- Email: [email protected]
Unified MTF for scintillator-coupled CMOS sensor
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238 www.elsevier.com/locate/nima
Unified MTF for scintillator-coupled CMOS sensor Kwang Hyun Kima,, Dong-Wan Kanga, Dong Ki Kima, Yong-Kyun Kimb a
College of Dentistry, Chosun University, Seosuk-Dong, Dong-Gu, Gwangju, South Korea Korea Atomic Energy Research Institute, Dukjin-Dong,Yusung-Gu, Daejeon, South Korea
b
Available online 6 April 2007
Abstract The spatial resolution of scintillator-coupled CMOS sensor has been investigated from intrinsic sensor Modulation Transfer Function (MTF) to system MTF for the conditions of the digital radiography. For the intrinsic sensor MTF, the geometric MTF (gMTF) and the unified MTF (uMTF) were compared by analytic calculations for various pixel sizes. The effects of the initial dark signal of the sensor were considered in the calculation of the uMTF and reflected in a newly developed semi-empirical model. The measured system MTF and the calculated system MTF including semi-empirical model were compared under radiography conditions of 28 and 80 kVp. From the results, the calculated system MTF reflecting the dark-signal contribution on the sensor resolution did fit for the measured system resolution, and the higher the fraction of an initial dark signal to an output signal in response to X-ray exposure showed more degradation of the system resolution even with same scintillator and sensor. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 87.59.Hp; 87.59.Ek Keywords: Unified MTF; Scintillator-coupled CMOS sensor; Digital radiography
1. Introduction In high-resolution two-dimensional (2D) digital radiography, an imaging system needs to have a small pixel pitch and thus the ability of discerning small objects. This requirement is even more stringent for dental radiography and mammography for which pixel pitches of below 50 mm are required. For an indirect detection imaging system that includes a scintillator coupled to a 2D matrix of individual sensors, the system Modulation Transfer Function (MTF), MTFsys may be expressed as MTFsys ¼ MTFsci MTFsen .
(1)
The scintillator resolution, MTFsci, reflects the blurring of scintillation light in the scintillator and the sensor resolution, MTFsen, represents the intrinsic sensor spatial resolution, respectively. When CMOS vision sensor is used, the unified MTF (uMTF) model has been applied as an intrinsic sensor resolution. A more detailed explanation of Corresponding author. Tel.: +82 62 230 6867; fax: +82 42 861 8779.
E-mail address: [email protected] (K.H. Kim). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.047
the sensor resolution as functions of the lateral diffusion length in the pixel, the fill factor (FF) of the pixel sensor, and the wavelength of the light incident onto the sensor are given in Refs. [1,2]. In this paper, considering the effects of the initial dark signal of the sensor on the uMTF and scintillator resolution, we evaluated the system resolution at the radiography conditions of 28 and 80 kVp, respectively, using 2D array CMOS Active Pixel Sensor (APS) which were coupled to CsI (Tl) coated Fiber Optic Plate (FOP) as an imaging system.
2. Unified MTF model considering dark signal and scintillation lights In the unified model of Ref. [1], by using the signal output S0 (f) of the sensor and the MTF (f), it was shown that the uMTF of the sensor is higher than the predicted by multiplying the diffusion and aperture MTF’s together and the discrepancy was greater in the smaller pixel aperture with a shorter wavelength. Here, the diffusion MTF means the resolution of the sensor by the lateral diffusion of charge carriers within the sensor array. The S0 (f) and the
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
236
MTF (f) are given respectively as S o ðf Þ ¼ aðlÞNa eaðlÞy ½K 1 ðf Þ þ
NaðlÞð1 eaðlÞy Þ ½K 2 ðf Þ 2
ð2Þ
where a(l) is the absorption coefficient as a function of the wavelength l of the incident light, N is incident photons, a is pixel aperture, and y is depletion depth. K1 (f) and K2 (f) were simplified here, but contain the information on sampling frequency, pixel pitch, and diffusion length as a function of sampling frequency f. MTF ¼ ðf Þ
So So
So max ðf Þ þ S o
max ðf Þ
min ðf Þ min ðf Þ
(3)
where So max (f) and So min (f) are each signal from the maximum signal generated in open aperture and the minimum signal generated in opaque aperture at the given pixel geometry for the incident light, respectively. Herein, the open and opaque aperture means each area in a pixel that the incident light enters or not. Each signal can be all calculated by the parameters used in Eq. (2) at the given pixel pitch and aperture size. Dark signal generated in the absence of incident light comes mainly from leakage current in the sensor. Such leakage current influences the dynamic range as well as contrast transfer function or modulation transfer function of the sensor. The influence of initial intrinsic dark signal on the spatial resolution using basic semiconductor physics and the uMTF was then investigated. The signals of So max (f) and So min (f) in Eq. (2) are the calculated values by the charge generated in the depletion region and diffusion from the bulk, using the assumption that the dark signal comes partially from the diffusion from bulk substrate [3]. Therefore, to relate the dark signal to Eq. (3), it is modeled for output signals as ^
S 0o max ðf ; F Þ ¼ aðlÞNaðSo max F Þ eaðlÞy ½K 1 ðf Þ ^
NaðlÞðS o max F Þð1 eaðlÞy Þ ½K 2 ðf Þ þ 2
ð4Þ
and ^
S 0o min ðf ; F Þ ¼ aðlÞNaðS o min F Þ eaðlÞy ½K 1 ðf Þ ^
NaðlÞðS o min F Þð1 eaðlÞy Þ þ 2 ^
½K 2 ðf Þ þ ðSo min F Þ.
ð5Þ
Eq. (4) was acquired from Eq. (2) multiplied by ^
S o max F Eq. (5) was also acquired from Eq. (2), multiplied ^
^
by S o min F and added to S o min F in the last term. ^
^
S o max and S o min each signal value, calculated by Eq. (2) using the same parameters from the maximum signal generated in open aperture and the minimum signal generated in opaque aperture, respectively. The new
parameter defined here, F, is the fraction of an initial dark signal to an output signal in response to X-ray exposure in ADC unit. For the given X-ray conditions such as an incident energy and a tube current of milliampere, the output signal from the scintillator-coupled CMOS APS can be measured in the ADC unit as can be X-ray linearity test of the digital radiography system. Therefore, these new maximum and minimum signals, reflecting the dark signal, are substituted into Eq. (3), which finally leads to the following equation as functions of frequency and dark signal: MTFðf ; F Þ ¼
S0o max ðf ; F Þ S 0o min ðf ; F Þ . S0o max ðf ; F Þ þ S 0o min ðf ; F Þ
(6)
We calculated the uMTF using Eqs. (2) and (3) and compared the outcome to the geometric MTF (gMTF). The gMTF used here was calculated using a sinc function, which corresponds to the pixel aperture, the fill factor being the fraction of the pixel aperture to the pixel pitch. The pixel sizes considered were from 10 to 48 mm. In the calculation of Eq. (2), the absorption coefficients of a(l) were 0.6 and 0.39 for the wavelengths of 550 and 610 nm from CsI (Tl) and ZnSe (Te) scintillators, respectively. The number of the incident photons of 450 was assumed for each scintillator light. The depletion depth and the diffusion length were 3.5 and 50 mm, respectively. The sensors of small pixel sizes and low FFs were applied to both the uMTF and the gMTF for each incident wavelength. The FFs of 30% and 40% for the pixel pitch of 10 mm at the wavelength of 550 nm and 40% and 50% for the pixel pitch of 15 mm at the wavelength of 610 nm were used. In Fig. 1a and b, even at small pixel pitch, the effect of the lateral diffusion of the minority carrier has not been revealed at the wavelength of 550 nm. However, at the wavelength of 610 nm, the higher frequency has shown a greater effect of diffusion, and degraded the spatial resolution as shown in Fig. 2a and b. The trends that followed are as explained in Ref. [1]. The higher the wavelength, the lower absorption in the sensor and the more pronounced the crosstalk of minority carriers. Thus, the more pronounced is the discrepancy between the uMTF and the gMTF. The discrepancy between the uMTF and the gMTF was relatively large at a larger FF than a smaller one in both wavelengths of 550 and 610 nm. For the large pixel pitch of 48 mm, the effects of the wavelength and FF on the uMTF with or without F value by two wavelengths of 550 and 610 nm were evaluated as shown in Fig. 3. In Fig. 3a, without F value in the calculation of the uMTF, the wavelength was not a parameter influence on the MTF, reflecting the F value on the calculation, increasing MTF degradation at short wavelength. The effect of FFs considering dark signal in MTF were calculated and showed that the degradation of the MTF was more severe in a large FF at all frequencies as shown in Fig. 3b.
ARTICLE IN PRESS
MTF
K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
1.0
1.0
0.8
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gM TF(FF-30) uM TF(FF-30) gM TF(FF-40) uM TF(FF-40)
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gM TF (FF-40) uM TF (FF-40) gM TF (FF-50) uM TF (FF-50)
0.6
0.6 0
10
20 30 cycle/mm
40
50
0
10
20 30 cycle/mm
40
MTF
Fig. 1. Comparison of geometric MTF and unified MTF by changing fill factors of 30% and 40% for the pixel pitch of 10 mm (a), and 40% and 50% for the pixel pitch of 15 mm (b), at the wavelength of 550 nm.
1.0
1.0
0.8
0.8
gM TF(FF-30) uM TF(FF-30) gM TF(FF-40) uM TF(FF-40)
gM TF(FF-40) uM TF(FF-40) gM TF(FF-50) uM TF(FF-50)
0.6
0.6 0
10
20 30 cycle/mm
40
50
0
10
20 30 cycle/mm
40
Fig. 2. Comparison of geometric MTF and unified MTF by changing fill factors of 30% and 40% for the pixel pitch of 10 mm (a), and 40% and 50% for the pixel pitch of 15 mm (b), at the wavelength of 610 nm.
The measured system MTFs were plotted and compared to the calculated system MTFs without and with F value. In the calculation of the system MTF, for the prismatic crystal structure of CsI (Tl) on FOP which has the frequency over 22 cycle/mm [4], the Coltman equation was used to convert the contrast transfer function to the modulation transfer function [5]. F values of 0.5 and 0.17 were used in the calculations of the intrinsic sensor uMTF. By using Eq. (3) without F value and Eq. (6) with F value, and by the substitutions of Eqs. (3) and (6) into Eq. (1), we
finally acquired the calculated system MTF and the measured system MTF, respectively, as shown in Fig. 4. 3. Summary and conclusions From the results, while the sensors having relatively small pixel pitch of 10 and 15 mm showed degradation of the intrinsic sensor resolution by the crosstalk at wavelength 610 nm, no change of sensor resolution was found in large pixel pitch of 50 mm for either wavelengths 550 and
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
1.0
1.0
0.8
0.8
MTF
238
0.6
0.6
Without F @ FF:0.8 Without F @ FF:0.9 With F:0.5 @ FF:0.8 With F:0.5 @ FF:0.9
Without F at 550nm Without F at 610nm With F:0.5 at 550nm With F:0.5 at 610nm 0.4
0.4 0
2
4 6 8 Frequency (cycle/mm)
0
10
2
4 6 8 Frequency (cycle/mm)
10
Fig. 3. The effect of wavelength for pixel pitch of 48 mm and fill factor of 0.85 (a), and fill factor (FF) for pixel pitch of 48 mm and incident wavelength of 550 nm on the unified MTF (b).
It is clear that reducing the FF also reduces efficiency for X-ray detection (thus increasing the statistical noise in the signal). It is believed that this would be revealed by a Detective Quantum Efficiency (DQE) analysis as described by other papers [6]. The semi-empirical new MTF model of the intrinsic sensor resolution was used in the calculation of the system MTF. It was shown that while the calculated system MTF without considering the dark-signal contribution did not predict the measured system resolution, the calculated system MTF reflecting the dark-signal contribution on the sensor resolution did fit the measured system resolution.
1.0 calculated sys MTF without F calculated sys MTF at 80kVp with F calculated sys MTF at 28kVp with F measured sys MTF at 80kVp measured sys MTF at 28kVp
MTF
0.8
0.6
0.4
0.2
Acknowledgment 0
2
4
6
8
10
12
Frequency (cycle/mm) Fig. 4. Comparison of the calculated and the measured system MTF for both radiography conditions considering the effect of F value.
610 nm. Therefore, for the small pixel pitch near 15 mm and the long wavelength of 610 nm, the uMTF model, instead of simple gMTF based on only pixel pitch and aperture, is more desirable to predict the intrinsic sensor resolution in the analytical evaluation.
This study was supported (in part) by research funds from Chosun University, 2006. References [1] [2] [3] [4]
E.G. Stevens, IEEE Trans. Electron. 40 (1992) 2621. I. Shcherback, et al., IEEE Trans. Electron. 48 (2001) 2710. N.V. Loukianova, et al., IEEE Trans. Electron. 50 (2003) 77. HAMAMATSU, FOS (Fiber Optic Plate with Scintillator for Digital X-ray Imaging), in: Technical Information, 1996, p. 4. [5] J.W. Coltman, et al., J. Optic. Soc. Am. 44 (1954) 468. [6] J.H. Siewerdsen, Med. Phys. 24 (1997) 71.
Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238 www.elsevier.com/locate/nima
Unified MTF for scintillator-coupled CMOS sensor Kwang Hyun Kima,, Dong-Wan Kanga, Dong Ki Kima, Yong-Kyun Kimb a
College of Dentistry, Chosun University, Seosuk-Dong, Dong-Gu, Gwangju, South Korea Korea Atomic Energy Research Institute, Dukjin-Dong,Yusung-Gu, Daejeon, South Korea
b
Available online 6 April 2007
Abstract The spatial resolution of scintillator-coupled CMOS sensor has been investigated from intrinsic sensor Modulation Transfer Function (MTF) to system MTF for the conditions of the digital radiography. For the intrinsic sensor MTF, the geometric MTF (gMTF) and the unified MTF (uMTF) were compared by analytic calculations for various pixel sizes. The effects of the initial dark signal of the sensor were considered in the calculation of the uMTF and reflected in a newly developed semi-empirical model. The measured system MTF and the calculated system MTF including semi-empirical model were compared under radiography conditions of 28 and 80 kVp. From the results, the calculated system MTF reflecting the dark-signal contribution on the sensor resolution did fit for the measured system resolution, and the higher the fraction of an initial dark signal to an output signal in response to X-ray exposure showed more degradation of the system resolution even with same scintillator and sensor. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 87.59.Hp; 87.59.Ek Keywords: Unified MTF; Scintillator-coupled CMOS sensor; Digital radiography
1. Introduction In high-resolution two-dimensional (2D) digital radiography, an imaging system needs to have a small pixel pitch and thus the ability of discerning small objects. This requirement is even more stringent for dental radiography and mammography for which pixel pitches of below 50 mm are required. For an indirect detection imaging system that includes a scintillator coupled to a 2D matrix of individual sensors, the system Modulation Transfer Function (MTF), MTFsys may be expressed as MTFsys ¼ MTFsci MTFsen .
(1)
The scintillator resolution, MTFsci, reflects the blurring of scintillation light in the scintillator and the sensor resolution, MTFsen, represents the intrinsic sensor spatial resolution, respectively. When CMOS vision sensor is used, the unified MTF (uMTF) model has been applied as an intrinsic sensor resolution. A more detailed explanation of Corresponding author. Tel.: +82 62 230 6867; fax: +82 42 861 8779.
E-mail address: [email protected] (K.H. Kim). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.047
the sensor resolution as functions of the lateral diffusion length in the pixel, the fill factor (FF) of the pixel sensor, and the wavelength of the light incident onto the sensor are given in Refs. [1,2]. In this paper, considering the effects of the initial dark signal of the sensor on the uMTF and scintillator resolution, we evaluated the system resolution at the radiography conditions of 28 and 80 kVp, respectively, using 2D array CMOS Active Pixel Sensor (APS) which were coupled to CsI (Tl) coated Fiber Optic Plate (FOP) as an imaging system.
2. Unified MTF model considering dark signal and scintillation lights In the unified model of Ref. [1], by using the signal output S0 (f) of the sensor and the MTF (f), it was shown that the uMTF of the sensor is higher than the predicted by multiplying the diffusion and aperture MTF’s together and the discrepancy was greater in the smaller pixel aperture with a shorter wavelength. Here, the diffusion MTF means the resolution of the sensor by the lateral diffusion of charge carriers within the sensor array. The S0 (f) and the
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
236
MTF (f) are given respectively as S o ðf Þ ¼ aðlÞNa eaðlÞy ½K 1 ðf Þ þ
NaðlÞð1 eaðlÞy Þ ½K 2 ðf Þ 2
ð2Þ
where a(l) is the absorption coefficient as a function of the wavelength l of the incident light, N is incident photons, a is pixel aperture, and y is depletion depth. K1 (f) and K2 (f) were simplified here, but contain the information on sampling frequency, pixel pitch, and diffusion length as a function of sampling frequency f. MTF ¼ ðf Þ
So So
So max ðf Þ þ S o
max ðf Þ
min ðf Þ min ðf Þ
(3)
where So max (f) and So min (f) are each signal from the maximum signal generated in open aperture and the minimum signal generated in opaque aperture at the given pixel geometry for the incident light, respectively. Herein, the open and opaque aperture means each area in a pixel that the incident light enters or not. Each signal can be all calculated by the parameters used in Eq. (2) at the given pixel pitch and aperture size. Dark signal generated in the absence of incident light comes mainly from leakage current in the sensor. Such leakage current influences the dynamic range as well as contrast transfer function or modulation transfer function of the sensor. The influence of initial intrinsic dark signal on the spatial resolution using basic semiconductor physics and the uMTF was then investigated. The signals of So max (f) and So min (f) in Eq. (2) are the calculated values by the charge generated in the depletion region and diffusion from the bulk, using the assumption that the dark signal comes partially from the diffusion from bulk substrate [3]. Therefore, to relate the dark signal to Eq. (3), it is modeled for output signals as ^
S 0o max ðf ; F Þ ¼ aðlÞNaðSo max F Þ eaðlÞy ½K 1 ðf Þ ^
NaðlÞðS o max F Þð1 eaðlÞy Þ ½K 2 ðf Þ þ 2
ð4Þ
and ^
S 0o min ðf ; F Þ ¼ aðlÞNaðS o min F Þ eaðlÞy ½K 1 ðf Þ ^
NaðlÞðS o min F Þð1 eaðlÞy Þ þ 2 ^
½K 2 ðf Þ þ ðSo min F Þ.
ð5Þ
Eq. (4) was acquired from Eq. (2) multiplied by ^
S o max F Eq. (5) was also acquired from Eq. (2), multiplied ^
^
by S o min F and added to S o min F in the last term. ^
^
S o max and S o min each signal value, calculated by Eq. (2) using the same parameters from the maximum signal generated in open aperture and the minimum signal generated in opaque aperture, respectively. The new
parameter defined here, F, is the fraction of an initial dark signal to an output signal in response to X-ray exposure in ADC unit. For the given X-ray conditions such as an incident energy and a tube current of milliampere, the output signal from the scintillator-coupled CMOS APS can be measured in the ADC unit as can be X-ray linearity test of the digital radiography system. Therefore, these new maximum and minimum signals, reflecting the dark signal, are substituted into Eq. (3), which finally leads to the following equation as functions of frequency and dark signal: MTFðf ; F Þ ¼
S0o max ðf ; F Þ S 0o min ðf ; F Þ . S0o max ðf ; F Þ þ S 0o min ðf ; F Þ
(6)
We calculated the uMTF using Eqs. (2) and (3) and compared the outcome to the geometric MTF (gMTF). The gMTF used here was calculated using a sinc function, which corresponds to the pixel aperture, the fill factor being the fraction of the pixel aperture to the pixel pitch. The pixel sizes considered were from 10 to 48 mm. In the calculation of Eq. (2), the absorption coefficients of a(l) were 0.6 and 0.39 for the wavelengths of 550 and 610 nm from CsI (Tl) and ZnSe (Te) scintillators, respectively. The number of the incident photons of 450 was assumed for each scintillator light. The depletion depth and the diffusion length were 3.5 and 50 mm, respectively. The sensors of small pixel sizes and low FFs were applied to both the uMTF and the gMTF for each incident wavelength. The FFs of 30% and 40% for the pixel pitch of 10 mm at the wavelength of 550 nm and 40% and 50% for the pixel pitch of 15 mm at the wavelength of 610 nm were used. In Fig. 1a and b, even at small pixel pitch, the effect of the lateral diffusion of the minority carrier has not been revealed at the wavelength of 550 nm. However, at the wavelength of 610 nm, the higher frequency has shown a greater effect of diffusion, and degraded the spatial resolution as shown in Fig. 2a and b. The trends that followed are as explained in Ref. [1]. The higher the wavelength, the lower absorption in the sensor and the more pronounced the crosstalk of minority carriers. Thus, the more pronounced is the discrepancy between the uMTF and the gMTF. The discrepancy between the uMTF and the gMTF was relatively large at a larger FF than a smaller one in both wavelengths of 550 and 610 nm. For the large pixel pitch of 48 mm, the effects of the wavelength and FF on the uMTF with or without F value by two wavelengths of 550 and 610 nm were evaluated as shown in Fig. 3. In Fig. 3a, without F value in the calculation of the uMTF, the wavelength was not a parameter influence on the MTF, reflecting the F value on the calculation, increasing MTF degradation at short wavelength. The effect of FFs considering dark signal in MTF were calculated and showed that the degradation of the MTF was more severe in a large FF at all frequencies as shown in Fig. 3b.
ARTICLE IN PRESS
MTF
K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
1.0
1.0
0.8
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gM TF(FF-30) uM TF(FF-30) gM TF(FF-40) uM TF(FF-40)
237
gM TF (FF-40) uM TF (FF-40) gM TF (FF-50) uM TF (FF-50)
0.6
0.6 0
10
20 30 cycle/mm
40
50
0
10
20 30 cycle/mm
40
MTF
Fig. 1. Comparison of geometric MTF and unified MTF by changing fill factors of 30% and 40% for the pixel pitch of 10 mm (a), and 40% and 50% for the pixel pitch of 15 mm (b), at the wavelength of 550 nm.
1.0
1.0
0.8
0.8
gM TF(FF-30) uM TF(FF-30) gM TF(FF-40) uM TF(FF-40)
gM TF(FF-40) uM TF(FF-40) gM TF(FF-50) uM TF(FF-50)
0.6
0.6 0
10
20 30 cycle/mm
40
50
0
10
20 30 cycle/mm
40
Fig. 2. Comparison of geometric MTF and unified MTF by changing fill factors of 30% and 40% for the pixel pitch of 10 mm (a), and 40% and 50% for the pixel pitch of 15 mm (b), at the wavelength of 610 nm.
The measured system MTFs were plotted and compared to the calculated system MTFs without and with F value. In the calculation of the system MTF, for the prismatic crystal structure of CsI (Tl) on FOP which has the frequency over 22 cycle/mm [4], the Coltman equation was used to convert the contrast transfer function to the modulation transfer function [5]. F values of 0.5 and 0.17 were used in the calculations of the intrinsic sensor uMTF. By using Eq. (3) without F value and Eq. (6) with F value, and by the substitutions of Eqs. (3) and (6) into Eq. (1), we
finally acquired the calculated system MTF and the measured system MTF, respectively, as shown in Fig. 4. 3. Summary and conclusions From the results, while the sensors having relatively small pixel pitch of 10 and 15 mm showed degradation of the intrinsic sensor resolution by the crosstalk at wavelength 610 nm, no change of sensor resolution was found in large pixel pitch of 50 mm for either wavelengths 550 and
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 579 (2007) 235–238
1.0
1.0
0.8
0.8
MTF
238
0.6
0.6
Without F @ FF:0.8 Without F @ FF:0.9 With F:0.5 @ FF:0.8 With F:0.5 @ FF:0.9
Without F at 550nm Without F at 610nm With F:0.5 at 550nm With F:0.5 at 610nm 0.4
0.4 0
2
4 6 8 Frequency (cycle/mm)
0
10
2
4 6 8 Frequency (cycle/mm)
10
Fig. 3. The effect of wavelength for pixel pitch of 48 mm and fill factor of 0.85 (a), and fill factor (FF) for pixel pitch of 48 mm and incident wavelength of 550 nm on the unified MTF (b).
It is clear that reducing the FF also reduces efficiency for X-ray detection (thus increasing the statistical noise in the signal). It is believed that this would be revealed by a Detective Quantum Efficiency (DQE) analysis as described by other papers [6]. The semi-empirical new MTF model of the intrinsic sensor resolution was used in the calculation of the system MTF. It was shown that while the calculated system MTF without considering the dark-signal contribution did not predict the measured system resolution, the calculated system MTF reflecting the dark-signal contribution on the sensor resolution did fit the measured system resolution.
1.0 calculated sys MTF without F calculated sys MTF at 80kVp with F calculated sys MTF at 28kVp with F measured sys MTF at 80kVp measured sys MTF at 28kVp
MTF
0.8
0.6
0.4
0.2
Acknowledgment 0
2
4
6
8
10
12
Frequency (cycle/mm) Fig. 4. Comparison of the calculated and the measured system MTF for both radiography conditions considering the effect of F value.
610 nm. Therefore, for the small pixel pitch near 15 mm and the long wavelength of 610 nm, the uMTF model, instead of simple gMTF based on only pixel pitch and aperture, is more desirable to predict the intrinsic sensor resolution in the analytical evaluation.
This study was supported (in part) by research funds from Chosun University, 2006. References [1] [2] [3] [4]
E.G. Stevens, IEEE Trans. Electron. 40 (1992) 2621. I. Shcherback, et al., IEEE Trans. Electron. 48 (2001) 2710. N.V. Loukianova, et al., IEEE Trans. Electron. 50 (2003) 77. HAMAMATSU, FOS (Fiber Optic Plate with Scintillator for Digital X-ray Imaging), in: Technical Information, 1996, p. 4. [5] J.W. Coltman, et al., J. Optic. Soc. Am. 44 (1954) 468. [6] J.H. Siewerdsen, Med. Phys. 24 (1997) 71.