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Design and image-quality performance of highresolution CMOS-based X-ray imaging detectorsfor digital mammographyTo cite this article B K Cha et al 2012 JINST 7 C04020

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2012 JINST 7 C04020

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED October 2 2011ACCEPTED March 15 2012PUBLISHED April 27 2012

13th INTERNATIONAL WORKSHOP ON RADIATION IMAGING DETECTORS3ndash7 JULY 2011ETH ZURICH SWITZERLAND

Design and image-quality performance of highresolution CMOS-based X-ray imaging detectors fordigital mammography

BK Chaa1 JY Kimb YJ Kimb S Yunc G Chob HK Kimc C-W Seoa S Jeona

and Y Huha

aKorea Electrotechnology Research InstituteAnsan 426-170 Republic of Korea

bKorea Advanced Institute of Science and TechnologyDaejeon 305-701 Republic of Korea

cPusan National UniversityBusan 609-735 Republic of Korea

E-mail Goldrain99gmailcom

ABSTRACT In digital X-ray imaging systems X-ray imaging detectors based on scintillatingscreens with electronic devices such as charge-coupled devices (CCDs) thin-film transistors (TFT)complementary metal oxide semiconductor (CMOS) flat panel imagers have been introduced forgeneral radiography dental mammography and non-destructive testing (NDT) applications Re-cently a large-area CMOS active-pixel sensor (APS) in combination with scintillation films hasbeen widely used in a variety of digital X-ray imaging applications We employed a scintillator-based CMOS APS image sensor for high-resolution mammography In this work both powder-typeGd2O2STb and a columnar structured CsITl scintillation screens with various thicknesses werefabricated and used as materials to convert X-ray into visible light These scintillating screenswere directly coupled to a CMOS flat panel imager with a 25 times 50 mm2 active area and a 48 micrompixel pitch for high spatial resolution acquisition We used a WAl mammographic X-ray sourcewith a 30 kVp energy condition The imaging characterization of the X-ray detector was measuredand analyzed in terms of linearity in incident X-ray dose modulation transfer function (MTF)noise-power spectrum (NPS) and detective quantum efficiency (DQE)

KEYWORDS X-ray detectors Scintillators scintillation and light emission processes (solid gasand liquid scintillators) Scintillators and scintillating fibres and light guides X-ray radiographyand digital radiography (DR)

1Corresponding author

ccopy 2012 IOP Publishing Ltd and Sissa Medialab srl doi1010881748-0221704C04020

2012 JINST 7 C04020

Contents

1 Introduction 1

2 Materials and methods 2

3 Results and discussion 2

4 Conclusions and future work 4

1 Introduction

In recent years digital mammography (DM) has been introduced into the mammography field toreplace conventional film-screen (FS) technology for the display of breast images There are twomethods such as indirect and direct detection to convert X-rays into electronic readout signals Inthe indirect method scintillation screens such as Gd2O2STb and CsITl materials are used to con-vert to the X-rays into visible light Solid-state imaging devices such as amorphous silicon (a-Si)TFT CMOS flat panel detectors and CCDs have widely been used to transform the incident visiblelight into an electric signal [1] In the direct conversion method photoconductors such as amor-phous selenium polycrystalline CdTe HgI2 PbI2 and PbO materials are used to convert incindentX-rays into proportional electric charges [2] Large area CMOS active-pixel sensor (APS) technol-ogy has been used recently in many digital X-ray imaging applications The CMOS-based X-raydetector has significant advantages such as the highly developed manufacturing process of thesemiconductor relative low manufacturing cost system-on-chip integration and compactness [3]

In this work both Gd2O2STb (Gadox) and CsITl scintillation screens were used as conver-sion materials from X-rays into visible light because of their high scintillation efficiency and goodspectral matching between the emission wavelength and silicon-based photo sensor A variety ofGd2O2STb and CsITl scintillating screens with different thicknesses were manufactured and di-rectly combined with a commercial CMOS flat panel imager with a 25 times 50 mm2 field-of-view(FOV) and a 48microm pixel pitch for potential use in mammographic applications Their X-ray imag-ing characterization was measured at 30 kVp of X-ray tube voltage using a W-anode and an Alfilter of 05 mm The X-ray imaging performance of the CMOS flat panel imager in conjunctionwith different scintillation screens was investigated in terms of signal response to exposure X-raydose modulation transfer function (MTF) noise-power spectrum (NPS) and detection quantum ef-ficiency (DQE) in the Fourier domain under the International Electrotechnical Commission (IEC)RQA 5 standard

ndash 1 ndash

2012 JINST 7 C04020

2 Materials and methods

In this experiment typical powdered Gadox (Gd2O2STb Kasei Optonix Ltd) materials were fab-ricated onto a 25 times 5 cm2 glass substrate through particle in binder (PIB) and screen printing(SP)methods to create the X-ray scintillation screens Furthermore CsITl films with columnar structurewere fabricated by the thermal evaporation method for the measurement of X-ray imaging perfor-mance The prepared scintillation screens and their microstructures are shown in figure 1 WhiteTiO2 reflective material was used in order to increase the light collection efficiency of isotropicallyemitted visible light from the scintillation screens employed The white TiO2 reflective layer wasdeposited on a glass substrate through a spin-coating process The detail manufacturing process anddescriptions are reported in [4] The fabricated scintillating screens were directly coupled on com-mercial CMOS photodiode pixel arrays which collect the emitted visible light from scintillatingscreens This CMOS flat panel detector consisted of a fiber-optic faceplate (FOP) on a photodiodearray surface with 512 times 1024 pixels and a 48 microm pixel pitch as well as a pixel fill factor of morethan 80 The FOP was 3 mm-thick with a fiber size of 6 microm and optical transmittance of 50ndash60 in order to prevent direct X-ray absorption on the photodiode array Details on the physicalspecifications of the commercial CMOS device are to be published in [5] A RadEye1T M CMOSAPS imager (RadEye1 Rad-icon Imaging Corp USA) with 25 times 50 mm2 sensitive area wasconnected to a Shad-o-Snap camera module (a digital frame grabber) Voltage signals from theCMOS APS photodiode array were digitized to 12 bit resolution through analog-to-digital con-verters (ADCs) The acquired 12-bit image data were transferred and saved through a USB cableto the corresponding ShadowCam software program Different 50ndash120microm-thick Gadox and 150ndash200microm-thick CsITl scintillating screens with and without reflectors were directly placed on theFOP surface of CMOS photodiode arrays as shown in figure 1 The readout time was 2000 ms forthe experimental measurements We used a 30 kVp tungsten-anode target with a 35 microm focal spot(Oxford Instruments Inc USA) and 05 mm-thick aluminum filtration for mammographic condi-tions The measured half-value layer (HVL) of aluminum was 1077 mm in thickness The distancebetween the X-ray source and the detector was 500 mm and X-ray exposure doses were varied bychanging the X-ray beam current (mA) from 01 to 09 mA at a fixed X-ray tube voltage of 30 kVp

3 Results and discussion

The linearity of the fabricated scintillating screens was acquired by measuring the average pixelvalue over the region of interest (ROI) of X-ray images as a function of exposure dose The resultsfor various scintillation screens are plotted in figure 2 These detectors were found to have linearresponse curves relation to the whole exposure dose A thicker Gadox scintillating screen showshigher light intensity and a larger slope than samples with thinner screens The signal to- noiseratio (SNR) was measured by dividing the average pixel value by the standard deviation over thesame ROI in the images The mean pixel value and SNR as a function of exposure dose are shownin figure 2

The MTF expressing the spatial resolution of the imaging detectors was measured using theslant slit method with a 10 microm width slit camera (IIE GmbH Aachen Germany) to avoid alias-ing The MTF curves were measured by the Fourier transform of the acquired line spread function

ndash 2 ndash

2012 JINST 7 C04020

Figure 1 Photographs of the CMOS flat panel detector (a) and 1 mm-thick graphite cover (b) fabricatedscintillation screens on glass substrates (c) SEM images of Gadox (d) and CsITl (e) scintillation screens

Figure 2 X-ray linearity (left) and SNR (right) of different scintillating screens as a function ofexposure dose

(LSF) curves The NPS as a function of the spatial frequency (f) was measured from the two-dimensional (2D) Fourier transform of white X-ray images for various samples under a 125 mR(Roentgens) exposure dose using 30 kVp The one-dimensional (1D) NPS was then obtained fromthe 2D NPS on a radial direction and subsequently normalized for the mean pixel value of the ROIresulting the normalized NPS (NNPS) With the measured MTF (f) and NNPS (f) values DQE (f)curves were calculated by the following equation [6 7]

DQE( f ) =SNR2

out

SNR2in

=MT F2( f )

NNPS( f ) middotq middotX

Where q is the number of incident X-ray photons per unit mR per mm2 and X is the exposure dosein units of mR Values of 72357mm2mR for the RQA 5 condition are used in these calculationsfor all samples

The MTF value of the Gadox screen with a 50 microm thickness was higher than that of otherscintillating samples because of less light spreading in the scintillating screen layer as shown in

ndash 3 ndash

2012 JINST 7 C04020

Figure 3 Measured MTF (left) and NPS (right) curves with various scintillating screens at a 125 mRexposure dose

figure 3 The MTF measurement of the Gadox samples with 50 and 120 microm thicknesses resultedin spatial frequencies of about 75 lpmm and 50 lpmm at an MTF value of 20 The extracted1D NPS curves along the radial direction with respect to various scintillating screens are plotted infigure 3 NPS curves have a similar form to the NPS falling over the whole spatial frequency re-gion as shown in figure 3 Figure 4 shows the calculated DQE (f) curves at a dose of 125 mR fromthe measured MTF and NPS curves for the RQA 5 beam quality condition The DQE (f) valueof the 50 microm-thick Gadox scintillating screen was higher than of the other samples in the highspatial frequency region because of the higher spatial resolution The estimated DQE values withdifferent scintillation screens at zero frequency were in the range of 015ndash03 at a dose of 125 mRThese values are significantly lower when than those of commercial detectors in use for currentmammography application Further improvement of DQE performance is still required throughmore uniformly fabricated scintillating screens with optimized thickness that do not demonstratesignificant loss of signal and spatial resolution

4 Conclusions and future work

In this work digital imaging detectors with various several microm-thick Gadox and CsITl scintillationscreens and a commercial CMOS APS image sensor were studied for mammography applicationTheir X-ray imaging performance characteristics in terms of X-ray linearity MTF NPS and DQEwere investigated under the RQA 5 beam quality condition with 30 kVp of X-ray tube voltage Themeasured results showed good linearity over the whole exposure dose and high MTF curves at highspatial frequencies Furthermore high spatial resolution and DQE at high spatial frequency wasachieved with a 50 microm-thick Gadox scintillating screen and a CMOS flat panel detector with a 48microm pixel pitch However the DQE performance of the detector is much lower than that of com-mercial digital mammographic detectors Therefore improvements in detectors are still requiredthrough a suitable choice among various scintillators as well as the structural design and fabrica-tion of uniform- and optimized scintillator thickness for high-quality mammographic imaging

ndash 4 ndash

2012 JINST 7 C04020

Figure 4 Measured DQE curves with various scintillating screens at a 125 mR exposure dose

Acknowledgments

This research was supported by research project of Korea Electrotechnology Research Institute(KERI) funded by the Ministry of Knowledge Economy (10-12-N0201-09)

References

[1] A Noel and F Thibault Digital detectors for mammography the technical challenges Eur Radiol 14(2004) 1990

[2] S Kasap et al Amorphous and polycrystalline photoconductors for direct conversion flat panel X-rayimage sensors Sensors 11 (2011) 5112

[3] D Scheffer A wafer scale active pixel CMOS image sensor for generic X-ray radiology Proc SPIE6510 (2007) 65100O-1

[4] BK Cha et al Fabrication and imaging characterization of high sensitive CsI(Tl) and Gd2O2S(Tb)scintllator screens for X-ray imaging detectors Radiation Measur 45 (2010) 742

[5] T Greave and GP Weckler High-resolution CMOS imaging detector Proc SPIE 4320 (2001) 68

[6] BK Cha et al Performance studies of a monolithic scintillator-CMOS image sensor for X-rayapplication Nucl Instrum Meth A 591 (2008) 113

[7] MK Cho et al Measurements of X-ray imaging performance of granular phosphors withdirect-coupled CMOS sensors IEEE Trans Nucl Sci 55 (2008) 1338

ndash 5 ndash

2012 JINST 7 C04020

2 Materials and methods

In this experiment typical powdered Gadox (Gd2O2STb Kasei Optonix Ltd) materials were fab-ricated onto a 25 times 5 cm2 glass substrate through particle in binder (PIB) and screen printing(SP)methods to create the X-ray scintillation screens Furthermore CsITl films with columnar structurewere fabricated by the thermal evaporation method for the measurement of X-ray imaging perfor-mance The prepared scintillation screens and their microstructures are shown in figure 1 WhiteTiO2 reflective material was used in order to increase the light collection efficiency of isotropicallyemitted visible light from the scintillation screens employed The white TiO2 reflective layer wasdeposited on a glass substrate through a spin-coating process The detail manufacturing process anddescriptions are reported in [4] The fabricated scintillating screens were directly coupled on com-mercial CMOS photodiode pixel arrays which collect the emitted visible light from scintillatingscreens This CMOS flat panel detector consisted of a fiber-optic faceplate (FOP) on a photodiodearray surface with 512 times 1024 pixels and a 48 microm pixel pitch as well as a pixel fill factor of morethan 80 The FOP was 3 mm-thick with a fiber size of 6 microm and optical transmittance of 50ndash60 in order to prevent direct X-ray absorption on the photodiode array Details on the physicalspecifications of the commercial CMOS device are to be published in [5] A RadEye1T M CMOSAPS imager (RadEye1 Rad-icon Imaging Corp USA) with 25 times 50 mm2 sensitive area wasconnected to a Shad-o-Snap camera module (a digital frame grabber) Voltage signals from theCMOS APS photodiode array were digitized to 12 bit resolution through analog-to-digital con-verters (ADCs) The acquired 12-bit image data were transferred and saved through a USB cableto the corresponding ShadowCam software program Different 50ndash120microm-thick Gadox and 150ndash200microm-thick CsITl scintillating screens with and without reflectors were directly placed on theFOP surface of CMOS photodiode arrays as shown in figure 1 The readout time was 2000 ms forthe experimental measurements We used a 30 kVp tungsten-anode target with a 35 microm focal spot(Oxford Instruments Inc USA) and 05 mm-thick aluminum filtration for mammographic condi-tions The measured half-value layer (HVL) of aluminum was 1077 mm in thickness The distancebetween the X-ray source and the detector was 500 mm and X-ray exposure doses were varied bychanging the X-ray beam current (mA) from 01 to 09 mA at a fixed X-ray tube voltage of 30 kVp

3 Results and discussion

The linearity of the fabricated scintillating screens was acquired by measuring the average pixelvalue over the region of interest (ROI) of X-ray images as a function of exposure dose The resultsfor various scintillation screens are plotted in figure 2 These detectors were found to have linearresponse curves relation to the whole exposure dose A thicker Gadox scintillating screen showshigher light intensity and a larger slope than samples with thinner screens The signal to- noiseratio (SNR) was measured by dividing the average pixel value by the standard deviation over thesame ROI in the images The mean pixel value and SNR as a function of exposure dose are shownin figure 2

The MTF expressing the spatial resolution of the imaging detectors was measured using theslant slit method with a 10 microm width slit camera (IIE GmbH Aachen Germany) to avoid alias-ing The MTF curves were measured by the Fourier transform of the acquired line spread function

ndash 2 ndash

2012 JINST 7 C04020

Figure 1 Photographs of the CMOS flat panel detector (a) and 1 mm-thick graphite cover (b) fabricatedscintillation screens on glass substrates (c) SEM images of Gadox (d) and CsITl (e) scintillation screens

Figure 2 X-ray linearity (left) and SNR (right) of different scintillating screens as a function ofexposure dose

(LSF) curves The NPS as a function of the spatial frequency (f) was measured from the two-dimensional (2D) Fourier transform of white X-ray images for various samples under a 125 mR(Roentgens) exposure dose using 30 kVp The one-dimensional (1D) NPS was then obtained fromthe 2D NPS on a radial direction and subsequently normalized for the mean pixel value of the ROIresulting the normalized NPS (NNPS) With the measured MTF (f) and NNPS (f) values DQE (f)curves were calculated by the following equation [6 7]

DQE( f ) =SNR2

out

SNR2in

=MT F2( f )

NNPS( f ) middotq middotX

Where q is the number of incident X-ray photons per unit mR per mm2 and X is the exposure dosein units of mR Values of 72357mm2mR for the RQA 5 condition are used in these calculationsfor all samples

The MTF value of the Gadox screen with a 50 microm thickness was higher than that of otherscintillating samples because of less light spreading in the scintillating screen layer as shown in

ndash 3 ndash

2012 JINST 7 C04020

Figure 3 Measured MTF (left) and NPS (right) curves with various scintillating screens at a 125 mRexposure dose

figure 3 The MTF measurement of the Gadox samples with 50 and 120 microm thicknesses resultedin spatial frequencies of about 75 lpmm and 50 lpmm at an MTF value of 20 The extracted1D NPS curves along the radial direction with respect to various scintillating screens are plotted infigure 3 NPS curves have a similar form to the NPS falling over the whole spatial frequency re-gion as shown in figure 3 Figure 4 shows the calculated DQE (f) curves at a dose of 125 mR fromthe measured MTF and NPS curves for the RQA 5 beam quality condition The DQE (f) valueof the 50 microm-thick Gadox scintillating screen was higher than of the other samples in the highspatial frequency region because of the higher spatial resolution The estimated DQE values withdifferent scintillation screens at zero frequency were in the range of 015ndash03 at a dose of 125 mRThese values are significantly lower when than those of commercial detectors in use for currentmammography application Further improvement of DQE performance is still required throughmore uniformly fabricated scintillating screens with optimized thickness that do not demonstratesignificant loss of signal and spatial resolution

4 Conclusions and future work

In this work digital imaging detectors with various several microm-thick Gadox and CsITl scintillationscreens and a commercial CMOS APS image sensor were studied for mammography applicationTheir X-ray imaging performance characteristics in terms of X-ray linearity MTF NPS and DQEwere investigated under the RQA 5 beam quality condition with 30 kVp of X-ray tube voltage Themeasured results showed good linearity over the whole exposure dose and high MTF curves at highspatial frequencies Furthermore high spatial resolution and DQE at high spatial frequency wasachieved with a 50 microm-thick Gadox scintillating screen and a CMOS flat panel detector with a 48microm pixel pitch However the DQE performance of the detector is much lower than that of com-mercial digital mammographic detectors Therefore improvements in detectors are still requiredthrough a suitable choice among various scintillators as well as the structural design and fabrica-tion of uniform- and optimized scintillator thickness for high-quality mammographic imaging

ndash 4 ndash

2012 JINST 7 C04020

Figure 4 Measured DQE curves with various scintillating screens at a 125 mR exposure dose

Acknowledgments

This research was supported by research project of Korea Electrotechnology Research Institute(KERI) funded by the Ministry of Knowledge Economy (10-12-N0201-09)

References

[1] A Noel and F Thibault Digital detectors for mammography the technical challenges Eur Radiol 14(2004) 1990

[2] S Kasap et al Amorphous and polycrystalline photoconductors for direct conversion flat panel X-rayimage sensors Sensors 11 (2011) 5112

[3] D Scheffer A wafer scale active pixel CMOS image sensor for generic X-ray radiology Proc SPIE6510 (2007) 65100O-1

[4] BK Cha et al Fabrication and imaging characterization of high sensitive CsI(Tl) and Gd2O2S(Tb)scintllator screens for X-ray imaging detectors Radiation Measur 45 (2010) 742

[5] T Greave and GP Weckler High-resolution CMOS imaging detector Proc SPIE 4320 (2001) 68

[6] BK Cha et al Performance studies of a monolithic scintillator-CMOS image sensor for X-rayapplication Nucl Instrum Meth A 591 (2008) 113

[7] MK Cho et al Measurements of X-ray imaging performance of granular phosphors withdirect-coupled CMOS sensors IEEE Trans Nucl Sci 55 (2008) 1338

ndash 5 ndash

2012 JINST 7 C04020

Figure 1 Photographs of the CMOS flat panel detector (a) and 1 mm-thick graphite cover (b) fabricatedscintillation screens on glass substrates (c) SEM images of Gadox (d) and CsITl (e) scintillation screens

Figure 2 X-ray linearity (left) and SNR (right) of different scintillating screens as a function ofexposure dose

(LSF) curves The NPS as a function of the spatial frequency (f) was measured from the two-dimensional (2D) Fourier transform of white X-ray images for various samples under a 125 mR(Roentgens) exposure dose using 30 kVp The one-dimensional (1D) NPS was then obtained fromthe 2D NPS on a radial direction and subsequently normalized for the mean pixel value of the ROIresulting the normalized NPS (NNPS) With the measured MTF (f) and NNPS (f) values DQE (f)curves were calculated by the following equation [6 7]

DQE( f ) =SNR2

out

SNR2in

=MT F2( f )

NNPS( f ) middotq middotX

Where q is the number of incident X-ray photons per unit mR per mm2 and X is the exposure dosein units of mR Values of 72357mm2mR for the RQA 5 condition are used in these calculationsfor all samples

The MTF value of the Gadox screen with a 50 microm thickness was higher than that of otherscintillating samples because of less light spreading in the scintillating screen layer as shown in

ndash 3 ndash

2012 JINST 7 C04020

Figure 3 Measured MTF (left) and NPS (right) curves with various scintillating screens at a 125 mRexposure dose

figure 3 The MTF measurement of the Gadox samples with 50 and 120 microm thicknesses resultedin spatial frequencies of about 75 lpmm and 50 lpmm at an MTF value of 20 The extracted1D NPS curves along the radial direction with respect to various scintillating screens are plotted infigure 3 NPS curves have a similar form to the NPS falling over the whole spatial frequency re-gion as shown in figure 3 Figure 4 shows the calculated DQE (f) curves at a dose of 125 mR fromthe measured MTF and NPS curves for the RQA 5 beam quality condition The DQE (f) valueof the 50 microm-thick Gadox scintillating screen was higher than of the other samples in the highspatial frequency region because of the higher spatial resolution The estimated DQE values withdifferent scintillation screens at zero frequency were in the range of 015ndash03 at a dose of 125 mRThese values are significantly lower when than those of commercial detectors in use for currentmammography application Further improvement of DQE performance is still required throughmore uniformly fabricated scintillating screens with optimized thickness that do not demonstratesignificant loss of signal and spatial resolution

4 Conclusions and future work

In this work digital imaging detectors with various several microm-thick Gadox and CsITl scintillationscreens and a commercial CMOS APS image sensor were studied for mammography applicationTheir X-ray imaging performance characteristics in terms of X-ray linearity MTF NPS and DQEwere investigated under the RQA 5 beam quality condition with 30 kVp of X-ray tube voltage Themeasured results showed good linearity over the whole exposure dose and high MTF curves at highspatial frequencies Furthermore high spatial resolution and DQE at high spatial frequency wasachieved with a 50 microm-thick Gadox scintillating screen and a CMOS flat panel detector with a 48microm pixel pitch However the DQE performance of the detector is much lower than that of com-mercial digital mammographic detectors Therefore improvements in detectors are still requiredthrough a suitable choice among various scintillators as well as the structural design and fabrica-tion of uniform- and optimized scintillator thickness for high-quality mammographic imaging

ndash 4 ndash

2012 JINST 7 C04020

Figure 4 Measured DQE curves with various scintillating screens at a 125 mR exposure dose

Acknowledgments

This research was supported by research project of Korea Electrotechnology Research Institute(KERI) funded by the Ministry of Knowledge Economy (10-12-N0201-09)

References

[1] A Noel and F Thibault Digital detectors for mammography the technical challenges Eur Radiol 14(2004) 1990

[2] S Kasap et al Amorphous and polycrystalline photoconductors for direct conversion flat panel X-rayimage sensors Sensors 11 (2011) 5112

[3] D Scheffer A wafer scale active pixel CMOS image sensor for generic X-ray radiology Proc SPIE6510 (2007) 65100O-1

[4] BK Cha et al Fabrication and imaging characterization of high sensitive CsI(Tl) and Gd2O2S(Tb)scintllator screens for X-ray imaging detectors Radiation Measur 45 (2010) 742

[5] T Greave and GP Weckler High-resolution CMOS imaging detector Proc SPIE 4320 (2001) 68

[6] BK Cha et al Performance studies of a monolithic scintillator-CMOS image sensor for X-rayapplication Nucl Instrum Meth A 591 (2008) 113

[7] MK Cho et al Measurements of X-ray imaging performance of granular phosphors withdirect-coupled CMOS sensors IEEE Trans Nucl Sci 55 (2008) 1338

ndash 5 ndash

2012 JINST 7 C04020

Figure 3 Measured MTF (left) and NPS (right) curves with various scintillating screens at a 125 mRexposure dose

figure 3 The MTF measurement of the Gadox samples with 50 and 120 microm thicknesses resultedin spatial frequencies of about 75 lpmm and 50 lpmm at an MTF value of 20 The extracted1D NPS curves along the radial direction with respect to various scintillating screens are plotted infigure 3 NPS curves have a similar form to the NPS falling over the whole spatial frequency re-gion as shown in figure 3 Figure 4 shows the calculated DQE (f) curves at a dose of 125 mR fromthe measured MTF and NPS curves for the RQA 5 beam quality condition The DQE (f) valueof the 50 microm-thick Gadox scintillating screen was higher than of the other samples in the highspatial frequency region because of the higher spatial resolution The estimated DQE values withdifferent scintillation screens at zero frequency were in the range of 015ndash03 at a dose of 125 mRThese values are significantly lower when than those of commercial detectors in use for currentmammography application Further improvement of DQE performance is still required throughmore uniformly fabricated scintillating screens with optimized thickness that do not demonstratesignificant loss of signal and spatial resolution

4 Conclusions and future work

In this work digital imaging detectors with various several microm-thick Gadox and CsITl scintillationscreens and a commercial CMOS APS image sensor were studied for mammography applicationTheir X-ray imaging performance characteristics in terms of X-ray linearity MTF NPS and DQEwere investigated under the RQA 5 beam quality condition with 30 kVp of X-ray tube voltage Themeasured results showed good linearity over the whole exposure dose and high MTF curves at highspatial frequencies Furthermore high spatial resolution and DQE at high spatial frequency wasachieved with a 50 microm-thick Gadox scintillating screen and a CMOS flat panel detector with a 48microm pixel pitch However the DQE performance of the detector is much lower than that of com-mercial digital mammographic detectors Therefore improvements in detectors are still requiredthrough a suitable choice among various scintillators as well as the structural design and fabrica-tion of uniform- and optimized scintillator thickness for high-quality mammographic imaging

ndash 4 ndash

2012 JINST 7 C04020

Figure 4 Measured DQE curves with various scintillating screens at a 125 mR exposure dose

Acknowledgments

This research was supported by research project of Korea Electrotechnology Research Institute(KERI) funded by the Ministry of Knowledge Economy (10-12-N0201-09)

References

[1] A Noel and F Thibault Digital detectors for mammography the technical challenges Eur Radiol 14(2004) 1990

[2] S Kasap et al Amorphous and polycrystalline photoconductors for direct conversion flat panel X-rayimage sensors Sensors 11 (2011) 5112

[3] D Scheffer A wafer scale active pixel CMOS image sensor for generic X-ray radiology Proc SPIE6510 (2007) 65100O-1

[4] BK Cha et al Fabrication and imaging characterization of high sensitive CsI(Tl) and Gd2O2S(Tb)scintllator screens for X-ray imaging detectors Radiation Measur 45 (2010) 742

[5] T Greave and GP Weckler High-resolution CMOS imaging detector Proc SPIE 4320 (2001) 68

[6] BK Cha et al Performance studies of a monolithic scintillator-CMOS image sensor for X-rayapplication Nucl Instrum Meth A 591 (2008) 113

[7] MK Cho et al Measurements of X-ray imaging performance of granular phosphors withdirect-coupled CMOS sensors IEEE Trans Nucl Sci 55 (2008) 1338

ndash 5 ndash

2012 JINST 7 C04020

Figure 4 Measured DQE curves with various scintillating screens at a 125 mR exposure dose

Acknowledgments

This research was supported by research project of Korea Electrotechnology Research Institute(KERI) funded by the Ministry of Knowledge Economy (10-12-N0201-09)

References

[1] A Noel and F Thibault Digital detectors for mammography the technical challenges Eur Radiol 14(2004) 1990

[2] S Kasap et al Amorphous and polycrystalline photoconductors for direct conversion flat panel X-rayimage sensors Sensors 11 (2011) 5112

[3] D Scheffer A wafer scale active pixel CMOS image sensor for generic X-ray radiology Proc SPIE6510 (2007) 65100O-1

[4] BK Cha et al Fabrication and imaging characterization of high sensitive CsI(Tl) and Gd2O2S(Tb)scintllator screens for X-ray imaging detectors Radiation Measur 45 (2010) 742

[5] T Greave and GP Weckler High-resolution CMOS imaging detector Proc SPIE 4320 (2001) 68

[6] BK Cha et al Performance studies of a monolithic scintillator-CMOS image sensor for X-rayapplication Nucl Instrum Meth A 591 (2008) 113

[7] MK Cho et al Measurements of X-ray imaging performance of granular phosphors withdirect-coupled CMOS sensors IEEE Trans Nucl Sci 55 (2008) 1338

ndash 5 ndash