Local model reconstruction for metal artifact reduction Katrien Van Slambrouck Johan Nuyts KU Leuven, Belgium October 2012, Lyon Content • Artifacts in computed tomography • Metal artifact reduction – Projection completion – Iterative reconstruction • Local models – Patchwork structure – Patchwork reconstruction • Measurement: hip phantom – Acquisition – Phantom – Results • Simulation: hip phantom with bone • Acceleration effect • Conclusion and future work Content • Artifacts in computed tomography • Metal artifact reduction – Projection completion – Iterative reconstruction • Local models – Patchwork structure – Patchwork reconstruction • Measurement: hip phantom – Acquisition – Phantom – Results • Simulation: hip phantom with bone • Acceleration effect • Conclusion and future work Artifacts in computed tomography (CT) • Important facility for non-destructive imaging • Metals in the scanned object → severe artifacts • Artifacts are often due to an incorrect or incomplete modeling of the acquisition • Most important causes of (metal) artifacts: – Beam hardening – (Non)-linear partial volume effects – Scatter – Noise – (Motion) Hip prostheses Double knee prosthesis Dental fillings Artifacts in CT I. Beam hardening Polychromatic spectrum, beam hardens when going through the object Low energy photons are more likely absorbed ) ) ) % % % ( ( ( y 10 cm y 10 cm y tisn water tisn water tisn e e e tn tn tn i d i d i d e e e z z z ila ila ila m m m r r r o o o N N N Energy (keV) Energy (keV) Energy (keV) Typical artifact appearance: dark streaks in between metals, dark shades around metals (and cupping) Iron in water Amalgam in PMMA Artifacts in CT II. (Non)-linear partial volume effects • Linear: voxels only partly filled with particular substance • Non-linear: averaging over beam width, focal spot, … I 0 µ µ 2 1 I Typical artifact appearance: dark and white streaks connecting edges Iron in water Amalgam in PMMA Artifacts in CT III. Scatter • Compton scatter: deviation form original trajectory • Scatter grids? µ I 0 Typical artifact appearance: dark streaks in between metals, dark shades around metals (and cupping) Iron in water Amalgam in PMMA Artifacts in CT IV. Noise • Quantum nature: Poisson distribution Typical artifact appearance: streaks around and in between metals Iron in water Amalgam in PMMA Content • Artifacts in computed tomography • Metal artifact reduction – Projection completion – Iterative reconstruction • Local models – Patchwork structure – Patchwork reconstruction • Measurement: hip phantom – Acquisition – Phantom – Results • Simulation: hip phantom with bone • Acceleration effect • Conclusion and future work Metal artifact reduction (MAR) Two important groups of MAR-methods: Projection completion Iterative reconstruction