Quantitative analysis of water distribution in human articular cartilage using terahertz time-domain spectroscopy.

Abstract

The water distribution in human osteoarthritic articular cartilage has been quantitatively characterized using terahertz time-domain spectroscopy (THz TDS). We measured the refractive index and absorption coefficient of cartilage tissue in the THz frequency range. Based on our measurements, the estimated water content was observed to decrease with increasing depth cartilage tissue, showing good agreement with a previous report based on destructive biochemical methods.

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Quantitative analysis of water distribution in
human articular cartilage using
terahertz time-domain spectroscopy
Euna Jung,1 Hyuck Jae Choi,2 Meehyun Lim,1 Hyeona Kang,1 Hongkyu Park,1
Haewook Han,1* Byung-hyun Min,3 Sangin Kim, 4 Ikmo Park4 and Hanjo Lim4
1National Research Lab for Nano-THz Photonics, Department of Electrical and Computer Engineering, POSTECH,
San 31, Hyoja-dong, Nam-gu, Pohang, Gyungbuk 790-784, Korea
2Department of Radiology and Research Institute of Radiology, Asan Medical Center, University of Ulsan,
388-1 Pungnap-2 dong, Songpa-gu, Seoul 138-736, Korea
3Department of Orthopedic Surgery, School of Medicine, Ajou University, 5 Wonchon-dong, Youngtong-gu,
Suwon, 443-749, Korea
4Department of Electrical and Computer Engineering, Ajou University, 5 Wonchon-dong, Youngtong-gu,
Suwon, 443-749, Korea
*hhan@postech.ac.kr
Abstract: The water distribution in human osteoarthritic articular cartilage
has been quantitatively characterized using terahertz time-domain
spectroscopy (THz TDS). We measured the refractive index and absorption
coefficient of cartilage tissue in the THz frequency range. Based on our
measurements, the estimated water content was observed to decrease with
increasing depth cartilage tissue, showing good agreement with a previous
report based on destructive biochemical methods.
© 2012 Optical Society of America
OCIS codes: (110.6795) Terahertz imaging; (000.1430) Biology and medicine.
References and links
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1. Introduction
Osteoarthritis (OA), one of the most prevalent chronic diseases in the elderly, is characterized
by progressive degeneration of cartilage. Cartilage degeneration is affected by biochemical
alterations, including an increase in water content and the loss of proteoglycans [1–3]. Several
studies have shown that the water content in osteoarthritic cartilage may increase by about
10% [4]. Therefore, a precise measurement of the water content in cartilage can aid in the
diagnosis of early-stage OA. However, changes in the water content in the early stages of OA
cannot be detected using current clinical techniques such as radiography and arthroscopy.
Only magnetic resonance imaging (MRI) has been used for the detection of water content in
the early stages of OA [5,6].
Terahertz time-domain spectroscopy (THz TDS) has recently been developed because of
recent advances in THz technology. THz TDS is a coherent and non-ionizing method that can
quantify the complex refractive index from both the phase and amplitude information of a
medium [7–9]. Moreover, this method can also probe low frequency vibrational modes of
biomolecules, thus providing structural and functional information about biological tissue
[10]. Because water has strong absorptions across the entire THz frequency range, THz
images will likely show a good image contrast dependent on the changes in medium water
content. This enables THz TDS to be used for spectroscopic investigation of a biological
medium.
To date, several biological tissues have been examined using this technique. For instance,
characterization of human dental tissues [11], basal cell carcinoma from both ex vivo and in
vivo samples [12,13], and human cortical bone [14] has been reported. More recently, human
breast tumors [15] and micro-metastic lymph nodes [16] have been successfully investigated
using THz TDS although the clinical application of THz TDS has not been demonstrated due
to the high water absorption. However, no literature is available on the quantitative analysis of
human articular cartilage in the THz region. Only THz reflection images of rabbit cartilage
have been reported [17]. Here we report on the THz characterization of water distribution in
human articular cartilage.
2. Materials and methods
Human osteoarthritic articular cartilage tissues were obtained from the Department of
Orthopedic Surgery at Ajou University Hospital, Korea. The tissue diagnosed as OA was
excised from a patient after total knee joint replacement. Appropriate consent was obtained
for the measurements and all materials were returned to the Ajou University Hospital for
disposal after the measurements. The articular surface of the cartilage tissue was visually
intact. Using a razor blade, the excised cartilage tissue was cut into a slice (622 ± 30 µm) to
study the depth information from the articular surface to the subchondral bone, as depicted in
Fig. 1(a), where the thickness was measured by a digital thickness gauge with an accuracy of
1 µm. The sliced cartilage was placed on a 150-µm-thick glass slide and covered with a 10-
µm-thick film of low density polyethylene (LDPE) to prevent desiccation (Fig. 1(b)).
The experimental setup was based on a conventional TDS system with transmission
geometry. The THz pulse was generated by an InAs wafer pumped by a Ti:sapphire laser with
a center wavelength of 790 nm, a pulse width of 100 fs, and a repetition rate of 80 MHz. The
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Cartilage
LDPE Cover glass
(b)
Sample
LDPE Cover glass
Articular surface Subchondral bone
(a)
Reference
01
x (mm)
3
1
2
0
y (mm)
2

Fig. 1. (a) Optical image of human articular cartilage tissue. (b) Schematic of reference and
cartilage samples.
generated THz pulse was collimated and focused by off-axis parabolic mirrors. The cartilage
sample was placed at the THz beam waist and moved on a motorized stage between two off-
axis parabolic mirrors. The focal length of a set of off-axis parabolic mirrors was 5 cm. The
scanned area was 3.5 × 2 mm2, and the scanning steps of the horizontal (x) and vertical (y)
directions were 0.3 and 1 mm, respectively. The transmitted THz signal was detected by a
photoconductive antenna fabricated on a low-temperature grown GaAs using standard optical
gating and phase-sensitive detection techniques.
3. Results and discussion
Figure 2 shows the THz pulse signals and amplitude spectra with and without cartilage tissue
with the transmitted THz pulses recorded at the center of cartilage sample (x = 1.0 and y = 1.0
mm). The transmitted THz pulse for the cartilage sample was significantly attenuated by
absorption and Fresnel loss, and was ~10 times smaller than that of the reference signal. As a
THz pulse propagates through an absorptive medium, such as a biological medium, the pulse
width broadens due to the dispersion. The spectral amplitude transmitted through the cartilage
tissue was found to be reduced over the entire THz frequency range (Fig. 2(b)).
Figure 3 shows the frequency-dependent refractive indices and absorption coefficients of
cartilage tissue along its depth. The dotted lines indicate the refractive index and absorption
coefficient of pure water, as reported in Ref. [18]. The refractive index and absorption
coefficients near the articular surface were not included in Fig. 3 because of the diffraction
effect at the edge of the sample. Over the entire frequency range, the refractive indices and

0.2 0.40.6 0.81.01.2 1.4
10
-5
10
-4
10
-3
10
-2
Transmitted amplitude (a. u.)
Frequency (THz)
Reference
Cartilage
05 10 1520253035
-4
-2
0
2
4
THz signal (nA)
Time (ps)
Reference
Cartilage(x10)
(a) (b)

Fig. 2. THz signals and transmitted amplitudes of reference and cartilage tissue.
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0.4 0.6 0.81.0
0
50
100
150
200
250
Absorption coefficient (cm
-1)
Frequency (THz)
0.40.6 0.81.0
1.0
1.5
2.0
2.5
3.0
3.5
Refractive index
Frequency (THz)
(a)(b)







x (mm)
water
0.4
0.7
1.0
1.3
1.6
1.9
2.2

Fig. 3. Frequency dependence of (a) the refractive index and (b) the power absorption
coefficient along the depth of the cartilage tissue. Each dotted line indicates the values for
liquid water as reported in a previous literature [18].
absorption coefficients of the cartilage tissue gradually decreased and increased, respectively.
Each absorption coefficient along the depth was lower than that of liquid water. In addition,
no significant change in the refractive indices along the depth of the cartilage was observed.
However, we found that the absorption coefficients decreased from the articular surface to the
subchondral bone. For the extraction of the complex refractive index, we used an iteration
method based on the transfer matrix theory where the effects of multiple reflections at the
interfaces between the slide, cartilage, and LDPE film are taken into account [19].
Figure 4 shows the refractive index images and absorption coefficient images of cartilage
tissue at 0.4 and 0.8 THz. The refractive index was relatively constant along the depth of
cartilage at both 0.4 and 0.8 THz with the exception of the surface of the cartilage because of
the diffraction. In the absorption coefficient image of the cartilage, the absorption was high at



012
0
0.5
1
1.4
1.6
1.8
2
2.2


012
0
0.5
1
1.4
1.6
1.8
2
2.2


012
0
0.5
1
0
50
100
150


012
0
0.5
1
0
50
100
150
y(mm)
x (mm)
y(mm)
y(mm)
y(mm)
x (mm)
x (mm)
(c)
x (mm)
(a)
(b)
(d)
0.4 THz 0.8 THz
0.4 THz 0.8 THz
nn
α (cm-1)
α (cm-1)

Fig. 4. Refractive index images and absorption coefficient images of articular cartilage at 0.4
and 0.8 THz. The dashed lines indicate the cartilage surface.
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0.00.51.0 1.52.0 2.5
0
50
100
150
200
Absorption coefficient (cm-1)
x (mm)
0.4 THz
0.6 THz
0.7 THz
1.0 THz
0.00.51.01.5 2.0 2.5
1.5
2.0
2.5
3.0
Refractive Index
x (mm)
0.4 THz
0.6 THz
0.7 THz
1.0 THz
(a) (b)

Fig. 5. (a) Refractive index profile and (b) absorption coefficient profile along the depth of
cartilage tissue at 0.4 and 0.8 THz.
the articular surface and gradually decreased along the depth of the cartilage. The refractive
indices and absorption coefficients along the depth of cartilage at specific frequencies are
shown in Fig. 5. At each frequency, the difference between the maximum and minimum
values of the refractive index was less than 5% along the depth. In contrast, the absorption
coefficient at each frequency significantly decreased from the articular surface to the
subchondral bone. It has been known that the cartilage tissue is spatially heterogeneous and
molecular composition of cartilage varies significantly in going from the articular surface to
subchondral bone [1–6]. Therefore we speculate that the alteration of absorption coefficient
along the depth of the cartilage matrix may result primarily from changes in water content
because water has a strong absorption in the THz frequency range.
The effective absorption coefficient of cartilage tissue is related to the absorption
coefficients of the components in the cartilage, including water, proteoglycans, and collagen.
To characterize the water distribution in cartilage tissue from the absorption coefficient, we
should in principle take into account all the effects of the biochemical components in
cartilage. However, we assumed that the absorption coefficient was determined predominantly
by the water content, and did not account for other components, since water has a much
higher THz absorption than the other biochemical components in cartilage. Consequently, we
also assumed that the absorption coefficient was almost proportional to the volume fraction of
0.40.6 0.8 1.0 1.21.4
0.0
0.2
0.4
0.6
0.8
1.0
Weight fraction of water
x (mm)
Our measurement
Ref. 20

Fig. 6. Weight-fractional distribution of water in cartilage. The red and black curves represent
the measurements by THz TDS and destructive biochemical method [20], respectively. The
first points in the two curves correspond to the cartilage surfaces.
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