This is a presentation by Nityanand Gopalika on Digital Radiograpgy. The presentation we given @ Digital Radiography workshop organized by GE at JFWTC, Bangalore.

1. Digital Radiography:
a-Si Array Detectors for Industrial
Applications
Nityanand Gopalika, D. Mishra, V. Manoharan & Greg Mohr*
Industrial Imaging and Modeling Laboratory
John F Welch Technology Center
Bangalore
*GE Inspection Technologies
One Neumann Way MD K207
Cincinnati, OH 45215

2. Presentation Outline:
• Technology Development
• Benefits of Digital Radiography
• Quantifying Image Quality
• Image Quality Metric for Digital Systems
• Comparison of Imaging Devices: CCD, CMOS
and a-Si
• Performance Study of Flat Panels

3. Technology Development
NDT World
Film radiography
Productivity Resolution
Image intensifiers
Computed radiography Cost Size
CCD technology
Direct digital radiography
Evolution of Direct Digital X-ray Detectors
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Industrial Imaging and Modeling Laboratory

4. Benefits of Digital Radiography
Productivity
• Faster response
• Elimination of chemical processing
• Automated inspection
• Elimination of retakes
Cost
• Elimination film and consumables
• ROI in two to three years
Quality
• Image processing and analysis
• Reduces operators fatigue
• Consistency
Advanced Application
• Volumetric CT for High Throughput
Productivity and cost benefits 4/
Industrial Imaging and Modeling Laboratory

5. Quantifying Image Quality
What is a good Physical Measure of
Increasing Contrast
Image Quality?
Contrast-to-Noise Ratio
Perceived Image Quality
Decreasing Noise
Measure of Image Quality 5/
Industrial Imaging and Modeling Laboratory

6. Image Quality Metric for Digital Systems
MTF: Same Response to Signal and Noise
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Industrial Imaging and Modeling Laboratory

7. Image Quality Metric for Digital Systems
MTF Limitations
Contrast
 Limiting Spatial Resolution (LSR), MTF measured at high contrast
• bar patterns » 100% input contrast
 MTF indicates fraction of signal that will be seen in image.
Noise
 MTF measured under noiseless conditions.
 MTF transfers noise in addition to signal.
 Image noise can interfere with object detectability.
Higher MTF Does Not Mean Better Imaging System
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Industrial Imaging and Modeling Laboratory

8. Image Quality Metric for Digital Systems
MTF: High Middle Low
SNR = 1
High
MTF Middle
Higher limiting resolution of smaller
Low pixels may not provide better
detectability in noisy images.
High MTF; But Poor Performance
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Industrial Imaging and Modeling Laboratory

9. Image Quality Metric for Digital Systems
• Quantum and electronic noise are unavoidable in digital imaging chain.
• SNR can vary widely across systems
• High SNR is key to better inspection power
• To increase SNR often the only way is to increase radiation dose,
unacceptable trade-off
• Achieving high SNR at lower dose: better imaging system
Traditional gauge used for quantifying image
quality cannot be used as a stand alone metric.
MTF is One Metric; But Not Enough
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Industrial Imaging and Modeling Laboratory

10. Image Quality Metric for Digital Systems
DQE: Detective Quantum Efficiency
SNR2 at detector output
DQE =
SNR2 at detector input
SNR = signal-to-noise ratio
Measure of SNR transmittance
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Industrial Imaging and Modeling Laboratory

11. Image Quality Metric for Digital Systems
Input SNR2 proportional to radiation dose
Image Quality
DQE α Input Radiation Dose
• Traditional measures such as MTF, LSR are not sufficient to
characterize detector performance
• Noise is a limiting factor for detectability, image processing, and
advanced applications
Doubling DQE means:
• Same output SNR (“image quality”) at half the dose
• 40% improvement in SNR at same dose
Less Dose and Better Image 11 /
Industrial Imaging and Modeling Laboratory

12. Image Quality Metric for Digital Systems
“Improved” “Standard”
“Object” DQE = 0.5 DQE = 0.25
SNR = 5 SNR = 3.5 SNR = 2.5
High DQE at Lower Dose
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Industrial Imaging and Modeling Laboratory

13. Image Quality Metric for Digital Systems
Film GE Detector
High DQE  Better Detectability
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14. Image Quality Metric for Digital Systems
Detector Design Keeping DQE in Mind
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15. Detector Design for High DQE
The Detector Measurements One
Properties and Requirements Image Quality
Measure
Pixel Size
MTF
Sampling, Fill
Factor, Aliasing High Resolution
(for small object
detection)
Scintillator/
Coupling
Signal (S) DQE
CsI, Lanex, Se
lens/Direct Efficient X-Ray Conversion
1 S 2 ⋅ MTF 2
(for minimum exposure) DQE =
Photodetector Q NPS
aSi, CCD,
CMOS
Noise (NPS)
Readout Low Noise
(for clear visualization)
Electronic Noise
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Industrial Imaging and Modeling Laboratory

16. Flat Panel Technology
Direct Conversion (Se) Indirect Conversion (CsI)
Photons Photons
Cesium Iodide (CsI)
Selenium
Light
Amorphous Silicon Panel
Electrons
Electrons
Read Out Electronics Read Out Electronics
Digital Data Digital Data
Flat Panel Technology Variation 16 /
Industrial Imaging and Modeling Laboratory

17. CsI vs. Se
Cesium Iodide Selenium
• Very high DQE; potential for high • Direct conversion of X-Ray into
image quality at low dose electrical signals
• Fluoro capable • Currently not capable of fluoro
• Advanced application capable • Low X-Ray absorption
• Mature technology: 25-year history • High sensitivity to temperature
with Image Intensifiers
Again Keep DQE in Mind!!
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18. Point Spread Function for Different
Detector Types
Electrons
Image Intensifiers CSI Flat panels Se Flat panels
MTF is One Part of the Story, DQE is the Other BIG Part
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19. CCD Technology
Photons
Scintillator
Light
Fiber Optic Taper
CCD
Electrons
Amorphous Silicon CCD
• Potentially high image quality at low • CCDs are easily available
dose (high DQE) • Low development costs
• Active Research on New Applications • “Transition” technology to
• Designed for X-Ray from the start flat panel
• Compact packaging • High CCD cost
• Very high development cost • Tiling and design complexity
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20. Silicon Imaging Devices, CCD, CMOS &
a-Si Imager Dimensions
• CCDs: 10 – 60-mm on a side
• CMOS: 50-mm on a side
• a-Si: 200 – 410-mm on a side
Size governed by silicon process
• CCDs and CMOS – 6” wafers
• Multiple chips/wafer – yield
• a-Si – Large area deposition/glass
Pixel Dimensions
• CCDs: 9 – 25 microns
• CMOS: 40 – 50 microns
• a-Si: 100 – 400 microns
• Pixel size governed by architecture
All will convert visible energies into an electronic charge
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Industrial Imaging and Modeling Laboratory

21. Silicon Imaging Devices, CCD, CMOS &
a-Si
Coupling device Phosphor or
Silicon Device Photoconductor
or Method
Fiber optic coupler
Requires 10X more Exposure
CCD or CMOS
Requires 100X more Exposure
Lens
Fiber optic scintillator and/or Shielding Cooled CCD
phosphor Glass Camera
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Industrial Imaging and Modeling Laboratory

22. GE Digital X-Ray Detectors
3 Types of GE Digital X-Ray Panels
All feature high efficiency & fast 14-bit
readout
Highest resolution (DXR-500)
• 7” x 9” (19 cm x 23 cm) @ 100
micron
• Over 20% MTF at 5 lp/mm
Highest efficiency (DXR-250)
• 16” x 16” (41 cm x 41 cm) @
200 micron
Fastest Imaging (DXR-250RT)
• Up to 30 Hz
• 8” x 8” (20 cm x 20 cm) @ 200
micron
Panels Optimized for Different Applications
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Industrial Imaging and Modeling Laboratory

23. Performance Study: Radiation Exposure
DXR-500 DR system
14000
12000
Signal(ADC)
10000
8000
6000
4000
2000
0
0 5 10 15 20 25 30 35
Relative exposure
Comparative study Useful exposure range
Industrial X-ray film
Characteristics DXR-500 Digital Detector
( Medium speed)
Min and Max exp ratio 2
In the order of few mR to get 1.3 R to get Optical Productivity:
Speed
signal level of 12000 density of 2
Useful minimum to • High Speed (mR vs. R)
Useful minimum to
Dynamic range maximum exposure • Minimized Rework
maximum exposure ratio 12
ratio 2 • High Latitude Coverage
Typical Exposure ratio requirements Advantages
• Less radiation field
X-ray tube Subject
Material Lattitude potential contrast
• Micro focus (Faster response
Ti 3 to 20 mm 160 kV 12.7
enables high definition
Steel 1 to 10 mm 160 kV 7.28 radiography)
Detector Characteristics Suitable for Industrial Applications 23 /
Industrial Imaging and Modeling Laboratory

24. Dynamic range
Ti step wedge image 2 –20 mm
• High latitude coverage
1
• ~ 10 times > image-
intensifier
2 • No blooming or
saturation
3
• Window leveling
2& 3 window level • No Lead masking
adjusted
Wide dynamic range enables- high latitude imaging
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Industrial Imaging and Modeling Laboratory

25. Artifacts
Source:GE Health care Source:GE Health care
Image Intensifier- distortion
Flat panel- no distortion
> No distortion Flat panel
XII
> No blooming
> Uniform sensitivity Source:GE Health care
over entire area
Brightness uniformity
> Brightness uniformity
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Industrial Imaging and Modeling Laboratory

26. Performance study-Spatial resolution
FS = 1.8 mm
FS = 0.4 mm
Observation:
• System can be designed to match with film MTF.
• Digital detector with mini/micro focal tube
outperforms film radiography with large focal spots.
• DQE of DXR-500 is comparatively good.
FS = 1.8mm
System Design for Meeting Requirements FS = 0.4 mm
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27. Detective quantum efficiency
Source: GE healthcare
DQE comparison- XII vs. DR
Better defect detectability in useful spatial resolution range
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28. Performance study-Noise response
• Poisson distributed noise
• Noise Quantum limited
• Averaging of frames reduces noise
Effect of no. of frames
5 frames
Effect of no. of X-ray photons
1 mAs 10 frames
20 frames
5 mAs
Quantum Limited Noise Performance 28 /
Industrial Imaging and Modeling Laboratory

29. Performance study-IQI sensitivity
Mat. – Ti Mat. – SS Mat. – Al
Mat. – Ti
Thick. – 25mm Thick. – 10mm Thick. – 40 mm
Thick. – 10mm
KVp – 120 KVp– 140 KVp – 120
KVp – 120
mAs – 1.0 mAs – 1.0 mAs – 1.0
mAs – 1.0
FS – 0.4 mm FS – 0.4 mm FS – 0.4 mm
FS – 0.4 mm
2-1T sensitivity over range of thickness
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Industrial Imaging and Modeling Laboratory

30. Performance study-Imaging
2-2T
Porosity
Sensitivity
Lack of
penetration
KVp – 125
mAs – 1.0
FS – 0.4
mm
Mat. – CSI
KVp – 125 Mat. – CS
mAs – 1.0
FS – 0.4 mm
Range of applications with 2-1T sensitivity Imaging and Modeling Laboratory/
Industrial
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31. Performance study-Imaging
Object – IC
KVp – 70
mAs – .5
FS – 10 microns
Mag. – 50X
Object – IC
Object –ceramics
KVp – 70
KVp – 70
mAs – .5
mAs – .5
FS – 10 microns
FS – 10 microns
Mag. – 50X
Mag. – 50X
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Enable high definition radiography Industrial Imaging and Modeling Laboratory

32. Welding defects-Lack of penetration
and spatter
Material: CS
Plate, 12 mm thk
SW SI – Offset
FS-0.4 mm
SDD-700 mm
KVp: 130, 2 mAs
Filter: Cu –0.4 mm
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33. Summary
• System can be designed to meet image
quality requirements
• Quantum limited noise performance
• Faster response and wide dynamic range
• Real time (fluoro)
• Range of applications with 2-1T sensitivity
• Enables high definition radiography
• Advanced image processing for image
optimization
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Industrial Imaging and Modeling Laboratory