Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–37

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Development of an energy-binned photon-counting detector for X-rayand gamma-ray imaging

Koichi Ogawa a,n, Toru Kobayashi a, Futoshi Kaibuki a, Tsutomu Yamakawa b, Tatsuya Nagano b,Daisuke Hashimoto b, Hideyuki Nagaoka b

a Department of Applied Informatics, Faculty of Science and Engineering, Hosei University, 3-7-2 Kajinocho, Koganei, Tokyo 184–8584, Japanb Telesystems Co., 2-8-19 Ebisu, Naniwa, Osaka 556-0003, Japan

a r t i c l e i n f o

Article history:

Received 27 February 2011

Received in revised form

8 October 2011

Accepted 11 October 2011Available online 29 October 2011

Keywords:

Photon Counting

Semiconductor Detector

X-Ray Imaging

Pixel Detector

Spectroscopic Imaging

CdTe

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.10.009

esponding author. Tel./fax: þ81 42 387 6189

ail address: ogawa@hosei.ac.jp (K. Ogawa).

a b s t r a c t

The aim of this study is to develop an energy-binned photon-counting (EBPC) detector that enables us

to provide energy information of x-rays with a reasonable count statistics. We used Al-pixel/CdTe/Pt

semiconductor detectors, which had an active area of 8 mm�144 mm and consisted of 18 modules

aligned linearly. The size of a CdTe detector module was 8 mm�8 mm and the thickness of the CdTe

crystal was 1 mm. Each module consisted of 40�40 pixels and the pixel size was 200 mm�200 mm.

We applied the bias voltage of �500 V to the Pt common electrode. The detector counted the number

of x-ray photons with four different energy windows, and output four energy-binned images with pixel

depths of 12, 12, 11 and 10 bits at a frame rate of 1200 Hz (300 Hz�4 energy bins). The basic

performance of the detector was evaluated in several experiments. The results showed that the detector

realized the photon counting rate of 0.4�106 counts/sec/pixel (107 counts/sec/mm2), energy resolution

4.4% FWHM at 122 keV. The integral uniformity of the detector was about 1% and the differential

uniformity was about 1%. In addition, the image quality was examined with a resolution chart and step-

wedge phantoms made of aluminum and polymethyl methacrylate. And we compared the quality of an

acquired image with that acquired with an energy integration detector. The results of these

experiments showed that the developed detector had desirable intrinsic characteristics for x-ray

photon counting imaging.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Recently, pixelated semiconductor detectors such as CdTe andCdZnTe, which work at room temperature, are being increasinglyused in the fields of medicine and non-destructive inspection.These semiconductor detectors have a high energy resolution andhigh intrinsic spatial resolution, which is basically determined bythe pixel size. With these advantages, the application of thesesemiconductor detectors has been studied by many researchers inthe field of nuclear medicine [1–8]. However, the photopeakenergy of the radionuclide mostly used in examinations isrelatively high (i.e. Tc-99 m: photopeak energy 140 keV), andthus the semiconductor material must be made thick. But thisintroduces a difficulty in manufacturing this kind of detector interms of the configuration of the electrodes enabling an efficientcarrier collection. At present, the implementation of the semi-conductor detector to commercially available clinical modalities

ll rights reserved.

.

is restricted to only a few specific applications such as myocardialSPECT imaging [9,10].

On the other hand, the development of a pixelated semicon-ductor detector, which is available for x-ray imaging, has alsobeen pursued by many researchers [11–13]. In the case of x-rayimaging the photon flux is very high, and thus most conventionaldetectors including ones used for x-ray CT adopt an energyintegration of detected photons. However, a detector that enablesa photon counting mode can yield the energy information of anindividual photon. And this kind of information can reduce thepatient dose [14] and increase the signal to noise ratio of objects[15,16]. In addition, this energy information is useful in removingbeam-hardening artifacts [17,18] and discriminating materials[19,20]. Thus many photon counting detectors for x-ray imaginghave been developed [21–35]. The requirement of a photoncounting detector for x-ray imaging is a high count rate such as106–108 counts/sec/mm2, and a spatial resolution of several ten tohundred micron meters [11]. If we realize these requirements, theapplication-specific integration circuits (ASICs) that count eachphoton become complicated, and much power is required tocount and readout. One practical solution to meet these require-ments is to simplify the circuit by reducing the number of

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–3730

components. Our previous work on improving the contrast of anobject and determining of unknown materials with a photoncounting approach [17,19] showed that the number of energywindows required was at least four, and so we simplified theASICs by reducing the number of discriminators to four. With thisdetector we can obtain four images having the information ofenergy acquired with these four energy windows with a reason-able statistics. A similar CdTe detector was realized with aMedipix2 readout chip [22] to increase the detection efficiencyof high energy x-rays, although the size of a pixel was 55 mm�55 mm with the thickness of 1 mm. This geometry increases themulti-pixel events caused by charge sharing. And so we enlargedthe size of a pixel and decreased the effect. Basolo et al. developeda similar photon counting detector with an XPAD chip [32] andthis detector achieves a very high counting rate. Hirono et al. alsodeveloped a CdTe pixel detector with a custom ASIC for thedetection of synchrotron radiation [33]. These detectors with awindow type discriminator can improve the quality of imagesusing the two energy windows. However we think that thenumber of discriminators is not sufficient for some applicationssuch as material discrimination, in which several numbers ofenergy-binned data are used. In this paper, we outlined the basicperformance of our newly developed energy-binned photon-counting (EBPC) detector for x- and gamma-ray imaging.

2. Materials and methods

2.1. Requirements of the EBPC detector

The basic requirements of the detector are as follows:

(1)

Fig.CdTe

Each

The photon counting rate is very important to acquirephotons under high-flux x-rays with a large tube-current.We targeted a pile up count-rate loss of 1% at a count rate of0.15�106 counts/sec/pixel with a pixel size of 200 mm�200 mm, which is acceptable for a flat panel detector. If adetector achieves this count-rate performance, the detectorcan be applied to most x-ray imaging.

8 mm40 pixels

8 mm40 pixels

Module structure Detector co18 mo

1mm

Top view

Side viewElectrodes

Bump bond

Photon CountingASIC

CdTe

PCB

Photo of a module

PtAl

1. Structure of a detector module and its photograph. The detector consisted of 1

crystal with the thickness of 1 mm and the size of 8�8 mm2 that was bump-bon

pixel has four comparators and makes four energy binned photon counting imag

(2)

nsisdule

8 mo

ded t

es w

The size of a pixel is very important to determine the imagingperformance of a pixelated detector in terms of the geome-trical efficiency and spatial resolution. In the case of amonolithic detector the pixel size is equivalent to an areaon which the ASICs are embedded just behind the bump ball.From the view point of a carrier collection the size of a pixeland the thickness of a crystal are important to reduce themulti-pixel events caused by charge sharing. We selected apixel size of 200�200 mm2, and a CdTe crystal thickness of1 mm. The maximum targeted energy of the x- and gamma-rays was 140 keV, which is required for industrial applica-tions such as nondestructive inspections and medical applica-tions including digital radiography and nuclear medicine.

(3)

The spectroscopic measurement of x-rays is important for theapplication of a material discrimination, nondestructiveinspection and clinical application. And the measurement ofenergy of an individual x-ray photon under a high flux ischallenging when designing ASICs. However, in most applica-tions the exact energy of an individual photon is not alwaysrequired, and so we limited the number of comparators tofour for each pixel. By restricting the number to four,difficulties in ASIC design and manufacture were reduced,and in the detection of x-rays we could use the energyinformation of detected photons roughly while keeping areasonable counting statistics.

(4)

The size of a detector module is also an important issue interms of the cost and yield rate of the crystal. We designed acompact module with a size of 8�8 mm2. With this modulewe can easily extend an active area of the detector byabutting the modules piece by piece. The distance betweenthe modules was 400 mm (2 pixels).

2.2. Realized detector and details of ASICs

We realized an EBPC detector consisting of 18 CdTe detectormodules as shown in Fig. 1. And we newly developed an ASIC thatis an integrated circuit for counting and reading out charge pulsesfrom a CdTe radiation detector. The ASIC chip was designed in

40 pixels

720 pixels

ts ofs Acquired images

bin0 bin1 bin2 bin3

dules resulting in an active area of 8�144 mm2. Each module consisted of a

o an ASIC, and making 40�40 pixels with the pixel size of 200 mm�200 mm.

ith a frame rate of 300 fps.

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–37 31

CMOS 0.25 mm technology. Each module consisted of 40�40pixels, with the size of a pixel on the detector 200 mm�200 mm.The pixel pitch on the readout was 196 mm�192 mm to allow thereadout to be narrower than the detector for three-side butt-ability. The gap between the pixel electrodes was 80 mm. Aroundthe module a guard ring electrode with a width of 15 mm wasattached. We adopted a Schottky contact with Aluminum (thick-ness: 20–50 nm) to improve the energy resolution and reduce thedark current. The low leakage current and the high energyresolution of the Schottky type CdTe diode are clearly describedin Ref. [34,40,41]. The reasons why we used the Al electrode aredescribed later. We used a wafer manufactured from a mono-crystal grown using the traveling heater method by ACRORAD(Japan) [36]. Our detector differs from the early work of the CdTephoton counting detector realized by Fisher et al. [24] in theconfiguration of the electrodes and the thickness of the crystal.The side onto which the radiation impinges was covered with acontinuous Pt electrode, and the pixel electrode was Al, and Sn–Biwas used for a bump-bonding material to the ASICs. The config-uration of the Al/CdTe/Pt detector was first developed by Toyamaet al. to reduce the leakage current and improve the energyresolution of the detector [37]. And the realized Al-pixelateddetector was evaluated by Watanabe et al. in a comparison withthe In/CdTe/Pt detector [38]. The energy resolution of In/CdTe/Ptdetector is better than that of Al/CdTe/Pt detector; however theAl/CdTe/Pt detector enables us to make electrodes on the anodeside to utilize the charge induced by electron movements ratherthan the hole movements. This configuration yields a pixelateddetector with a good energy resolution, and so we adopted thisconfiguration in our EBPC detector. We collected only electrons asa carrier. The bias voltage �500 V was applied to the Pt commonelectrode on the CdTe crystal with a thickness of 1 mm. Wethought that the bias voltage was sufficiently high to activatealmost the whole volume. The activation of a CdTe detector isdescribed in the literature [40,41,34]. The low leakage current ofthe Schottky type CdTe detectors enables us to apply high biasvoltage onto a relatively thin device. This treatment makes thedetector a fully active device [40,41,34] and also realizes a hightiming response (high carrier speed), which are important char-acteristics for imaging detectors when we use CdTe or CdZnTe.The theoretical absorption probability of the crystal is 82.9% at80 keV and 62.4% at 100 keV [13]. These figures were calculatedwith the photon cross-sections’ database [49]. This Schottky CdTediode detector exerts a polarization effect [35,39–42], and so weturned the bias voltage off every several seconds as well as thedetector was inactive. The interval of the switching off of the biasvoltage depends on the application of this EBPC detector. Eachpixel had an integrating preamp with a continuous resistive reset.There were two gain ranges (14–150 keV and 9–100 keV) thatwere selectable via a control bit. Following the preamp was ashaper with adjustable shaping time (500 or 300 nsec). Theoutput of the shaper then went into a bank of four comparatorswith adjustable thresholds. Each comparator output went to acounter of 10, 11, or 12 bits. From the lower energy bin to thehigher energy bin we set 12, 12, 11 and 10 bits/pixel. Uponexternal command, counting was inhibited and the countercontents were read out through five outputs. The readout opera-tion resets the counters and the counting process began again. Allbiases and control functions were programmable through a serialport. Additionally, there was the capability to monitor criticalsignals in each pixel through a programmable diagnostic port. Thepower dissipation of our detector was very small, and the ASICoperated with a power of 151 mW at 300 nsec shaping time, soeach pixel enabled counting with four energy windows with apower of 94.4 mW. A sleep mode, in which the power dissipationwas kept the lowest level, was also available when the detector

was inactive. A block diagram of the ASIC and the implementationof the discriminators in a pixel are shown in Fig. 2, and specifica-tions of the EBPC detector are shown in Table 1.

2.3. Evaluation of the EWPC detector

The realized detector was evaluated in terms of its physicalproperties and imaging qualities. For the physical properties wetested the count rate performance, energy resolution and uni-formity of the detector. We set four threshold energies in theacquisition of x-rays, and measured the number of x-ray photonswith four energy windows. We used the x-ray tubes L9121(Hamamatsu Photonics, Japan) and TRIX-150S (Toreck, Japan) inthis experiment. We first measured the energy spectra ofour x-ray tubes with a CdTe semiconductor detector XR-100T(Amptek, USA). The calibration of the EBPC detector was con-ducted by referring to the highest end of an energy spectrum ofx-rays for different tube voltages. The accuracy of this calibrationmethod was confirmed with standard radioisotopes commonlyused for energy calibration. We set four thresholds (30, 40, 50 and60 keV) and acquired EBPC images of the following energy bins:bin0 (30–39 keV), bin1 (40–49 keV), bin2 (50–59 keV) and bin3(60–79 keV).

2.3.1. Physical properties

In the measurement of the count rate performance of thedetector, we first confirmed the linearity between the tubecurrent and the number of counted photons with a standarddetector XRIX-150S. In this experiment we used an x-ray tubeL9121 and changed the tube current from 0 to 60 mA. The tubevoltage was 80 kV. An aluminum filter with a thickness of 0.5 mmwas used. The distance between the x-ray focal spot and detectorwas 12 cm. The data acquisition time was 1 s. We measured themean count of the detector module located at the center of theactive area with a size of 40�640 pixels.

In the measurement of the energy resolution we operated the Al-pixel/CdTe/Pt detector in a thermostatic chamber at 25 1C. Theapplied bias voltage was �500 V for the Pt common cathode. Weused the standard sources of Am-241 (photopeak energy: 60 keV)and Co-57 (photopeak energy: 122 keV). We scanned the thresholdto determine the noise level and decided the lowest energy thresh-old of the detector beforehand. The low energy threshold was about15 keV. The number of pixels used in this measurement was 1600,that is, we used one module without the calibration of the gain ofeach pixel. The measurement of the energy spectra was conductedby changing the threshold level of the ASIC, which was equivalent tothe threshold energy, and measuring the number of photons withonly one energy window. The distance between the position of thesource and detector was 2.5 mm, and each data acquisition timewas 1 min. We subtracted the total measured count of the specifiedthreshold energy with the succeeding measured count with thedifferent threshold energy, and obtained the energy spectra ofAm-241 and Co-57. The energy resolution was calculated with thefull width at half maximum (FWHM) of these two photopeaks.

In the measurement of the uniformity we used an x-ray tubeTRIX-150S. We set a tube current of 1.0 mA, and tube voltage of80 kV. Data acquisition time was 8 s. And we repeated thismeasurement 48 times and added those count data pixel by pixel.The filtration material was aluminum with a width of 10 mm.We detected dead pixels in which photons are abnormallymeasured beforehand and replaced the count with the averageof the neighboring pixels. The uniformity was evaluated numeri-cally with an integral uniformity and differential uniformity inthe active area. No standards are available for x-ray semiconduc-tor detectors, and so we used the uniformity measures described

Clock

Clock Data

Data

Out 4Out 1Out 0 Out 3Out 2

8:1 FastMux 8:1 FastMux 8:1 FastMux 8:1 FastMux 8:1 FastMux

Pixel Control Register

Mux

Voltage & Current Monitor

Threshold DACs Bias DACs

Chip Control Register

Key Voltages& Current

40 x 40 Pixel Array

Con

trol B

its

Fig. 2. (A) Block diagram of the ASIC and (B) implementation of discriminators for a pixel. For each pixel there is a 4-bits threshold trim DAC for each energy bin. The pixel

depth of energy bin bin0, bin1, bin2 and bin4 is 12, 12, 11 and 10 bits, respectively.

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–3732

in the NEMA standard [43]. The distance between the position ofthe source and detector was 56 cm.

2.3.2. Imaging performance

To evaluate the quality of images acquired with the EBPCdetector, we conducted experiments with several phantoms. Inthese experiments we used an x-ray tube TRIX-150S. The tubevoltage was 80 kV and the tube current was 1 mA. The dataacquisition time was 4 s. The filter used was aluminum with awidth of 10 mm. The distance between the x-ray tube anddetector was 56 cm.

To evaluate the spatial resolution we used a resolution chart‘‘x-ray test chart (type 7)’’ (Mitsubishi Chemical Co., Japan), inwhich there were eight slit patterns (0.5, 1.0, 1.5, 2.0, 2.5, 3.0,4.0 and 5.0 lp/mm). To avoid Moire patterns caused by inter-ference between the slit patterns and the pixel alignment we setthis chart with a small tilting angle to the pixel alignment in thedetector. The resolution chart was located just in front of thedetector.

To evaluate the number of transmitting photons in an objectwith each energy bin, we conducted experiments with two step-wedge phantoms. One was an aluminum step-wedge phantom,and the thickness of the aluminum increased from 4.5 mm to

Fig. 3. Count rate characteristics of the detector. The count rate increased linearly

up to 0.4 Mcps/pixel (10 Mcps/mm2) and saturated around 0.5 Mcps/pixel

(12.5 Mcps/mm2).

Table 1Specifications of the ASICs.

Specification Realized Value

Array configuration 40�40 (8 mm�8 mm)

Pixel size 200 mm�200 mm

Input charge polarity Negative (collecting electrons)

Energy ranges 9–100 keV or 14–150 keV

Shaping time 300 nsec or 500 nsec

Pileup o1% loss at 150 kcps for 500 nsec shaping time

RMS Noise 0.3–0.4 keV (not including dark current shot noise)

Power Dissipation 116 mW (@500 nsec shaping time)

151 mW (@300 nsec shaping time)

Number of energy ranges 4

Counter bits (energy range) 12 (bin0), 12 (bin1), 11 (bin2), 10 (bin3)

Threshold accuracy less than 70.5 keV

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–37 33

22.5 mm in 3 mm increments (nine steps). The other one was apolymethyl methacrylate (PMMA) step-wedge phantom, and thethickness of PMMA increased from 15 mm to 60 mm in 15 mmincrements (four steps).

2.4. Comparison of the EBPC detector with an energy integrating

detector

To evaluate the performance of the EBPC detector we conductedan experiment with a detector that had almost the same specifica-tions of a CdTe crystal except for the data acquisition mode. Thecompared detector was SCAN300F (Ajat Oy, Finland) [44]. The activearea of this detector was 6.4�151.0 mm and the detector consistedof six hybrids. The size of a hybrid was 6.4�25 mm and this moduleconsisted of 64�250 pixels with the size of a pixel on the detector100 mm�100 mm. The thickness of the detector was 0.75 mm.

We compared the performance of our EBPC detector andSCAN300F with a phantom. The phantom consisted of fourcylinders (12 mm|, volume: 2.4 ml) made of PMMA and a watercontainer with the size of 30 mm (width) �160 mm (length)�30 mm (height). We put these four cylinders in the container,and in these four cylinders we put iodine solution with differentconcentrations of Iopamidol (3%, 4%, 5% and 6%). The x-ray tubevoltage was 70 kV, and data acquisition time was 5 s for bothdetectors. The tube current was 1.2 mA and we used an Al filterwith a width of 1 mm. For the EBPC detector we set four energythresholds (35, 40, 45 and 50 keV), and acquired four energy-binned images (bin0: 35–39 keV, bin1: 40–44 keV, bin2:45–49 keV, and bin3: 50–70 keV). To make a composite image withthe above energy-binned images, we used the following equation:

composite_image¼imagebin0ð1=37:5Þ3þ imagebin1ð1=42:5Þ3þ imagebin2ð1=47:5Þ3

ð1=37:5Þ3þð1=42:5Þ3þð1=47:5Þ3

ð1Þ

In this experiment we used only three energy-binned images(bin0–bin2). The weight of each image was E�3, where E was thecentral energy of the corresponding energy bin [15].

3. Results

3.1. Physical properties

Fig. 3 shows the relationship between the tube current andoutput count rate. This graph shows a good linearity between thetube current and the count rate up to a count rate of0.4�106 counts/sec/pixel (107 counts/sec/mm2).

Fig. 4 shows the energy resolution of the detector. This graphshows that the energy resolution of the EBPC detector was 4.4%FWHM at 122 keV and 7.5% FWHM at 60 keV. These energy

resolutions represent moderate values for a typical CdTe semi-conductor area detector.

Table 2 shows the integral uniformity and differential uni-formity of the detector. The averaged integral uniformity of thefour bins was 1.26%, and the averaged differential uniformity was0.97%. The numbers of photons in a pixel in bin0, bin1, bin2 andbin3 were around 2,100,000, 2,500,000, 2,100,000 and 980,000,respectively.

3.2. Imaging performance

Fig. 5 shows the image of the resolution chart acquired withbin0–bin3, and the density profile curves of the x-ray chart imagealong the horizontal line (25th line from the top). The profiles areshown for bin0 and bin3. To avoid the artifact caused by theinterpolation process of a gap between two modules, we showedthe original images without gap interpolation. These images showthat the spatial resolution of the detector was 2–2.5 lp/mm,which is the theoretical upper limit of the resolution.

Fig. 6 shows the logarithmic display of images of the alumi-num step-wedge phantom acquired with energy bins (bin0–bin3)and the count profile curves of these images at the center position(averaged counts of 11 lines) of each image. And Fig. 7 showsthose of the PMMA step-wedge phantom. From these graphs wecan confirm that there is a wide linearity of the detector for thenumber of detected photons passing through the objects.

Fig. 4. Energy resolution of the detector. The energy resolution was measured

with Am-241(photopeak: 60 keV) and Co-57(photopeak: 122 keV) point sources.

The sum of energy spectra of 1600 pixels without calibration for each radioisotope

is shown.

Table 2Uniformity of the EBPC detector.

bin0 bin1 bin2 bin3

Integral uniformitya 1.11% 1.27% 1.01% 1.68%

Differential uniformityb 0.90% 1.00% 0.76% 1.23%

a Integral uniformity is defined by the difference between the maximum and

the minimum divided by the sum of these two values in the whole active area.b Differential uniformity is calculated with the difference between the max-

imum and the minimum divided by the sum of these two values in a set of five

contiguous pixels. Above difference is compared among all sets in the active area,

and the maximum difference becomes the differential uniformity.

bin0

bin1

bin2

bin3

Spatial resolu1.522.535 4

Fig. 5. Acquired images (bin0–bin3) of a resolution chart and their density profiles. Acq

of the detector was about 2.5 lines/mm. The profiles of images (bin0 and bin3) along a

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–3734

3.3. Comparison of the EBPC detector with an energy integrating

detector

Fig. 8 shows the obtained images of the phantom with theEBPC detector and SCAN300F. In these images the count values in

0.5

(3000,12000)

(2000,7000)

Display range(min,max)

(3000,12000)

(3000,12000)

bin0

bin3

tion (lpm)1

uired images of the resolution chart for bin0–bin3 show that the spatial resolution

center line indicate the variation of counted photons quantitatively.

bin0: 30 39 keV

bin1: 40 49 keV

bin2: 50 59 keV

bin3: 60 79 keV

Thickness of Aluminum (mm)4.57.510.513.519.522.525.528.5 16.5

Fig. 6. Acquired images (bin0–bin3) of the aluminum step-wedge phantom and

their density profiles. Acquired images show the uniformity of the detector

sensitivity of four energy windows. The density profile of each image along the

center line shows that the beam hardening effect decreased the number of many

photons in the acquisition of the low energy bin (bin0). That is, the slope of the

step curve of bin0 is steeper than that of bin3.

bin3: 60 79 keV

15304560Thickness of PMMA (mm)

bin0: 30 39 keV

bin1: 40 49 keV

bin2: 50 59 keV

Fig. 7. Acquired images (bin0–bin3) of the PMMA step-wedge phantom and their

density profiles. As compared with the results of the aluminum step wedge

phantom the effect of the beam hardening effect was small, and so the slope of the

step curves of the four bins was also small.

SCAN300F

EBPC

water

water with Iopamidole

Fig. 8. Images of the iodine-solution cylinder phantom acquired with the EBPC

detector and SCAN300F, and their density profiles. The concentration of Iopamidol

differs from these four cylinders (from left to right: 3, 4, 5 and 6%).

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–37 35

each image are normalized with the count at the part of water.And to evaluate the effect of the energy weighting quantitativelywe showed profiles along the horizontal line of the images. Wecould enhance the contrast of the object by multiplying the largeweight corresponding to the attenuation coefficient determined

by the photoelectric absorption to the images collected in the binwith the lower energy.

4. Discussion

Semiconductor detectors are used mainly in the field of nuclearmedicine where the photon flux is low, because a collimatordecreases the geometrical efficiency, and the amount of radioactivityadministered to the patient is restricted. On the other hand, as arequirement for an x-ray detector, the detector must work under ahigh photon flux. And so if we develop a monolithic semiconductordetector, sophisticated technology is required in photon countingASICs and manufacture of the electrodes. In this paper we showed ournewly developed detector that was available for measuring photonswith four specified energy windows. The results of the experimentsshowed that our EBPC detector had the following sufficient photoncounting characteristics for x-ray imaging. Our EBPC detector showslinearity between the tube current and output counts up to a countrate of 0.4�106 counts/sec/pixel (107 counts/sec/mm2). Beyond thiscount rate the peak pulse pile-up effect [45] occurred. It is verydifficult to measure the exact count-rate loss in our EBPC detector;however we can surmise the performance of the counting ability ofthe detector in Fig. 3.

As for the energy resolution of the detector, the resultantresolution was acceptable for the CdTe semiconductor detectordeveloped for an area detector. The design concept of the detectordiffers from those developed by the other researchers. And insome aspects, the performance of our detector is lower than thatof ordinary detectors, which maximize the performance in termsof the energy resolution. In our detector the temperature is 30 1Cnear the ASICs that are bump-bonded to the crystal. If wedecrease the temperature of the detector, we can obtain a verygood energy resolution. However, a large heat sink and additionalequipment are needed to cool down the detector. And so wedecided to operate the detector module under a less than idealcondition in terms of the energy resolution. In our detector thethreshold accuracy was controlled to within70.5 keV for eachpixel, so that the theoretical energy resolution of a pixel will behigher than the measured value, if the energy is well calibrated.As regards the low energy threshold, which was the upper limit ofthe noise level, the value was about 15 keV with a bias voltage of�500 V. In this research we could not measure the leakagecurrent, but we consider that the low energy threshold is a usefulfigure to evaluate the amount of the leakage current. Thus, weusually set the lowest threshold above 25 keV avoiding themarkedly increased multi-pixel event caused by charge sharingeffect including the leakage current. The capacitance of thedetector is also an important factor that affects the energyresolution of the detector. We did not measure the capacitanceof each pixel actually, but it is estimated to be 5 fF referring toRef. [46]. A detailed analysis of the energy resolution for eachsmall pixel is one of our future research tasks. The uniformity ofthe detector is also very important in the semiconductor detector.Fig. 6 shows one of the features of our EBPC detector. Theuniformity for several count levels is quite good for each energybin. Generally, such a small number of pixels do not work orcorrectly count photons. However the number of dead pixels inour EBPC detector was quite small, and the total number of deadpixels was 200 of 27,200 pixels (0.74%). And so in the case of anydead pixels we could correct their value by referring to thesurrounding pixels. Most of the dead pixels are located at theedges of a module, and this is caused by the manufacturingprocess of bump ball bonding or electrodes including a guard ring.The dead pixels are attributable to the following factors. Smallcracks or an insufficient polish on a crystal causes dead pixels and

K. Ogawa et al. / Nuclear Instruments and Methods in Physics Research A 664 (2012) 29–3736

so these pixels may occur everywhere on a module. The physicalforce, which is applied when a module wafer is manufactured or amodule is bump-bonded to the ASICs is also one of the causes ofthe dead pixels at the periphery of the module. Imperfect bump-bonding may also cause a dead pixel. The temperature andhumidity also promote the formation of the dead pixels, byaffecting the amount of leakage current. Similar reasons are alsodiscussed in [7,22,32,50,51]. The bias voltage of the detector was�500 V, which was selected in consideration of the energyresolution and uniformity, that is, if we increase the bias voltage,the energy resolution increases, but the uniformity of pixelsensitivity decreases especially at the periphery of a module.The integral uniformity and differential uniformity after the deadpixel correction showed considerably good figures, mainly ascrib-able to the homogeneity of a CdTe crystal. In this calculation theedge pixels are extracted beforehand as described in the stan-dards. And so the original physical characteristics of our detectorwere superior to those of the detectors that we have hithertoevaluated in the development of a semiconductor detector forclinical use. In the EBPC detector the temperature of the detectorhybrid was controlled with a Peltier device attached to a heatsink, and the temperature of the crystal and ASICs was kept atabout 16 1C.

The quality of acquired images is very important to an imagingdetector. With regard to the spatial resolution, the theoretical resolu-tion limit was 2.5 lp/mm, because the pixel size was 0.2�0.2 mm2. InFig. 5 line patterns of 2 lp/mm are clearly resolved. If we decrease thesize of a pixel to increase the spatial resolution, multi-pixel eventscaused by charge sharing effects [47,48] decrease the spatial resolu-tion and energy resolution. If we compare our EBPC detector with theMedipix2 detector, the size of a pixel was relatively large and theaspect ratio of a pixel defined by the size of a pixel and thickness ofthe crystal was large, as a result of which the multi-pixel eventscaused by charge sharing did not markedly affect the image quality.However, there is some amount of multi-pixel events with which wehave to cope. Various reduction methods of the charge sharing havebeen proposed by researchers. We will investigate the amount of themulti-pixel events caused by charge sharing effect in the next stageand develop a reduction method, which is applicable even under highflux x-rays.

Figs. 6 and 7 show images acquired with our EBPC detector andtheir profiles along a horizontal line. These images show the numberof transmitted photons passing through an aluminum or PMMA withdifferent thicknesses. The count at each thickness decreases linearlyin a logarithmic scale. But the behavior of bin0 differs from that of theothers in Fig. 6. We attributed this to the effect of multi-pixel eventscaused by charge sharing and increase in the number of low energyphotons.

Fig. 8 shows the results of a comparison between the photoncounting detector (EBPC) and energy integration detector(SCAN300F). The results show that we can enhance the contrast ofa medium by multiplying predefined weights to acquired images andmaking a composite image by adding these images. Another advan-tage is that if we can acquire these energy-binned images withdifferent energy windows, we can identify an unknown material,which is not feasible with the data acquired with the energyintegration detector.

The results of these experiments showed that the overallperformances of the EBPC detector were acceptable for practicalapplication with a photon counting method.

5. Conclusion

We developed a CdTe EBPC detector that measured x-ray photonswith four energy bins, and evaluated the physical performance and

imaging quality of the detector with several phantoms. Moreover, wecompared the quality of an acquired image with that acquired withan energy integration detector. The results of the experimentsshowed that the photon counting rate was linear up to a count rateof 0.4�106 counts/sec/pixel, the sensitivity of the detector wasconsiderably uniform, and the energy resolution was 4.4% FWHM at122 keV. The image acquired with a resolution test chart showed thatthe spatial resolution of the detector was 2.5 lp/mm. The images ofstep-wedge phantoms made of aluminum and PMMA showed thewide dynamic range of the EBPC detector. And compared withthe energy integration detector, we could enhance the contrast ofthe image acquired with our EBPC detector after multiplying aweighting factor. The results of these experiments showed that thedeveloped detector had desirable intrinsic characteristics for x-rayphoton counting imaging.

Acknowledgment

This work was supported in part by the Ministry of Education,Culture, Sports, Science and Technology, Grant-in-Aid for Scien-tific Research (B) 23390307, 2011.

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