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1
May 2011,Zscherpel et al.
Film Replacement in Radiographic Weld Inspection
The New ISO Standard 17636-2
Requests and information to:uwez@bam.de
Uwe Ewert, Uwe Zscherpel, Mirko Jechow
1
BerlinBAM
Film Replacement in Radiography
May 2011,Zscherpel et al.
Outline
- The 3 essential parameters for image quality in Digital Radiology (DR)
- Signal to noise ratio (SNR) vs. contrast to noise ratio (CNR)
- Specific contrast (µeff)
- Compensation (I) : contrast vs. SNR
- Basic spatial resolution
- Compensation (II): basic spatial resolution vs. SNR
- IQI visibility, calculated from measured essential parameters.
- Minimum requirements of EN ISO/DIS 17636-2: NDT of welds — RT—Part 2: X- and gamma ray techniques with digital detectors.
- Requirements for system unsharpness in accordance with requirements for geometrical unsharpness of film radiography in ISO 5579
- Increased SNRN requirements
- 3 compensation principles
- Recent developments in standardization of DR techniques
2Film Replacement in Radiography
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May 2011,Zscherpel et al.
All film based standards require:
• Minimum optical Density (e.g. D > 2.0)
• Maximum film system class (e.g. ISO C4)
• Maximum unsharpness (< 0.1 mm, FFD/FOD)
• Minimum IQI perception (e.g. > W14, < H3)
Basic Requirements for Film Radiography in all National and International Standards
What are the correct requirements for film replacement?
Film Replacement in Radiography
May 2011,Zscherpel et al.
Effect of System Class in Film Radiography
D
Lost of information / perception of flaws
Film system C1 C5 C6
Opt. Density = 2
Fine grained film Coarse grained film
Film Replacement in Radiography
3
May 2011,Zscherpel et al.
Image Quality in Digital Radiology
5
The 3 essential parameters: Signal to noise ratio (SNR)
Specific contrast (µeff)
Basic spatial resolution (SRb)
Contrast to noise ratio (CNR)
Normalized signal to noise ratio (SNRN)
Film Replacement in Radiography
May 2011,Zscherpel et al.
Influence of Image Noise on Detail Visibility
Length
Inte
nsi
ty Contrast
Signal(base material)
Length
Inte
nsi
ty Contrast
Signal(base material)
Notch visible!
Contrast/Noise is highSignal/Noise is high
Notch not visible!
Contrast/Noise is lowSignal/Noise is low
6Film Replacement in Radiography
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May 2011,Zscherpel et al.
Basics of Digital Radiography
effITotalSNR
w
CNR
Material, keV, Source typeScattered radiationScreens and filters
{Exposure timeTube current, ActivityDetector efficiencySource-to-Detector Distance
{
Image Quality for small flaws or IQI of given size
Specific Contrast-to-Noise Ratio
Specific contrast
7Film Replacement in Radiography
May 2011,Zscherpel et al.
Typical noise sources in digital radiography:
1. EXPOSURE CONDITIONS: Photon noise, depending on exposure dose (e.g. mAs or GBqmin). This is the main factor! SNR increases with higher exposure dose.
2. Limitation for the maximum achievable SNR:
1. DETECTOR: Structural noise of DDAs and Imaging Plates also called fixed pattern noise (due to variations in pixel to pixel response and inhomogeneities in the phosphor layer).
2. OBJECT:
1. Crystalline structure of material (e.g. nickel based steel, mottling)
2. Surface roughness of test object
Noise Sources in Radiographic Images
8Film Replacement in Radiography
5
May 2011,Zscherpel et al.
CNR and SNR increase with increasing exposure time due to improved photon statistics
Noise in Dependence on Exposure Time
Normalized at constant contrast
9Film Replacement in Radiography
May 2011,Zscherpel et al.
Compensation Principle (I)Specific Contrast vs. SNR (Exposure Dose)
• The test object was exposed with different X-ray energies and mAs settings
• Operators did read the visible IQI hole for evaluation, and indicated it in the following graph with 1 for a visible 1T hole, 2 and 4 for visible 2T and 4T holes.
• The test object was a ferritic step wedge with thickness (t) steps from 0.05 to 0.25 inch (1,27 – 6,4 mm) step height.
• The relative IQI thickness (2% t) and the absolute hole diameters were kept constant.
• The best image quality was achieved at highest voltage and reduced mAs and, therefore, testing is most economic at highest tube voltage here.
Compensation Principle ICompensation for Reduced Contrast by Increased SNR
10
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May 2011,Zscherpel et al.
Compensation Principle (I)
Visibility of IQIs depends on exposure dose and tube voltage for steel.
- Increasing tube voltage reduces the specific contrast µeff but increases the exposure dose on the detector.
- The increase of SNR by the improved quantum statistic compensates the loss of contrast.
- Note: IQI hole diameters do not change here.
effITotalSNR
w
CNR
IQI-perception (wires, plate holes)
contrast
11Film Replacement in Radiography
May 2011,Zscherpel et al.
Digital Detector Arrays Exceed Film Qualitywith
High Contrast Sensitivity TechniqueHCS-RT
12Film Replacement in Radiography
DDA
7
May 2011,Zscherpel et al.
FujiFilm IX25SNRnorm~ 265
PerkinElmer 1620SNRnorm~ 1500Magn. = 3.5
DDA Technology provides better image quality than film with aspecial calibration procedure!
Best (slowest) NDT film
DDA exposure
Images high pass filtered for better presentation
BAM 5, 8mm steel
May 2011,Zscherpel et al. 14
IQI Visibility
Calculation from essential parameters and its measurement
Film Replacement in Radiography
8
May 2011,Zscherpel et al.
Measurement of Contrast to Noise Ratio
• CNR shall be measured in the 4T hole for proof of image quality.
• A minimum CNR of 2.5 is required by ASTM E 2698.
• This value needs to be revised!
15
CNR = 6.7C = I = 473Noise = 71SNR = 155
ASTM E 1025
I
I=contrastNoise
by ASTM E 2698
Film Replacement in Radiography
May 2011,Zscherpel et al.
The Visibility of indications is based on the principle of „Scene Brightness“
The threshold (PT), at which we see (percept) an indication is given below:
PT = d ▪ CNR
with d = Diameter
Diameter of flat bottom holeCon
tras
t of
the
rad
iogr
aphi
c im
age
of a
fla
t bo
ttom
hol
e
Operator Based Visibility of IQIsand Influence of Image Sharpness
and Hole Diameter
16Film Replacement in Radiography
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May 2011,Zscherpel et al.
Measurement of Basic Spatial Resolution
• Determination of the basic spatial resolution in each production radiograph is not required but recommended.
• SNRN controls sufficiently the image quality at a given pixel size.
• The detector unsharpness udetector shall be controlled by reference exposures with the duplex wire IQI.
• The basic spatial resolution is
Duplex wire IQIEN 462-5ISO 19232-5 ASTM E 2002
totalb uSR 21
3 33det geometryectortotal uuu
17Film Replacement in Radiography
May 2011,Zscherpel et al.
ASTM DDA practice, E 2698-10, requires measurement of CNR in production radiographs for visibility evaluation of plate hole IQIs.
The revised CNRmin for just visible IQI holes (2-2T) is given by:
SRbimage is the value measured in the object plane if magnification is used!
The constant value of CNRmin = 2.5 of ASTM E 2698-10 is correct for thin objects (2T hole for testing of t < ½ inch or < 13 mm) and typical DDAs in the range of SRb of about 100 to 150 µm, measured in the image,
but CNRmin = 2.5 is wrong, if SRbimage ¼ diameterhole is not fulfilled. The
standard should be revised for correctness.
Revised Minimum Requirement forCNRmin for IQI Hole Visibility
hole
imageb
diameter
SRCNR
10min
18Film Replacement in Radiography
10
May 2011,Zscherpel et al.
EPS – Procedure of proposed CR qualification in the new ASTM E 2033 draft
- The EPS (equivalent penetrameter sensitivity) measurement is based on E 746
- A smooth ¾ inch (19 mm) steel plate with a set of plate holes is radiographed at 200 kV in ≥ 1 m distance
- Other IQIs are on the plate to increase the information on image response.
The exposure is performed with different mAs settings
Two graphs are generated, see next page
Calculation of the Visibility of IQIs from µeff, SNR and SRb?
SNR
SRPTd
eff
imageb
visible
PT depends slightly on operator and viewing conditionsµeff for 200 kV and 19 mm Fe is about 0,05 mm-1
19
E 746
New formula
Film Replacement in Radiography
May 2011,Zscherpel et al.
Example: EPS test with DÜRR HD CR scanner at 20 µm pixel size
Do you see the holes?
¾” (19 mm) steel plate, 200 kV
CNR 0.8
20Film Replacement in Radiography
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May 2011,Zscherpel et al.
New Formula for Conversion of SNRN
Measurements to EPS Values
0
0,5
1
1,5
2
2,5
3
3,5
4
0 10000 20000 30000 40000 50000 60000 70000
PT/sqrt(SNR)
EPS
Pixel value
EPS vs.SNR method with 3/4" Fe
Measured EPS, UR 1, DynamIx HR
Measured EPS, ST VI, HD‐CR 35
Calc. EPS from SNR, ST VI, HD‐CR 35
Calc. EPS from SNR, UR 1, DynamIx HR
SNRµ
SR
t
PTEPS
eff
imageb
testplate
'
PT’ is about 2 for visibility of the 2 T hole of IQIs corr. to ASTM E 1025 EPS by ASTM E 746 with 200 kV, t = 19 mm Fe plate and µeff = 0.05 mm-1
21Film Replacement in Radiography
May 2011,Zscherpel et al.
Requirements of ISO/DIS 17636-2
NDT of welds — RT— Part 2: X- and gamma ray techniques with digital detectors
22Film Replacement in Radiography
12
May 2011,Zscherpel et al.
Key Technologies for Film Replacement• Computed Radiography (CR) with storage
phosphor imaging plates
• Digital Detector Arrays (DDA)
CR DDA
23Film Replacement in Radiography
May 2011,Zscherpel et al.
New Standard Proposal EN ISO 17636-2
Radiographic testing of welds with digital detectors.
Most important parameters that are regulated by this standard:
I. Minimum score for wire type or step-hole type Image Quality Indicators (IQI’s) in function of test technique (similar to ASME)
II. Maximum image unsharpness requirements (SRb = Basic Spatial Resolution, determined using a duplex wire gauge)
III. Choice of tube voltage or gamma source in function of object composition, penetrated thickness and detector
IV. Minimum normalized Signal-to-Noise ratio (SNRN) requirements
– Determine SNR (Signal-to-Noise Ratio) and SRb
– Anti-scatter filter type & thickness
– Source-to-Detector Distance requirements
– New testing geometries
V. New compensation principles
24Film Replacement in Radiography
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May 2011,Zscherpel et al.
Minimum IQI score for wire type or step-hole type IQI
IQI scores takenfrom EN1435
Potential Issues
• Same requirements for wire type and step hole IQI’s as in EN 1435 and ISO 19232-3
• New exceptions for isotopes for double wall inspections
– 10 mm < w 25 mm : 1 wire or step-hole value less for Ir192
– 5 mm < w 12 mm: 1 wire or step-hole value less for Se75
25Film Replacement in Radiography
May 2011,Zscherpel et al.
New European standard proposal ISO 17636-2Choice of tube voltage or gamma source
NEW: Compensation principle (I):• Low SNR: IQI score not reached use lower X-ray voltage (increase
contrast sensitivity) longer exposure times
• High SNR use higher X-ray voltage IQI score reached
Max. X-ray voltage as given in EN 1435 or ISO 17636-1 or gamma sourceis given in function of object composition and penetrated thickness
ISO 17636-2
26Film Replacement in Radiography
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May 2011,Zscherpel et al.
To maintain a good flaw sensitivity, the X-ray tube voltage should be as low as possible. The recommended maximum values of tube voltage versus thickness are given in figure shown before.
These maximum values are best practice values for film radiography.
DDAs provide sufficient image quality at significant higher voltages too.
Highly sensitive imaging plates with high structure noise of plate crystals (coarse grained) should be applied with about 20 % less X-ray energy as indicated in the figure shown before.
High definition imaging plates, which are exposed similar to X-ray films and having low structure noise (fine grained) can be exposed with X-ray energies of figure shown before or significantly higher if the SNR is sufficiently increased.
Selection of Exposure Voltage of X-ray Tube
Consequences of Compensation Principle (I)
27Film Replacement in Radiography
May 2011,Zscherpel et al.
New European standard proposal ISO 17636-2
Minimum SNRN and anti-scatter filter
• Concept SNRN
was taken from EN14784-1.
• Minimum values are tabulated in function of radiation energy and object thickness, similar to table 2 of EN14784-2.
• Front lead screens are reduced to maximum values only except for high energy.
Higher SNRN requirements than in EN14784-2
15
May 2011,Zscherpel et al.
New European standard proposal ISO 17636-2Maximum detector or image unsharpness (SRb)
SRb concept taken from EN14784-1IQI scores relaxed compared with EN14784-2
0.030
0.030
0.040
0.040
0.063
May 2011,Zscherpel et al.
Compensation Principle (II)
Compensation of high detector unsharpness by increased SNR
• Unsharp digital systems may be applied for NDT if they enable to compensate the missing sharpness by increased SNR.
• That means, achieves a digital system not the required visibility of the separated duplex wires, it can be used for NDT, if one or two single wires more than required (see tables B.1 –B.12 of ISO/DIS 17636-2) can be seen clearly in the digital image for one or two missing duplex wire pairs. Compensation of 3 wires vs. wire pairs requires agreement of contracting parties.
• Compensation principle (II):
• High detector unsharpness can be compensated by increased SNR
Compensation Principle IICompensation for Higher Unsharpness by Increased SNR
30
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May 2011,Zscherpel et al.
13 14 15 16 17 18 19 13 14 15 16 17 18 19
highpass filtered
200µm pixel size!
C1 film:wire ~16 visible100µm contrast resolution
DDA (magnification = 1):W19 = 50µm contrast resolution
Compensation Principle (II)
Wire O EN 462-1
W13 200µmW14 160µmW15 130µmW16 100µmW17 80µmW18 63µmW19 50µm
class B
Detection of fine flaws with sub-pixel resolution
Test sample BAM 5: 8 mm steel
31Film Replacement in Radiography
May 2011,Zscherpel et al.
New European standard proposal ISO/DIS 17636-2
Example : new compensation principle II
Compensate missing spatial resolution by increased single wire sensitivity:
• A lower spatial resolution i.e. a lower double wire score (D) may be compensated by a higher single wire sensitivity i.e. higher single wire score (W).
• Max. two (or three) single/double wire scores may be exchanged.
Required:
D12W14
NotOKD10W15
OK:
D11W15
OK
D10W16
NotOKD9
W17Duplex wire score
Single wire score
Interesting for detectors with higher unsharpness
32Film Replacement in Radiography
P. Willems, ICS
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May 2011,Zscherpel et al.
Measurement of Basic Spatial Resolution and SNRN vs. grey value (GV)
Annex C: Determination of basic spatial resolution SRb
Annex D: Determination of normalized SNRN from SNRmeasured
33Film Replacement in Radiography
May 2011,Zscherpel et al.
SNR and Grey values by Measurements and Calculations for
Qualification of imaging plate - scanner systems
- The qualification is based on the fixed correlation of SNR and Grey Values of a CR system (IP and scanner) with fixed scanner settings.
Scanner parameters as gain, scan speed, laser intensity, scan pixel resolution and others shall not be modified for qualification and usage in the field.
Grey value (GV)
SN
RN
SNRnorm = SNR * 88,6µm/SRb
34Film Replacement in Radiography
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May 2011,Zscherpel et al. 35
Exposure Chart for CR
Film Replacement in Radiography
SNRmin=100
May 2011,Zscherpel et al.
Testing with flat Detectors and flat Cassettes is Possible
for Effective Testing with DDAs and Imaging Plates
3/1t
ba
d
f Class A: a = 7,5
Class B: a = 15
f – source –object distance (SOD)d – focal spot sozet – wall thickness (nominal)
Film or flexible IP DDA or Cassette
36Film Replacement in Radiography
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May 2011,Zscherpel et al.
EN 13068 Radioscopy
EN 14096, ISO 14096 Film Digitization
EN 14784 CR (2005)Goes to ISO, revision required
Part 1: Classification of Systems, Part 2: General principles, becomes ISO 16371
ISO 10893-7 (2010) Steel tubes – NDT of welds with DDA and (CR)
New ISO/DIS 17636-2 NDT of welds: CR and DDA to substitute EN 1435
New ISO draft: corrosion and wall thickness measurement
Practice with film, CR and DDA for double wall and tangential technique
ASME (BPVC, S.V, XI) CR Radiography (CR) with Phosphor Imaging Plates
ASTM CR (2005)Revision required
Classification (E 2446-05), Long term stability (E2445-05), Guide (E 2007-10), Practice (E 2033-06)
ASTM DDA (2010) Characterization (E 2597-07), Guide (E 2736-10), Practice (E 2698-10), Long Term Stability (E 2737-10)
ASTM DICONDE (2010)
(data format)
Standard Practice for Digital Imaging and Communication Non-destructive Evaluation (DICONDE)(E 2663-08, E 2699-10, E 2669-10, E 2738-10, E 2767-10 )
ASTM E 2422-05, E 2660-10, E 2669-10
Digital reference image catalogues,
light alloy, titanium and steel castings
Overview of Standards on Digital Industrial Radiology
May 2011,Zscherpel et al.
• Digital radiography with CR and DDAs will substitute film radiography similar to digital photography.
• Image quality depends on (specific) contrast µeff, achieved SNR and basic spatial resolution SRb.
• SNR increases with exposure time but it does not exceed a SNRmax value which is limited by the design of the imaging plate (fixed pattern noise) or DDA calibration.
• The operator increases the contrast sensitivity by the exposure time and tube current.
• DDAs achieve a significant higher contrast sensitivity with correct detector calibration than film radiography.
• A practice for CR and DR with DDA’s is proposed in one standard document for weld inspection in ISO 17636-2.
• SNRN or grey value are used as equivalent value for film system class and opt. density.
• Usage of duplex wire for system qualification and system selection is mandatory.
• Mandatory usage of duplex wire is required for magnification technique only.
• Usage of flat cassettes and DDAs for curved objects with new formula for calculation of SDD will be accepted.
• New revised unsharpness tables enable correct hardware selection.
• 3 compensation principles (3rd one for DDAs only) are described in the standard.
Conclusions
38Film Replacement in Radiography
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May 2011,Zscherpel et al.
“HEDRad”High Energy Digital Radiography2009 – 2011
European Projects:
“FilmFree”with 33 Partners, 11 countries2005 – 2009
www.filmfree.eu.comAcknowledgement
39Film Replacement in Radiography
May 2011,Zscherpel et al.
End
uwe.ewert@bam.deuwez@bam.de
40Film Replacement in Radiography
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