Minimum Requirements for Digital Radiography Equipment and Measurement Procedures by Different Industries and Standard Organizations

Transcription

1 Minimum Requirements for Digital Radiography Equipment and Measurement Procedures by Different Industries and Standard Organizations Uwe Ewert and Uwe Zscherpel BAM Federal Institute for Materials Research and Testing, Berlin, Germany 1

2 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. 2

3 Basic Requirements in Radiography Requirement Film Digital Standards Exposure Detector requirements Maximum image unsharpness Image quality ISO, CEN, ASTM, ASME Exceed minimum optical density Use required film system class or better Do not exceed the geometrical unsharpness limits Achieve required IQI contrast sensitivity ISO, CEN Exceed minimum required SNR N (Normalized Signal to Noise Ratio) Use detectors which achieve the required SNR N and do not exceed the detector unsharpness limit Do not exceed the image unsharpness limits Achieve IQI contrast sensitivity + duplex wire resolution 3 ASTM, ASME, MAI DDAs: Exceed minimum required CNR CR: expose to EPS plateau (MAI) Do not exceed image unsharpness limits Do not exceed the image unsharpness limits Achieve required IQI contrast sensitivity

4 Key Technologies for Film Replacement Computed Radiography (CR) with storage phosphor imaging plates Digital Detector Arrays (DDA) CR DDA 4

5 Image Quality in Radiography - Influence of SNR and CNR - Intensity Signal (base material) Contrast Intensity Signal (base material) Contrast Notch visible! Contrast/Noise is high Signal/Noise is high Length Notch not visible! Contrast/Noise is low Signal/Noise is low 5 Length Notch is visible in the profile if C > 2.5 Noise

6 Noise Sources in Radiographic Images Typical noise sources in digital radiography: 1. EXPOSURE CONDITIONS: Photon noise, depending on exposure dose (e.g. ma s or GBq min). 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 6

7 Measurement of Contrast to Noise Ratio in DDA Practice by ASTM E 2698 Noise I=contrast CNR shall be measured in the 4T hole for proof of image quality. A minimum CNR of 2.5 is required by ASTM E This value needs to be revised! ASTM E 1025 I CNR = 6.7 C = I = 473 Noise = 71 SNR = 155 7

8 Human Observer Model: Perception Threshold PT 50 µm pixel size Diameter mm Flat bottom holes of different depth and diameter SR b image = 50 µm Noise = 1000 Signal = CNR = 2.5 d - diameter of hole CNR = 1.25 CNR = ASTM E 2698 requires CNR > 2.5 Needs revision! Large area flaws are better visible than small ones at same Contrast-to-Noise Ratio Each row has indications with same CNR = C depth / Each column has holes with same diameter PT const d SR b Rose approach, 1946 CNR PT const - constant human perception threshold 10 d diameter of just visible hole CNR - contrast to noise ratio SR b basic spatial resolution (effective pixel size) 8

9 System Selection for ASTM Standard Practice 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 (SR b detector ) if images have been taken without magnification or a minimum basic spatial resolution in the magnified image SR b image ). image Effective pixel size of digital image ( SR b ) 9 9

10 Verification of CNR min 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 require for digital images with DDAs in the 2T IQI hole: CNR min 2.5 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 < SR b < 130 µm. Generally, CNR min for all digital images can be described more accurate by the equation: image SRb CNR 10 min diameteriqi hole Based on old Rose equation as shown before for PT constant 10

11 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. 11

12 Basic Requirements for IQI Visibility of International Standards in Comparison ISO Step Holes, ISO , 2013 Plate Holes, ASME BPVC SC V Ar. 2 Plate Holes, ASTM E 1742, E 2104 ASME BPVC SC V, Ar. 2, and ISO class A are about equivalent ASTM E1742, E 2104, ISO class B are about equivalent below t=12 mm (1/2 ) ASTM E 1025 ISO

13 Additional Requirements for Unsharpness and Basic Spatial Resolution In all film radiography standards the permitted geometric unsharpness (U g ) is limited and the film unsharpness is neglected. In digital radiography the detector unsharpness (U detector ) contributes significantly to the image unsharpness (U im ) or total unsharpness (U T ). Therefore, the permitted detector unsharpness (U detector ) is limited in different standard systems, which is relevant for detector selection. The detector unsharpness shall be always smaller than the permitted geometric unsharpness (U detector < U g ). The basic spatial resolution is defined as ½ unsharpness in digital radiography (SR b = ½ U). 13

14 Measurement of Basic Spatial Resolution Duplex wire IQI ISO ASTM E 2002 ASTM E 2002 Two new wire pairs The detector unsharpness u detector shall be controlled by reference exposures with the duplex wire IQI. Duplex Wire IQI, 90 kv, 1m SDD, small focus 14

15 Standard ISO (2013): Table B.13, B.14 Maximum detector or image unsharpness (2 SR b ) DDA CR 15

16 Unsharpness Requirements of Different International Standards No International Harmonization - Unsharpness requirements 0,08 0,07 E 1030, 1032 Unsharpness in inch 0,06 0,05 0,04 0,03 0,02 0,01 E 1742 E 1030 (2004) MAI E 2104 ISO class A ISO class B 0,00 0,0 1,0 2,0 3,0 4,0 5,0 6,0 Thickness in inch Co-60, High Energy 16

17 Unsharpness Requirements in Different Int. Standards 17

18 EPS= equivalent penetrameter sensitivity (see ASTM E 746, E 747, E 1025) EPS Procedure is proposed accepted for CR qualification in ASTM 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 mas settings A graph is generated, see next pages The calculation of just visible hole diameter is given by: d visible PT New formula Qualification of CR-Systems - The EPS Concept - image SRb SNR eff ASTM E 746 PT depends slightly on operator and viewing conditions µ eff for 200 kv and 19 mm Fe is about 0,05 mm -1 18

19 Example: EPS Test with HD CR Scanner at 20 µm Pixel Size Do you see the holes? ¾ (19 mm) steel plate, 200 kv 1 mm steel step for measurement of µ eff CNR

20 Visibility of EPS holes (E 746) Do you see the holes? CNR

21 New Formula for Conversion of SNR N Measurements to EPS Values and Working Range for CR (ASTM Draft E 2033) PT is about 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 µ eff = 0.05 mm -1 EPS method for determination of performance levels Measured EPS,IP 1, Scanner 2 Best fit Level I level II 2,6 2,4 2,2 140 EPS PT' t testplate image SRb µ SNR eff EPS 2 1,8 350 Level II: PV ,6 1,4 1,2 Level I: PV aeps = Dose / mgy EPS= equivalent penetrameter sensitivity (see ASTM E 746, E 747, E 1025) Plateau range Pixel value 90% PVmax

22 Old CR Classification Scheme 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 They are calculated by SNR= log(e) (Gradient/Granularity) of Table 1 in EN The measured SNR values are calculated from linearised signal data. Classification scheme of ASTM; e 2446, EN and ISO by system classes. The CR systems are classified by the maximum achievable SNR N (IP 1 6) and the basic spatial detector resolution SR b detector (Y). 22

23 New CR Classification Scheme New 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 SNR N (normalized to SR b =88.6 µm) Maximum isr b detector value [µm] CR Special CR Level I CR Level II CR Level III Maximum achieved EPS by E 746 [%] 23

24 Qualification Spidernet Graph and Classification Statement Qualification of CR System XY Interpolated SR b detector for E 2795, ISO users SRbdetector = 145µm Achievable SNR N for ISO user asnrn = 88 Speed as defined for film ISO Speed@SNRN130 = Csa = 0,8 aeps@mag 1 = 1,65 Achievable CS for E 2597 user Achievable EPS Specific for CR Performance level II Efficiency@1mGy = 115 Efficiency as defined for DDAs in E

25 New Qualification Procedures Required Test for ASTM E 2445/2446 Required Result Geometric Distortion Fail if distortion > 2% Laser Jitter Not permitted, Laser Beam Scan Line Integrity Not permitted Scan column dropout Not permitted Scanner Slippage Not permitted Imaging plate Artifacts Not permitted Erasure Fail if > 2% Shading or banding Fail if more than ±10% Test results shall be reported, Result to report classification is possible in case of exceeding the limits 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 report Imaging Plate Fading (no test object required), optional test Report fading in %, 25

26 Requirements for Digital Detector Arrays DDA Qualification by ASTM E 2597M 26

27 BAM 5, 8mm steel FujiFilm IX25 SNRN ~ 265 DDA Technology provides better image quality than film with a special calibration procedure! Best (slowest) NDT film Images high pass filtered for better presentation Radiological DDA exposure 8.3 PerkinElmer 1620 SNRN ~ 1500 Magn. =

28 Qualification of Digital Detector Arrays - Management of Underperforming Pixels - A bad pixel can be corrected if it has at least 5 good neighbors ASTM E 2597M Definition and Test of Bad Pixels: Dead Pixel Over Responding Pixel Under Responding Pixel Noisy Pixel Non-Uniform Pixel Persistence/Lag Pixel Bad Neighborhood Pixel Types or Groups of Bad Pixels: Single Bad Pixel Cluster of Bad Pixels A cluster without any CKP is well correctable A cluster with CKP is labeled a relevant cluster A single bad line segment is a special irrelevant cluster.

29 Qualification of Digital Detector Arrays - Pixel Coverage Recommendations - ASTM E

30 Qualification of Digital Detector Arrays - Management of Underperforming Pixels - ASTM E 2736 Compensation Principles 30

31 Compensation Principle (II) of ISO 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 ) 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 31

32 Example : Compensation Principle (II) of ISO Interesting for detectors with higher unsharpness 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. Duplex wire score Single wire score Not OK D13 W13 Required: D12 W14 OK: D11 W15 OK D10 W16 OK by agreement D9 W17 32

33 Qualification of Digital Detector Arrays - Efficiency Measurement kv, 40 mm Al 90 kv 160 kv 120 kv, 3 mm Cu 50 kv 220 kv, 40 mm Al Qualification by ASTM E 2597M 33

34 Qualification of Digital Detector Arrays - Image Lag - Qualification by ASTM E 2597M Important for CT and fast measurements as e.g. automated defect recognition (ADR). 34

35 Qualification of Digital Detector Arrays - Contrast Sensitivity by CNR-Step Wedge Measurement - Qualification by ASTM E 2597M Step wedge with notch Measure contrast and noise per step Contrast sensitivity (CA) at 5% notch in a step wedge by 1/CNR 35

36 Qualification of Digital Detector Arrays - Contrast Sensitivity by CNR-Step Wedge Measurement - Qualification by ASTM E 2597M Working range (4s) 36

37 Qualification of Digital Detector Arrays - SMTR by SNR-Step Wedge Measurement - Qualification by ASTM E 2597M SNR = 250 SNR = 130 Specific Material Thickness Range by SNR limits Qualification by ASTM E 2597M 37

38 Qualification of Digital Detector Arrays DDAs are qualified by different tests of ASTM E Five relevant parameters shall be provided: Basic spatial resolution (SRb) Specific Material Thickness Range Efficiency Qualification by ASTM E 2597M Image lag Contrast sensitivity 38 38

39 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 µ eff, SNR and basic spatial resolution SR b. CR is accepted as film replacement technology: The prove of image quality requires SNR N measurements and IQI visibility (ISO ). CR classification and qualification is under major revision. Classification will consider SNR N, SR detector b 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. 39

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