Hybrid Visualization for White Matter Tracts using

Triangle Strips and Point Sprites

D. Merhof1,2, M. Sonntag1, F. Enders1,2,C. Nimsky2, P. Hastreiter1,2, G. Greiner1

1Computer Graphics Group2Neurocenter, Dept. of Neurosurgery

University of Erlangen-Nuremberg

IEEE Vis 20062 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Outline

Motivation

Diffusion tensor imaging

Fiber tracking

3D visualization approaches

Results

Conclusion

Acknowledgements

IEEE Vis 20063 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Motivation

White matter tractsNeural structures within thehuman brain

Transmission of motor, sensor and other impulses

Tract systems:Pyramidal tract

Corpus callosum

Optic tract

IEEE Vis 20064 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Motivation

Reconstruction and visualization of white matter tracts of interest for different research areas:

Development of the brain

Study of brain diseases

Neurosurgery

IEEE Vis 20066 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Diffusion tensor imaging (DTI)

MR imaging sequence (echo planar imaging)

Measures diffusion of water molecules

White matter consists of long, cylindrical cells

Diffusion within white matter is anisotropic

Diffusion tensor: Gaussian diffusion per voxel

AnisotropicdiffusionAnisotropicdiffusion

IsotropicdiffusionIsotropicdiffusion

IEEE Vis 20067 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Basic Idea:Reduce tensor field to vector field of major eigenvector

Vector field of main diffusion

Perform streamline propagation

Recostruction - Fiber Tracking

IEEE Vis 20068 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Visualization

Lines+ Fast rendering- Gaps, if lines wider than one pixel- Width fix, independent of distance

Tubes+ Significantly improved Visualization- Slow rendering, due to geometric

complexity

Lines

Tubes

IEEE Vis 20069 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Visualization - Idea

Rectangular cross section of tube for v ┴ t

Use triangle strips

Spherical cross section of tube for v || t Use point sprites

Hybrid combination of both techniques

IEEE Vis 200610 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Triangle strips - construction

IEEE Vis 200611 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Triangle strips - texturing

1D texture with luminance and alpha channelTextures for ambient und specular illuminationDecreasing value when approaching the border(sine function)

diffusediffuse specularspecular

1D texture1D texture

channel 1channel 1 channel 2channel 2

IEEE Vis 200612 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Flipping artifacts

Occur when viewing and segment direction are nearly parallelUse point sprites in this case

textured quads, rotated towards cameraresemble cross section of tube

IEEE Vis 200613 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Point Sprites - Texturing

2D-RGB-Texture: draw and illuminate point spritesR channel: alpha channelG / B channel: diffuse / specular illumination

IEEE Vis 200614 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Point Sprites - Rotation

Rotate point sprite according to segment direction:

Project fiber tangent t to image plane tp

Normalize tp tp0

Alpha provides rotation of segment

IEEE Vis 200615 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Point Sprites

When v close to t:Staggered point sprites

Results in natural appearance of a tube

vvPoint Sprites

IEEE Vis 200616 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Visualization – Hybrid approach

Triangle StripsRepresent rectangular cross section

Displayed for v t < 0.98

Point SpritesRepresent circular cross section

Displayed for v t > 0.93

IEEE Vis 200617 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Terminating textured quads

Problem – Beginning and end of each fiber No tube-like appearance

SolutionTerminating textured quads

IEEE Vis 200618 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Results

Hybrid visualization approach based on triangle strips and point sprites provides

High visual quality (similar to tubes)

Visual drawbacks of lines are circumvented

High performance (due to reduced complexity)

Lines HybridTubes

IEEE Vis 200619 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Results

Tracking of whole brain (4 500 fibers, 300 000 segments)Tubes : 16 ∆ / seg. ca. 4.8 million ∆Triangle strips : 2 ∆ / seg. ca. 0.6 million ∆ Graphics card : Nvidia Geforce 6800 Ultra, 256 MBViewport : 600x800 Pixel

IEEE Vis 200620 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Results

Clinical relevance:Combination with Volume Rendering

Realtime visualization improvesinspection of data

Improved analysis of the data

Tumor

IEEE Vis 200622 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Conclusion

Improved visualization forstreamlines based on DT-MRI data

Technical aspectsHybrid approach instead of tubes/lines

Rendering performance ≈ Lines

Visual quality ≈ Tubes

Medical aspectsImproved analysis in real-time

IEEE Vis 200623 Computer Graphics Group + Neurocenter, Dept. of Neurosurgery • University of Erlangen-Nuremberg

Fundings

Graduiertenkolleg „Dreidimensionale Bildanalyse und –synthese“

Deutsche Forschungsgemeinschaft (DFG), SFB 603, Project C9

Thank you!Questions?Thank you!Questions?