Hybrid Visualization for White Matter Tracts using Triangle Strips and Point Sprites

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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 Group
• 2Neurocenter, Dept. of Neurosurgery
• University of Erlangen-Nuremberg

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Outline

• Motivation
• Diffusion tensor imaging
• Fiber tracking
• 3D visualization approaches
• Results
• Conclusion
• Acknowledgements

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Motivation

• White matter tracts
• Neural structures within thehuman brain
• Transmission of motor, sensor and other impulses
• Tract systems
• Pyramidal tract
• Corpus callosum
• Optic tract

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Motivation

• Reconstruction and visualization of white matter
  tracts of interest for different research areas
• Development of the brain
• Study of brain diseases
• Neurosurgery

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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

Isotropic diffusion
Anisotropic diffusion
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Recostruction - Fiber Tracking

• Basic Idea
• Reduce tensor field to vector field of major
  eigenvector
• Vector field of main diffusion
• Perform streamline propagation

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Visualization

• Lines
• Fast rendering
• Gaps, if lines wider than one pixel
• Width fix, independent of distance
• Tubes
• Significantly improved Visualization
• Slow rendering, due to geometriccomplexity

Lines
Tubes
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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

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Triangle strips - construction
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Triangle strips - texturing

• 1D texture with luminance and alpha channel
• Textures for ambient und specular illumination
• Decreasing value when approaching the
  border(sine function)

channel 1
channel 2
diffuse
specular
1D texture
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Flipping artifacts

• Occur when viewing and segment direction are
  nearly parallel
• Use point sprites in this case
• textured quads, rotated towards camera
• resemble cross section of tube

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Point Sprites - Texturing

• 2D-RGB-Texture draw and illuminate point sprites
• R channel alpha channel
• G / B channel diffuse / specular illumination

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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

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Point Sprites

• When v close to t
• Staggered point sprites
• Results in natural appearance of a tube

Point Sprites
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Visualization Hybrid approach

• Triangle Strips
• Represent rectangular cross section
• Displayed for v t lt 0.98
• Point Sprites
• Represent circular cross section
• Displayed for v t gt 0.93

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Terminating textured quads

• Problem Beginning and end of each fiber
• No tube-like appearance
• Solution
• Terminating textured quads

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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
Hybrid
Tubes
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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
  MB
• Viewport 600x800 Pixel

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Results

• Clinical relevance
• Combination with Volume Rendering
• Realtime visualization improvesinspection of
  data
• Improved analysis of the data

Tumor
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Conclusion

• Improved visualization forstreamlines based on
  DT-MRI data
• Technical aspects
• Hybrid approach instead of tubes/lines
• Rendering performance Lines
• Visual quality Tubes
• Medical aspects
• Improved analysis in real-time

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Fundings

• Graduiertenkolleg Dreidimensionale Bildanalyse
  und synthese
• Deutsche Forschungsgemeinschaft (DFG), SFB 603,
  Project C9

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Thank you! Questions?