A case study on digital detectors performance study

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:
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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
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Industrial Imaging and Modeling Laboratory

5. Quantifying Image Quality
Increasing Contrast
What is a good Physical Measure of
Image Quality?
Contrast-to-Noise Ratio
Perceived Image Quality
Decreasing Noise
Measure of Image Quality
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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
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Limiting Spatial Resolution (LSR), MTF measured at high contrast
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bar patterns » 100% input contrast
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MTF indicates fraction of signal that will be seen in image.
Noise
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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
Low
Middle
MTF: High
SNR = 1
MTF
High
Middle
Low
Higher limiting resolution of smaller
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
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Quantum and electronic noise are unavoidable in digital imaging chain.
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SNR can vary widely across systems
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High SNR is key to better inspection power
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To increase SNR often the only way is to increase radiation dose,
unacceptable trade-off
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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
DQE =
SNR2 at detector output
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
DQE
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α
Image Quality
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:
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Same output SNR (“image quality”) at half the dose
40% improvement in SNR at same dose
Less Dose and Better Image
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Industrial Imaging and Modeling Laboratory

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

14. Image Quality Metric for Digital Systems
Detector Design Keeping DQE in Mind
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Industrial Imaging and Modeling Laboratory

15. Detector Design for High DQE
The Detector
Properties
Pixel Size
Sampling, Fill
Factor, Aliasing
Scintillator/
Coupling
CsI, Lanex, Se
lens/Direct
Measurements
and Requirements
MTF
High Resolution
(for small object
detection)
Signal (S)
Efficient X-Ray Conversion
(for minimum exposure)
Photodetector
aSi, CCD,
CMOS
Readout
One
Image Quality
Measure
DQE
1 S 2 ⋅ MTF 2
DQE =
Q NPS
Noise (NPS)
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
Read Out Electronics
Digital Data
Electrons
Read Out Electronics
Digital Data
Flat Panel Technology Variation
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Industrial Imaging and Modeling Laboratory

17. CsI vs. Se
Cesium Iodide
Selenium
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Very high DQE; potential for high
image quality at low dose
Fluoro capable
Advanced application capable
Mature technology: 25-year history
with Image Intensifiers
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Direct conversion of X-Ray into
electrical signals
Currently not capable of fluoro
Low X-Ray absorption
High sensitivity to temperature
Again Keep DQE in Mind!!
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Industrial Imaging and Modeling Laboratory

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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Industrial Imaging and Modeling Laboratory

19. CCD Technology
Photons
Scintillator
Light
Fiber Optic Taper
CCD
Electrons
CCD
Amorphous Silicon
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Potentially high image quality at low
dose (high DQE)
Active Research on New Applications
Designed for X-Ray from the start
Compact packaging
Very high development cost
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CCDs are easily available
Low development costs
“Transition” technology to
flat panel
High CCD cost
Tiling and design complexity
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Industrial Imaging and Modeling Laboratory

20. Silicon Imaging Devices, CCD, CMOS &
a-Si
Imager Dimensions
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CCDs:
CMOS:
a-Si:
10 – 60-mm on a side
50-mm on a side
200 – 410-mm on a side
Size governed by silicon process
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CCDs and CMOS – 6” wafers
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Multiple chips/wafer – yield
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a-Si – Large area deposition/glass
Pixel Dimensions
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CCDs:
9 – 25 microns
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CMOS:
40 – 50 microns
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a-Si:
100 – 400 microns
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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
or Method
Silicon Device
Phosphor or
Photoconductor
Fiber optic coupler
Requires 10X more Exposure
CCD or CMOS
Requires 100X more Exposure
Lens
Fiber optic scintillator and/or Shielding
phosphor
Glass
Cooled CCD
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
Signal(ADC)
14000
12000
10000
8000
6000
4000
2000
0
0
5
10
15
20
25
30
35
Relative exposure
Comparative study
Industrial X-ray film
Characteristics DXR-500 Digital Detector
( Medium speed)
In the order of few mR to get 1.3 R to get Optical
Speed
signal level of 12000
density of 2
Useful minimum to
Useful minimum to
Dynamic range
maximum exposure
maximum exposure ratio 12
ratio 2
Typical Exposure ratio requirements
Material
Ti
Steel
X-ray tube
Lattitude
potential
3 to 20 mm
160 kV
1 to 10 mm
160 kV
Subject
contrast
12.7
7.28
Useful exposure range
Min and Max exp ratio 2
Productivity:
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High Speed (mR vs. R)
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Minimized Rework
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High Latitude Coverage
Advantages
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Less radiation field
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Micro focus (Faster response
enables high definition
radiography)
Detector Characteristics Suitable for Industrial Applications
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Industrial Imaging and Modeling Laboratory

24. Dynamic range
Ti step wedge image 2 –20 mm
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2& 3 window level
adjusted
High latitude coverage
~ 10 times > imageintensifier
No blooming or
saturation
Window leveling
No Lead masking
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
Flat panel- no distortion
Image Intensifier- distortion
Flat panel
> No distortion
XII
> No blooming
> Uniform sensitivity
over entire area
> Brightness uniformity
Source:GE Health care
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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Industrial Imaging and Modeling Laboratory

27. Detective quantum efficiency
Source: GE healthcare
DQE comparison- XII vs. DR
Better defect detectability in useful spatial resolution range
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Industrial Imaging and Modeling Laboratory

28. Performance study-Noise response
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Poisson distributed noise
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Noise Quantum limited
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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
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Industrial Imaging and Modeling Laboratory

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

31. Performance study-Imaging
Object – IC
KVp – 70
mAs – .5
FS – 10 microns
Mag. – 50X
Object – IC
KVp – 70
mAs – .5
FS – 10 microns
Mag. – 50X
Object –ceramics
KVp – 70
mAs – .5
FS – 10 microns
Mag. – 50X
Enable high definition radiography
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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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Industrial Imaging and Modeling Laboratory

33. Summary
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System can be designed to meet image
quality requirements
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Quantum limited noise performance
Faster response and wide dynamic range
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Real time (fluoro)
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Range of applications with 2-1T sensitivity
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Enables high definition radiography
Advanced image processing for image
optimization
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Industrial Imaging and Modeling Laboratory