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Accurate Measurement of Cartilage Morphology Using a 3D Laser Scanner

Accurate Measurement of Cartilage Morphology Using a 3D Laser Scanner. Nhon H. Trinh 1 , Jonathan Lester 2 , Braden C. Fleming 1,2,3 , Glenn Tung 2,3 , and Benjamin Kimia 1 1 Division of Engineering, Brown University, Providence, RI 02912

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Accurate Measurement of Cartilage Morphology Using a 3D Laser Scanner

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  1. Accurate Measurement of Cartilage Morphology Using a 3D Laser Scanner Nhon H. Trinh1, Jonathan Lester2, Braden C. Fleming1,2,3, Glenn Tung2,3, and Benjamin Kimia1 1Division of Engineering, Brown University, Providence, RI 02912 2Department of Orthopedics, Brown Medical School, Providence, RI 02903 3Department of Diagnostic Imaging, Brown Medical School, Providence, RI 02903 4Rhode Island Hospital, Providence, RI 02903 2nd International Workshop onComputer Vision Approaches to Medical Image Analysis (CVAMIA 2006) May 12, 2006

  2. Why measure cartilage morphology? • Osteoarthritis (OA) • common and a leading cause of disability in elderly people. • associated with degeneration of cartilage in articulating joints. • To monitor OA: use morphology measurements of cartilage, e.g. overall volume and thickness. CVAMIA 2006

  3. The challenges • MRI is imaging modality of choice for knee cartilage • Non-invasive • Able to differentiate soft tissues • Quantification challenges: • Typical voxel size: 0.3–1.0 mm • Average knee cartilage thickness: 1.3-2.5 mm • Change in thickness due to OA can be overcome by one pixel error (≈ 25%). • It is imperative to know the accuracy of morphology measurements. • The need for ground truth data. CVAMIA 2006

  4. Obtaining ground truth of knee cartilage is difficult • Not possible to obtain the cartilage in one piece • Thin curved structure, thickness typically less than 6mm. Thinner for OA patient. • Strong bond with knee bones. • The cartilage cannot be left outside for a long time • its content is mostly water http://www.uchospitals.edu CVAMIA 2006

  5. Existing methods 1. Water displacement of surgically removed cartilage tissue • Only measure volume • Prone to error and requires a highly skilled technician 2. High resolution scans of anatomical sections with high precision saws. • Only measure “thickness” in the sectioning direction CVAMIA 2006

  6. Existing methods 3. Computed tomography (CT) arthrography: • Same resolution problem as MR images 4. Stereophotogrammetry • Extensive work to calibrate cameras • Specimen is attached to a calibration frame, thus limiting number of views 5. Laser scanner: • Major differences with the proposed method CVAMIA 2006

  7. 3D Laser scanner • Create a 3D point cloud sampling of the specimen's surface • Highly accurate • Established technology: commercial products and technical support available • Wide range of algorithms available ShapeGrabber® PLM 300 with scan head SG-1000 (depth resolution: 5.0μm) CVAMIA 2006

  8. outer surface inner surface cartilage The approach • The cartilage volume is the difference between two bone surfaces: one with cartilage and one without cartilage. • To find the cartilage volume, we find those two surfaces and “subtract” them. CVAMIA 2006

  9. Equipment • Laser scanner • Shape Grabber ® PLM300 with scan head SG-1000 (Vitana Corporation, Ottawa, Ontario, Canada) • Depth range: 250-900 mm, depth accuracy: 5.0 μm • PolyWorks® IMAlign and IMMerge: • software packages to process 3D point clouds CVAMIA 2006

  10. Specimen • Cadavers: • 5 intact fresh frozen human cadavers from the right limb (age 51-59, 3 males/2 females) • MR-scanned before dissected Femur Tibia CVAMIA 2006

  11. Specimen (cont’d) 2. Synthetic knee bone and cartilage model: • Synthetic knee cartilage constructed in our lab, firm and water-proof. • Used to validate the proposed • 16 fiducial points are marked on the cartilage surface for thickness validation. Tibia Femur CVAMIA 2006

  12. The plan • Method • Validation experiments with synthetic models • Obtain ground truth of the cadavers’ cartilage CVAMIA 2006

  13. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  14. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  15. Scanning the bone surface • Scanning procedure • 20 scans for each bone. • Time ≈ 1 hour/bone. • Properties • Easy to set up • At least 30% overlap among adjacent scans. • Redundant coverage of the cartilage. CVAMIA 2006

  16. Example CVAMIA 2006

  17. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  18. Dissolving cartilage off the bone • Immerse bones in Clorox® bleach 5.25% sodium hypochlorite. • Tibia: 4-5 hours • Femur: • Regularly rotated to prevent the bleach from dissolving the bone and soft tissue • Time: 8-9 hours CVAMIA 2006

  19. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  20. Reconstruct bone surfaces Align the range images Merge the aligned images Smooth and fix topology problems PolyMender (Ju SIGGRAPH 2004) • PolyWorks® IMAlign • manual aligment interface • ICP • PolyWorks® IMMerge • handle outliers CVAMIA 2006

  21. Surface reconstruction example CVAMIA 2006

  22. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  23. Reconstruct the cartilage volume • Goal: a triangular mesh of the knee cartilage from the two bone surfaces. • Procedure: • Align the two surfaces and construct an error map The two reconstructed bone surfaces do not overlap completely on the bone body. CVAMIA 2006

  24. Reconstruct the cartilage volume • Procedure: (cont’d) • Manually outlines the cartilage area. • Project the outline orthogonally to both bone surfaces to segment the cartilage regions. CVAMIA 2006

  25. Reconstruct the cartilage volume • Procedure: (cont’d) • Connect the two segmented surface patches with a band-like triangular mesh CVAMIA 2006

  26. Cartilage reconstruction examples CVAMIA 2006

  27. Method Scan the bone surface with cartilage intact Dissolve cartilage off the bone Scan the bone surface without cartilage Compute cartilage morphology from the mesh Reconstruct cartilage surface mesh Reconstruct two bone surfaces CVAMIA 2006

  28. Quantify knee cartilage morphology • Volume: Vertices Faces CVAMIA 2006

  29. Quantify knee cartilage morphology • Thickness: • Use “outer” surface as reference surface and compute closest point on the “inner” surface. • Algorithm: spatial partitioning algorithm by Aspert et al. (ICME 2002) CVAMIA 2006

  30. Validation experiments • Synthetic cartilage models: • Scanned as if they were from real cadavers. • Reconstruct a triangular mesh of the cartilage • Compute volume and thickness at fiducial points. • Compare results with ground truth obtained from current standard methods: • volume using water displacement • thickness using a caliper CVAMIA 2006

  31. Volume measurements • Synthetic volume measurements within accuracy range • Average discrepancy: 5% CVAMIA 2006

  32. Thickness measurements • Laser-scanner thickness measurements are within error range of caliper’s. • Discrepancy: less than 5% error on average CVAMIA 2006

  33. Work in progress • Overall goal : Validate segmentation algorithms on MR images of the cadavers • Volume estimate from manual segmentation: • Average discrepancy: 14.7% • Using measurements from laser-scanning method as ground truth is appropriate. CVAMIA 2006

  34. Related work: Koo et al. Osteoarthritis and Cartilage 2005 • Use a 3D laser scanner to scan femurs of porcine knees (with and without cartilage) • Align using the attached frame • Construct thickness map • Advantages of our method: • Construct a 3D mesh instead of thickness map • Specimen not attached to a frame • Provide validation CVAMIA 2006

  35. Summary • A method using a 3D laser scanner to reconstruct the triangular mesh of the knee cartilage. • Able to measure multiple morphological properties • Validation using synthetic knee bone and cartilage models. • Can be used to validate MR measurements. Future directions • Automate the cartilage boundary outlining process. • Validate for accuracy using a more reliable methods, e.g. coordinate measuring machine. • Validate the method for reproducibility CVAMIA 2006

  36. Acknowledgements • NSF grant IIS-0413215 • NIH grant AR047910S1 • Professor Richard Fishman, Visual Arts Department, Brown University CVAMIA 2006

  37. Thank you

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