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SORMA WEST 2008
Seung Man Yun, Chang Hwy Lim, Min Kook Cho, Ho Kyung Kim*
School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea
Thorsten GraeveRad-icon Imaging Corp., Belick street, Santa Clara, CA 95045-2404, USA
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Motivation & Objective
Experimental
Image data analysis
Modeling
Monte Carlo simulation
Results
Conclusion
Contents
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Motivation & Objective Investigated noise-power spectrum of an indirect-conversion detector
– A thin phosphor screen + CMOS photodiode array
NPS degradation due to the additional white spectral noise
– Directly absorbed x-ray photons unattenuated from the overlying scintillator???
Purpose of this study
– Measurements of signal and noise characteristics induced by direct x-ray absorption in photodiode array
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Expected: NPS(f)
Measured
White spectral noiseExpected: NPS(f)
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Experimental (1/3) Detector preparation
– Photodiode arrays manufactured by CMOS process (Rad-icon Imaging Corp., USA)
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RadEyeTM
Phosphor screenRad-icon RadEye1TM
Array format 512 × 1024 pixels
Pixel pitch 48 mm
Field of view 25 × 50 mm2
Dynamic range 85 dB (>14 bits)
ADC bit-depth 12 bits
Pixel fill factor 0.87
Saturation 2,800,000 electrons
Dark current (each pixel) < 104 electrons/sec
Read Noise (at 1 fps) < 200 electrons
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Experimental (2/3) Two phosphor screen
– Terbium-doped gadolinium oxysulfide phosphor screen (Eastman Kodak Co., USA)
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Min-R2000TM
Composition Gd2O2S:Tb
Coverage 33.91 mg/cm2
Thickness 84 mm
Density 4.04 g/cm3
Lanex MediumTM
Composition Gd2O2S:Tb
Coverage 59.20 mg/cm2
Thickness 160 mm
Density 3.70 g/cm3
Thickness
Thickness
84 mm
160 mm
Input dose
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
X-ray quality in IEC standard
Technique W/Al RQA5
IEC filtration
mmAl
2.5 21
IEC HVL 0.83 6.8
Measured HVL 0.85 6.78
IEC tube voltagekVp
28 70
Adjusted voltage 28 69
Calculated q#/mm2/mGy
6,575 30,174
Measured q 6,527 29,707
Percent difference % 0.73 1.55
In order to isolate the signal and noise due to the direct x-rays from those due to the optical photons
– Insert a light-absorbing blackout material between a phosphor screen and photodiode array
Experimental (3/3)
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IEC 61267, 62220-1-2 reports
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Image data analysis Signals due to absorption of optical photons or direct x-rays are uncorrelated each other
– Mean and variance of an x-ray white image Iwhite
– Mean and variance of a direct x-ray image Idirect
– Mean and variance of a dark image Ioffset
– Magnitude of each signal and noise
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darkdirectoptwhite SSSIMean
blackwhiteopt IMeanIMeanS
offsetblackdirect IMeanIMeanS
2222
adddarkdirectoptwhiteIVar
blackwhiteopt IVarIVar 2
offsetblackdirect IVarIVar 2
darkdirectdirect SSIMean 222
adddarkdirectdirectIVar
darkoffset SIMean 22
adddarkdirectIVar
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Interaction mechanism to produce electronic signals in an indirect-conversion detect
Modeling (1/3)
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Optical Direct
Gd2O2S:Tb screen
Photodiode array
e-
e-
x-ray photons
light photons
SiO2 passivationp-n junction
Si substrate
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Modeling signal and noise characteristics using the cascaded linear-systems theory
Modeling (2/3)
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Cascaded model due to the optical photon
Cascaded model due to the direct x-ray absorption
H. K. Kim, "Generalized cascaded model to assess noise transfer in scintillator-based x-ray imaging detectors," Appl. Phys. Lett., vol. 89, iss. 23, pp. 233504-1–233504-3, 2006.
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Cascaded model due to the optical photon
Cascaded model due to the direct x-ray absorption
Modeling (3/3)
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pdscnscnopt aqn γβα2
pdpdscndirect aqn βα)α1(2
1
βγ1γβαδ 22
scn
scnpdpdscnscnopt
Iaq
pd
pdpdscn
directI
aq 22
2β)αα(1
δ
Notations
n mean number of electrons
d2 Variance
q average incident x-ray fluence
a photodiode aperture size
a quantum absorption efficiency
b mean gain of secondary quanta
gpd quantum efficiency of photodiode
I Swank or statistical factor
scn in phosphor screen
pd in photodiode array
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Monte Carlo simulation In order to avoid the contamination of the other interactions such as Compton scattered X-rays
– We removed the phosphor layer in Monte Carlo geometry when simulate the direct absorption X-rays
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (1/6) Measured signal mean-variance curve due to the direct x-ray photon absorption
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (2/6) Measured signal mean-variance curve due to the optical photon absorption
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (3/6) Comparing with calculation and measured results (RQA5, Min-R2000TM screen)
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (4/6) This discrepancy comes from Compton scattered x-ray s in p-n junction of diffusion of electrons
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phosphor screen
SiO2 passivation
p-n junction
Si substrate
SiO2 passivation
p-n junction
Si substrate
SiO2 passivation
p-n junction
Spectrum F(E)
F(E)emLscn
F(E)emLscn
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (5/6) Energy dependency noise power spectrum (kerma normalized)
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (6/6) Screen dependency noise power spectrum (kerma normalized)
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Conclusion
From the measurement results
– Although the magnitude of direct signal is small, but its variance is considerably large
– It gives rise to the reduction of signal-to-noise ratio
From the theoretical estimation
– The effect of direct x-ray absorption is negligible in radiographic imaging condition
Main cause of the degradation in noise characteristics and thus the reduction of SNR is not by the direct x-rays but by the other sources
– such as the Compton scattered x-rays from a phosphor screen
It should be noted that the magnitude of signal and noise due to the direct x-ray absorption is considerable in mammographic imaging condition (PE is dominant process in this condition)
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SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Results (1/6)
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Measured signal and noise magnitudes with respect to the optical and direct x-ray photons
X-rayquality
Phosphorscreen
Kerma(mGy)
Signal (ADU) Variance (ADU2)
optical direct optical direct
W/Al Min-R20008.8 58.328 ± 0.448 5.495 ± 0.280 9.772 ± 2.198 23.580 ± 1.248
11.2 77.116 ± 0.502 5.649 ± 0.195 11.474 ± 1.660 30.391 ± 1.102
Slope (ADU/mGy or ADU2/mGy) 6.728 ± 0.190 0.565 ± 0.087 1.061 ± 0.057 2.686 ± 0.024
RQA5 Min-R2000
16.9 323.851 ± 8.554 45.081 ± 3.594 144.770 ± 20.694 312.464 ± 19.120
20.9 428.108 ± 5.038 57.548 ± 2.950 207.951 ± 9.270 400.229 ± 10.165
26.5 561.390 ± 15.065 76.220 ± 1.186 274.729 ± 22.456 550.722 ± 18.609
32.4 737.061 ± 8.939 95.273 ± 1.312 433.184 ± 10.615 660.602 ± 14.136
Slope (ADU/mGy or ADU2/mGy) 20.921 ± 1.510 2.813 ± 0.122 10.568 ± 2.012 19.772 ± 1.099
RQA5 Medium
16.9 813.002 ± 9.988 43.005 ± 0.356 80.226 ± 10.357 288.343 ± 4.900
20.9 1063.376 ± 8.598 54.812 ± 0.412 103.279 ± 13.148 372.294 ± 4.920
26.5 1378.908 ± 19.373 70.117 ± 0.681 146.894 ± 6.326 479.240 ± 11.816
32.4 1769.243 ± 13.073 87.467 ± 1.179 153.691 ± 22.018 639.821 ± 19.715
Slope (ADU/mGy or ADU2/mGy) 51.485 ± 2.710 2.631 ± 0.064 4.953 ± 0.423 18.213 ± 1.159
SORMA WEST, June 2-5, 2008, Berkeley, CA, USA
Light absorbing blackout paper
– For light trapping, baffling, beam stopping in visible ray wavelength band
100 mm thickness, Velvet like black material (Flock paper #40, Edmond Optics, USA)
Experimental
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Visible light transparency test (Hitachi U2010, Japan)
Gd2O2S:TbEmission peak