Film Replacement in Radiographic Weld Inspection The New ISO Standard

Transcription

1 BAM Berlin Film Replacement in Radiographic Weld Inspection The New ISO Standard Uwe Ewert, Uwe Zscherpel, Mirko Jechow Requests and information to: 1 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 (µ eff ) - 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 : 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 Increased SNR N requirements - 3 compensation principles - Recent developments in standardization of DR techniques 2 1

2 Basic Requirements for Film Radiography in all National and International Standards 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) What are the correct requirements for film replacement? Effect of System Class in Film Radiography Lost of information / perception of flaws D Fine grained film Coarse grained film Opt. Density = 2 Film system C1 C5 C6 2

3 Image Quality in Digital Radiology The 3 essential parameters: Signal to noise ratio (SNR) Specific contrast (µ eff ) Basic spatial resolution (SR b ) Contrast to noise ratio (CNR) Normalized signal to noise ratio (SNR N ) 5 Influence of Image Noise on Detail Visibility Intensity Signal (base material) Contrast Intensity Signal (base material) Contrast Length Length Notch visible! Notch not visible! Contrast/Noise is high Signal/Noise is high Contrast/Noise is low Signal/Noise is low 6 3

4 Basics of Digital Radiography Image Quality for small flaws or IQI of given size CNR w SNR ITotal Specific contrast eff { Screens Material, kev, Source type Scattered radiation and filters Specific Contrast-to-Noise Ratio Exposure time Tube current, Activity efficiency {Source-to-Detector Distance 7 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 8 4

5 Noise in Dependence on Exposure Time Normalized at constant contrast CNR and SNR increase with increasing exposure time due to improved photon statistics 9 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 I Compensation for Reduced Contrast by Increased SNR 10 5

6 Compensation Principle (I) Visibility of IQIs depends on exposure dose and tube voltage for steel. - Increasing tube voltage reduces the specific contrast µ eff 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. contrast IQI-perception (wires, plate holes) CNR w 11 SNR ITotal eff Digital Detector Arrays Exceed Film Quality with High Contrast Sensitivity Technique HCS-RT DDA 12 6

7 BAM 5, 8mm steel FujiFilm IX25 SNR norm ~ 265 Best (slowest) NDT film DDA Technology provides better image quality than film with a special calibration procedure! Images high pass filtered for better presentation DDA exposure PerkinElmer 1620 SNR norm ~ 1500 Magn. = 3.5 IQI Visibility Calculation from essential parameters and its measurement 14 7

8 Measurement of Contrast to Noise Ratio 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 = Operator Based Visibility of IQIs and Influence of Image Sharpness and Hole Diameter Contrast of the radiographic image of a flat bottom hole Diameter of flat bottom hole 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: with PT = d CNR d = Diameter 16 8

9 Duplex wire IQI EN ISO ASTM E 2002 Measurement of Basic Spatial Resolution Determination of the basic spatial resolution in each production radiograph is not required but recommended. SNR N controls sufficiently the image quality at a given pixel size. The detector unsharpness u detector shall be controlled by reference exposures with the duplex wire IQI. The basic spatial resolution is u total SR 1 b 2 u total u u detector geometry 17 Revised Minimum Requirement for CNR min for IQI Hole Visibility ASTM DDA practice, E , requires measurement of CNR in production radiographs for visibility evaluation of plate hole IQIs. The revised CNR min for just visible IQI holes (2-2T) is given by: CNR min SR 10 diameter image b hole SR image b is the value measured in the object plane if magnification is used! The constant value of CNR min = 2.5 of ASTM E is correct for thin objects (2T hole for testing of t < ½ inch or < 13 mm) and typical DDAs in the range of SR b of about 100 to 150 µm, measured in the image, but CNR min = 2.5 is wrong, if SR image b ¼ diameter hole is not fulfilled. The standard should be revised for correctness. 18 9

10 Calculation of the Visibility of IQIs from µ eff, SNR and SR b? EPS Procedure of proposed CR qualification in the new ASTM E 2033 draft - The EPS (equivalent penetrameter sensitivity) measurement is based on E 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 d visible PT New formula image SRb SNR eff E 746 PT depends slightly on operator and viewing conditions µ eff for 200 kv and 19 mm Fe is about 0,05 mm 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

11 New Formula for Conversion of SNR N Measurements to EPS Values 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 µ eff = 0.05 mm ,5 3 EPS vs.snr method with 3/4" Fe PT' EPS t testplate image SRb µ SNR eff PT/sqrt(SNR) EPS 2,5 2 1,5 1 0,5 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 Pixel value 21 Requirements of ISO/DIS NDT of welds RT Part 2: X- and gamma ray techniques with digital detectors 22 11

12 Key Technologies for Film Replacement Computed Radiography (CR) with storage phosphor imaging plates Digital Detector Arrays (DDA) CR DDA 23 New Standard Proposal EN ISO 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. III. IV. Maximum image unsharpness requirements (SR b = Basic Spatial Resolution, determined using a duplex wire gauge) Choice of tube voltage or gamma source in function of object composition, penetrated thickness and detector Minimum normalized Signal-to-Noise ratio (SNR N ) requirements Determine SNR (Signal-to-Noise Ratio) and SR b Anti-scatter filter type & thickness Source-to-Detector Distance requirements New testing geometries V. New compensation principles 24 12

13 Minimum IQI score for wire type or step-hole type IQI IQI scores taken from EN1435 Potential Issues Same requirements for wire type and step hole IQI s as in EN 1435 and ISO 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 25 New European standard proposal ISO Choice of tube voltage or gamma source Max. X-ray voltage as given in EN 1435 or ISO or gamma source is given in function of object composition and penetrated thickness NEW: Compensation principle (I): ISO 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 26 13

14 Consequences of Compensation Principle (I) Selection of Exposure Voltage of X-ray Tube 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. 27 New European standard proposal ISO Minimum SNR N and anti-scatter filter Concept SNR N was taken from EN Minimum values are tabulated in function of radiation energy and object thickness, similar to table 2 of EN Front lead screens are reduced to maximum values only except for high energy. Higher SNR N requirements than in EN

15 New European standard proposal ISO Maximum detector or image unsharpness (SR b ) SR b concept taken from EN IQI scores relaxed compared with EN 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 ) 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 30 Compensation Principle II Compensation for Higher Unsharpness by Increased SNR 15

16 Compensation Principle (II) Test sample BAM 5: 8 mm steel Detection of fine flaws with sub-pixel resolution highpass filtered C1 film: wire ~16 visible 100µm contrast resolution class B Wire O EN W13 200µm W14 160µm W15 130µm W16 100µm W17 80µm W18 63µm W19 50µm DDA (magnification = 1): W19 = 50µm contrast resolution 200µm pixel size! 31 New European standard proposal ISO/DIS Example : new compensation principle II 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 D9 W17 Not OK D10 W15 Required: D12 W14 OK: D11 W15 32 OK D10 W16 P. Willems, ICS 16

17 Measurement of Basic Spatial Resolution and SNR N vs. grey value (GV) Annex C: Determination of basic spatial resolution SR b Annex D: Determination of normalized SNR N from SNR measured 33 SNR N 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) SNRnorm = SNR * 88,6µm/SR b 34 17

18 Exposure Chart for CR SNR min = Testing with flat Detectors and flat Cassettes is Possible for Effective Testing with DDAs and Imaging Plates Film or flexible IP DDA or Cassette f source object distance (SOD) d focal spot soze t wall thickness (nominal) f d b a t 1/3 Class A: a = 7,5 Class B: a =

19 Overview of Standards on Digital Industrial Radiology EN Radioscopy EN 14096, ISO Film Digitization EN CR (2005) Goes to ISO, revision required Part 1: Classification of Systems, Part 2: General principles, becomes ISO ISO (2010) Steel tubes NDT of welds with DDA and (CR) New ISO/DIS NDT of welds: CR and DDA to substitute EN 1435 New ISO draft: corrosion and wall thickness measurement ASME (BPVC, S.V, XI) CR ASTM CR (2005) Revision required Practice with film, CR and DDA for double wall and tangential technique Radiography (CR) with Phosphor Imaging Plates Classification (E ), Long term stability (E ), Guide (E ), Practice (E ) ASTM DDA (2010) Characterization (E ), Guide (E ), Practice (E ), Long Term Stability (E ) ASTM DICONDE (2010) (data format) ASTM E , E , E Standard Practice for Digital Imaging and Communication Non-destructive Evaluation (DICONDE) (E , E , E , E , E ) Digital reference image catalogues, light alloy, titanium and steel castings Conclusions Digital radiography with CR and DDAs will substitute film radiography similar to digital photography. Image quality depends on (specific) contrast µ eff, achieved SNR and basic spatial resolution SR b. SNR increases with exposure time but it does not exceed a SNR max 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 SNR N 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 (3 rd one for DDAs only) are described in the standard

20 Acknowledgement European Projects: FilmFree with 33 Partners, 11 countries HEDRad High Energy Digital Radiography End