June 1, 2011 / Vol. 36, No. 11 / OPTICS LETTERS

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Real-time line projection for fast terahertz
spectral computed tomography
Emmanuel Abraham,1 Yoshiyuki Ohgi,2 Masa-aki Minami,2 Mukesh Jewariya,3Masaya Nagai,2 Tsutomu Araki,2 and Takeshi Yasui2,4,*
1

2
3

Laboratoire Ondes et Matière d’Aquitaine (LOMA), University of Bordeaux-CNRS, UMR 5798,
351 Cours de la Libération, 33405 Talence, France

Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

Renovation Center of Instruments for Science Education and Technology, Osaka University, Toyonaka, Osaka 560-0043, Japan4
Institute of Technology and Science, University of Tokushima, Tokushima 560-8531, Japan*Corresponding author: yasui@me.tokushima‐u.ac.jp
Received March 23, 2011; revised April 29, 2011; accepted May 9, 2011;
posted May 11, 2011 (Doc. ID 144716); published June 1, 2011
We demonstrated fast terahertz spectral computed tomography by using real-time line projection of a terahertz
beam. Two types of cross-sectional images of continuously rotating samples have been measured in only a few
seconds. From temporal data, a peak-to-peak sinogram and cross sections have been reconstructed using a filteredbackprojection algorithm. Using fast Fourier transform from temporal data, spectral cross sections of the sample
have been obtained. © 2011 Optical Society of America
OCIS codes: 110.6795, 110.6955, 120.3620, 320.7090.

In the field of three-dimensional (3D) imaging, x-ray computed tomography (CT) is a ubiquitous technique, which
provides cross-sectional images of an object by analyzing
the radiation transmitted by the sample at different projection angles. Then a filtered backprojection (FBP) algorithm using a Radon inverse transform makes it possible
to reconstruct the cross-sectional images [1]. However,
X-ray CT cannot be easily applied to soft materials suchas plastics, papers, or paintings because of low absorption of the radiation. In this case, terahertz (THz) radiation can provide an alternative CT method due to its
moderate penetration in dry, nonpolar, and soft materials, insensitivity to optical scattering, free-space propagation, and low photon energy. Furthermore, THz waves
effectively provide spectral images of the test object in
the THz range for material characterization. Therefore,
THz CT has been developed to visualize 3D objects composed by opaque soft materials such as plastics, papers,composites, etc. [2–7].
Ferguson et al. demonstrated that cross-sectional
images could be obtained by measuring the transmitted
amplitude and phase of broadband THz pulses at multiple
projection angles [2]. The reconstruction algorithm used
for the visualization of the cross-sectional images usuallyconsists of application of the FBP algorithm from the projection data. However, a strong limitation in practical use
is the long acquisition time required by a pulsed THz system based on a time-domain spectrometer (TDS), since
it uses point-by-point scanning to record a THz image.
A simple way to decrease the acquisition time of thetemporal waveform is to use a fast scanning mechanicaltime delay. Another effective way to realize rapid dataacquisition is the use of an asynchronous optical sampling THz-TDS [8]. Nonmechanical time-delay scanning
up to 250 Hz has been effectively applied to THz CT [6].
However, signal integration is usually required to getgood dynamic range. An alternative method is the development of depth-resolving THz imaging with tomosynthesis, which is similar to CT except that the number ofprojections is much smaller [9]. In the above case, only
0146-9592/11/112119-03$15.00/0

five projections were used instead of generally 15 to 20 inTHz CT. However, the efficiency of this system is mainly
limited to thin and wide samples (50 sheets of Post-it
notes). Furthermore, those methods were still based on
point-by-point scanning measurements. If the pixel data
could be measured in parallel simultaneously, the dataacquisition time would be greatly reduced.
Recently, real-time two-dimensional spatiotemporal(2D-ST) imaging in the THz region has been achieved by
a combination of noncollinear electro-optical time-tospace conversion [10] and line focusing of a THz beam,
which has been effectively applied to THz reflective
tomography [11] and spectral imaging of moving objects[12,13]. In this Letter, to significantly decrease the acquisition time of THz CT, we propose an experimental arrangement in which the sample is illuminated by aline-focused THz beam to achieve real-time line projection across the sample without scanning. 2D-ST lineprojection data of continuously rotating samples havebeen measured with a 100 Hz acquisition rate and used
for spectral imaging of the sample cross section.
With a 1 kHz femtosecond Ti:sapphire regenerative
amplifier (900 μJ=pulse, 150 fs, 800 nm), an intense THz
pulse is generated using the tilted pulse front excitation
technique in a LiNbO3 crystal [14]. The accessible spectral bandwidth of the THz pulse extends from 0 to 2 THz,limited by the THz absorption in the LiNbO3 crystal.
The THz beam is line-focused onto the sample with aTHz cylindrical lens (THz-CL1, f 0 ¼ 100 mm, Tsurupica),
resulting in a line of illumination (20 mm long) acrossthe sample, as shown in Fig. 1. Then, the THz projection
line was imaged onto a ZnTe crystal by a combinationof a THz plano–convex lens (THz-L, f 0 ¼ 100 mm, Tsurupica) and a THz cylindrical lens (THz-CL2, f 0 ¼ 100 mm,Tsurupica). For detection, the THz pulse passing throughthe sample and the infrared probe light were noncollinearly incident on the ZnTe crystal for a time-to-spaceconversion of the pulsed THz electric field and line
imaging of the sample along the THz focal line via 2D
free-space electro-optical sampling. The spatial intensity© 2011 Optical Society of America

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OPTICS LETTERS / Vol. 36, No. 11 / June 1, 2011

Fig. 1. (Color online) Experimental setup. THz-CL1 andTHz-CL2, THz cylindrical lenses; THz-L, THz plano–convex
lens; P, polarizer; A, analyzer; L, plano–convex lens.

distribution of the probe beam was finally imaged with aplano–convex lens (L, f 0 ¼ 80 mm) onto a high-speed
complementary metal-oxide semiconductor (CMOS)
camera (Hamamatsu Photonic Intelligent Vision System,Hamamatsu, Japan, 12 bits, 232 × 232 pixels, 1000frames=s) synchronized with the 1 kHz laser pulse. A
2D-ST THz image was acquired at a frame rate of
500 Hz by the CMOS camera working in the dynamic subtraction mode [15], in which the temporal profile of theTHz pulse (temporal range ¼ 37 ps) and the THz lineimage of the projection (spatial range ¼ 20 mm) developed along the horizontal and vertical dimensions,respectively. Finally, owing to the 10 ms integration timeof the CMOS camera, the 2D-ST image is acquired witha 100 Hz acquisition rate. In this case, the measured dynamic range is 945, defined as the ratio of the signal amplitude (peak-to-peak) in the presence of the THz beam
to that in the absence of the THz beam. The transverse
resolution along the line projection attains the diffraction
limit of the imaging system [12].
To perform fast THz CT, the sample was continuously
rotated across the THz focal line as indicated in Fig. 1.
The rotation time for a 360° revolution of the sample
was set to 6 s. During this time, owing to the 10 ms continuous acquisition time of the THz line projection, up to
600 line-projection data were collected corresponding toan angular sampling of only 0:6°. This represents a large
improvement for THz CT, where usually only 18 (anglestep of 10°) or 36 (angle step of 5°) projections angles
are selected owing to the long acquisition time of theexperiment (point-by-point scanning measurement ismore than 1 h).
Finally, the combination of line projections and theFBP algorithm provide the sinogram and corresponding
cross section of the sample at the tightly focusing position of the THz line. From the analysis of the temporal
data, the peak-to-peak amplitude of the main THz pulse
will provide information about global transmission of thesample in the 0–2 THz spectral range. Using a fast Fouriertransform (FFT) from the temporal data, spectral crosssections can be visualized at the spectral resolution of
27 GHz in order to identify specific THz absorptions
and perform THz spectral CT [5,6].
To demonstrate the ability of fast cross-section reconstruction, we first investigated a simple sample consi-Fig. 2. Four metallic bars (2 mm diameter). (a) Peak-to-peaksinogram. (b) Peak-to-peak cross section. (c) Amplitude sinogram at 0:6 THz. (d) Amplitude cross section at 0:6 THz.

sting of four horizontal metallic bars (2 mm diameter)
illuminated by the vertical THz line. As explained previously, up to 600 projection data were recorded, corresponding to the visualization of the sample through 600
different angles in only 6 s, which represents a significant
increase of the acquisition rate for THz CT. From theseraw data, the first analysis consists in measuring thepeak-to-peak amplitude of each THz waveform. Consequently, we can build the sinogram of the sample corresponding to the evolution of the THz line projection as afunction of the rotation angle. Figure 2(a) shows thesinogram of the peak-to-peak amplitude for this sample,
whose horizontal and vertical coordinates respectively
give the rotation angle (600 points from 0 to 360°) and theTHz projection-line image (232 points from 0 to 20 mm).
The white areas indicate a decrease in the peak-to-peakamplitude. Isolated distribution of the four metallic bars
generates four overlapping sinusoidal waveforms in thesinogram with different original phases of the rotation
angle. Using the standard FBP algorithm [1], we reconstructed the cross-sectional image corresponding to
the portion of the object illuminated by the THz line
as shown in Fig. 2(b). Although some artifacts are visible(diffraction pattern around the metallic bars), the positions of the four metallic bars in the region corresponding to the illuminating THz vertical line are clearlyidentified, demonstrating the ability of the system toproperly reconstruct in only 6 s cross-sectional imagesof simple samples.
Figures 2(c) and 2(d) illustrate the amplitude sinogramand cross section measured at 0:6 THz after FFT of thetemporal data (the white area indicates a decrease in theTHz amplitude). Although no spectroscopic information
can be obtained from this metallic sample, this sinogramand cross section exhibit a higher transverse resolution compared to temporal data owing to the shorterwavelength of the THz radiation (500 μm). Also, the diffraction pattern around the four metallic bars has almost
disappeared in the 0:6 THz cross section. This indicates
that, in the peak-to-peak cross section of Fig. 2(b), thediffraction pattern mainly comes from the interaction
between the sample structure with a diameter of 2 mm

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to the 10 ms acquisition time of 2D-ST THz images, lineprojection data have been recorded in real time associated with the continuous rotation of the sample. Withthis system, by FFT of the time dimension in the 2D-ST
THz image, fast THz spectral CT has been achieved at theacquisition time of 6 s. Further developments include
additional horizontal scanning of the sample associatedwith the continuous rotation to perform full 3D reconstruction of volumetric objects.
Fig. 3. Foam cylinder (16 mm diameter) with one off-axis
cylindrical hole (4 mm diameter) filled with lactose andpolyethylene powder. (a) Amplitude sinogram at 0:53 THz.
(b) Spectral cross section at 0:53 THz and intensity profile along
the vertical white line.

and the low frequency components of the THz radiation (0–0:5 THz).
The second sample is a foam cylinder (16 mm diameter) drilled with an off-axis cylindrical hole (4 mm diameter). To demonstrate THz spectral CT, the hole wasfilled with α-lactose monohydrate sugar mixed with polyethylene powder (30% lactose and 70% polyethylene).
According to previous studies, lactose exhibits a THz
spectral fingerprint at 0:53 THz [13]. With the same experimental conditions (rotation from 0 to 360° in 6 s, 600
projections, horizontal sample illuminated by the verticalTHz line), Figs. 3(a) and 3(b) represent the amplitude
sinogram measured at 0:53 THz and the corresponding
reconstructed spectral cross section of the sample, respectively. First, the shape of the foam cylinder is notvisible, owing to the low refractive index change from
air to foam and the low absorption coefficient of the foam[Rohacell, (Evonik Industries, Germany) α ¼ 0:1 cm−1 ,refractive index ¼ 1:15]. In Figs. 3(a) and 3(b), the position and the size of the inner cylindrical hole is revealed
due to the different transmission of the THz radiation inthis part of the sample. The lower transmission is due toboth refraction losses and lactose absorption at 0:53 THz.
In the cross-section image, we can observe that the shapeof the hole is not regular, perhaps due to diffraction
artifacts. The vertical intensity profile measured along
the white line drawn in Fig. 3(b) is also presented, inwhich the pixel values have been plotted in logarithmic
scale. The data reflect the sample absorbance, classicallydefined as the logarithmic ratio between the transmittedTHz signal measured out of the sample and the THz signal
measured along the vertical line drawn in Fig. 3(b). In theregion of the lactose cylinder, the absorbance at 0:53 THz
has been evaluated to 0.5 in agreement with lactose concentration and sample thickness. In the future, we plan toextend the spectral bandwidth of the THz emission toperform broadband spectral CT up to several THz.
In summary, we demonstrated the potential of realtime THz line projection for fast THz spectral CT. Owing

E. Abraham is grateful to the Invitation Fellowship
Programs for Research, Japan Society for the Promotionof Science (JSPS), Japan. This work was supported by
Grants-in-Aid for Scientific Research 21650111 from
the Ministry of Education, Culture, Sports, Science &Technology (MEXT), Japan. We also acknowledge theRenovation Center of Instruments for Science Education
and Technology (Osaka University), and Tateishi Scienceand Technology Foundation for financial support.
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