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New Standard Practices on Digital RTNew Standard Practices on Digital RT - essential parameters determining image quality -

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Uwe Ewert, Uwe Zscherpel, Mirko Jechow

April 2012,Ewert et al. Standards in Digital Radiology

[email protected]

1

Outline- New standards on Computed Radiography and radiography with Digital

Detector Arrays have been developed and partly published.- Some key requirements are discussed for the major standard organizations:

ASME, ASTM, EN ISO. - The proof of minimum image quality by image quality indicators in radiography

is the basis for technical safety and contractual agreements in industry and will y g ynot be changed.

- The essential parameters for optimization and prediction of step hole visibility are introduced and discussedare introduced and discussed.

- Three essential parameters for image quality in Digital Radiology (DR) are:- Signal to noise ratio (SNR)

S ifi t t ( )- Specific contrast (µeff)- Basic spatial resolution (SRb).

- Discussion of the requirements for detector qualification and selection.- Information on the implementation of the essential parameter concept in the new

standard practice EN ISO/DIS 17636-2: “NDT of welds — RT— Part 2: X- and gamma ray techniques with digital detectors” and in the ASTM E 2033 (draft: CR


g y q g (practice) and E 2597 (DDA qualification) and E 2698 (DDA practice).

2

EN 13068 Radioscopy

Overview of Standards on Digital Industrial RadiologyEN 13068 Radioscopy

EN 14096, ISO 14096 Film Digitization

EN 14784 CR (2005) Part 1: Classification of Systems, Part 2: General ( )Goes to ISO, revision required

y ,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 techniquewall thickness measurement tangential technique

ASME (BPVC, S.V, XI) CR Radiography (CR) with Phosphor Imaging Plates

ASTM CR (2005) Classification (E 2446-05), Long term stability (E2445-Revision required 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)act ce ( 698 0), o g e Stab ty ( 3 0)

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 )


(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

Basic Requirements for Film Radiography g yin all National and International Standards

All film based standards require:• Minimum optical Density (e.g. D > 2.0)

M i fil t l ( ISO C4)• Maximum film system class (e.g. ISO C4)

• Maximum unsharpness (< 0.1 mm, FFD/FOD)p ( , )

• Minimum IQI perception (e.g. > W14, < H3)

What are the correct requirements for film replacement by Digital Radiology?


Digital Radiology?

4

ASME BPVC Section V (NDE)Article 2 (Radiographic Examination)Article 2 (Radiographic Examination)

Mandatory AppendicesI In-Motion RadiographyII R l Ti R di i E i iII Real-Time Radioscopic ExaminationIII Digital Image Acquisition, Display, and Storage for Radiography and RadioscopyIV Interpretation, Evaluation, and Disposition of Radiographic and Radioscopic Examination

Test Results Produced by the Digital Image Acquisition and Display ProcessV Glossary of Terms for Radiographic ExaminationVI Digital Image Acquisition, Display, Interpretation, and Storage of Radiographs for

Nuclear ApplicationsVII Radiographic Examination of Metallic CastingsVIII Radiography Using Phosphor Imaging Plateg p y g p g gIX Application of Digital Radiography

April 2012,Ewert et al. Standards in Digital Radiology 5

ASME Section V Article 2 Radiographic Examination 2007

W 13W 12W 11W 10W 09W 08W 07W 06W 05W 05W 04W 02W 01

CEN wires

ASME requires basically the


q yminimum IQI-Visibility for digital radiography only!

6

Basic Requirements for IQI Visibility of International Standards in ComparisonInternational Standards in Comparisony = 17,687x-0,483 Step Holes, ISO/DIS 17636-2, EN 1435

y = 15,711x-0,48

y = 11,112x-0,505

on [%

]

p , ,Plate Holes, ASME BPVC SC V Ar. 2Plate Holes, ASTM E 1742, E 2104,

E 1025

3,00

s Res

olut

io E 1025

ASME SC V A 2

Thic

knes

s ASME SC V, A. 2, EN 1435-class A and ISO 17636-class A are about equivalent

y = 5,851x-0,402

y = 4,2561x-0,406

Wal

l T

Wi ISO/DIS 1 636 2 EN 143

ASTM E1742, E 2104, EN 1435-class B and ISO 17636-class b are

0,301,00 10,00 100,00

Penetrated Wall Thickness [mm]

Wires ISO/DIS 17636-2, EN 1435 ISO 17636 class b are about equivalent below t=12 mm (1/2“)


Penetrated Wall Thickness [mm]

7

Influence of Image Noise on Detail VisibilityDetail Visibility

Inte

nsity Contrast

Signal(base material)

Inte

nsity Contrast

Signal(base material)

Length

(base material)

Length

(base material)

Notch visible!

Contrast/Noise is high

Notch not visible!

Contrast/Noise is low


gSignal/Noise is high Signal/Noise is low

8

Effect of System Class in Film Radiography

Lost of information / perception of flaws

D Fine grained film, slow Imaging plates

Coarse grained film, fast imaging plates


Film system C1 C5 C6

9

Noise Sources in Radiographic Images

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 in Dependence on Exposure Time

CNR and SNR increase with increasing exposure time due to

Normalized at constant contrast


g pimproved photon statistics

11

Wh t d ? Detector = 50 µm pixel size

Human Observer Model: Perception Threshold PT

What do you see?

0 5T 1T 2T 4T 8TNoise = 1000Signal = 30000

Detector = 50 µm pixel sizeor magnification technique

Large area flaws are betterCNR = 2.5

0.5T 1T 2T 4T 8T Signal = 30000 Large area flaws are better visible than small ones at same Contrast-to-Noise RatioAlateral

CNR = 1.25

Ratio

Each row has indications with same CNR = Cd th/

lateral

CNR = 0.625

with same CNR Cdepth/

Each column has holes with same diameterFlat bottom holes of different depth and diameter

2T = 0.5 mm / 0.02”SRb image = 50 µmCalculated CNRmin = 1

CA PT human perception threshold

with same diameterp

b

depthlateralconst SR

CAPT

PT - human perception thresholdAlateral – area of just visible indicationCdepth – mean contrast of just visible indication - noise of image (N)


g ( )SRb – basic spatial resolution (effective pixel size)Rose approach, 1946

12

Image Quality in Digital Radiology- the 3 Essential Parameters for Visibility of IQIs -- the 3 Essential Parameters for Visibility of IQIs -

The 3 essential parameters: Si l t i ti (SNR) Contrast to noise Signal to noise ratio (SNR) Specific contrast (µeff)

Contrast to noise ratio (CNR)

Normalized signal to Basic spatial resolution (SRb)Normalized signal to noise ratio (SNRN)


Essential Parameters for Calculation of Just Visible IQI 1 T Hole Diameter

The essential parameters are: µeff, SNR and SRb?

New FormulaDepends on Hardware: effective pixel sizeMagnificationFocal spot size, source size{

SRPTdimageb

visible

p{SNR - Signal to noise ratio µeff – specific contrast,

effective attenuation coefficientSNReffvisible

}M t i l

effective attenuation coefficientSRb – effective pixel size in the image,

basic spatial resolution of image

{}Material, keV, Source typeScattered radiationScreens and filters

Exposure timeTube current, ActivityDetector efficiencyS t D t t Di t

PT 2 8 for 1T-holes ; slightly dependent on

{}Screens and filters Source-to-Detector Distance


PT 2,8 for 1T holes ; slightly dependent onviewing conditions and operator

14

Comparison of Calculated and Measured Image Quality

0,17Berechnete und gemessene Bildqualität

d = PT*/square_root(µ(kV,w) ▪ SNR(GV))

Calculated and Measured Image Quality

Dependence on tube kV

0,15

s (m

m)

Calculated IQI visibility from measured CNR

Diameter of Visible wires

Diameter of visible wires (digital filter)

W 14dnt

in m

m

SR = const0,13

bare

n D

raht

e

Diameter of Visible hole normalised by 2.5

Diameter of Visible hole normalised by 2.5 (digital filter)

Calculated IQI visibility from calc CNR

W 15

e IQ

I ele

me SRb = const.

0,09

0,11

ser d

es s

icht

Calculated IQI visibility from calc CNR

W 16Calculated just visible wire di tus

t vis

ible

0,07Dur

chm

ess

W 17

Just visible wire diameter and of the just visible1T-hole, determined by 3 operators with and without image processing

diameter

eter

of t

he j

132 mAmin1 m SDD

0,0560 80 100 120 140 160 180 200 220 240kV

determined by 3 operators, with and without image processing (high pass filter)

Dia

me 19 mm Fe

UR 1, DynamIx HR


Qualification of Imaging Plate Systems by the Visibility of IQIs and/or EPS calculation from µeff, SNR and SRb

E 10

25)

EPS – Procedure is proposed for CR qualification in the new ASTM E 2033 draft and the Draft on “Practice for the Use of … Computed Radiology for Aerospace Casting Inspections” (USA: MAI – group)46

, E 7

47, E

- 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 e

AST

M E

74

- Other IQIs are on the plate to increase the information on image response. si

tivity

(see

g p

The exposure is performed with different mAs settings

Two graphs are generatedamet

erse

ns

Two graphs are generated, see next page

Verification of equation:

ilent

pen

etra

SNRSRPTdeff

imageb

visible

PT depends slightly on operator and viewing conditions

E 746

N f lPS=

equi

val


PT depends slightly on operator and viewing conditionsµeff for 200 kV and 19 mm Fe is about 0,05 mm-1

16

New formulaEP

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 0.81 mm steel step for

t fmeasurement of µeff


Visibility of EPS holes (E 746)

Do you see the holes?

CNR 0.8


New Formula for Conversion of SNRN Measurements to EPS Values and Working Range for CR (ASTM Draft E 2033)

PT’ is about 2▪100 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

Plateau area defines the ki SNRµ

SRtPTEPS

ff

imageb

l

'

PT EPSmax

working range SNRµt efftestplate

Level IIASTME 2033 Draft

EPS = 15% Level I

Level II E 2033 Draft


EPS= equivalent penetrameter sensitivity (see ASTM E 746, E 747, E 1025)

Requirements for Digital Detector Arrays

Exceeding Film Quality withHigh Contrast Sensitivity Technique

HCS-RT

DDA


Qualification of Digital Detector Arrays- Management of Underperforming Pixels -- Management of Underperforming Pixels -

6.2.1 Definition and Test of Bad Pixels:

6 2 1 1 Dead Pixel

A bad pixel can be corrected if it has at least 5 good neighbors 6.2.1.1 Dead Pixel

6.2.1.2 Over Responding Pixel

6.2.1.3 Under Responding Pixel

6.2.1.4 Noisy Pixel y

6.2.1.5 Non-Uniform Pixel

6.2.1.6 Persistence/Lag Pixel

6.2.1.7 Bad Neighborhood Pixel

6.2.2 Types or Groups of Bad Pixels:

6 2 2 1 Single Bad Pixel6.2.2.1 Single Bad Pixel

6.2.2.2 Cluster of Bad Pixels

6.2.2.3 A cluster without any CKP is well ycorrectable.

6.2.2.4 A cluster with CKP is labeled a relevant cluster.

6 2 2 5 A single bad line segment is a


6.2.2.5 A single bad line segment is a special irrelevant cluster.

21

Qualification of Digital Detector ArraysDDAs are qualified by different tests of ASTM E 2597.Five relevant parameters shall be provided: Basic spatial resolution (SRb) Specific Material Thickness Range Efficiency Image lag Contrast sensitivity


Measurement of Contrast to Noise Ratio in DDA Practiceb ASTM E 2698

• CNR shall be measured in the f f fI=contrastNoise

by ASTM E 2698

4T hole for proof of image quality.• A minimum CNR of 2.5 is

required by ASTM E 2698.

I=contrast

• This value needs to be revised!

ASTM E 1025ASTM E 1025

CNR 6 7I CNR = 6.7C = I = 473Noise = 71

I


SNR = 155

Verification of CNRmin for ASTM Standard Practice

ASTM Draft, “Practice for the Use of Digital Detector Arrays and Computed Radiology for Aerospace Casting Inspections” and E 2698 “Standard Practice for Radiological Examination Using Digital Detector Arrays” requirePractice for Radiological Examination Using Digital Detector Arrays require for digital images with DDAs in the 2T IQI hole:

52CNR But CNRmin for all digital images can be described more accurate by the

5.2min CNR

minequation:

imagebSRCNR 10

holeIQI

b

diameterCNR

min

Based on old Rose equation

The ASTM E 2698 value (CNR = 2.5) is optimized for testing of thin objects with thickness < ½” and typical DDAs in the range of 0.1 mm < SRb < 130 µm.

Based on old Rose equation


BAM 5, 8mm steel

FujiFilm IX25SNRN ~ 265

DDA Technology provides better imageprovides better image quality than film with aspecial calibration

Best (slowest) NDT filmspec a ca b at oprocedure!

Images high pass filtered for better presentation

PerkinElmer 1620

presentation


SNRN ~ 1500Magn. = 3.5DDA exposure 25

Requirements of ISO/DIS 17636-2

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


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

phosphor imaging plates

• Digital Detector Arrays (DDA)• Digital Detector Arrays (DDA)

CR DDA


New Standard Proposal EN ISO 17636-2EN ISO 17636 2

Radiographic testing of welds with digital detectors.

Most important parameters that are regulated by this standard: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 (SNR ) requirementsIV. Minimum normalized Signal-to-Noise ratio (SNRN) requirements– Determine SNR (Signal-to-Noise Ratio) and SRb

– Anti-scatter filter type & thicknessyp– Source-to-Detector Distance requirements– New testing geometries


V. New compensation principles

28

Minimum IQI score for wire type or step-hole type IQItype IQI

IQI scores takenfrom EN1435

Potential Issues• Same requirements for q

wire type and step hole IQI’s as in EN 1435 and ISO 19232-3N ti f• New exceptions for isotopes for double wall inspections

– 10 mm < w 25 mm :10 mm < w 25 mm : 1 wire or step-hole value less for Ir192

– 5 mm < w 12 mm: 1 wire or step hole1 wire or step-hole value less for Se75


Selection of X-Ray Tube Voltage and Influence of µeff

Compensation principle I:Image Quality = f (µ ▪ SNR)

Di l ti f

Image Quality = f (µeff ▪ SNR)

Diagram on selection of maximum tube voltage

applies for film only (ISOapplies for film only (ISO 17636)

Selection of maximum tube voltage for application of digital detectors:application of digital detectors:- CR, Class B: reduce kV by 20%


- Well calibrated DDAs: increase kV

30

New European standard proposal ISO 17636-2

Mi i SNR d ti tt filtMinimum SNRN and anti-scatter filter

• Concept SNRN was taken from EN14784-1.

• Minimum values are tabulated in function of radiation energy and gyobject thickness, similar to table 2 of EN14784-2.

• Front lead screens are• Front lead screens are reduced to maximum values only except for hi hhigh energy.

April 2012,Ewert et al. Standards in Digital RadiologyHigher SNRN requirements than in EN14784-2

New European standard proposal ISO 17636-2Maximum detector or image unsharpness (SRb)

0.0300.0300 0400.0400.040

0.063


SRb concept taken from EN14784-1IQI scores relaxed compared with EN14784-2

Measurement of Interpolated Basic Spatial Resolution

Duplex wire IQIEN 462-5ISO 19232-5ISO 19232 5 ASTM E 2002

• Determination of the basic spatial resolution in each production radiograph is not required but recommended.SNR t l ffi i tl

• SNRN controls sufficiently the image quality at a given pixel size.

• The detector unsharpness QI,

90 k

V,

l foc

us

The detector unsharpnessudetector shall be controlled by reference exposures with the duplex wire IQI.

ex W

ire IQ

DD

, sm

all

• The interpolated basic spatial resolution is

uiSR 1

Dup

le1m

SD


totalb uiSR 2

33

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.

Th t hi di it l t t th i d• 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 p p q gof contracting parties.

• Compensation principle (II):• High detector unsharpness can be compensated by

increased SNR 34


Compensation Principle IICompensation for Higher Unsharpness by Increased SNR

New European standard proposal ISO/DIS 17636-2

Example : new compensation principle IIInteresting for detectors with higher unsharpness

Compensate missing spatial resolution by increased single

Interesting for detectors with higher unsharpness

wire sensitivity:• A lower spatial resolution i.e. a lower double wire score (D)

may be compensated by a higher single wire sensitivity i emay 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:NotOK

OK: OK OKby agreement

D12W14

OKD13W13

D11W15

D10W16

by agreementD9

W17Duplex wire score

Single wire score


Conclusions• New standards were developer for digital industrial radiography for detector qualification, quality

assurance and standard practices. • Image quality depends on the essential parameters: (specific) contrast µeff, achieved SNR and

basic spatial resolution SRbasic spatial resolution SRb. • For practical application further parameters have been included for DDAs, as contrast sensitivity,

efficiency, image lag and material thickness range. • SNR and EPS improve with exposure time but they are limited by a maximum achievable SNR• SNR and EPS improve with exposure time but they are limited by a maximum achievable 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 up to a

certain EPS plateau, which defines the recommended grey value working range for CR (ASTM E p , g y g g (2033, new draft).

• DDAs achieve a significant better contrast sensitivity with correct detector calibration than film radiography. The prove of image quality requires CNR measurements (ASTM practice E 2798).

• New standard practices have been developed for CR and Radiography with DDAs. Basic requirements are defined by EPS, SNR, SRb and CNR depending on the technology.

• ISO/DIS 17636-2 defines additionally compensation principles for the standard practice and minimum requirements for wire and step hole IQIsminimum requirements for wire and step hole IQIs.• 3 compensation principles (3rd one for DDAs only) are described in the standard.

• ISO/DIS 17636-2 defines minimum requirements for detector SRb, depending on the material thickness to inspect


thickness to inspect.

36

www.filmfree.eu.comAcknowledgement

European Projects:

“FilmFree”FilmFreewith 33 Partners, 11 countries2005 – 2009

“HEDRad”High Energy Digital RadiographyHigh Energy Digital Radiography2009 – 2011


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