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May 2011,Zscherpel et al.

Film Replacement in Radiographic Weld Inspection

The New ISO Standard 17636-2

Requests and information to:[email protected]

Uwe Ewert, Uwe Zscherpel, Mirko Jechow

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BerlinBAM

Film Replacement in Radiography


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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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?



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


3


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)



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


4


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



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


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CNR and SNR increase with increasing exposure time due to improved photon statistics

Noise in Dependence on Exposure Time

Normalized at constant contrast



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

6


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



Digital Detector Arrays Exceed Film Qualitywith

High Contrast Sensitivity TechniqueHCS-RT


DDA

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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


8


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



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


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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



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


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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



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


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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



Requirements of ISO/DIS 17636-2

NDT of welds — RT— Part 2: X- and gamma ray techniques with digital detectors


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Key Technologies for Film Replacement• Computed Radiography (CR) with storage

phosphor imaging plates

• Digital Detector Arrays (DDA)

CR DDA



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


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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



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


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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)



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

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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


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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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



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


P. Willems, ICS

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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



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


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May 2011,Zscherpel et al. 35

Exposure Chart for CR


SNRmin=100


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


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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


• 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


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“HEDRad”High Energy Digital Radiography2009 – 2011

European Projects:

“FilmFree”with 33 Partners, 11 countries2005 – 2009

www.filmfree.eu.comAcknowledgement



End

[email protected]@bam.de


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