CT Traceability - Calibration and Accuracy Prof. Wim Dewulf, Group T - KU Leuven
Outline Introduction: terminology and procedures Voxel size calibration Edge offset calibration Conclusions
Outline Introduction: terminology and procedures Voxel size calibration Edge offset calibration Conclusions
CT metrology: no resolution, but accuracy - Medical and material applications are satisfied with resolution - Dimensional metrology requires accuracy and traceability to the unit of length (the meter) Required accuracy: - Medical and material applications: none or 10-1 -10-2 (1..5%) - Dimensional metrology: CMM 10-5 -10-6 (laserinterferometry 10-7 -10-8 ) 1µm 1µm 5µm X-ray image with Nanotech transmission target 5µm Electron microscopy image of test pattern Outer bar width = 5 µm Inner bar width = 1 µm Thickness 1 µm gold Image at 80kV
Resolution
Traceability
CT metrology procedure CT scan Data Processing 3D Reconstruction (CT Pro) 3D grey voxel model Object Data Analysis Dimensional measurement (VolumeGraphics) 2D X-ray images Thresholding Edge detection (VolumeGraphics) Dimensions Surface model
Influencing parameters
Two main questions Voxel size? Edge
Outline Introduction: terminology and procedures Voxel size calibration Edge offset calibration Conclusions
Voxel size calibration Influence of workpiece position 2 µm 1 µm
Voxel size calibration Calibration with traceable reference objects
Voxel size calibration Calibration with traceable reference objects Excellent repeatability
Voxel size calibration Calibration with traceable reference objects Repeatability Distance between spheres [mm] 4,002 4,001 Repeatability of distance between spheres 0 2 4 6 8
Voxel size calibration Calibration with traceable reference objects Repeatability 2.999 Sphere center distance [mm] 2.998 2.997 2.996 2.995 0 1 2 3 4 5 Measurement n
Voxel size calibration Calibration with traceable reference objects Repeatability
Voxel size calibration Calibration by combining CT and CMM
Voxel size calibration Calibration by combining CT and CMM
Voxel size calibration Calibration by combining CT and CMM
Voxel size calibration Calibration by combining CT and CMM
Voxel size calibration Calibration by combining CT and CMM Repeatability Repeatability of data Spreiding processing: van 30 metingen 2D-3D reconstruction in software (VG)(CT-pro), automatic edge detection and plane-plane measurement (Volume Graphics) mean value measurements -1 sigma +1 sigma 4,9640 (30 measurements in software) 4,9630 Measured value (mm) Meetwaarde [mm] 4,9620 4,9610 4,9600 4,9590 4,9580 4,9570 σ 1µm Range 5.7µm ±3σ Repeatability including data acquisition: Range: 15 µm (at 35µA, 180kV) 30 µm (for 45µA and voltage varying from 160 to 190 kv) (5 measurements in approx. same conditions: temperature, time of day and machine warm-up, but no temperature controlled room)
Influences on the voxel size Drift of X-ray source Source: Vogeler et al., 2011
Influences on the voxel size Drift of X-ray source Source: Vogeler et al., 2011
Influences on the voxel size Tilt of detector and/or rotation stage
Influences on the voxel size Tilt of detector and/or rotation stage
Outline Introduction: terminology and procedures Voxel size calibration Edge offset calibration Conclusions
Edge detection Partial volume effect
Edge detection Background Material ISO50%
Edge detection
32
33
Edge detection Edge Background Material ISO50%
Deviation Afwijking compared t.o.v. meting to CMM met measurements CMM for i.f.v. grijswaarde different grey voor values de rand 0.060 automatischeranddetectie Automatic edge (= ISO50%) detection 0.040 0.020 0.000 56D 47D 39D 48D 27500 28000 28500 29000 29500 30000 30500 31000 31500 32000-0.020 Deviation Afwijking t.o.v. compared meting met to CMM CMM (CMM measurements -CT) [mm] [mm] -0.040-0.060 Grey value for edge detection Grijswaarde voor randdetectie
Calibration by combining CT and CMM
Calibration by combining CT and CMM Source: Carmignato et al., 2009
Calibration by combining CT and CMM 38
Advanced edge detection
Deviation Afwijking compared t.o.v. meting to CMM met measurements CMM for i.f.v. grijswaarde different grey voor values de rand 0.060 automatischeranddetectie Automatic edge (= ISO50%) detection 0.040 0.020 0.000 56D 47D 39D 48D 40 27500 28000 28500 29000 29500 30000 30500 31000 31500 32000-0.020 Deviation Afwijking t.o.v. compared meting met to CMM CMM (CMM measurements -CT) [mm] [mm] -0.040-0.060 Grey value for edge detection Grijswaarde voor randdetectie
Calibration workpieces
Calibration workpieces
Influence of workpiece properties (e.g. material, roughness, ) Tactile Optical
Influence of spot size and workpiece position
Influence of feature orientation
Influence of source settings No BHC
Influence of beam hardening Initial spectrum After e.g. 1mm
Influence of beam hardening
Influence of beam hardening
Influence of beam hardening Reconstructed slice
Influence of beam hardening
Influence of beam hardening Source: Wenig et al., 2006
Influence of beam hardening Source: Wenig et al., 2006
Influence of beam hardening
Influence of beam hardening
Influence of beam hardening Inwards offset
Influence of beam hardening
Influence of beam hardening
Multi-material objects Number (Source: Shammaa et al., 2010) Grey value
Multi-material objects Steel Steel Ceramic Ceramic Advanced calibration method (local thresholding) Thresholding on 1 greyvalue
Outline Introduction: terminology and procedures Voxel size calibration Edge offset calibration Conclusions
Conclusions Traceability of CT is challenging Influencing factors relate to CT device, measurement procedure, data processing, object, and environment Good measurement procedures, including calibration of both voxel size and edge offset, can reduce measurement uncertainties by a factor 5-10