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System performance of phosphor screen coupled CMOS imager for long-term radiation exposure
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 607 (2009) 218–220
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
System performance of phosphor screen coupled CMOS imager for long-term radiation exposure Kwang Hyun Kim College of Dentistry, Chosun University, Seosuk-Dong, Dong-Gu, Gwangju, South Korea
a r t i c l e in f o
a b s t r a c t
Available online 25 March 2009
We present the performances of a scintillator coupled CMOS APS imager under cumulative X-ray exposure conditions in terms of the modulation transfer function (MTF), noise power spectrum, and detective quantum efficiency (DQE). An industrial X-ray generator with the condition of 50 kVp/500 mA was used to take cumulative exposure conditions. The experimental results show that the MTF and DQE exponentially degraded because of direct X-ray exposure of the imager through the scintillator. For the given scintillator and radiation exposure, the system performance of the scintillator coupled CMOS APS imager can be predicted. & 2009 Elsevier B.V. All rights reserved.
Keywords: NDT Long-term X-ray exposure Scintillator coupled CMOS sensor DQE
1. Introduction In digital X-ray imaging, Scintillator Coupled CMOS Active Pixel Sensor (SC CMOS APS) imagers are used as one of the approaches for non-destructive test (NDT) applications. For that application, X-ray machines delivering high doses in short times are used. This results in high exposures of the imager by the transmitted X-rays through the scintillator (direct interaction with the sensor). It has been shown that, overall radiation effects on the imager result in an increase of the leakage current with increasing radiation dose [1]. In our previous research, the effects of the transmitted X-rays on the SC CMOS APS imager were investigated [2]. However, a generalization of the system performance for long-term irradiation condition with X-rays was not possible. In this paper, we present the characterization of the relationship between signal to noise ratio (SNR) and dynamic range (DR), and the derived generalized equation for the modulation transfer function (MTF) and detective quantum efficiency (DQE) under long-term exposures.
window was fixed at the tube voltage of 50 kVp and the tube current of 500 mA. In order to measure the cumulative X-ray exposure impinging on the detector, an ionization chamber from RAD CHEKTM PLUS (model 06-526) was located at the same distance of 300 mm from the X-ray source. For single X-ray exposures of 50 kVp and 500 mA, the signal to noise ratio of the detector was determined by the ratio of the mean total signal and the root mean square of the total signal. In addition, a workable definition of dynamic range the ratio of the highest signal which a detector can record to the lowest (actually, noise level in dark) signal, was used. The MTF of the SC CMOS APS imager was derived from the measured line spread-function (LSF) by using a Tantalum phantom with the thickness of 1.5 mm and a slit width of 10 mm [3]. NPS measurements were performed for the detector employing each scintillator up to 50% of pixel saturation. Flat field correction was performed by a method recommended in the literature [4]. DQE describes the transfer of SNR through the imaging chain and is regarded as the most useful measure of sensitivity and noise performance of an imaging system and is measured as a function of the spatial frequency of the object [5].
2. Materials and methods We used a LanexTM Fine scintillator, CMOS APS imagers from RadEyeTM, and a Fein focusTM X-ray machine. The density and thickness of the scintillator were 7.34 g/cm3 and 34 mg/cm2, respectively. The CMOS APS has a pixel size of 48 mm, noise floor of 150 electrons rms, and a digitization of 250–300 electrons rms. The X-ray machine with a tungsten target and a beryllium Corresponding author. Tel.: +82 62 230 6867; fax: +82 42 8618779.
E-mail address: [email protected] 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.03.157
3. Results and discussions Using the definitions of SNR and DR, we plotted the results of the SNR and the DR as a function of accumulated dose in Fig. 1. They were all normalized to 1 at zero exposure. In this figure, there is a linear increase of SNR while an exponential decrease of DR as function of increasing X-ray dose. As we discussed in a previous paper, the direct X-ray detection in the image sensor through the scintillator gives cause for both a
ARTICLE IN PRESS K.H. Kim / Nuclear Instruments and Methods in Physics Research A 607 (2009) 218–220
1.0
1.1 Normalized value (arb.unit)
Normalized value (arb. unit)
1.2
219
1.0 0.9 DR SNR
0.8 0.7 0.6 0.5 0.4
0.9
First Order Exponential Fitting
0.8 0.7 0.6 0.5
0.3 0
1
2 3 4 Cumulative exposure (kR)
5
6 0
Fig. 1. Normalized SNR and DR as function of cumulated X-ray dose.
1 2 3 Cumulative exposure (kR)
4
Fig. 3. Degradation of DQE as function of cumulated dose.
1.0
0.9
Normalized value (arb. unit)
Normalized value (arb.unit)
1.0
First Order Exponential Fitting
0.8 0.7
Dark Signal Dynamic Range MTF DQE
0.8
0.6
0.4
0.2
0.6 0.0
0.5
0
0
1 2 3 Cumulative exposure (kR)
4
Fig. 2. Degradation of the MTF from an initial as function of cumulated dose. These data were obtained by Eq. (1).
generation of photocurrent and increase of dark current in the imager [2]. The measured MTF(f)s for each cumulative exposure were normalized to an initial value at 0 kR by using the following equation: R f Nyq MTFðf Þdf jR ¼ var MTFdeg ¼ R0 f Nyq MTFðf Þdf jR ¼ 0 0
(1)
where MTFdeg means normalized MTF but degraded by the cumulative exposure, fNyq is the Nyquist frequency, and R is a cumulative exposure level ranged from 0 to any level. The results are plotted in Fig. 2, which showed an exponential decrease of the MTF with increasing cumulative X-ray exposure. The NPS (f), which depends on the incident X-ray flux, was constant for successive exposure. As the cumulative exposure increases, DQE (f) increases in the low-frequency region, but decreased in the high-frequency region. The overall change of the DQE, integrated over all frequencies and normalized by the same method as described for the MTF above, is plotted in Fig. 3. Fig. 4 shows all the influences of the cumulative X-ray exposure on the performances of the detector. The X-rays that
1 2 3 Cumulative exposure (kR)
4
Fig. 4. All expressions for the influences of cumulative X-ray exposure on SC CMOS APS.
are transmitted through scintillator cause the dark signal increase in the detector as function of accumulated dose. The increased dark signal again degrades dynamic range and the resolution of the detector. Since there is a correlation between MTF and DQE as expressed in DQE itself, the degradation of DQE eventually follows the trend of MTF for a given NPS. System performance of phosphor screen coupled CMOS imager for longterm cumulative X-ray exposure can be derived as a final form as follows: DQEðRÞ ¼ DQER¼0 eaR
(2)
where ‘‘a’’ is a constant and ‘‘R’’ the dose seen directly by the imager, and which depends on the given scintillator properties such as attenuation coefficient and thickness. The fit presented in Fig. 3, gives a value of a ¼ 0.83 kR1. This generalized DQE equation is useful to predict the system performance for all scintillator coupled two-dimensional array detectors.
Acknowledgements This study was supported (in part) by research funds from Chosun University, 2008.
ARTICLE IN PRESS 220
K.H. Kim / Nuclear Instruments and Methods in Physics Research A 607 (2009) 218–220
References [1] G.R. Hopkinson, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 2490. [2] K.H. Kim, et al., Nucl. Instr. and Meth. A 537 (2005) 454.
[3] K. Rossmann, et al., Radio 93 (1969) 257. [4] S. Vedantham, et al., Med. Phys. 27 (3) (2000) 558. [5] J.M. Boone, et al., Handbook of Medical Imaging (SPIE), vol. I, 2000, p. 50.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
System performance of phosphor screen coupled CMOS imager for long-term radiation exposure Kwang Hyun Kim College of Dentistry, Chosun University, Seosuk-Dong, Dong-Gu, Gwangju, South Korea
a r t i c l e in f o
a b s t r a c t
Available online 25 March 2009
We present the performances of a scintillator coupled CMOS APS imager under cumulative X-ray exposure conditions in terms of the modulation transfer function (MTF), noise power spectrum, and detective quantum efficiency (DQE). An industrial X-ray generator with the condition of 50 kVp/500 mA was used to take cumulative exposure conditions. The experimental results show that the MTF and DQE exponentially degraded because of direct X-ray exposure of the imager through the scintillator. For the given scintillator and radiation exposure, the system performance of the scintillator coupled CMOS APS imager can be predicted. & 2009 Elsevier B.V. All rights reserved.
Keywords: NDT Long-term X-ray exposure Scintillator coupled CMOS sensor DQE
1. Introduction In digital X-ray imaging, Scintillator Coupled CMOS Active Pixel Sensor (SC CMOS APS) imagers are used as one of the approaches for non-destructive test (NDT) applications. For that application, X-ray machines delivering high doses in short times are used. This results in high exposures of the imager by the transmitted X-rays through the scintillator (direct interaction with the sensor). It has been shown that, overall radiation effects on the imager result in an increase of the leakage current with increasing radiation dose [1]. In our previous research, the effects of the transmitted X-rays on the SC CMOS APS imager were investigated [2]. However, a generalization of the system performance for long-term irradiation condition with X-rays was not possible. In this paper, we present the characterization of the relationship between signal to noise ratio (SNR) and dynamic range (DR), and the derived generalized equation for the modulation transfer function (MTF) and detective quantum efficiency (DQE) under long-term exposures.
window was fixed at the tube voltage of 50 kVp and the tube current of 500 mA. In order to measure the cumulative X-ray exposure impinging on the detector, an ionization chamber from RAD CHEKTM PLUS (model 06-526) was located at the same distance of 300 mm from the X-ray source. For single X-ray exposures of 50 kVp and 500 mA, the signal to noise ratio of the detector was determined by the ratio of the mean total signal and the root mean square of the total signal. In addition, a workable definition of dynamic range the ratio of the highest signal which a detector can record to the lowest (actually, noise level in dark) signal, was used. The MTF of the SC CMOS APS imager was derived from the measured line spread-function (LSF) by using a Tantalum phantom with the thickness of 1.5 mm and a slit width of 10 mm [3]. NPS measurements were performed for the detector employing each scintillator up to 50% of pixel saturation. Flat field correction was performed by a method recommended in the literature [4]. DQE describes the transfer of SNR through the imaging chain and is regarded as the most useful measure of sensitivity and noise performance of an imaging system and is measured as a function of the spatial frequency of the object [5].
2. Materials and methods We used a LanexTM Fine scintillator, CMOS APS imagers from RadEyeTM, and a Fein focusTM X-ray machine. The density and thickness of the scintillator were 7.34 g/cm3 and 34 mg/cm2, respectively. The CMOS APS has a pixel size of 48 mm, noise floor of 150 electrons rms, and a digitization of 250–300 electrons rms. The X-ray machine with a tungsten target and a beryllium Corresponding author. Tel.: +82 62 230 6867; fax: +82 42 8618779.
E-mail address: [email protected] 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.03.157
3. Results and discussions Using the definitions of SNR and DR, we plotted the results of the SNR and the DR as a function of accumulated dose in Fig. 1. They were all normalized to 1 at zero exposure. In this figure, there is a linear increase of SNR while an exponential decrease of DR as function of increasing X-ray dose. As we discussed in a previous paper, the direct X-ray detection in the image sensor through the scintillator gives cause for both a
ARTICLE IN PRESS K.H. Kim / Nuclear Instruments and Methods in Physics Research A 607 (2009) 218–220
1.0
1.1 Normalized value (arb.unit)
Normalized value (arb. unit)
1.2
219
1.0 0.9 DR SNR
0.8 0.7 0.6 0.5 0.4
0.9
First Order Exponential Fitting
0.8 0.7 0.6 0.5
0.3 0
1
2 3 4 Cumulative exposure (kR)
5
6 0
Fig. 1. Normalized SNR and DR as function of cumulated X-ray dose.
1 2 3 Cumulative exposure (kR)
4
Fig. 3. Degradation of DQE as function of cumulated dose.
1.0
0.9
Normalized value (arb. unit)
Normalized value (arb.unit)
1.0
First Order Exponential Fitting
0.8 0.7
Dark Signal Dynamic Range MTF DQE
0.8
0.6
0.4
0.2
0.6 0.0
0.5
0
0
1 2 3 Cumulative exposure (kR)
4
Fig. 2. Degradation of the MTF from an initial as function of cumulated dose. These data were obtained by Eq. (1).
generation of photocurrent and increase of dark current in the imager [2]. The measured MTF(f)s for each cumulative exposure were normalized to an initial value at 0 kR by using the following equation: R f Nyq MTFðf Þdf jR ¼ var MTFdeg ¼ R0 f Nyq MTFðf Þdf jR ¼ 0 0
(1)
where MTFdeg means normalized MTF but degraded by the cumulative exposure, fNyq is the Nyquist frequency, and R is a cumulative exposure level ranged from 0 to any level. The results are plotted in Fig. 2, which showed an exponential decrease of the MTF with increasing cumulative X-ray exposure. The NPS (f), which depends on the incident X-ray flux, was constant for successive exposure. As the cumulative exposure increases, DQE (f) increases in the low-frequency region, but decreased in the high-frequency region. The overall change of the DQE, integrated over all frequencies and normalized by the same method as described for the MTF above, is plotted in Fig. 3. Fig. 4 shows all the influences of the cumulative X-ray exposure on the performances of the detector. The X-rays that
1 2 3 Cumulative exposure (kR)
4
Fig. 4. All expressions for the influences of cumulative X-ray exposure on SC CMOS APS.
are transmitted through scintillator cause the dark signal increase in the detector as function of accumulated dose. The increased dark signal again degrades dynamic range and the resolution of the detector. Since there is a correlation between MTF and DQE as expressed in DQE itself, the degradation of DQE eventually follows the trend of MTF for a given NPS. System performance of phosphor screen coupled CMOS imager for longterm cumulative X-ray exposure can be derived as a final form as follows: DQEðRÞ ¼ DQER¼0 eaR
(2)
where ‘‘a’’ is a constant and ‘‘R’’ the dose seen directly by the imager, and which depends on the given scintillator properties such as attenuation coefficient and thickness. The fit presented in Fig. 3, gives a value of a ¼ 0.83 kR1. This generalized DQE equation is useful to predict the system performance for all scintillator coupled two-dimensional array detectors.
Acknowledgements This study was supported (in part) by research funds from Chosun University, 2008.
ARTICLE IN PRESS 220
K.H. Kim / Nuclear Instruments and Methods in Physics Research A 607 (2009) 218–220
References [1] G.R. Hopkinson, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 2490. [2] K.H. Kim, et al., Nucl. Instr. and Meth. A 537 (2005) 454.
[3] K. Rossmann, et al., Radio 93 (1969) 257. [4] S. Vedantham, et al., Med. Phys. 27 (3) (2000) 558. [5] J.M. Boone, et al., Handbook of Medical Imaging (SPIE), vol. I, 2000, p. 50.