1 Interactive Thickness Visualization of Articular Cartilage Author :Matej Mlejnek, Anna Vilanova,Meister Eduard GröllerMatej MlejnekAnna VilanovaMeister Eduard Gröller Source :Proceedings of Visualization 2004, pages October 2004 Speaker : Ren-LI Shen Advisor : Ku-Yaw Chang

2 Outline   Introduction   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

3 Introduction   Articular cartilage   Surfaces of knee joints are covered by tissue   Is a curved structure   difficult to read the thickness changes

4 Introduction   Main functions of the cartilage   Distribution of weight   Frictionless motion   Shock absorption

5 Introduction   Height field   Unfolding and depicting   Eliminates the complexity of the 3D shape   concentrate solely on the inspection   Offers several visualization modes   Color mapping   Scaling   Glyphs   Iso-lines

6 Introduction

7   Distortion   Minimize the distortion, in a user-defined area   Flattening   Requires parameterization of the surface

8 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

9 Pipeline for thickness visualization   Consists of the following steps

10 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

11 Cartilage segmentation   Two main classes of segmentation methods   Manual segmentation   Time-consuming   Requires an experienced user   Semi-automatic segmentation   use thresholding, region growing, snakes, or edge detection filters   In this paper they use an active contour model (snake)   controlled by internal and external forces

12 Cartilage segmentation

13 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

14 Thickness measurement   Several possibilities to calculate   Vertical distance   Is not appropriate for curved surfaces   Proximity method   This paper uses   Normal distance

15 Thickness measurement   Euclidean distance   Optimizations of distance transforms   Chamfer distance transforms   propagates the local distance by adding the neighborhood values   Vector distance transforms   propagates the distance vector to the nearest sample point of the object surface

16 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

17 Flattening of articular cartilage   Flattening cartilage into the corresponding 2D plane should fulfill the following criteria   Need a parameterization   Minimizes area distortion   Local and global intersections have to be prevented   Common problem in the area on surface parameterization

18 Flattening of articular cartilage   Do not allow multiple patches   In order to keep spatial relations.   Parameterization has to be fast

19 Flattening of articular cartilage   Efficiently prevent local as well as global intersections   Align all points onto a line   Reduces the distortion minimization issue

20 Flattening of articular cartilage   In order to meet all of the constraints   Grow a planar patch   First, the focal triangle includes the focal point is rigidly transformed into the 2D plane   The distance is defined by the height of the focal triangle (height = 2·area / |p2−p1|)   Next step, patch is iteratively flattened by adding active points ai to the patch

21 Flattening of articular cartilage

22 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

23 Operations on the height field   Slight changes in the thickness on the reconstructed surface may, however, not be noticeable   In order to enhance the thickness information   Propose a non-uniform scaling only in the height direction

24 Operations on the height field

25 Operations on the height field

26 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

27 Thresholded non-linear scaling

28 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

29 Non-linear scaling on interval   Thresholded scaling

30 Non-linear scaling on interval

31 Non-linear scaling on interval   Show the extraction of thickness information enhanced by color coding

32 Non-linear scaling on interval   By iso-lines

33 Non-linear scaling on interval   By glyphs

34 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

35 Scale transfer function   Need a tool enables detection of subtle thickness changes on each range of the thickness values   Using non-linear scaling approach, interesting features may be occluded by other scaled areas   Can be overcome by thresholded non-linear scaling

36 Scale transfer function   Define a continuous linear scaling transfer function   Maps the original thickness values   Assigned to each vertex   To the scaled values   Thickness preservation is performed on intervals, where x = y

37 Scale transfer function

38 Outline   Introduction and medical background   Pipeline for thickness visualization   Cartilage segmentation   Thickness measurement   Flattening of articular cartilage   Operations on the height field   Thresholded non-linear scaling   Non-linear scaling on interval   Scale transfer function   Summary and conclusions

39 Summary and conclusions   The approach has been illustrated on the visualization of articular cartilage   Detection of slight thickness changes is vital for diagnosis   Has been shown that unfolding of anatomic organs is promising   Future work   Continue with a broader clinical study on a variety of datasets