz (THz) imaging has developed as an attractive and powerful
means to study the properties of various materials in many practical applications, such as
the inspection of packaging (Morita et al 2005) and artwork, analyzing chemical composition
(Tonouchi 2007).THz-wave radiation has merits of being nonionizing, nondestructive,
and low scattering. It can be used to excite large amplitude vibrational modes in various
macromolecules and probe the weak interactions between them. More recently, THz imaging
has been successfully applied to biomedical and biotissue diagnostics (Mittleman et al 1996,
Wallace et al 2004, Fitzgerald et al 2006, Sy et al 2010, Taylor et al 2008, L¨ offler et al 2001,
Siebert et al 2002, Kleine-Ostmann et al 2001, Hoshina et al 2009). In most of the biological
0031-9155/11/144517+11$33.00© 2011 Institute of Physics and Engineering in MedicinePrinted in the UK4517
4518Y Y Wang et al
applications, owing to the high absorptivity of water in the THz region, it limits the sensing
and imaging in water-rich samples and prohibits transmission-mode imaging through a thick
tissue. For this reason, reflective THz imaging (Wallace et al 2004, Fitzgerald et al 2006, Sy
et al 2010, Taylor et al 2008) is employed, but it increases the system complexity. Moreover,
specific sample preparations like dehydration, chemical process (L¨ offler et al 2001, Siebert et
al 2002, Kleine-Ostmann et al 2001) and low temperature cooling (Hoshina et al 2009) are
required in order to decrease water absorption, which are time consuming and of high cost.
Although much effort has focused on removing water in tissue, water, one of the most
important materials in science, has great significance for biologically functional dynamics.
with water content changes (Wallace et al 2004, Fitzgerald et al 2006, Sy et al 2010), and THz
spectroscopy provides a sensitive probe of high water content change (Reid et al 2010). In
principle, water content in biological tissue can be used as a marker for different soft tissues,
benignandmalignanttissues. Thus, itisanticipatedthatduetotheextremesensitivitytowater
content of THz-wave, THz imaging can produce images with ‘water content and distribution’
enabling an analysis of the structure and composition change of tissues.
Recently, water concentration and distribution measurement in thin tissues using THz-
wave (Wang et al 2010) has been proposed as an innovative sensing and imaging technology
that can provide distinct water information unavailable through conventional methods, such
as the drying method and the chemical method. The chosen basis for biosample thickness
and THz measuring frequency was presented theoretically. However, an essential but often
unmentioned premise of biotissue imaging studies is that repeatable and reliable measurement
should be available in order to draw adequate conclusions from the data. In this paper,
we presented a detailed study with the purpose of evaluating the stability and reliability of
water content and distribution measurement using THz-wave. A novel sample preparation
technique was introduced as a viable solution for tissue freshness preservation at room
temperature. The stability of such a method was demonstrated by measuring the transmittance
spectroscopy and imaging many times within a certain time. The reliability of the water
volume concentration measurements using THz-wave is estimated by comparison to volume
concentration measurements based on other quantitative methods.
2.1. Sample preparation
Biological sample preparation is a key technique in such an imaging experiment. In this
study, chicken tissues were used. To ensure the moisture content of the fresh specimens
remaining constant, they were stored in a refrigerated, humidified environment. A sample for
the THz imaging measurement was prepared by cutting uniform slices of tissue at −20◦C
using a microtome (CM1900; Leica). This temperature can be adjusted according to different
kinds of tissues. The sample cutting thickness can be continuously chosen in the range of
40–60 μm based on the analysis (Wang et al 2010). The sample holder consists of two
Tsurupica plates, which is separated by a spacer of chosen thickness to match the thickness
of the sliced sample. An improved technique is to cover the tissue by oleic acid under lower
during the experiment, as shown in figure 1. This is the technique employed in this study.
Oleic acid has a melting point of 13.4◦C and a refractive index of 1.46 at room temperature
(Bernardo-Gil et al 1990), which is very close to the refractive index of the Tsurupica plate
(about 1.52). Moreover, oleic acid and Tsurupica plate both have high transmission in the
Terahertz-wave water concentration and distribution measurement in thin biotissue 4519
Frozen oleic acid
Thawing oleic acid and sample
Figure 1. Schematic of sample preparation.
THz band of interest. Thus, marginal Fresnel loss due to their low refractive index can be
neglected. And when the plates are fixed together there was no deforming of the tissue and no
air gap between the sample and Tsurupica plates. Then, the sample is measured using the THz
imaging system at a controlled room temperature after sample thawing. The whole process is
purged with dry nitrogen to keep water vapor away from the sample.
2.2. Experiment setup
The experimental setup of the THz imaging and spectroscopy system used in this work is the
same as that described in our previous paper (Wang et al 2010). It is based on transmission
geometry using the ring-cavity THz-wave parametric oscillator (TPO) source (Minamide
et al 2009). The output tunable range of THz-wave is 0.9–2.5 THz. The spectral resolution
was about 30 GHz at 1.5 THz. The THz-wave was separated into two beams by a wire-grid
beam splitter, a signal beam for imaging and a reference beam. The signal beam is reflected
and normally focused on the sample by using an aspherical lens. The sample is moved by
a computer-controlled x–y linear motor stages. The transmitted THz beam and reference
beam were measured by a liquid helium cooled 4.2 K Si bolometer, and two-dimensional
water distribution images can be obtained. The use of two-channel bolometer detection can
effectively decrease the random noise. The signal-to-noise ratio of the present imaging setup
was about 21 dB. The whole imaging set-up was enclosed in a box and purged with dry
nitrogen to reduce absorption induced by water vapor in the air.
2.3. Theoretical background
Water volume concentration analysis of biotissue was undertaken using the Lambert–Beer
law. Considering that the THz wavelength is much longer than that of optical and infrared,
THz radiation is less susceptible to scattering within freshly biological tissue and it can be
negligible. The transmission of light through substance is
where Iinand Ioutare the intensities of the incident and transmitted pulses, respectively, and d
is the sample thickness. We consider that the THz-wave in water-rich tissues is absorbed by
water and the other components. The absorption coefficient is described as follows:
α = αwνw0+ αnwνnw0= αwνw0+ αnw0(1 − νw0).
Here, αwand αnwrepresent the absorption coefficient of water and the other components, and
νw0and νnw0indicate the volume concentration of water and other components, respectively.
They satisfy νw0+ νnw0= 1. The transmittance can be expressed as
T = exp[−(αwνw0+ αnwνnw0)d].
4520Y Y Wang et al
Figure 2. The absolute error ?νwversus true value of the water volume concentration.
Assuming that the water absorption coefficient is much higher than that of other components,
absorption was mainly dominated by the water content in tissue (Png et al 2008). In water-
rich tissues, αwνw0? αnwνnw0is reasonable. Thus, the water volume concentration can be
νw0≈ νw= −lnT
Here, νwis the measured value of the water volume concentration with THz-wave, which is
the upper limit of the water volume concentration νw0.
From equation (3), the theoretically true value for the water volume concentration νw0is
Combining equations (4) and (5), we can obtain the absolute error between the true value and
measured value for water volume concentration:
Since the absorption coefficients of many dehydrated tissues are less than 40 cm−1below
2 THz (Png et al 2008, Zhang and Durbin 2006) and the absorption coefficient of pure water
is about 400 cm−1at 2 THz (Bertie and Lan 1996), figure 2 shows the absolute error ?νw
changing with the true value of the water volume concentration. Here, it should be pointed
out that the absorption coefficients of many dehydrated tissues in references were measured
with THz–TDS source, the upper frequency limit of which is about 2 THz. Furthermore, the
increasing rate of water absorption coefficient with THz frequency is much faster than that
of dehydrated tissues. Therefore, the parameters αnw/αware assumed to be 0.05 and 0.1 in
figure 2. It is seen that ?νwdecreases with increasing νw0. ?νwcan be less than 5% for the
water volume concentration more than 0.5 while αnw/αw= 0.1. For the tissue with low water
concentration, the lower THz frequency should be chosen to reduce the ratio value, which can
be satisfied using a continuously tunable TPO source. Moreover, when αnw/αwis smaller
than 0.05, ?νwis less than 5% even though the water volume concentration in tissue is very
small. In such a case, the error between the theoretical value and the experimental value using
THz-wave measurement is very small for water-rich tissue.
(1 − νw0).