digital detector arrays (flat panel detectors) · 2007-07-31 · digital detector arrays (flat...
TRANSCRIPT
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Digital Detector Arrays
Digital Detector Arrays(Flat Panel Detectors)
Klaus Bavendiek,YXLON International, Hamburg, GermanyUwe Ewert, Uwe Zscherpel, BAM, Berlin, Germany
April 2007
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Digital Detector Arrays
>110 years:
X-Ray film was developed formedicial applications
- High Speed Films for minimumdose for patient with flourecentsscreens
~50 years:
X-Ray film was adapted toindustrial applications (Pb screens):
- Higher spatial Resolution
- Higher Energy Range
- Lower Noise (higher dose)
~ 20 years:
DDAs* were developed formedicial applications
- High Speed DDAs for even lessdose for patient
- Highly optimized for a limitedrange of energy and resolution
*DDA = Digital Detector Array or Flat Panel Detector
~10 years:
- DDAs were taken as there are for industrial applications
~ 3 years:
- DDAs were adapted to industrialrequirements
History of X-Ray applications
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Digital Detector Arrays
Radiographic Image Quality is the basis for all comparisons
New Table in ASTM E94
Radiographic Image Quality
Radiographic Contrast
Film Contrast
Subject Contrast
Film System GranularityRadiographic Definition
Inherent Unsharpness
Geometrical Unsharpness
DDA Image Quality
QuantumNoise
DetectorContrast
Subject Contrast
DetectorUnsharpness
Projected Unsharpness
DetectorNoise
BasicNoise
StructualNoise
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Digital Detector Arrays
Image Quality Parameter with DDAs
QuantumNoise
Detector Contrast
Subject Contrast
DetectorUnsharpness
Projected Unsharpness
DetectorNoise
Basic Spatial ResolutionSRB (effective Pixel Size)
CNR
SNR
CNR = Contrast to Noise RatioImportant Image Quality Paramter for DDA:
1. SRB Basic Spatial Resolution
2. SNR Signal to Noise Ratio / Efficiency
3. CS Contrast Sensitivity
4. DR Dynamic Range
5. LAG Behaviour in time
6. IScR Internal Scatter Radiation
7. BP Bad Pixel Distribution
SNR = Signal to Noise Ratio
CS = Contrast Sensitivity= 1 / CNR
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Digital Detector Arrays
Image Quality Parameter: Basic Spatial Resolution
SRB
Spatial detector resolution of the image in x- and y-direction
Smallest detail perpendicular to the X-ray beam resolved in the image
Also known as effective pixel size
Corresponds to ½ of the detector unsharpness
Measured with the Duplex Wire (EN 462-5 or ASTM E2002)
May be calculated out of the visibility of duplex wires
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Digital Detector Arrays
Measurement of the SRB with the Duplex Wire (here: D2 Film)
wire distance wire distanceD4 400µm D9 130µmD5 320µm D10 100µmD6 250µm D11 80µmD7 200µm D12 63µmD8 160µm D13 50µm
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Digital Detector Arrays
SRB of film is much better than with DDAs without magnification
SRB of a 200µm detector. The SRB is 200µm.
With magnification & small focal spot similar SR with a 200µm detector:
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Digital Detector Arrays
Film like SR with DDA using µ-focus system and magnification
Spatial Resolution of 50µm using a of a 200µm SRB detector and µ-Focus tube
But: The very high SRB is not necessary for most applications
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Digital Detector Arrays
σDLoss of information / perceptibility
Example of different SNR at film
Image Quality Parameter: Signal to Noise Ratio (SNR)B
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Signal to Noise Ratio (SNR) = 2
Examples for different SNR in the Image
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Signal to Noise Ratio (SNR) = 4
Examples for different SNR in the ImageB
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Digital Detector Arrays
Signal to Noise Ratio (SNR) = 8
Examples for different SNR in the Image
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Digital Detector Arrays
Signal to Noise Ratio (SNR) = 20
Examples for different SNR in the ImageB
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Digital Detector Arrays
Signal to Noise Ratio (SNR) = 80
Examples for different SNR in the Image
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Digital Detector Arrays
D-D
0
Signal
2*Noise== 2 SRMS
SNR = S
SRMS
Det
ecto
r U
nits
1024
2048
Image Quality Parameter: Signal to Noise Ratio (SNR)
Noise is in every image. Reasons for noise are:
Quantum Noise (X-Ray) Quantum Noise (X-Ray)Grain Distribution Structure NoiseFog Contribution Electronic Noise
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Digital Detector Arrays
Noise in the Film
Optical Densitiy above fog and base SignalGradient GD @ D=2 and D=4 above fog and base Dose ResponseGranularity σD @ D=2 Noise
Most important parameter for the perception of fine flaws == G2 / σD
Conversion to SNR possible by (linear systems)
SNR = (G2/σD) / ln(10) = (G2/σD) / 2,3026
Diaphragm area (aperture) converted to square shaped area of 88.6 x 88.6µmfor comparison of digitized films with DDAs and different SRB
Measured SNR has to be corrected by
SNRnorm = SNRmeas * 88,6µm / SRB
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Digital Detector Arrays
The limit in SNRnorm for film is some-where in the range of 150 - 200.
With more dose the SNR would
become better but film with densities >4.8 is not readable.
Normalized SNR in the Standards (at Film Density D-D0=2; EN 584-1)
EN 14784-1 Computed Radiography
ASTM E 2446 Computed Radiography
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Digital Detector Arrays
DDA: Electronic Noise Quantum Noise Structure Noise
Reasons for Noise in the film and in digital Systems
Film: Base and fog Quantum Noise too dark for readout
Is this the samewith a DDA?
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Digital Detector Arrays
Two important differences between Film DDAs at Noise:
Quantum Noise (X-Ray) Quantum Noise (X-Ray)Film Granularity Structure NoiseFog Contribution Elektronic Noise
FIXED WITH THE SE-LECTION OF THE FILM
With Image Integration in a Computerand very long exposure times ( )it can be reduced to ~0
8
Can be compensated with a propperdetector calibration
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300s integration time
300s integration time
Conditions: 1000mm Focus DDA Distance, SNRnorm calculated by substracted images
1500s integration time
SNRnorm= 6600
Film:SNRnorm= 150
SNR at a good DDA is mainly limited by Photon Statistics and the Efficiency differs for different Energies:
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Digital Detector Arrays
The Efficiency can be used to select the best DDAfor a dedicated Energy level:
Normalized SNR forcomparison of different systems
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Image Quality Parameter: Contrast Sensitivity
Contrast Sensitivity (CS) indicates the smallest material difference which can be resolved in the image
Contrast= ΔS
CS = = SRMS
ΔS1
CNR
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Digital Detector Arrays
Measurement of Contrast Sensitivity
15mmBase material
1.25mm
2.5mm
5.0mm
7.5mm
10mm
12.5mm
15mm
2% Groove
CS measured with
Duplex Stepwedge
(stepwedge with grooves)
CS is calculated of
difference in signal
devided by
mean noise level
per step.
Example: Stainless Steel
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Measurement of Contrast Sensitivity
Three different Materials:Aluminium, Titanium and Stainless Steel
Old version: still 2 grooves ...
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Result of Contrast Sensitivity Measurement
In the range of 1.25 to 12.5 mm stainless steel a 0.2% material thicknesscan be resolved.Film level is >1% and only possible within a smaller material thickness range
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Digital Detector Arrays
Image Quality Parameter: Dynamic Range
Three different Materials:Aluminium, Titanium and Stainless Steel
SNR on each step is calculated for different exposure times ...
... also called in ASTM: „Specific Material Thickness Range“
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Digital Detector Arrays
Measurement of the Dynamic Range B
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Measurement of the Dynamic Range
Dynamic Range from 10 to 72mm for 1% applications with 4s exposure timeDynamic Range from 10 to 90mm for 2% applications with 4s exposure time
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Digital Detector Arrays
Image LAG or Ghosting
Image LAG is caused by the electronic of a DDA
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Digital Detector Arrays
Image LAG or Ghosting
Image LAG is caused by the electronic of a DDA
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Digital Detector Arrays
LAG and Burn In
Negative image of a turbine blade two days ago. Corrected one day before
Lead letter on the step wedge from the day before. No new calibration was done ...
Example for „Burn In“
Burn In is a long term effect in the Scintillator of a DDA
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Internal ScatterRadiation
a
b
Internal Scatter Radiation increases with the energy.
Important Parameter for CT machines ...
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Digital Detector Arrays
Bad Pixel
... only a dead pixel is a bad pixel?
Not really ...
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Bad Pixel
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Groups of Bad Pixel (Cluster)B
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Digital Detector Arrays
Main Difference between Film and DDAs
Two unique property of DDAs:
Take multiple images at identical pixel positions and cumulate.
Calibrate on pixel basis with
suitable energy and
material in the beam similar to the test object
The key for best image quality with contrast sensitivity and superior SNR is
the exact calibration of each detector pixel
to obtain the same radiation response as the neighbourhood detector pixel.
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Digital Detector Arrays
Basic Functionality of all Types of Detectors
X-Rays reaching the sensitive area of the detector are converted toelectrical charge.
The amount of charge is measured.
It is proportional to the Dose.
Different Technologies for Flat Panel Detectors are available
intrinsic Method(„direct Method“) - very inefficient, no products
direct Method with Photoconductor (Amourphous Selen or CdTe ) - example: AJAT DICT100 CdTe
indirect Method (Converting X-Ray to Light with Scintillator, „normal“ Photodiodes)
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Matrix-Structure of all Flat Panel Detectors
.. with Photo Diode for „Light Capture“ and TFT for selective Read out
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Digital Detector Arrays
Basic Principle: Indirect Method with Scintillator
X-ray quantum
Scintillator (screen) Coating
Photo Diode Layer
Pixel width (indicated geometrical resolution)
A thick scintillator results in many photons, but with a loss of geometric resolution(noise vs. resolution)
The effective geometric resolution depends on various parameters, these must be measured!
Cro
ss S
ectio
nal V
iew
of D
etec
tor light quantum
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Digital Detector Arrays
Basic Principle: Indirect Method with Scintillator
Scintillator (screen) Coating
Photo Diode Layer
Pixel width (indicated geometrical resolution)
Cro
ss S
ectio
nal V
iew
of D
etec
tor
light quantum
X-ray quantum
Cesium iodide can be formed into needles on top of the diode structure to direct the lightto the photodiodes without significant light scatter. The needles ensures a high total internal reflection of the light and direct it to the photodiode.
Disadvantage of CsJ: ImageLag and Burn In
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Photodiode
TFT
~8pF
U+
TFTSwitch
LineSelection
Read-OutAmplifier
U
Photo-Diode
2. Incoming Light discharges the capacity;more light means less charge left in theCapacity
3. Now the cell should be read out:The TFT becomes conductive and the capacity is recharged through the Read-Out Amplifier up to U+ ...
TFT
1. Capacity of Photodiode is charged up to U+
3a. The amount of Charge for Rechargingin measured in the Read-Out Amplifier.It is transfered as „U“ to the ADC
What happens:
What happens during operation?B
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Digital Detector Arrays
1. Capture the Dark Image
Uncorrected Image with Flaws, Patterns, Bad Pixel, ...without X-Ray
==> Offset Image
How do we do the calibration
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Offset corrected Image without X-Ray
1. Substract the Dark Image from any further Image
How do we do the calibrationB
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2. Capture the Gain Images
Offset corrected Image with X-Ray andwithout Gain Correction
==> Gain Image
How do we do the calibration
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2. Multiply each Pixel with Gain Image for further Images
Offset and Gain corrected Image with X-Ray
How do we do the calibrationB
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Digital Detector Arrays
energy
Det
ecto
r O
utpu
t Sig
nal
[AD
U]
full dose
reduced dose during calibration
real dose withmaterial in the beam
2b. Correct Non-Linearities with Energy adapted Gain Images
How do we do the calibration
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Signal after Correction with Offset and Multi-Gain
Signal after Correction with „Multi-Gain“
„Gain Line“, calculated of Correction Points 1 to 3
Real (non-linear) Detector Output Signal
Raw Detector Signal
Cor
rect
ed D
etec
tor
Out
put
r4
Offs
etn 1
r1
Correction Point 1
r2
Correction Point 2
r3
Correction Point 3
How do we do the calibrationB
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Digital Detector Arrays
(SNR is limited to ~1000 because of the Structure of the Material)
With 16s same quality like
with 500s andnormal calibration
A High SNR is optained with Optimized Calibration
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Digital Detector Arrays
Aluminum, 10 - 110 mm, PerkinElmer RID 512 / AF1 and Image 3500 DD
in %
Inspection wide Range of Material with optimal CalibrationB
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With a high SNR details can be seen more easily and even objects smaller than the pixel size can be detected
800µm
200µm
127µm
400µm
1200µm
Aluminum wedge with 5 long holes inside;
Image from 400µm pixel pitch panel PerkinElmer RID512/400 AF1
40mm Al
5mm Al
with Highpass Filter
Can a High SNR Compensate a Lower SRB?
5mm
40mm
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800µm
200µm
127µm
400µm
1200µm
This is just a third of the pixelsize.How is this possible?
With a high SNR details can be seen more easily and even objects smaller than the pixel size can be detected
Can a High SNR Compensate a Lower SRB?
with Highpass Filter
Aluminum wedge with 5 long holes inside;
Image from 400µm pixel pitch panel PerkinElmer RID512/400 AF1
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With a high SNR it is enough that just a part of a pixel gets the information.
The long hole, just a quarter of a pixel width
Example:
The hole has 2% less density than the rest.
The hole covers ¼ of the pixel.
The visibility would be 2% * ¼ = 0.5%.
With a SNR > 200 the 0.5% is visable.
Pixel Grid of the Detector
Yes, a High SNR can Compensate a Lower SRB
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Compensate a lower SRB with a higher SNR
Another example with 8mm steel weld (test weld BAM5)
Film AGFA D2, class C1 EN 584-1
330s exposure time
PE XRD1620 detector (200µm SRB), magnification of 3.0 100s exposure time only
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Compensate a lower SRB with a higher SNR
Film AGFA D2, class C1 EN 584-1
330s exposure time
PE XRD1620 detector (200µm SRB), magnification of 3.0 100s exposure time only
Another example with 8mm steel weld (test weld BAM5)
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Advantages of Digital X-Ray
Time of Inspection will be reducedExample (from Boeing): 6“ diameter Inconel, 10 welds
Film Process: 2-3 hoursDigital X-Ray: 12 minutes 10 seconds
Avoiding high Recurring Costs (Chemicals and Film)
No Chemical Processing incl. associated facilities + waste water handling
Lower Storage Costs - cheaper Archiving
Faster Access to an Image in the Database - if needed
Image Transfer from one Location to another in few seconds (network)
Very short set-up time with manipulation system (no film transport)
Ability to use Digital Image Processing like Filter, Zoom, Measurement
Smaller Footprint
Inspection of larger Range of Material Thicknesses within one Image
Higher recognition rate of defects in production ...
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Comparison of Film (single wall, double wall), RT und DR
done by Bill Meade, Clay Kidwell, Greg Warren (Boeing Commercial Aircraft)
100%
Digital System: Thales FS35 with YXLON Image 3500 DD
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Result of the POD Study by Flaw „Volume“ (Depth x Length)B
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Saving time using DDA systems
Necessary time for inspection of a part:
2 – 3 hours with Radiography using the current film process
35-40 minutes with current radioscopy inspection
12 minutes are sufficient for the same task with the DDA system
Hint:
The POD was not done withthe best available DDA.
New gereration of DDAs increase SNR and CS byfactor of 2.5.
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Example: Small Turbine Blade with ASTM Penetramter „5“
Film Agfa D2,
420s exp. timedigitized with50µm and 16 Bit
DDA + µ-Focus
PE XRD1620,
60s exp. time
Imag
es h
ighp
ass
filte
red
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Cutout of Penetrameter with the 3 Holes
Film Agfa D2,
420s exp. timedigitized with50µm and 16 Bit
DDA + µ-Focus
PE XRD1620,
60s exp. time
254µm
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Digital Detector Arrays
Example: Small Titanium Duct with Penetramter „5“
Film Agfa D2
100s exposure timeDDA System
16s exposure time
Imag
es h
ighp
ass
filte
red
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Cutout of the defects in the weld
Film Agfa D2
100s exposure time
DDA System
16s exposure time
50µm
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Inspection of Ducts, potential for ADR ?B
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Inspection of Ducts, potential for ADR ?
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Diameter 0.05mm
Inspection of Ducts, potential for ADR ?B
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Digital Detector Arrays
The new DDA systems are suitable for film replacement, ADR and CT
Basic Parameters for Digital Radiology are- Basic Spatial Resolution (SRB)- Contrast Sensitivity (CS)- Dynamic Range Specific Material Thickness Range- Normalized SNR (SNRNorm) Efficiency
SNRNorm of DDA system depends on - integration time, - calibration procedure and - radiation quality
Conclusion:
DDA systems can exceed Film systems by factor >20 in SNRNorm
Very high SNR gives a superior Contrast Sensitivity CS
A high Contrast Sensitivity can compensate a lower SRB
With > CS fine details <1 pixel size give enough signal to become visible
Adapted Filters increase the Visibility of Inhomogenities