Reconstruction Methods for Coplanar Translational Laminography Applications

Transcription

1 Reconstruction Methods for Coplanar Translational Laminography Applications U. EWERT, K.-U. THIESSENHUSEN, A. DERESCH, C. BELLON, S. HOHENDORF, S. KOLKOORI, N. WROBEL, B. REDMER, M. TSCHAIKNER, BAM, Berlin Methods Laminography & Reconstruction 1 uwe.ewert@bam.de 1

2 Content - Different laminographic methods - Coplanar translational laminographic techniques - Reconstruction techniques -Fast filtered shift average technique (also FBP) - Algebraic reconstruction techniques (ART) - Applications of coplanar translational laminography (CTL) - Welding - Security - Concrete impact measurements - Artifacts of coplanar translational laminography - Cross artifacts - Overshoot at edges - New ART technique: Next to Base Plane MART for reduction of artifacts Methods Laminography & Reconstruction 2 2

3 Laminographic Techniques Tomosynthesis - Describes reconstruction algorithm from incomplete set of projections. Laminography Describes acquisition geometry. Laminographic methods: Translational laminography Planar Tomography Swing laminography Limited view CT Rotational laminography Tomosynthesis with free trajectories Methods Laminography & Reconstruction 3 3

4 Coplanar Translational Laminography Manipulator Test object Detector Shift of X-ray tube parallel to detector plane Acquisition of projections Reconstruction Methods Laminography & Reconstruction 4 4

5 Coplanar Translational Laminography A classic algorithm of Computed Laminography is the fast Shift Average Algorithm Each projection is shifted digitally and the backprojection is performed by averaging of the related projection elements. Averaging Projection number Profiles after Average Reconstruction Methods Laminography & Reconstruction 5 Spatial coordinate 5

6 Co-Planar Translational Laminography Filtered Shift Average Algorithm as used in our applications The X-ray tube is shifted with constant speed and projections are taken in equidistant steps. Consequently, at longer SDD more projections are acquired for improved SNR The projections are weighted considering the nonlinear angle stepping from projection to projection and the variation of the beam length. A directional high pass filter is applied to achieve a filtered backprojection. The reconstruction is performed in parallel beam geometry for fast reconstruction via GPU A cone beam correction after reconstruction is used for geometric correction. Number of reconstruction layer Position in reconstruction space Methods Laminography & Reconstruction 6 6

7 Translational Laminography: Artifacts Filtered Shift Average Algorithm (FBP) vs. Algebraic Reconstruction Technique (ART) Limited View reconstruction (±45 ) : No information from dead angle, high projection number Artifacts Cross artifacts are almost the same for FBP and ART. Overshoots of intensities are similar for FBP and ART. Overshot Cross artefakts Reconstruction of 3 lead balls Reconstruction of simulated data set, ± 45, 500 projections Filtered Shift Average reconstruction Very fast ART Methods Laminography & Reconstruction 7 7

8 Coplanar Translational Laminography: Star Artefacts Filtered shift average reconstruction (FBP) and effect of dead angle artist modeling of a star crack phantom in a weld (16 mm thickness) Crack notches 150 x 0,2 x 1 mm³ Reconstruction angle ± projections All notches are visible Notches at 2 and 3 to reconstr uction direction are visible with reduced contrast Methods Laminography & Reconstruction 8 8

9 Applications: TomoWELD Pipe Scanner 15 tilted to pipe axis X-ray tube 360 rotation Photon counting DDA New pipe scanner for in-service inspection of circumferential welds with overview RT and cross section laminography Manipulator: AREVA X-ray tube: rtw Detector: XCounter/Ajat Methods Laminography & Reconstruction 9 9

10 Field Test in a Nuclear Power Station Test of TomoWELD-Scanner in NPP at secondary circuit pipe, Block I Mounting of scanner at pipe Block I is not in use since Block II will be operated until 2022 latest. Methods Laminography & Reconstruction 10 10

11 Field Test in a Nuclear Power Station Test of TomoWELD-Scanner in NPP at secondary circuit pipe, Block I Evaluation in three views permits the evaluation by different standards Slag inclusion Length 1.6 mm Slag inclusion KTA in service inspection ISO 5817 cross section evaluation DIN : 2015 RT-practice Outer surface Digital radiograph Undercut Slag inclusion depth width Inner surface t = 16 mm Slag inclusion Indications permissible and in agreement with indications found after manufacturing Methods Laminography & Reconstruction 11 11

12 Co-Planar Translational Laminography Applications for Large Objects Laminography with High Energy X-raY Technology Scanning source Scanning DDA or LDA Source Object Detector Phi S Phi O Phi D Laminographic Focussing with DDA Scan Methods Laminography & Reconstruction 12 12

13 Aviation Security (Project SiLuFra) High energy X ray imaging of air freight containers Air freight container Detector tower Source tower Matrix detector Matrix detector Rotary table X ray source X ray source(7.5 MV) High resolution (400µm) X ray imaging Detection of complex patterns of materials Flexible rotation of source and detector Container content Computer screen Electronic printer Steel pipe Automobile parts Simulated dangerous objects Methods Laminography & Reconstruction 13 13

14 Mobile High-Energy X-ray Imaging of Air Freight Containers R2 R3 R1 CTL withscanning DDA Region of interest (R1) DARC alarm (insufficient penetration) 8.3 Radiological Methods Region of interest (R3) Region of interest (R2) bottle filled with dangerous material pattern of simulated IED mock-up Laminography & Reconstruction 14 14

15 Concrete Impact Sample with 2 Layers of Steel Reinforcement, 1.5 x 1,5 m², 7.5 MV Front view Side view HEXY-Tech and laminogrphy of first enforcement layer in the concrete block and orthogonal views. Measured with scanning DDA. Cross view Cross view Methods Laminography & Reconstruction 15 15

16 Coplanar Translational Laminography: Artefacts Fast filtered Shift Average method (modified FBP) vs. Simultaneous-Algebraic Reconstruction Technique (S-ART) Reconstruction angle ± 8 Cracks in a concrete block after impact 1.5 x 1.5 x 0,3 m³ Fast filtered Shift Average method Horizontal steel reinforcement missing Simultaneous-Algebraic Reconstruction Technique Horizontal steel reinforcement visible, but without depth information, Better visibility of volumetric indications and better CNR Methods Laminography & Reconstruction 16 16

17 Translational Laminography: Artifacts Reduction of artifacts by next to base plane multiplicative S-ART Limited View reconstruction (±45 ) : No information from dead angle, high projection number Artifacts Artifact reduction by multiplicative S-ART next to base plane. Prior condition: Do not allow that indication intensities cross the base plane 1 st Reconstruction result cannot provide values below base plane 2 nd Reconstruction result cannot provide values above base plane Combination of both reconstructions if required. S-ART Methods Laminography & Reconstruction 17 New: NBP MART 17

18 Next-to-Baseplane Multiplikative ART Measurement and preprocessing of projections Initial assumption, e.g. unfiltered backprojection Logarithmization, subtract base plane P ( i x, y, ) Calculate projections Forward projection Iterated reconstruction matrix Result Q ( i x, y, ) Correction by comparison with measured data Multiplicative; consider base plane, no change of sign in projection abs[ P( x, y, i)] sh K( x, y, i) abs[ Q( x, y, i)] sh sh K( x, y, i) abs[ Q( x, y, i)] sh Correction term at same sign of P und Q Correction if different signs will be permitted Methods Laminography & Reconstruction 18 18

19 NBP-MART NBP next to base plane MART Special reconstruction for objects with low amount of indications as e.g. welds. An indication size is selected which is extracted with a modified Median high-pass. The NBP-MART can be overlaid onto the FBP reconstruction for better visualization. Shift average reconstruction NBP-MART Combination of NBP-MART for the short scaled fraction with an FBP Methods Laminography & Reconstruction 19 19

20 Summary - Laminography is used if CT is not applicable. - The fast shift average reconstruction and ART-based reconstruction have proven to be successful techniques for measurements with high number of projections and limited view conditions (e.g. ± 45 ). - ART-based reconstructions provide acceptable reconstruction results if a limited number of projections is used, but fine cracks may be missed. - Both construction techniques generate cross artefacts and edge overshoots.. - The new ART-technique, next to base plane-mart, was developed and tested to reduce cross artefacts and overshoots. - A base plane is calculated by a Median based high pass filter. - The NBP-MART iteration is controlled in a way that the sign of a structure indication will be maintained in relation to the base plane. - Coplanar translational laminography was successfully tested for evaluation of - Welded cross sections, - Complex cargo structures and - Large concrete impact samples. Methods Laminography & Reconstruction 20 20

21 FBP ART NBP - MART z z z z z Acknowledgement: We thank A. Rogge, F. Hille and M. Grunwald for concrete samples and interesting discussions. x x x uwe.ewert@bam.de End Methods Laminography & Reconstruction 21 21