Computed Radiography

Transcription

1 BAM Berlin Computed Radiography --INDE 2007, Kalpakkam, India -- Uwe Zscherpel, Uwe Ewert BAM Berlin, Division VIII.3 Requests Requests and and information information to: to: Dr. Dr. U. U. Zscherpel Zscherpel // BAM BAM 1

2 New Technologies and Film Replacement Imaging Plates Scanner for Imaging Plates CR: Mobile Orex Scanner Radiography with Imaging Plates (CR) Agfa Hard Copy Grayscale Printer CR:FujiFilm XG1 NDT 2

3 Exposure Imaging Plate Cycle Computed Radiography Re-usage Erasure Read-out Lumisys ACR2000 Laser high sensitivity medium resolution high dynamic range re-usable 3

4 Storage Principle of of Luminescence Imaging Plates BaFCl:Eu scintillator BaFBr:Eu storage phosphor 4

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6 IP BAS 2000 Comparison of of step wedge exposures a) a) Imaging plate plate system system with with log. log. amplifier. The The grey grey level level is is proportional to to the the wall wall thickness. b) b) Digitized film. film. The The grey grey level level is is proportional to to the the light light intensity behind behind the the film. film. X-ray tube Comparison: a) Imaging plates (IP) and b) Film Film 6

7 Image Quality Parameters Radiographic Image Quality is the basis for all comparisons Digital Detector Subject Contrast Detector Contrast Noise Detector Unsharpness Projected Unsharpness CNR CNR - Contrast/Noise Ratio SNR - Signal/Noise Ratio SNR CNR = SNR(I Tot ) Δw µ/(1 + k) k = I Scatter / I Primary BSR Detector Contrast: Signal for linear detectors Basic Spatial Resolution (effektive pixel size) Detectors can be qualified and classified by its SNR and BSR values! 7

8 D-D 0 Signal/Noise Ratio Definition of Signal/Noise Ratio and Contrast/Noise Ratio Signal proportional to radiation dose! 2*Noise Signal 0 Contrast 8

9 Standards, Regulations Welding Casting Comparison of Systems Digital Industrial Radiology Film Replacement Standards EN for CR were developed to be comparable with EN 444, EN 584-1, EN 462-5! All film based standards require: Minimum optical Density (e.g. > 2.0) Maximum film system class (e.g. ASTM special) Maximum unsharpness (> 0.1 mm, FFD/FOD) 9

10 Film replacement: What is the Optical Density? Film reading from light box, D=2: Light source Film Sensor Light intensity measurement 6000 Luxmeter I o Optical Density = log (I o / I 1 ) The opt. density is a measure of light absorption by film No digital X-ray detector yields an optical density 60 Luxmeter I 1 There exist software on the market which provides wrong equivalent values for the optical density of CR or DR systems! 10

11 NDT film as as linear radiation detector (D-D 00 )) K: K: steel, 160 kv 11

12 Film system classes have different SNR values at at opt. density = 2 for different dose values (EN 584-1, ASTM E 1815) all NDT film systems of all vendors along one line! AGFA films In mixed systems SNR film film (D-D 00 )) SNR Max Max (class C x x,, D<4.5) square root (dose / mgy) 12

13 Film System Classes are the Basis for CR Classification and all Digital Detectors Tab. 1: Overview about the film system classes in different standards and the corresponding SNR values and G 2 /σ D values. SNR = log(e) (G 2 /σ D ) for linear detectors only System class Minimum gradientnoise ratio at Signal to Noise Ratio World ISO T1 T2 Europe CEN USA ASTM E Japan K D=2 above D 0 D=2 above D 0 G 2 /σ D SNR C1 Special C2 T ! ! C3 I C4 T T3 C5 II T T4 C6 III T W-A W-A 135 W-B W-B 110 W-C W-C 80 EN Part 1: Classification of Systems 13

14 Classification by - SNR and - Spatial resolution e.g.: IP-3/200 CR System classes Table 1: IP-scanner system classes in dependence on the minimum SNR IP System classes System class CEN System class ISO System class ASTM IP 1 IP-AS Special 130 IP 2 IP-T IP 3 78 IP 4 IP-T2 IP-AS 1 65 IP 5 IP-T3 IP-AS 2 52 IP 6 IP-T4 IP-AS 3 43 Minimum Signal-noise ratio New table 1 with 6 classes! NOTE:The SNR-values of table 1 are equivalent to EN 584-1, ISO , ASTM E They are calculated by: SNR = log (e) (Gradient/Granularity) of table 1 of the corresponding standards and up-rounded. The calculation of measured SNR values shall be performed from dose proportional data. Part 1: Classification of Systems 14

15 FUJI XG-1 CR System Classification (BSR = 130 µm) CR problem: SNR limited by structure noise of imaging plates! SNR SNR 0.1mm Pb f&b versus SQRT(Dose/mGy) 180,00 160,00 140,00 y = -0,7114x ,315x 3-63,635x x R 2 = 0, ,00 100,00 80,00 60,00 40,00 20,00 01Pb f&b 130µm Polynomisch (01Pb f&b 130µm) 0,00 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 Wurzel (Dosis) 15

16 BAM 5 test weld, 8 mm Steel, 120 kv, different exposure time: Exp. Time 1x Exp. Time 4.3x Exp. Time 43x magnification CR-System: FujiFilm DynamiX (XG 1) 16

17 Basic differences Film Imaging Plate: Film systems are characterized by sensitivity (speed) and the SNR at D=2. The spatial resolution is high, rarely used and typically limited by geometrical unsharpness of exposure set-up (shortest exposure time!). Digital detectors have a higher dynamic range, the SNR depends on exposure. The spatial resolution is limited (digital file size!). 17

18 Status and Situation for CR: : Computed Radiology standards were completed and published in USA and Europe! CR Procedure CEN EN EN ASTM E The CR procedure is different from the film radiography procedure - The optical impression of digital radiographic images is not different from film images in its structure (if not digitally processed, except brightness and contrast control) - RT-trained personal can interpret digital images in analogy to film - Digital images need a PC for presentation and may be altered by specialized image processing - A basic training in image processing is essential to avoid miss interpretation - Quantitative assessment of flaw sizes is improved by digital processing but the results may differ from those ones of film interpretation - Electronic reference catalogues may support correct image assessment 18

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21 Achievable Contrast Sensitivity and Exposure Time - The contrast sensitivity improves with the increasing exposure time (better SNR, weaker details becomes visible!) - The structural noise of the IPs limits the max. achievable SNR - Step hole, plate hole and wire image quality indicators measure the increase of CNR and improvement of contrast sensitivity - Unsharp imaging plates achieve a higher max. SNR and need less exposure time - The normalized SNR N (with BSR) is independent on the unsharpness, but depends on the efficiency and plate homogeneity - Only unsharp IPs for radiography with higher energy (about > 250 kev) yield a considerable reduction of exposure time in comparison to film. 21

22 Required Image Quality Indicators for CR See also ASTM E but different to EN 462 Description of Wire -IQI For For contrast only only!! Both IQI s are are required for for CR! For unsharpness only!! Wire IQI EN Step-Hole IQI EN Lead marks Plastic Duplex wire IQI / EN / ASTM E

23 CR System Selection European Standard EN

24 EN

25 FujiFilm NDT Correct IP-Type X-ray X-ray tube: tube: Seifert Seifert Isovolt Isovolt kv, kv, 1 ma, ma, 1 min, min, small small focus focus SDD SDD = mm mm Agfa NDT Agfa Prototype (blue) digitised grey values scanning: Agfa DPS, pixel distance: 28.2 µm (900 dpi) double wire mm unsharpness 25

26 High Definition CR Dürr Germany -- Important Advantages: - flexible imaging plates (IP) in direct contact with inner canister wall - arbitrary formats possible (length not limited, width max. 14 (35cm) - could even be tried with existing setup (colimator, rotating canister) - small Laser spot (8 µm) allows small BSR for high resolution plates Disadvantages: - No automated inspection - Manual handling for IP read-out - IP based on BaFBr have limited absorption efficiency scan direction Flexible IP (14 x17 ) on mobile CR scanner (Dürr) 50 k High resolution mode (BSR=40 µm with suitable Imaging plate) available, with thicker IP high Sensitivity achievable (BSR µm) 26

27 High Definition CR Systems Systems available down to 12 µm pixel pitch and 40 µm basic spatial resolution weld quality HD CR 35 NDT: of Dürr, Germany HD-IP, light blue 27

28 BAM Certification

29 Under Discussion: - Intermediate filters - Pre filters - Max tube voltage Improvement of CNR Lead screens may damage the IPs! EN

30 Improvement of CNR Under Discussion: - Intermediate filters - Pre filters Draft - Max tube voltage (lower than for film!) Designation: E 2033 xx TABLE 1 Lead Screen and Filter Thickness Lead Thickness A KV Range Front Screen Maximum, in. Back Screen Minimum, in. Filter at Tube port > 40kV None. None. <2mm Be Tube Port 40 to 55kV None. None Alum. (1mm) 55 to 90kV None (25µm) None (25µm) 0.04 Alum. (1mm) 90 to 120 kv (50µm) (50µm) 0.08 Alum. (2mm) 120 to 160kV (125µm) (125µm) 0.01 Copper (250µm) 160 to 200 kv 0.01 (0.25 mm) 0.01 (0.25 mm) 0.04 Copper (1mm) 200 to 320kV 0.02 (0.5mm) 0.02 (0.5mm) Copper (3mm) 320kV-420kV 0.04 (1mm) 0.04 (1mm) Copper (6mm) 1MeV to <? 0.04 (1mm)?0.04 (1mm) None Se (0.25mm) 0.01 (0.25mm) None Ir (0.5mm) 0.02 (0.5mm) None Co (1mm) 0.04 (1mm) None Lead screens may damage the IPs! 30

31 CR Phantom - long term stability tests - EN , ASTM E 2445 Evaluation of Phantom: spatial resolution (by duplex-wire method, optional converging line pairs) contrast (recognized contrast percentage of the material to examine) slipping (yes/no) jitter (yes/no) MTF (European standard only) shading (percentage at selected distance) SNR N (Intensity) near BAM-snail 20x400 window recommended 31

32 EN Appendix B: 32

33 Photograph: 35x43 cm² Protoype Available from: - Fluke Biomedical, Part/No , 14 x CIT (8 x10 ) in U.K. 33

34 CR radiograph kv, 2m, 4mAmin 34

35 Duplex wire IQI Pair 9 = 260 µm BAM-snail for test of source position 20% dip EN 462-5: unsharpness 0.26 mm BSR = 0.13 mm 35

36 BAM-snail for test of source position Converging line pair IQI 3.6 lp/mm Unsharpness = 280 µm Basic Spatial Resolution = 140 µm Problems arise arise for for high high resolution systems; aliasing aliasing effects effects 36

37 Linearity test - Check rulers - Slipping IP was detected by a line structure in the image. Scanner needs repair! 37

38 Profile for MTF - Jitter Test: no problem - MTF calculation from pofile: Zero Frequency drop indicates internal scatter MTF MTF 20 = 1.6 lp/mm Unsharpness = 0.62 mm (too high!) 38

39 SNR N (Intensity) near BAM-snail 20x400 window recommended 39

40 Documentation of Long Term Stability Tests 40

41 Summary: The practice for CR (EN ) is based on EN 444 and ISO 5579 for NDT film. Film can be substituted by IPs The exposure conditions have to be selected to achieve the required SNR and CNR 2 IQIs are required for CNR and BSR Tab. 4 in EN regulates the device selection, especially the required basic spatial resolution (BSR) The max tube voltage should be reduced in comparison to film application Long term stability can be checked by CR Phantom 41

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