minimum requirements for digital radiography …€¦ · ewert and zscherpel 1 ecndt requirements...

Ewert and Zscherpel 1Requirements for RT-DECNDTPraha, Oct. 20148.3Radiological

Methods

Minimum Requirements for Digital Radiography Equipment and Measurement Procedures by

Different Industries and Standard Organizations

[email protected]

Uwe Ewert and Uwe ZscherpelBAM Federal Institute for Materials Research and Testing,

Berlin, Germany


Methods

Introduction

- Film Radiography has been developed for quality assurance in industry over decades.

- It is basis for contractual agreements of Supplier and Purchaser.- Digital Radiography shall achieve the same image quality or better, but

with higher efficiency.

- Selected image quality requirements of different standards as ASTM, ASME and ISO are compared.

- The essential parameters for optimization and prediction of IQI visibility are discussed.

- Requirements for equipment selection are derived for CR and DDAs.

- Newly developed classification procedures for RT-D equipment are introduced.


Methods

Basic Requirements in Radiography Requirement Film DigitalStandards ISO, CEN, ASTM,

ASME ISO, CEN ASTM, ASME, MAI

Exposure Exceed minimum optical density

Exceed minimum required SNRN(Normalized Signal to Noise Ratio)

DDAs: Exceed minimum required CNRCR: expose to EPS plateau (MAI)

Detector requirements

Use required film system class or better

Use detectors which achieve the required SNRN anddo not exceed the detector unsharpness limit

Do not exceed image unsharpness limits

Maximum image unsharpness

Do not exceed the geometrical unsharpness limits

Do not exceed the image unsharpness limits

Do not exceed the image unsharpness limits

Image quality Achieve required IQI contrast sensitivity

Achieve IQI contrast sensitivity + duplex wire resolution

Achieve required IQI contrast sensitivity


Methods

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

phosphor imaging plates

• Digital Detector Arrays (DDA)

CR DDA


Methods

Image Quality in Radiography- Influence of SNR and CNR -

Length

Inte

nsity Contrast

Signal(base material)

LengthIn

tens

ity Contrast

Signal(base material)

Notch visible!

Contrast/Noise is highSignal/Noise is high

Notch not visible!

Contrast/Noise is lowSignal/Noise is low

Notch is visible in the profile if C > 2.5 Noise


Methods

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


Methods

Measurement of Contrast to Noise Ratio in DDA Practice

• 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!

CNR = 6.7C = I = 473Noise = 71SNR = 155

ASTM E 1025

I

I=contrastNoise

by ASTM E 2698


Methods

CNR = 2.5

CNR = 1.25

CNR = 0.625

Diameter0.12 0.25 0.5 1 2 mm

Noise = 1000Signal = 30000

50 µm pixel size

SRb image = 50 µm

Human Observer Model: Perception Threshold PT

CNRb

const SRdPT PTconst - constant human perception threshold 10

d – diameter of just visible holeCNR - contrast to noise ratioSRb – basic spatial resolution (effective pixel size)

Large area flaws are better visible than small ones at same Contrast-to-Noise Ratio

Each row has indications with same CNR = Cdepth/

Each column has holes with same diameter

Flat bottom holes of different depth and diameter

d - diameterof hole

Rose approach, 1946

ASTM E 2698 requires CNR > 2.5Needs revision!


Methods

ASTM Draft, “Practice for the Use of Digital Detector Arrays and Computed Radiology for Aerospace Casting Inspections” requires a minimum basic spatial resolution of the detector (SRb

detector) if images have been taken without magnification or

a minimum basic spatial resolution in the magnified image SRbimage).

System Selection for ASTM Standard Practice

9

imageb

SREffective pixel size of digital image ( )


Methods

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” require for digital images with DDAs in the 2T IQI hole:

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.

Generally, CNRmin for all digital images can be described more accurate by the equation:

Verification of CNRmin for ASTM Standard Practice

holeIQI

imageb

diameterSRCNR

10min

5.2min CNR

Based on “old” Rose equation as shown before for PTconstant


Methods

Contrast Sensitivity as Required by Different Standards

Almost all standards require image quality indicators on each production radiograph.

The operator has to evaluate if the required image quality has been achieved.

He decides about acceptance or rejection of the production radiographs.

Different international standards require different contrast sensitivities, which yields different inspection quality.

The thickness sensitivity improves with material thickness.


Methods

Basic Requirements for IQI Visibility of International Standards in Comparison

ASME BPVC SC V, Ar. 2, andISO 17636-class A are about equivalent

ASTM E1742, E 2104, ISO 17636-class B are about equivalent below t=12 mm (1/2“)

Step Holes, ISO 17636-2, 2013Plate Holes, ASME BPVC SC V Ar. 2Plate Holes, ASTM E 1742, E 2104

ISO 19232-3

ISO 19232-2

ASTM E 1025


Methods

Additional Requirements for Unsharpness and Basic Spatial Resolution

In all film radiography standards the permitted geometric unsharpness (Ug) is limited and the film unsharpness is neglected.

In digital radiography the detector unsharpness (Udetector) contributes significantly to the image unsharpness (Uim) or total unsharpness(UT).

Therefore, the permitted detector unsharpness (Udetector) is limited in different standard systems, which is relevant for detector selection.

The detector unsharpness shall be always smaller than the permitted geometric unsharpness (Udetector < Ug).

The basic spatial resolution is defined as ½ unsharpness in digital radiography (SRb = ½ U).


Methods

Measurement of Basic Spatial Resolution

Duplex wire IQIISO 19232-5 ASTM E 2002

• The detector unsharpness udetector shall be controlled by reference exposures with the duplex wire IQI.

Dup

lex

Wire

IQI,

90 k

V,

1m S

DD

, sm

all f

ocus

ASTM E 2002Two new wire pairs


Methods

Standard ISO 17636-2 (2013): Table B.13, B.14 Maximum detector or image unsharpness (2SRb)

DD

A

CR


Methods

Unsharpness Requirements of Different International Standards – No International Harmonization -

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,0 1,0 2,0 3,0 4,0 5,0 6,0

Unsh

arpn

ess

in in

ch

Thickness in inch

Unsharpness requirements

E 1742 ISO 17636 class B ISO 17636 class A

MAI

E 1030 (2004)

E 1030, 1032

E 2104

Co-60, High Energy


Methods

Unsharpness Requirements in Different Int. Standards


Methods

EPS – Procedure is proposed accepted for CR qualification inASTM E 20445/6 draft and the draft on “Practice for the Use of … Computed Radiology for Aerospace Casting Inspections” (USA: MAI – group)

- The EPS (equivalent penetrameter sensitivity) measurement is based on ASTM 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

The exposure is performed with different mAssettings

A graph is generated, see next pages The calculation of just visible hole diameter is

given by:

Qualification of CR-Systems- The EPS Concept -

SNRSRPTdeff

imageb

visible

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

ASTM E 746

New formulaEPS=

equ

ival

ent p

enet

ram

eter

sens

itivi

ty (s

ee A

STM

E 7

46, E

747

, E 1

025)


Methods

Example: EPS Test with 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 measurement of µeff


Methods

Visibility of EPS holes (E 746)

Do you see the holes?

CNR 0.8


Methods

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

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

1

1,2

1,4

1,6

1,8

2

2,2

2,4

2,6

0 500 1000 1500 2000 2500 3000 3500 4000 450

EPS

Pixel value

EPS method for determination of performance levelsMeasured EPS,IP 1, Scanner 2 Best fit Level I level II

350

140

0 23 46 69 91 114 137 160 183 206

Dose / mGy

90% PVmax

aEPS = 1.16

SNRµSR

tPTEPS

eff

imageb

testplate

'

Level II: PV 140 – 4095

Level I: PV 350 – 4095

Plateau range


Methods

Old CR Classification Scheme

Classification scheme of ASTM; e 2446, EN

14784-1 and ISO 16371-1 by system classes.

The CR systems are classified by the

maximum achievable SNRN (IP 1 – 6) and the basic spatial detector

resolution SRbdetector (Y).

System class CEN

Minimum normalised SNR

IP 1/Y 130

IP 2/Y 117

IP 3/Y 78

IP 4/Y 65

IP 5/Y 52

IP 6/Y 43 Note 1: The normalized SNR values of Table 1 are similar to those of EN 584-1. They are calculated by SNR= log(e) (Gradient/Granularity) of Table 1 in EN 584-1. The measured SNR values are calculated from linearised signal data.


Methods

New CR Classification SchemeNew classification by performance levels as given in the

draft revision of ASTM E 2446 (2014).

Additionally, a specified EPS performance is required.

CR System Classification

Minimum SNRN (normalized to SRb=88.6 µm)

Maximum iSRb

detector

value [µm]

Maximum achieved EPS by E 746 [%]

CR Special 200 50 1.00 CR Level I 100 100 1.41CR Level II 70 160 1.66CR Level III 50 200 1.92


Methods

Qualification “Spidernet” Graph and Classification Statement

02468

101214

SRbdetector = 145µm

Csa = 0,8

aEPS@mag 1 = 1,65

Efficiency@1mGy = 115

ISO Speed@SNRN130 = 400

aSNRN = 88

Qualification of CR‐System XY

Performance level II

Achievable EPSSpecific for CR

Efficiency as defined for DDAs in E 2597

Speed as defined for film

Achievable SNRN for ISO 17636-2 user

Achievable CS for E 2597 user

Interpolated SRbdetector for

E 2795, ISO 17636-2 users


Methods

Required Test for ASTM E 2445/2446 Required ResultGeometric Distortion Fail if distortion > 2%Laser Jitter Not permitted, Laser Beam Scan Line Integrity Not permittedScan column dropout Not permittedScanner Slippage Not permittedImaging plate Artifacts Not permittedErasure Fail if > 2%Shading or banding Fail if more than ±10%

Test results shall be reported,classification is possible in case of exceeding the limits

Result to report

PMT Non-linearity Report if > 2%Burn-In Report if > 2%Spatial Linearity < 2%Imaging plate response variation Report if > ±10%

Optional test on request Result to reportImaging Plate Fading (no test object required), optional test Report fading in %,

New Qualification Procedures


Methods

Requirements for Digital Detector Arrays

DDAQualification by ASTM E 2597M


Methods

FujiFilm IX25SNRN ~ 265

PerkinElmer 1620SNRN ~ 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


Methods

Qualification of Digital Detector Arrays- Management of Underperforming Pixels -

Definition and Test of Bad Pixels:6.2.1.1 Dead Pixel6.2.1.2 Over Responding Pixel6.2.1.3 Under Responding Pixel6.2.1.4 Noisy Pixel 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 Pixel6.2.2.2 Cluster of Bad Pixels 6.2.2.3 A cluster without any CKP is well correctable. 6.2.2.4 A cluster with CKP is labeled a relevant cluster. 6.2.2.5 A single bad line segment is a special irrelevant cluster.

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

ASTM E 2597M


Methods

Qualification of Digital Detector Arrays- Pixel Coverage Recommendations -

ASTM E 2736


Methods

Qualification of Digital Detector Arrays- Management of Underperforming Pixels -

ASTM E 2736

Compensation Principles


Methods

Compensation Principle (II) of ISO 17636-2

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 31


Methods

Example : Compensation Principle (II) of ISO 17636-2

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

NotOKD13W13

OK:

D11W15

OK

D10W16

OKby agreement

D9W17

Duplex wire score

Single wire score

Interesting for detectors with higher unsharpness


Methods

Qualification of Digital Detector Arrays- Efficiency Measurement -

Qualification by ASTM E 2597M

120 kV, 40 mm Al

220 kV, 40 mm Al

90 kV

50 kV160 kV

120 kV, 3 mm Cu


Methods

Qualification of Digital Detector Arrays- Image Lag -


Important for CT and fast measurements as e.g. automated defect recognition (ADR).


Methods

Qualification of Digital Detector Arrays- Contrast Sensitivity by CNR-Step Wedge Measurement -


Contrast sensitivity (CA) at 5% notch in a step wedge by 1/CNR

Step wedge with notch

Measure contrast and noise per step


Methods

Qualification of Digital Detector Arrays- Contrast Sensitivity by CNR-Step Wedge Measurement -


Working range (4s)


Methods

Qualification of Digital Detector Arrays- SMTR by SNR-Step Wedge Measurement -


Specific Material Thickness Range by SNR limits

SNR = 250

SNR = 130



Methods

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

38



Methods

Conclusions Replacement of film radiography by digital techniques in NDT requires careful selection of

suitable digital detectors. Standards on digital radiography were published and revised since 2005 in ISO, CEN, ASTM

and ASME, defining requirements for image quality and detector selection. International standards define different requirements for image quality and detector selection. The image quality depends on the essential parameters: specific contrast µeff, SNR and basic

spatial resolution SRb. CR is accepted as film replacement technology:

The prove of image quality requires SNRN measurements and IQI visibility (ISO 17636-2).

CR classification and qualification is under major revision. Classification will consider SNRN, SRb

detector and EPS DDAs achieve a significantly better contrast sensitivity with suitable detector calibration than

film radiography. The prove of image quality requires CNR measurements and IQI visibility (ASTM practice

E 2798). DDAs are qualified by different but similar procedures than CR systems, because the DDA

detector calibration and image integration influences seriously the qualification, and DDAs provide currently better image quality than CR.


Methods

[email protected]

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minimum requirements for digital radiography …€¦ · ewert and zscherpel 1 ecndt requirements...

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