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A new gamma camera with a Gas ElectronMultiplierTo cite this article T Koike et al 2012 JINST 7 C01078

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2012 JINST 7 C01078

PUBLISHED BY IOP PUBLISHING FOR SISSA

RECEIVED November 14 2011ACCEPTED January 4 2012

PUBLISHED January 23 2012

2nd INTERNATIONAL CONFERENCE ON MICRO PATTERN GASEOUS DETECTORS29 AUGUST ndash 1 SEPTEMBER 2011KOBE JAPAN

A new gamma camera with a Gas Electron Multiplier

T Koikea1 S Unob T Uchidab M Sekimotob T Murakamib M Shojic

F Nagashimad K Yamamotoe and E Nakanoe

aInternational University of Health and WelfareOtawara-shi Tochigi 324-8501 Japan

bHigh Energy Accelerator Research Organization (KEK)Tsukuba-shi Ibaraki 350-0801 Japan

cThe Graduate University for Advanced StudiesTsukuba-shi Ibaraki 350-080 Japan

dTohoku Gakuin UniversityTagajyo-shi Miyagi 985-8537 Japan

eOsaka City UniversitySumiyoshi-ku Osaka 558-8585 Japan

E-mail takahisaiuhwacjp

ABSTRACT We have developed a prototype gaseous gamma camera with a Gas Electron Multiplier(GEM) for biomolecular analysis and medical applications The system consists of three devicesa detector an Ethernet hub and a PC The detector consists of a GEM-chamber and integratedreadout electronics The GEM-chamber consists of three parts the gamma-ray conversion andamplification layers consisting of GEM foils and a readout component To increase gamma-ray-detection efficiency we constructed a new type of GEM foil The gamma-ray conversion layersare plated with gold 3 microm thick on both surfaces In order to test the performance of the prototypecamera several measurements were performed with a phantom filled with 99mTc (141 keV) as theradioactive source and two-dimensional (2D) images were obtained using a pinhole collimatorThe camera showed good capability to resolve objects of a few mm2 We believe that this systemwill offer new knowledge in biomolecular analysis and medical applications

KEYWORDS Gamma camera SPECT PET PETCT coronary CT angiography (CTA) Detectordesign and construction technologies and materials Gamma detectors (scintillators CZT HPGHgI etc)

1Corresponding author

ccopy 2012 IOP Publishing Ltd and SISSA doi1010881748-0221701C01078

2012 JINST 7 C01078

Contents

1 Introduction 1

2 Gamma camera systems with GEM 221 GEM chamber 222 Readout electronics module 423 Image processing software 5

3 Basic Experiment 631 Dependence of counting rate on applied voltage 632 Dependence of spatial resolution on drift gap 733 Linearity of radioactivity 834 Phantom studies 8

4 Conclusion 9

1 Introduction

The power of nuclear medicine lies in its ability to provide exquisitely sensitive measurements ofvarious biological processes by radionuclide imaging which is one of the most important appli-cations of radioactivity in nuclear medicine Small animals such as mice or rats are widely usedbecause of their generic resemblance to humans and ability to mimic their healthy and diseasedstates Gamma cameras have proven to be one of the most useful tools in the field of medical di-agnostics and new pharmaceuticals development because they are capable of imaging low-energygamma-ray emitters such as 99mTc (141 keV) which is distributed within a human body or smallanimals to study molecular process in vivo

Conventional gamma cameras using NaI(Tl)PMT scintillation detectors have inherent limita-tions in spatial resolution and the detector system is very expensive The system spatial resolutionof our prototype is 2ndash3 mm The basic detector element of a clinical gamma camera system isbased on the Anger camera (NaI(Tl) scintillation counter) which offers a system spatial resolutionof 6ndash10 mm Gamma cameras using a gas electron multiplier (GEM) have been shown to offerpotential improvements in basic performance relative to conventional gamma cameras in terms ofspatial resolution and system costs

The GEM which was first introduced in 1996 [1] has been used to detect uncharged particlessuch as X-rays and gamma rays [2] A GEM foil consists of a thin metal-clad polymer foil piercedwith a high density of holes by plasma etching Upon application of a potential difference betweenthe two electrodes electrons released by radiation in the gas drift to the holes of the following foilaccording to the potential grading multiply and finally move to a collection region The multiplier

ndash 1 ndash

2012 JINST 7 C01078

can be used as a detector on its own or as a preamplifier in a multiple structure in either case itpermits the attainment of large overall gains in a harsh radiation environment [3]

However detectors based on GEMs are gas-sensitive with typically low efficiencies of gamma-ray-detection hence Xenon which is an expensive gas is generally used to fill the detector to ob-tain a high detection efficiency [4] Unfortunately the efficiency thus obtained remains insufficientOur approach has thus been to develop a novel GEM foil to increase the efficiency of gamma-ray-detection in these detectors A novel GEM foil is plated on both surfaces by gold with whichgamma rays interact easily The GEM foils of the conversion layer are plated by a layer of gold3 microm thick on both surfaces [5] In order to obtain higher detection efficiency several gold-platedGEM foils are stacked in a chamber

We have constructed a small field-of-view (FOV) prototype gamma camera with 100 mm times100 mm GEM foils Here in order to test its performance we performed several measurementswith a phantom filled with 99mTc (141 keV) as the radioactive source and obtained 2-D imagesusing a pinhole collimator Our system enables gamma-ray imaging to be used in various fieldswhere it has not previously been possible

2 Gamma camera systems with GEM

Our gamma camera system consists of a GEM-chamber four readout electronics modules low andhigh voltage supplies an Ethernet hub and a PC [6] The detector and PC are linked by an Ethernetconnection The GEM chamber detects the gamma-rays and the readout electronics process signalsfrom the chamber and generate event data on an event-by-event basis The PC located remotelyfrom the detector receives the event data via Ethernet The event data which consist of a timestamp and the detected position information is stored and analyzed using the PC

Figure 1 shows a photograph of the detector The readout electronics module employs inte-grated circuit technologies an Application Specific Integrated Circuit (ASIC) for analog signalprocessing and a Field Programmable Gate Array (FPGA) for digital signal processing [6]

21 GEM chamber

Figure 2 shows the structure of the GEM chamber which consists of three parts a converter anamplifier and a read out component The gamma-ray converter consists of four gold-plated GEMfoils and a gold-plated cathode Each converter converts incoming gamma rays into electrons Bothsurfaces of the GEM foil are plated by gold with which gamma rays interact easily because of itslarge atomic number The cross section of the photoelectric absorption of gold for gamma rays isalmost equal to that of lead

The gold-plated GEM foil was developed by plating gold on a standard GEM foil 100 mm times100 mm in size and about 50 microm thick For the insulator a liquid crystal polymer (LCP) was usedCopper layers 1 microm thick were deposited on both sides of the LCP and a 3-microm-thick gold layer wasplated atop the copper The holes in the GEM and gold-plated GEM foils are 70 microm in diameterand have a pitch of 140 microm Ionization is caused by the interaction of gamma rays with gold andthe released electron is amplified by avalanching which occurs in the presence of a large electricfield in the holes In our gamma camera GEM foils are used as both converters and amplifiers

ndash 2 ndash

2012 JINST 7 C01078

200mm 200mm

95mm500mm

GEM Chamber

Readout electronics(ASIC + FPGA)times4

Figure 1 Photograph of the detector The GEM chamber and readout electronics for detecting gamma raysare integrated in one device The direction of gamma rays is indicated by the yellow arrow A PC obtainsthe 2-D position data of detected gamma rays from the readout board with an Ethernet link

Figure 2 Schematic of GEM chamber structure

Given that a GEM foil allows most of the electrons to pass through its holes to the next regiongold-plated GEM foils can be used for gamma-ray detection as well as for multiple converters

The amplifier uses a standard GEM foil [7] an LCP insulator 50 microm thick and copper elec-trodes 5 microm thick The foil has holes of the same shape as those of the gold-plated GEM foils Forthe three amplifier GEM foils comprising the amplifier (ie GEMs 1 2 and 3) we used 100 micromGEM foils for GEM 1 and 50 microm GEM foils for GEMs 2 and 3 The 100 microm GEM foils resultin higher effective gas gain for the amplifier [8] The width of the sensitive gas regions between

ndash 3 ndash

2012 JINST 7 C01078

Figure 3 Block diagram of the readout system All signals and transform the readout strips are processedand transferred to the PC as event data

the cathode mesh and the gold-plated GEM and between the gold-plated GEM and GEM 1 were1 mm the transfer gaps between the amplifier GEMs were 2 mm and the induction gap betweenGEM 3 and the readout strips were 2 mm For stable operation the voltage applied to the ampli-fier GEMs (∆VGEM) differed between GEM 1 (600 V) GEM 2 (400 V) and GEM 3 (400 V) Toprevent electric discharge the effective gas gain of GEM 1 was large and those of GEMs 2 and 3were suppressed because there is less total electric charge in GEM 1 and a higher voltage can beapplied (larger effective gas gain) However total electric charge increases in GEM 2 and GEM 3and electric discharges occur more frequently Therefore reduced voltages are supplied in GEM2 and GEM 3 (smaller effective gas gain) In this setting higher effective gas gain in total can beobtained without electric discharges A rather lower voltage was supplied to the gold-plated GEMfoil (240 V) so that the effective gas gain becomes 1 Then average pulse heights for the signalsoriginating in both the upper and lower regions of the gold-plated GEM foil were the same Theelectric fields in the drift region (ED) transfer region (ET ) and induction region (EI) were 15 20and 40 kVcm respectively The highest negative voltage was supplied on the gold cathode and aresistance chain was used to provide the appropriate voltage to each region

The 2-D readout component consists of 120 X and 120 Y strips with a 08 mm pitch on onesurface To realize this strips a 08 mm square pixel is divided to two triangle pads which areassigned to X pad and Y pad respectively X pads are connected on the frond surface and Y padsare connected on the rear surface with through hole technology The effective detection area of thereadout board is 96 mm times 96 mm We used a gas mixture of Ar and CO2 (70 30) at atmosphericpressure which resulted in stable operation The effective gas gain was calculated from the positionof the main peak of a 59-keV X-ray emitted from a 55Fe radioactive source Effective gas gains ofmore than 50000 were obtained

22 Readout electronics module

Figure 3 shows a block diagram of the readout electronics system consisting of four readout mod-ules an Ethernet hub and a PC The readout electronics are integrated into the detector and linked

ndash 4 ndash

2012 JINST 7 C01078

Figure 4 Snapshot of a graphical user interface of the data acquisition program

to a PC with an Ethernet connection All signals from the readout strips are processed and trans-ferred to the PC as event data The readout module consists of an ASIC and an FPGA The ASICsprocesses analog signals and the FPGA processes digital signals The detector signals are digi-tized with discriminators in the analog component The digital component generates 2-D positiondata with timestamps and transfers them to a PC using transmission control protocol (TCP) via anEthernet link [6]

The FPGA can be controlled by the PC to set threshold voltages for the comparators Giventhat none of the energy of electrons discharged because of the interaction is spent in the sensitivegas regions our gamma camera dose not have a pulse height analyzer (PHA) for incoming gammarays This lack of a PHA is a minor problem in small animals such as mice and rats where littlescatter arises from inside the animals For larger species scatter from inside and outside the FOVwill certainly degrade the image quality

The detector is connected with Ethernet links aggregated by an Ethernet Hub The raw dataand hit pattern data generated by the detector can be processed by a PC

23 Image processing software

We have developed a simple application program for data acquisition The program runs on LinuxOS and has a number of Graphical User Interfaces (GUIs) Figure 4 shows a snapshot of the GUIsof the program The figure shows control panels and the 2-D image online monitor The controlpanels are used to configure the data acquisition and the online monitors Images and values of themonitors can be saved in PNG and CSV format files respectively Existing software is used fordetailed image analysis

ndash 5 ndash

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

can be used as a detector on its own or as a preamplifier in a multiple structure in either case itpermits the attainment of large overall gains in a harsh radiation environment [3]

However detectors based on GEMs are gas-sensitive with typically low efficiencies of gamma-ray-detection hence Xenon which is an expensive gas is generally used to fill the detector to ob-tain a high detection efficiency [4] Unfortunately the efficiency thus obtained remains insufficientOur approach has thus been to develop a novel GEM foil to increase the efficiency of gamma-ray-detection in these detectors A novel GEM foil is plated on both surfaces by gold with whichgamma rays interact easily The GEM foils of the conversion layer are plated by a layer of gold3 microm thick on both surfaces [5] In order to obtain higher detection efficiency several gold-platedGEM foils are stacked in a chamber

We have constructed a small field-of-view (FOV) prototype gamma camera with 100 mm times100 mm GEM foils Here in order to test its performance we performed several measurementswith a phantom filled with 99mTc (141 keV) as the radioactive source and obtained 2-D imagesusing a pinhole collimator Our system enables gamma-ray imaging to be used in various fieldswhere it has not previously been possible

2 Gamma camera systems with GEM

Our gamma camera system consists of a GEM-chamber four readout electronics modules low andhigh voltage supplies an Ethernet hub and a PC [6] The detector and PC are linked by an Ethernetconnection The GEM chamber detects the gamma-rays and the readout electronics process signalsfrom the chamber and generate event data on an event-by-event basis The PC located remotelyfrom the detector receives the event data via Ethernet The event data which consist of a timestamp and the detected position information is stored and analyzed using the PC

Figure 1 shows a photograph of the detector The readout electronics module employs inte-grated circuit technologies an Application Specific Integrated Circuit (ASIC) for analog signalprocessing and a Field Programmable Gate Array (FPGA) for digital signal processing [6]

21 GEM chamber

Figure 2 shows the structure of the GEM chamber which consists of three parts a converter anamplifier and a read out component The gamma-ray converter consists of four gold-plated GEMfoils and a gold-plated cathode Each converter converts incoming gamma rays into electrons Bothsurfaces of the GEM foil are plated by gold with which gamma rays interact easily because of itslarge atomic number The cross section of the photoelectric absorption of gold for gamma rays isalmost equal to that of lead

The gold-plated GEM foil was developed by plating gold on a standard GEM foil 100 mm times100 mm in size and about 50 microm thick For the insulator a liquid crystal polymer (LCP) was usedCopper layers 1 microm thick were deposited on both sides of the LCP and a 3-microm-thick gold layer wasplated atop the copper The holes in the GEM and gold-plated GEM foils are 70 microm in diameterand have a pitch of 140 microm Ionization is caused by the interaction of gamma rays with gold andthe released electron is amplified by avalanching which occurs in the presence of a large electricfield in the holes In our gamma camera GEM foils are used as both converters and amplifiers

ndash 2 ndash

2012 JINST 7 C01078

200mm 200mm

95mm500mm

GEM Chamber

Readout electronics(ASIC + FPGA)times4

Figure 1 Photograph of the detector The GEM chamber and readout electronics for detecting gamma raysare integrated in one device The direction of gamma rays is indicated by the yellow arrow A PC obtainsthe 2-D position data of detected gamma rays from the readout board with an Ethernet link

Figure 2 Schematic of GEM chamber structure

Given that a GEM foil allows most of the electrons to pass through its holes to the next regiongold-plated GEM foils can be used for gamma-ray detection as well as for multiple converters

The amplifier uses a standard GEM foil [7] an LCP insulator 50 microm thick and copper elec-trodes 5 microm thick The foil has holes of the same shape as those of the gold-plated GEM foils Forthe three amplifier GEM foils comprising the amplifier (ie GEMs 1 2 and 3) we used 100 micromGEM foils for GEM 1 and 50 microm GEM foils for GEMs 2 and 3 The 100 microm GEM foils resultin higher effective gas gain for the amplifier [8] The width of the sensitive gas regions between

ndash 3 ndash

2012 JINST 7 C01078

Figure 3 Block diagram of the readout system All signals and transform the readout strips are processedand transferred to the PC as event data

the cathode mesh and the gold-plated GEM and between the gold-plated GEM and GEM 1 were1 mm the transfer gaps between the amplifier GEMs were 2 mm and the induction gap betweenGEM 3 and the readout strips were 2 mm For stable operation the voltage applied to the ampli-fier GEMs (∆VGEM) differed between GEM 1 (600 V) GEM 2 (400 V) and GEM 3 (400 V) Toprevent electric discharge the effective gas gain of GEM 1 was large and those of GEMs 2 and 3were suppressed because there is less total electric charge in GEM 1 and a higher voltage can beapplied (larger effective gas gain) However total electric charge increases in GEM 2 and GEM 3and electric discharges occur more frequently Therefore reduced voltages are supplied in GEM2 and GEM 3 (smaller effective gas gain) In this setting higher effective gas gain in total can beobtained without electric discharges A rather lower voltage was supplied to the gold-plated GEMfoil (240 V) so that the effective gas gain becomes 1 Then average pulse heights for the signalsoriginating in both the upper and lower regions of the gold-plated GEM foil were the same Theelectric fields in the drift region (ED) transfer region (ET ) and induction region (EI) were 15 20and 40 kVcm respectively The highest negative voltage was supplied on the gold cathode and aresistance chain was used to provide the appropriate voltage to each region

The 2-D readout component consists of 120 X and 120 Y strips with a 08 mm pitch on onesurface To realize this strips a 08 mm square pixel is divided to two triangle pads which areassigned to X pad and Y pad respectively X pads are connected on the frond surface and Y padsare connected on the rear surface with through hole technology The effective detection area of thereadout board is 96 mm times 96 mm We used a gas mixture of Ar and CO2 (70 30) at atmosphericpressure which resulted in stable operation The effective gas gain was calculated from the positionof the main peak of a 59-keV X-ray emitted from a 55Fe radioactive source Effective gas gains ofmore than 50000 were obtained

22 Readout electronics module

Figure 3 shows a block diagram of the readout electronics system consisting of four readout mod-ules an Ethernet hub and a PC The readout electronics are integrated into the detector and linked

ndash 4 ndash

2012 JINST 7 C01078

Figure 4 Snapshot of a graphical user interface of the data acquisition program

to a PC with an Ethernet connection All signals from the readout strips are processed and trans-ferred to the PC as event data The readout module consists of an ASIC and an FPGA The ASICsprocesses analog signals and the FPGA processes digital signals The detector signals are digi-tized with discriminators in the analog component The digital component generates 2-D positiondata with timestamps and transfers them to a PC using transmission control protocol (TCP) via anEthernet link [6]

The FPGA can be controlled by the PC to set threshold voltages for the comparators Giventhat none of the energy of electrons discharged because of the interaction is spent in the sensitivegas regions our gamma camera dose not have a pulse height analyzer (PHA) for incoming gammarays This lack of a PHA is a minor problem in small animals such as mice and rats where littlescatter arises from inside the animals For larger species scatter from inside and outside the FOVwill certainly degrade the image quality

The detector is connected with Ethernet links aggregated by an Ethernet Hub The raw dataand hit pattern data generated by the detector can be processed by a PC

23 Image processing software

We have developed a simple application program for data acquisition The program runs on LinuxOS and has a number of Graphical User Interfaces (GUIs) Figure 4 shows a snapshot of the GUIsof the program The figure shows control panels and the 2-D image online monitor The controlpanels are used to configure the data acquisition and the online monitors Images and values of themonitors can be saved in PNG and CSV format files respectively Existing software is used fordetailed image analysis

ndash 5 ndash

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

200mm 200mm

95mm500mm

GEM Chamber

Readout electronics(ASIC + FPGA)times4

Figure 1 Photograph of the detector The GEM chamber and readout electronics for detecting gamma raysare integrated in one device The direction of gamma rays is indicated by the yellow arrow A PC obtainsthe 2-D position data of detected gamma rays from the readout board with an Ethernet link

Figure 2 Schematic of GEM chamber structure

Given that a GEM foil allows most of the electrons to pass through its holes to the next regiongold-plated GEM foils can be used for gamma-ray detection as well as for multiple converters

The amplifier uses a standard GEM foil [7] an LCP insulator 50 microm thick and copper elec-trodes 5 microm thick The foil has holes of the same shape as those of the gold-plated GEM foils Forthe three amplifier GEM foils comprising the amplifier (ie GEMs 1 2 and 3) we used 100 micromGEM foils for GEM 1 and 50 microm GEM foils for GEMs 2 and 3 The 100 microm GEM foils resultin higher effective gas gain for the amplifier [8] The width of the sensitive gas regions between

ndash 3 ndash

2012 JINST 7 C01078

Figure 3 Block diagram of the readout system All signals and transform the readout strips are processedand transferred to the PC as event data

the cathode mesh and the gold-plated GEM and between the gold-plated GEM and GEM 1 were1 mm the transfer gaps between the amplifier GEMs were 2 mm and the induction gap betweenGEM 3 and the readout strips were 2 mm For stable operation the voltage applied to the ampli-fier GEMs (∆VGEM) differed between GEM 1 (600 V) GEM 2 (400 V) and GEM 3 (400 V) Toprevent electric discharge the effective gas gain of GEM 1 was large and those of GEMs 2 and 3were suppressed because there is less total electric charge in GEM 1 and a higher voltage can beapplied (larger effective gas gain) However total electric charge increases in GEM 2 and GEM 3and electric discharges occur more frequently Therefore reduced voltages are supplied in GEM2 and GEM 3 (smaller effective gas gain) In this setting higher effective gas gain in total can beobtained without electric discharges A rather lower voltage was supplied to the gold-plated GEMfoil (240 V) so that the effective gas gain becomes 1 Then average pulse heights for the signalsoriginating in both the upper and lower regions of the gold-plated GEM foil were the same Theelectric fields in the drift region (ED) transfer region (ET ) and induction region (EI) were 15 20and 40 kVcm respectively The highest negative voltage was supplied on the gold cathode and aresistance chain was used to provide the appropriate voltage to each region

The 2-D readout component consists of 120 X and 120 Y strips with a 08 mm pitch on onesurface To realize this strips a 08 mm square pixel is divided to two triangle pads which areassigned to X pad and Y pad respectively X pads are connected on the frond surface and Y padsare connected on the rear surface with through hole technology The effective detection area of thereadout board is 96 mm times 96 mm We used a gas mixture of Ar and CO2 (70 30) at atmosphericpressure which resulted in stable operation The effective gas gain was calculated from the positionof the main peak of a 59-keV X-ray emitted from a 55Fe radioactive source Effective gas gains ofmore than 50000 were obtained

22 Readout electronics module

Figure 3 shows a block diagram of the readout electronics system consisting of four readout mod-ules an Ethernet hub and a PC The readout electronics are integrated into the detector and linked

ndash 4 ndash

2012 JINST 7 C01078

Figure 4 Snapshot of a graphical user interface of the data acquisition program

to a PC with an Ethernet connection All signals from the readout strips are processed and trans-ferred to the PC as event data The readout module consists of an ASIC and an FPGA The ASICsprocesses analog signals and the FPGA processes digital signals The detector signals are digi-tized with discriminators in the analog component The digital component generates 2-D positiondata with timestamps and transfers them to a PC using transmission control protocol (TCP) via anEthernet link [6]

The FPGA can be controlled by the PC to set threshold voltages for the comparators Giventhat none of the energy of electrons discharged because of the interaction is spent in the sensitivegas regions our gamma camera dose not have a pulse height analyzer (PHA) for incoming gammarays This lack of a PHA is a minor problem in small animals such as mice and rats where littlescatter arises from inside the animals For larger species scatter from inside and outside the FOVwill certainly degrade the image quality

The detector is connected with Ethernet links aggregated by an Ethernet Hub The raw dataand hit pattern data generated by the detector can be processed by a PC

23 Image processing software

We have developed a simple application program for data acquisition The program runs on LinuxOS and has a number of Graphical User Interfaces (GUIs) Figure 4 shows a snapshot of the GUIsof the program The figure shows control panels and the 2-D image online monitor The controlpanels are used to configure the data acquisition and the online monitors Images and values of themonitors can be saved in PNG and CSV format files respectively Existing software is used fordetailed image analysis

ndash 5 ndash

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Figure 3 Block diagram of the readout system All signals and transform the readout strips are processedand transferred to the PC as event data

the cathode mesh and the gold-plated GEM and between the gold-plated GEM and GEM 1 were1 mm the transfer gaps between the amplifier GEMs were 2 mm and the induction gap betweenGEM 3 and the readout strips were 2 mm For stable operation the voltage applied to the ampli-fier GEMs (∆VGEM) differed between GEM 1 (600 V) GEM 2 (400 V) and GEM 3 (400 V) Toprevent electric discharge the effective gas gain of GEM 1 was large and those of GEMs 2 and 3were suppressed because there is less total electric charge in GEM 1 and a higher voltage can beapplied (larger effective gas gain) However total electric charge increases in GEM 2 and GEM 3and electric discharges occur more frequently Therefore reduced voltages are supplied in GEM2 and GEM 3 (smaller effective gas gain) In this setting higher effective gas gain in total can beobtained without electric discharges A rather lower voltage was supplied to the gold-plated GEMfoil (240 V) so that the effective gas gain becomes 1 Then average pulse heights for the signalsoriginating in both the upper and lower regions of the gold-plated GEM foil were the same Theelectric fields in the drift region (ED) transfer region (ET ) and induction region (EI) were 15 20and 40 kVcm respectively The highest negative voltage was supplied on the gold cathode and aresistance chain was used to provide the appropriate voltage to each region

The 2-D readout component consists of 120 X and 120 Y strips with a 08 mm pitch on onesurface To realize this strips a 08 mm square pixel is divided to two triangle pads which areassigned to X pad and Y pad respectively X pads are connected on the frond surface and Y padsare connected on the rear surface with through hole technology The effective detection area of thereadout board is 96 mm times 96 mm We used a gas mixture of Ar and CO2 (70 30) at atmosphericpressure which resulted in stable operation The effective gas gain was calculated from the positionof the main peak of a 59-keV X-ray emitted from a 55Fe radioactive source Effective gas gains ofmore than 50000 were obtained

22 Readout electronics module

Figure 3 shows a block diagram of the readout electronics system consisting of four readout mod-ules an Ethernet hub and a PC The readout electronics are integrated into the detector and linked

ndash 4 ndash

2012 JINST 7 C01078

Figure 4 Snapshot of a graphical user interface of the data acquisition program

to a PC with an Ethernet connection All signals from the readout strips are processed and trans-ferred to the PC as event data The readout module consists of an ASIC and an FPGA The ASICsprocesses analog signals and the FPGA processes digital signals The detector signals are digi-tized with discriminators in the analog component The digital component generates 2-D positiondata with timestamps and transfers them to a PC using transmission control protocol (TCP) via anEthernet link [6]

The FPGA can be controlled by the PC to set threshold voltages for the comparators Giventhat none of the energy of electrons discharged because of the interaction is spent in the sensitivegas regions our gamma camera dose not have a pulse height analyzer (PHA) for incoming gammarays This lack of a PHA is a minor problem in small animals such as mice and rats where littlescatter arises from inside the animals For larger species scatter from inside and outside the FOVwill certainly degrade the image quality

The detector is connected with Ethernet links aggregated by an Ethernet Hub The raw dataand hit pattern data generated by the detector can be processed by a PC

23 Image processing software

We have developed a simple application program for data acquisition The program runs on LinuxOS and has a number of Graphical User Interfaces (GUIs) Figure 4 shows a snapshot of the GUIsof the program The figure shows control panels and the 2-D image online monitor The controlpanels are used to configure the data acquisition and the online monitors Images and values of themonitors can be saved in PNG and CSV format files respectively Existing software is used fordetailed image analysis

ndash 5 ndash

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Figure 4 Snapshot of a graphical user interface of the data acquisition program

to a PC with an Ethernet connection All signals from the readout strips are processed and trans-ferred to the PC as event data The readout module consists of an ASIC and an FPGA The ASICsprocesses analog signals and the FPGA processes digital signals The detector signals are digi-tized with discriminators in the analog component The digital component generates 2-D positiondata with timestamps and transfers them to a PC using transmission control protocol (TCP) via anEthernet link [6]

The FPGA can be controlled by the PC to set threshold voltages for the comparators Giventhat none of the energy of electrons discharged because of the interaction is spent in the sensitivegas regions our gamma camera dose not have a pulse height analyzer (PHA) for incoming gammarays This lack of a PHA is a minor problem in small animals such as mice and rats where littlescatter arises from inside the animals For larger species scatter from inside and outside the FOVwill certainly degrade the image quality

The detector is connected with Ethernet links aggregated by an Ethernet Hub The raw dataand hit pattern data generated by the detector can be processed by a PC

23 Image processing software

We have developed a simple application program for data acquisition The program runs on LinuxOS and has a number of Graphical User Interfaces (GUIs) Figure 4 shows a snapshot of the GUIsof the program The figure shows control panels and the 2-D image online monitor The controlpanels are used to configure the data acquisition and the online monitors Images and values of themonitors can be saved in PNG and CSV format files respectively Existing software is used fordetailed image analysis

ndash 5 ndash

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Figure 5 Dependence of counting rate on applied voltage The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counting rate

3 Basic Experiment

In the field of small animal imaging gamma cameras have been one of the most useful tools tovisualize gamma-ray emitters A high spatial resolution is required for small animal imagingTherefore gamma camera systems with a pinhole collimator have been proposed and have demon-strated good spatial resolution The 2-D detector with high spatial resolution allows the pinholes tofunction as a small pinhole gamma camera In order to test the performance of our prototype weconducted several measurements using a phantom filled with 99mTc (141 keV) as the radioactivesource The measurements with pinhole collimator the distances between the radioactive sourceand the collimator and between the collimator and the detector were both 100 mm respectivelyand imaging was performed with a pinhole collimator of 2 mm in diameter

31 Dependence of counting rate on applied voltage

The dependence of the counting rate on the applied voltage is shown in figure 5 A 20-mm-thicklead block with a 4-mm-diameter hole placed in front of the radioactive sources ensured a smallirradiation area Given that increasing the applied voltage leads to an increase in the effective gasgain signals that exceed the voltage threshold increase and the counting rate saturates slowly Withan adequate effective gas gain it is possible to make the most electrons produced by the interactionof gamma rays with the gold-plated GEM foil contribute to the detected signal From such a pointof view there is no electrical discharge

The efficiency of gamma-ray-detection for a 141 keV gamma ray at a stable operating voltage(4900 V) was 257plusmn 005 On the other hand the efficiency calculated by the GEANT4 [9]simulation was 278plusmn002 for a similar detector setup This indicates that we achieved a detectorsetup with sufficient effective gas gain

ndash 6 ndash

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Gap04mm

Gap10mm

Gap20mm

20mm

20mm

(a)

(b)

Figure 6 Dependence of spatial resolution on drift gap (a) source images and (b) FWHM as a function ofthe drift gap The spatial resolution improves as the drift gap becomes narrow although the probability ofelectric discharge increases

32 Dependence of spatial resolution on drift gap

Our gamma camera uses electrons produced by the interaction of gamma rays with the gold-platedGEM foil and detects the source position using a pinhole collimator To increase the efficiency ofgamma-ray detection we used four gold-plated GEM foils and a gold-plated cathode Howeverthe use of many converters causes degradation of spatial resolution such that the place wheregamma rays are converted into electrons for a certain collimator aperture by a particular incidentgamma-ray angle changes (the parallax problem) In addition larger drift gap of the convertermakes this effect more significant We measured changes in the spatial resolution for a drift gapto find the range of gaps that could achieve stable operation The distances of drift gaps uses were04 10 and 20 mm respectively and the imaged radioactive source was 7 mm in diameter Thespatial resolution was then evaluated by measuring the Full Width at Half Maximum (FWHM)on an image

Figure 6 shows the spatial resolution as a function of the drift gap The former depends onthe latter the FWHM increases as the drift gap widens A fluctuation of an electron pass in the

ndash 7 ndash

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Figure 7 Changes in the image count for radioactivity The acquisition time was set to 10 min and theregion of interest (ROI 20 times 20 mm) was set to the area of the radiation source in the obtained images tomeasure the counts The prototype gamma camera shows good radioactive linearity

gas region affects the spatial resolution The influence of parallax is small in the four gold-platedGEM foils and also the diffusion in the gas region of an electron is not so large On this point it isdesirable to make the drift gap narrow However stable detector operation is not secured becausean electric discharge can easily occur (gap 04 mm) When considering the spatial resolution withthe most stable operation a drift gap of 10 mm is most appropriate

33 Linearity of radioactivity

We evaluated whether the relationship between radioactivity and acquisition counts was linearWe used three different sources of radioactivity (194 885 119 MBq) with a diameter of 7 mmfor the measurements This evaluation revealed that the gamma camera showed good linearity ofradioactivity (figure 7)

34 Phantom studies

An original resolution phantom was used to asses the resolution capability of the scanner Thephantom consists of a 10-mm-thick acrylic disk with a diameter of 95 mm The disk is divided intosix sections each containing holes 10 mm in depth with different diameters (figure 8) The holediameters in millimeters are (center-to-center spacing given in parentheses) 05 (10) 10 (20) 15(30) 20 (40) 25 (50) and 30 (60) The holes were filled with a 99mTc solution (80 MBqml)and the phantom was placed 100 mm from a collimator Image profiles were obtained by filteringthe data with a Butterworth filter (cutoff frequency 02 cyclecm)

The image of an original resolution phantom is displayed in figure 8(b) The holes are clearlyvisible down to a diameter of 25 mm For the case where the 25-mm-hole diameter source waslocated at the center of FOV the FWHM values in the x and y directions are 27 mm and 28 mmrespectively In addition image distortion is seen along the border portions of the FOV We considerthis to be a general characteristic of the pinhole collimator

ndash 8 ndash

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

(a)

A

B

25mm 20mm

10mm

30mm 15mm

05mm

(b)

BA

Cou

nt

Position

FWHM（ 25mm ）=27mm

(c)

Figure 8 (a) Imaging result of a 99mTc-filled original resolution phantom constructed from an acrylic diskin which holes of different sizes (05 10 15 20 25 and 30 mm) were drilled (b c) Holes are clearlyvisible down to a size of 25 mm with sharp signals in the profile line

4 Conclusion

We are developing a novel gamma camera based on a gaseous detector that uses a GEM forbiomolecular analysis and nuclear medicine A performance study carried out with a radioac-tive source of 99mTc (141 keV) shows linearity of radiation measurements and a good ability toresolve objects a few mm2 in size Although good spatial resolution is obtained under the presentconditions the efficiency of gamma-ray detection is insufficient To overcome this problem weare currently using the image acquisition and reconstruction method with a multi-pinhole collima-tor and obtaining results judged satisfactory by simulation Further measurements will be madewith this new system both to finalize the development of the Single Photon Emission ComputedTomography (SPECT) system and to characterize various aspects of its performance

ndash 9 ndash

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion

2012 JINST 7 C01078

Acknowledgments

We thank Dr Ohshita for his suggestions concerning the performance measurements as well asfor the useful discussions This study was performed within the framework of the KEK DetectorTechnology Project (KEKDTP) of the High Energy Accelerator Research Organization (KEK)Japan

References

[1] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[2] S Bachmann et al High rate X-ray imaging using multi-GEM detectors with a novel readout designNucl Instrum Meth A 478 (2002) 104

[3] R Higgons PSD Technologies at CERN 9th Sep 2004

[4] A Buzulutskov et al GEM operation in pure noble gases and the avalanche confinement NuclInstrum Meth A 433 (1999) 471

[5] T Koike et al A new gamma-ray detector with gold-plated gas electron multiplier Nucl InstrumMeth A 648 (2011) 180

[6] T Uchida et al Prototype of a Compact Imaging System for GEM Detectors IEEE NSS 55 (2008)2698

[7] T Tamagawa et al Development of thick-foil and fine-pitch GEMs with a laser etching techniqueNucl Instrum Meth A 608 (2009) 390

[8] S Uno et al Performance study of new thicker GEM Nucl Instrum Meth A 581 (2007) 271

[9] S Agostinelli et al Geant4 mdasha simulation toolkit Nucl Instrum Meth A 506 (2003) 250

ndash 10 ndash

• Introduction
• Gamma camera systems with GEM
• • GEM chamber
  • Readout electronics module
  • Image processing software
  • • Basic Experiment
    • • Dependence of counting rate on applied voltage
      • Dependence of spatial resolution on drift gap
      • Linearity of radioactivity
      • Phantom studies
      • • Conclusion