Timing performance measurements of Si-PM-based LGSO phoswich detectors

Author’s Accepted Manuscript Timing performance measurements of Si-PM-based LGSO phoswich detectors Seiichi Yamamoto, Takahiro Kobayashi, Satoshi Okumura, Jung Yeol Yeom www.elsevier.com/locate/nima

PII: DOI: Reference:

S0168-9002(16)30072-9 http://dx.doi.org/10.1016/j.nima.2016.03.053 NIMA58720

To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 30 January 2016 Revised date: 15 March 2016 Accepted date: 15 March 2016 Cite this article as: Seiichi Yamamoto, Takahiro Kobayashi, Satoshi Okumura and Jung Yeol Yeom, Timing performance measurements of Si-PM-based LGSO phoswich detectors, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.03.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Timing performance measurements of Si-PM-based LGSO phoswich detectors

Seiichi Yamamoto1, Takahiro Kobayashi1,2, Satoshi Okumura1, Jung Yeol Yeom3 1

Radiological and Medical Laboratory Sciences, Nagoya University Graduate School of Medicine, Nagoya, Japan 2 Department of Radiology, Daiyukai General Hospital, Ichinomiya, Japan 3 Department of Biomedical Engineering, Korea University, Seoul, South Korea ABSTRACT Since the timing resolution was significantly improved using silicon photomultipliers (Si-PMs) combined with fast scintillators, we expect that phoswich detectors will be used in future TOF-PET systems. However, no practical phoswich detector has been proposed for TOF-PET detectors. We conducted timing performance measurements of phoswich detectors comprised of two types of Ce-doped LGSO scintillators with different decay times coupled to Si-PMs and digitized the output signals using a high bandwidth digital oscilloscope. We prepared three types of LGSOs (LGSO-fast, LGSO-standard, and LGSO-slow) with different Ce concentrations. After measuring the decay time, the energy performance, and the timing performance of each LGSO, we conducted pulse shape analysis and timing resolution measurements for two versions of phoswich LGSOs: LGSO-standard/LGSOfast and LGSO-slow/LGSO-fast combinations. The pulse shape spectra for a 10-mm-long crystal LGSOslow/LGSO-fast combination showed good separation of the front and back crystals with a peak-to-valley ratio of 2.0. The timing resolutions for the 20-mm-long crystal LGSO-slow/LGSO-fast combination were ~300 ps FWHM. The timing resolutions for the phoswich LGSOs were slightly inferior than that measured with the individual LGSO fast, but the acquired timing resolution for the phoswich configuration, ~300 ps with a LGSO-slow/LGSOfast combination, is adequate for TOF-PET systems. We conclude that LGSO phoswich detectors are promising for TOF-DOI-PET systems.

Keywords: TOF, DOI, phoswich, LGSO

I. INTRODUCTION The phoswich technique [1] is a common method to realize depth-of-interaction (DOI) detectors for improving the spatial resolution in the peripheral area of PET scanners, and several works have reported on detectors and non-timeof-flight (non-TOF) PET systems using such a phoswich technique [2-5]. The time-of-flight (TOF) technique is a 1

valuable method for improving the signal-to-noise (S/N) of the reconstructed images of PET systems with TOF information [6-7]. This has led to the development of commercial TOF-PET systems for clinical use [8-10]. Since the timing resolution was significantly improved by silicon photomultipliers (Si-PMs) combined with fast scintillators [11-13], we expect that phoswich detectors will be used in future TOF-PET systems. With the improved timing resolution by the used of the Si-PMs, DOI-TOF detectors will be used for small detector ring high resolution brain PET systems or positron emission mammography (PEM) systems which have more serious parallax errors in their systems. There is a simulation study reported that DOI detection improved the noise equivalent count rate (NECR) for a long axial field-of-view PET system [14]. Although DOI discrimination using rise-time measurements for TOF-PET was reported [15-16], no timing measurements of Si-PM-based LGSO phoswich detectors have been reported to date. Phoswich Ce-doped Lu1.8Gd0.2SiO5 (LGSO) scintillators were used for a Si-PM-based small animal PET system [5], but it was not a TOF PET system with timing resolution of nanosecond order that was determined by the data acquisition system. Thus we conducted timing performance measurements of dual-layer phoswich detectors comprised of three types of LGSO scintillators with different decay times coupled to Si-PMs, digitized the output signals using a high bandwidth digital oscilloscope, and compared the performance with individual LGSO pixels.

II. MATERIALS AND METHODS We used three types of LGSOs with different Ce concentrations (Table 1). The decay times of LGSOs with different Ce concentrations from 0.025 to 0.75 mol% Ce have been reported to range from 34.2 to 44.8 ns, and LGSOs with higher Ce concentrations have a slower decay time [17]. Table 1 Composition of three types of LGSOs used for experiments LGSO type

Fast

Standard

Slow

Lu1.9Gd0.1SiO5

Lu1.9Gd0.1SiO5

Lu1.9Gd0.1SiO5

(0.025 mol%Ce)

(0.15 mol%Ce)

(0.75 mol%Ce)

Composition

The sizes of the LGSO crystal used for each measurement setup are listed in Table 2. The sizes in the individual performance measurements of the single elements were 2.9 x 2.9 x 20 mm3. All the sides of the LGSOs were chemically etched. A single element crystal was used to measure the timing performance for practical clinical 2

PET/CT systems [8-10] as well as PET/MR [18] systems that typically use ~20-mm-long crystals. The length of the phoswich crystals was 20 mm (10 mm x 2) to compare the performance with individual crystals.

Table 2 Sizes of LGSOs used for measurements Dimension single element

2.9 x 2.9 x 20 mm3

phoswich

2.9 x 2.9 x 20 mm3 (10 mm x 2)

The phoswich LGSOs are comprised of two 10-mm-long LGSOs of different decay times optically coupled in the depth direction for evaluating the timing performance for practical PET systems. For phoswich LGSOs, we prepared two types of LGSO crystal combinations: LGSO-standard/LSGO-fast and LGSO-slow/LGSO-fast. We set the LGSO-standard or LGSO-slow on the upper layer (further from Si-PM) and the LGSO-fast on the lower layer (contacting the Si-PM). We arranged them in this way (slower LGSO on the upper and LGSO-fast on the lower layer) because the upper region of the crystals provides better timing performance than the lower (closer to the photo-sensor) region. This is because the scintillation photons in the upper region reach Si-PM more simultaneously [13]. Thus we assumed that positioning the slower LGSO on the upper and the faster crystal on the lower will provide a uniform timing resolution along the depth direction.

A. Energy performance Energy performances were measured by optically coupling each LGSO or phoswich LGSO element to a Si-PM (S12572-050C, Hamamatsu Photonics, Japan) with a 3 x 3 mm active area and 50-µm microcell pixels, and gamma photons from Na-22 (511keV) were irradiated from the top of the scintillator. Signals from the Si-PM were amplified by a standard NIM module and fed to a multi-channel analyzer (MCA) (Clear Pulse, 1125, Japan). The resolution of the energy spectrum was evaluated with Gaussian fit software in the MCA.

B. Decay times The pulse shape was measured by optically coupling each LGSO element to a 2-inch round Photomultiplier tube (PMT) (Hamamatsu Photonics, R9779). We used a PMT for the decay time measurements because the pulse shape from the Si-PM is usually smoothed by the Si-PM’s high capacitance. Thus it is difficult to measure the precise 3

pulse shape from the Si-PM signals. To measure the precise decay times of the scintillators, we selected a PMT with fast response. Gamma photons from Cs-137 (662keV) were irradiated from the top of each 2.9-mm-long LGSO, and the output signals were fed to a digital oscilloscope (Yokogawa DLM2052: maximum sampling rate 500 MHz, 2.5 GS/s) with a 50-ohm resistor for termination. We plotted the data on a graph and evaluated the decay times. The curve fittings were conducted using the exponential fitting function of spread sheet software (EXCEL) and conducted five times at different time points. The average and standard deviation (SD) were also calculated.

C. Pulse shape spectra The pulse shape spectra were measured to evaluate their separation for phoswich LGSOs. Each 10-mm-long LGSO was optically coupled to a 4 x 4 Si-PM array (Hamamatsu MPPC S11064–050P) with 1-mm-thick light guide and gamma photons from Cs-137 (662keV) were irradiated. The size of the Si-PM array was 18 x 16 mm, composed of 16 (in a 4 x 4 matrix), 3 x 3 mm Si-PM elements. We used a 4 x 4 Si-PM array for the pulse shape spectra measurements because we already had a data acquisition system that used the array for acquiring the pulse shape spectra [5]. With this system, the Si-PM array signals were weight summed and digitized with 100-MHz free running analog-digital (A-D) converters and integrated with two different integration times: 120 ns (partial) and 320 ns (full). We computed the ratio of these two values to obtain the pulse shape spectra [5]. The data were read out by all 16 channels, but only the area below the crystals was used for calculating the pulse shape spectra. The pulse shape spectra for the phoswich configurations were derived by adding a pair of the acquired individual pulse shape spectra, and we also measured the pulse shape spectra for the phoswich configurations. These measurements were conducted with side-irradiations.

D. Timing resolution measurements with Si-PM Schematic drawings of the LGSO arrangements of the timing resolution measurements with Si-PM are shown in Figs. 1(A) and (B). For measurement with the 20-mm-long single-element LGSOs, the Na-22 point source was positioned at the vertical center of two LGSOs and measured with side-irradiations. We also measured the timing resolution at 5-mm-upper (+5 mm) and 5-mm-lower (-5 mm) positions relative to the vertical center to assess the timing resolution as a function of the source position (Fig. 1(A)-1). In addition, we measured the timing resolution with head-on irradiation (Fig. 1(A)-2).

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In the setup with a 20-mm-long phoswich with different decay times, the Na-22 point source was positioned at the vertical center of the LGSOs for measuring the timing resolution with side-irradiations. The timing resolution 5-mm upper (+5 mm) and 5-mm-lower (-5 mm) positions relative to the vertical center was also measured in this setup to assess the timing resolution as a function of the source height (Fig. 1(B)-1). From the side-irradiation measurements with different heights, we obtained the basic information of the timing performance for a phoswich DOI detector. We expected to find inferior timing resolution in the vertical center because the coincidence events between the fast and slow LGSOs have larger fractions. In addition, we measured the timing resolution with head-on irradiation (Fig. 1(B)-2), which reflects the actual timing resolution for the development of TOF-DOI-PET systems. We also measured the head-on condition with the LGSO-fast on the upper layer (further from Si-PM) and the LGSO-slow on the lower side (contacting the Si-PM) (LGSO-fast/LGSO-slow) and compared the performance with LGSO-slow on the upper layer (further from Si-PM) and the LGSO-fast on the lower side (contacting the Si-PM) (LGSO-slow/LGSO-fast) to test the difference of these two configurations.

(A)-1

(A)-2

(B)-1

(B)-2

Fig. 1 Schematic drawing of LGSO arrangements for timing resolution measurements with different sizes and LGSO configurations

A photo of our timing resolution evaluating system is shown in Fig. 2. The Si-PM used for the evaluation (Hamamatsu Photonics, S12572-050C) had a 3 x 3 mm active area and 50-µm microcell pixels. Each Si-PM signal was read out with a high-speed RF transimpedance amplifier (Mini-circuits TB-408-3+). Leading edge triggering

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was performed on the signals acquired with a digital oscilloscope (Agilent Technologies, DSO9104A, 1 GHz, 20 GSa/s).

Fig. 2 Photo of detector part of timing resolution evaluating system

The leading edge time differences between two LGSO phoswich block detector modules were saved offline, and the timing resolution was calculated by Gaussian fitting of the timing spectrum. No other algorithms or filtering was performed to reduce the effect of dark counts or to improve the timing. The setup and evaluation method are basically the same as previously reported [12-13]. For all the measurements, a Silicone compound (Shin-netsu Silicone, KE420, Tokyo) was used for the optical coupling of each LGSO scintillator and the Si-PM, and Teflon tape was used as the reflector. For each setup, the timing resolution measurements were conducted more than six times and the average and standard deviation were computed.

III RESULTS A. Energy performance The energy spectra of a 20-mm-long LGSO-fast, a LGSO-standard, and a LGSO-slow are given in Fig. 3(A). The photo-peak pulse-height channel number and energy resolution are listed in Table 3. The pulse heights and energy resolutions are almost the same for LGSO-slow and LGSO-standard, and those for LGSO-fast were slightly lower and worse. We also show the energy spectra for the LGSO-standard/LGSO-fast and LGSO-slow/LGSO-fast phoswich elements in Fig. 3(B). The energy distribution for the former showed a single photo-peak while the latter showed a double peak. The acquired photo-peak pulse-height channel number and energy resolution are also included in Table 3. The energy resolution for the LGSO-slow/LGSO-fast phoswich element was not evaluated because it had two peaks. 6

(A)

(B)

Fig. 3 Energy spectra of 20-mm-long LGSO-fast, LGSO-standard, and LGSO-slow for individually measured LGSO elements (A) and phoswich elements (B)

Table 3 Pulse height and energy resolution for 20–mm-long LGSOs

LGSO-fast

Pulse height (ch)

LGSO-standard

LGSO-slow

LGSO-standard

LGSO-slow

/LGSO-fast

/LGSO-fast

phoswich

phoswich

243

288

288

254

9.5

8.1

7.7

10.0

250/287

Energy resolution (% FWHM)

-

B. Decay times We show the LGSO-fast, LGSO-standard, and LGSO-slow pulse shapes in Fig. 4. The decay times were evaluated as listed in Table 4. The decay time was fastest for LGSO-fast and slowest for LGSO-slow. The decay time difference between LGSO-fast and LGSO-standard was 5 ns, and it was 14.2 ns between LGSO-fast and LGSOslow.

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Fig. 4 Pulse shape of individual LGSO-fast, LGSO-standard, and LGSO-slow

Table 4 Decay times for three types of LGSOs

Decay time (ns)

LGSO-fast

LGSO-standard

LGSO-slow

33.1 ±0.7

38.3±1.2

47.3±1.8

C. Pulse shape spectra The averaged pulse shapes for LGSO with Si-PM are shown in Fig. 5(A). The partial and full integration widths are schematically shown. The pulse shape spectra, which were individually measured for LGSO-fast, LGSO-standard, and LGSO, are shown in Fig. 5(B). The horizontal axis in Fig. 5 represents the ratio of partial to full integration to evaluate the pulse shape spectra, and the calculated value was multiplied by 128. Although each pulse shape spectrum overlaps its adjacent spectrum, the peak position shifted toward a higher channel with a shorter decay time.

(A)

(B)

Fig. 5 Average pulse shape (A) and pulse shape spectra for LGSO-fast, LGSO-standard, and LGSO-slow (B)

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Figure 6(A) shows the pulse shape spectra for the LGSO-standard/LGSO-fast phoswich detector measured individually and summed using 10-mm-long LGSOs. We identified two peaks in the distribution with a peak-tovalley ratio of 1.24. For the pulse shape spectra with a LGSO-standard/LGSO-fast phoswich detector (Fig. 6(A)), the calculated identification error of the LGSO-fast component in the LGSO-standard area was 14%, and the LGSOstandard component in the LGSO-fast area was 4.6%. Fig. 6(B) shows the pulse shape spectra for the LGSOstandard/LGSO-fast phoswich detector measured with phoswich configuration with 20-mm-long (10 mm x 2) LGSOs. We failed to observe a clear distinction between the two peaks with this configuration.

(A)

(B)

Fig. 6 Pulse shape spectra for LGSO-standard/LGSO-fast phoswich detector: individually measured with 10-mmlong LGSOs and summed pulse shape spectra (A), and measured with phoswich configuration (B) using 20-mmlong (10 mm x 2) LGSOs

The pulse shape spectra for the LGSO-slow/LGSO-fast phoswich detector measured individually and summed using 10-mm-long LGSOs are shown in Fig. 7(A). We identified two peaks in the distribution with a peak-to-valley ratio of 3.2. The calculated identification error of the LGSO-fast component in the LGSO-slow area was 1.3%, and the LGSO-slow component in the LGSO-fast area was 2.1%. The pulse shape spectra for the LGSO-slow/LGSO-fast phoswich detector measured with phoswich configuration using 20-mm-long (10 mm x 2) LGSOs are shown in Fig. 7(B). The separation of these two peaks slightly worsened when the peak-to-valley ratio degraded to 2.0. The calculated identification error was. 6.1%.

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(A)

(B)

Fig. 7 Pulse shape spectra for LGSO-slow/LGSO-fast phoswich detector: individually measured with 10-mm-long LGSOs and summed pulse shape spectra (A), and measured with phoswich configuration (B) using 20-mm-long (10 mm x 2) LGSOs

D. Timing resolution measurements with Si-PM We show the measured timing resolution of the LGSO-fast, LGSO-standard, and LGSO-slow for 20-mm-long scintillators in Fig. 8. As expected, the timing resolution was best for LGSO-fast, and those for LGSO-standard and LGSO-slow were similar.

Fig. 8 Timing resolution for LGSO-fast, LGSO-standard, and LGSO-slow for 20-mm-long LGSOs measured with side-irradiations

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The measured timing resolution of LGSO-fast, LGSO-standard, and LGSO-slow for 20-mm-long crystals at three different source positions and head-on irradiations are shown in Fig. 9. The timing resolution was best at +5 mm position (upper part of the scintillator) for all three types of LGSOs. The timing resolution of the head-on irradiation was almost the same or slightly worse than that measured at +5 mm position.

Fig. 9 Timing resolution of 20-mm-long LGSO-fast, LGSO-standard, and LGSO-slow at three different source positions measured with side- and head-on irradiations

The timing resolution of the LGSO-standard/LGSO-fast phoswich detector acquired with side-irradiations is plotted in Fig. 10, which also shows the timing resolutions for the individually measured LGSO-fast and LGSOstandard. The timing resolution for the LGSO-standard/LGSO-fast phoswich detector was slightly (~10-20 ps) worse than that of the LGSO-standard.

Fig. 10 Timing resolutions of 20-mm-long LGSO-fast, LGSO-standard, and LGSO-standard/LGSO-fast phoswich detectors acquired with side-irradiations 11

The timing resolutions of the LGSO-standard/LGSO-fast for a 20-mm-long phoswich configuration at three different source positions and head-on irradiation are shown in Fig. 11. The timing resolution was worst at the center position, and those for the upper and lower positions were similar to each other. The timing resolution of the head-on irradiation was almost the same as those of the upper and lower positions.

Fig.11 Timing resolution of 20-mm-long LGSO-standard/LGSO-fast phoswich detector at three different source positions LGSOs measured with side- and head-on irradiations

The timing resolution of the LGSO-slow/LGSO-fast phoswich detector is plotted in Fig. 12. The timing resolutions for the individually measured LGSO-fast and LGSO-slow are also given. The timing resolution for the LGSOslow/LGSO-fast phoswich detector is ~30-80 ps worse than that of the LGSO-slow.

Fig. 12 Timing resolution of 20-mm-long LGSO-slow/LGSO-fast and LGSO-slow/LGSO-fast phoswich detector measured with side-irradiations 12

As above, the measured timing resolution of the LGSO-slow/LGSO-fast for 20-mm-long detectors at three different source positions and head-on setup is shown in Fig. 13. It was worst at the center, and those of the upper and lower positions were similar. The timing resolution of the head-on condition was slightly worse than those of the upper or lower positions. The timing resolutions for the LGSO-fast on the upper layer (further from Si-PM) and the LGSO-slow on the lower side (contacting the Si-PM) (LGSO-fast/LGSO-slow) and the LGSO-slow on the upper layer (further from Si-PM) and the LGSO-fast on the lower side (contacting the Si-PM) (LGSO-slow/LGSO-fast) were almost the same, 305±10 ps and 305±11 ps, respectively.

Fig. 13 Timing resolution of 20-mm-long LGSO-slow/LGSO-fast phoswich detector at three different source positions measured with side- and head-on irradiation

IV. DISCUSSION For Si-PM-based LGSO phoswich detectors, we successfully acquired timing resolutions, which were slightly worse than those for individual LGSOs. However, since the timing resolutions of these phoswich detectors were ~300 ps FWHM, they are sufficient for TOF-PET systems because the timing resolution of commercial PET systems is around 500 ps FWHM, despite having no DOI information [8-10]. Si-PM-based phoswich detectors are a promising approach to realize a TOF-DOI-PET system. Si-PM-based phoswich detectors are also advantageous to realize a one-to-one coupling of phoswich scintillators and Si-PM channels. One-to-one coupling will improve the system’s spatial resolution because it does not require the Anger principle that may degrade the spatial resolution of the PET system. 13

We tested two types of LGSO-based phoswich detectors: LGSO-standard/LGSO-fast and LGSO-slow/LGSO-fast. Comparing their performances, the LGSO-slow/LGSO-fast combination is a better selection for a TOF-DOI-PET system because the pulse shape spectra had better separation for this detector (Fig.7). The separation of the pulse shape spectra of the LGSO-slow/LGSO-fast combination resembled that used for our Si-PM-based PET system [5]. Although the LGSO-standard/LGSO-fast phoswich detector had slightly better timing resolution, the separation of the pulse shape spectra (Fig. 6) is not appropriate for DOI detectors. The differences of the pulse shape spectra between the summed (Fig. 7(A)) and phoswich setup (Fig. 7(B)) were mainly caused by the inter-crystal scatter. The scattered events in the phoswich setup produced both decay curves that are positioned between the two peaks of spectra, decreasing the separation. In the pulse shape spectra from the summed data, no scattered events between the two types of LGSO were included. The timing resolutions at the upper region of the individual LGSOs were better than the lower side (Fig. 9). This is consistent with the results reported by Yeom et al. [13]. On the other hand, the timing resolution for the phoswich detectors was worse at the center, while those at the upper and lower sides were similar because we positioned the LGSO-fast at the lower layer of the phoswich detectors, thus improving the timing resolution at their lower side. We also set slower LGSOs (LGSO-standard or LGSO-slow) on the upper side of the phoswich detector; the timing resolution at the upper side of the detector was not much better. The timing resolution at the middle of the scintillators (Fig. 9) was worse than at the upper and lower positions because the Na-22 source was not collimated. The coincidences at the middle position are the sum of the results of the upper-lower, middle-middle, and lower-upper positions. Within these coincidences, lower-upper and upper-lower had the largest timing differences, leading to the worst timing resolution in the middle position. This phenomenon is more clearly observed in the phoswich detectors at the 0-mm position (Fig.13). We observed some coupling loss of scintillation light especially for the phoswich configuration, but it was reduced by positioning the LGSO with larger light output ((LGSO-standard or LGSO-slow) on the upper and smaller light output (LGSO-fast) at the lower. This configuration has an advantage for producing similar light outputs of the two types of LGSOs in phoswich detectors. We also measured the timing resolution for a head-on configuration and found that its timing resolution was as good as that measured for the upper side (Figs. 11 and 13). The reason was probably that a larger fraction of gamma photons (detected in the head-on condition) was located in the upper side of the detector under a similar condition as

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measured for the upper side (+5 mm position in Figs. 11 and 13). Because head-on is the most similar condition to the actual PET scanners, this is a promising result for the development of TOF-DOI-PET systems. From the measured results for LGSO-slow/fast phoswich detectors, we will be able to develop a high-resolution TOF-DOI-PET system with almost the same TOF resolution as a non-DOI-PET system because similar timing resolution was obtained with that of the LGSO-fast in the head-on condition. However, one disadvantage of the phoswich DOI detector is the cost increase due to additional detector elements. Another possible approach to realize TOF-DOI-PET detectors is the use of the offset arrangement of the scintillators combined with the Anger principle. With this approach, there is an advantage of using the same type crystals for fabricating the detectors. Compare to this offset arranged TOF-DOI-PET detectors; our phoswich TOFDOI-PET detectors have an advantage of realizing one-to-one coupling detectors which can improve the timing resolution. Another advantage of our phoswich TOF-DOI-PET detectors is the high resolution capability when we combined with the Anger principle because there will be wider margins between the calculated scintillator positions than those of the offset arranged detectors in 2-dimensional distribution.

V. CONCLUSION We measured the timing resolution for phoswich LGSO elements. The timing resolutions for the phoswich LGSOs were slightly worse than those measured between individual LGSOs, but a timing resolution of ~300 ps for 20-mm-long phoswich LGSOs is sufficient for TOF-PET systems. We conclude that LGSO phoswich detectors are promising for TOF-DOI-PET systems.

ACKNOWLEDGEMENTS This study was supported in part by the Ministry of Education, Science, Sports and Culture, Japan and National Research Foundation of Korea (NRF-2014R1A1A1005242) and “Creative ICT Convergence Human Resource Development” support program (NIPA-2014-H7501-14-1002) supervised by the National IT Industry Promotion Agency (NIPA).

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