1Laminography & Reconstruction
Ewert et al. - June 20158.3 RadiologicalMethods 1
Reconstruction Methods for Coplanar TranslationalLaminography Applications
U. EWERT, K.-U. THIESSENHUSEN, A. DERESCH, C. BELLON, S. HOHENDORF, S. KOLKOORI, N. WROBEL, B. REDMER, M. TSCHAIKNER, BAM, Berlin
www.bam.de [email protected]
Digital Industrial Radiology and Computed Tomography (DIR 2015) 22-25 June 2015, Belgium, Ghent - www.ndt.net/app.DIR2015
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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
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Laminographic methods:
• Translational laminography
• Planar Tomography
• Swing laminography
• Limited view CT
• Rotational laminography
• Tomosynthesis with free trajectories
Laminographic TechniquesTomosynthesis - Describes
reconstruction algorithm from incomplete set of projections.
Laminography – Describes acquisition geometry.
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• Shift of X-ray tube parallel to detector plane• Acquisition of projections• Reconstruction
Coplanar Translational Laminography
Detector
Manipulator
Test object
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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.
Profiles after Average Reconstruction
Pro
ject
ion
num
ber
Spatial coordinate
Averaging
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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.
Num
ber
of r
econ
stru
ctio
n la
yer
Position in reconstruction space
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Translational Laminography: Artifacts
Cross artefakts
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 .
Filtered Shift Average Algorithm (FBP) vs. Algebraic Reconstruction Technique (ART)
Filtered Shift Average reconstructionVery fast
ART
Reconstruction ofsimulated data set,
± 45°, 500 projections
Overshot Reconstruction of 3 lead balls
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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
All notches are visible Notches at 2°and 3°to reconstr uction direction are visible
with reduced contrast
Reconstruction angle ± 45°200 projections
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New pipe scanner for in-service inspection of circumferential welds withoverview RT and cross section laminography
Applications: TomoWELD Pipe Scanner
X-ray tube
Manipulator: AREVA X-ray tube: rtw Detector: XCounter/Ajat
Photon counting DDA
360°rotation
15°tilted to pipe axis
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Test of TomoWELD-Scanner in NPP at secondary circuit pipe, Block I
Field Test in a Nuclear Power Station
Mounting of scanner at pipe
Block I is not in use since 2011.
Block II will be operated until 2022 latest.
www.enbw.com/kernkraft
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Evaluation in three views permits the evaluation by different standards
KTA 3211.4 in service inspectionISO 5817 cross section evaluationDIN 25435-7: 2015 RT-practice
Inner surface t = 16 mm
Slag inclusion
Undercut
Digital radiograph
Indications permissible andin agreement with indications found after manufacturing
Field Test in a Nuclear Power StationTest of TomoWELD-Scanner in NPP at secondary circuit pipe, Block I
Slag inclusion
Slag inclusion
Length 1.6 mm
Slag inclusion
depth
width
Outer surface
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Co-Planar Translational Laminography
Applications for Large Objects
Laminography withHigh Energy X-raY Technology
Scanning DDA or LDAScanning source
PhiS PhiO
Source Object Detector
PhiD Laminographic Focussing with DDA Scan
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Aviation Security (Project SiLuFra) High‐energy X‐ray imaging
of air freight containers
Matrix
detector
Air freight container
X‐ray source (7.5 MV)
Container content
•Computer screen•Electronic printer•Steel pipe•Automobile parts•Simulated dangerous objects
High‐resolution (400µm) X‐ray imaging
Detection of complex patterns of materials
Flexible rotation of source and detector
Matrix detector
X‐raysource
Detector
tower
Source tower
Rotary table
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Region of interest (R1) Region of interest (R2) Region of interest (R3)
DARC alarm (insufficient penetration) bottle filled with dangerous material
R1
R2
R3
pattern of simulated IED mock-up
Mobile High-Energy X-ray Imaging of Air Freight Containers
CTL withScanning DDA
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Concrete Impact Sample with 2 Layers of Steel Reinforcement, 1.5 x 1,5 m² , 7.5 MV
HEXY-Tech and laminogrphy offirst enforcement layer in the concrete blockand orthogonal views.Measured with scanning DDA.
Front view
Side view
Cross view
Cro
ss v
iew
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Coplanar Translational Laminography: Artefacts
Fast filtered Shift Average method (modified FBP) vs.
Simultaneous-Algebraic Reconstruction Technique (S-ART)
Cracks in a concrete block after impact 1.5 x 1.5 x 0,3 m³
Fast filtered Shift Average method
Simultaneous-Algebraic Reconstruction Technique
Horizontal steel reinforcement visible, but without depth information,Better visibility of volumetric indications and better CNR
Horizontal steel reinforcement missing
Reconstruction angle ± 8°
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Translational Laminography: Artifacts
Reduction of artifacts by „next to base plane“ multiplicative S-ART
S-ARTNew:NBP – MART
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 plane1st Reconstruction result cannot provide values below base plane2nd Reconstruction result cannot provide values above base plane Combination of both reconstructions if required.
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„Next-to-Baseplane“ Multiplikative ART
Measurement and preprocessing of projections
Initial assumption,e.g. unfilteredbackprojection
Calculate projections
Iteratedreconstruction
matrix
Correction bycomparison with measured data
Result
Forward projection
Multiplicative; consider base plane, no change of sign in projection
Logarithmization, subtract base plane
),,( iyxQ
),,( iyxP
shyxQabs
shyxPabsyxK
i
ii )],,([
)],,([),,(
shyxQabs
shyxK
ii )],,([),,(
Correction term at same sign of P und Q
Correction if different signs will be permitted
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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
Combination of NBP-MART for the short scaled fraction with an FBP
NBP-MART
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- 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.
Summary
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End
z
x
zz
x
z
xx
z
FBP ART NBP - MART
Acknowledgement:We thank A. Rogge, F. Hille and M. Grunwaldfor concrete samples and interesting discussions.