MR Diffusion Tensor Imaging, Tractography

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MR Diffusion Tensor Imaging, Tractography

• Richard Watts, D.Phil.
• Citigroup Biomedical Imaging Center
• Weill Medical College of Cornell University
• Box 234, 1300 York Avenue, New York, NY 10021
• Email riw2004_at_med.cornell.edu, Telephone 212
  746-5781

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Acknowledgements

• Weill Medical College of Cornell University
• Department of Radiology
• Aziz Ulug, Linda Heier.
• Citigroup Biomedical Imaging Center
• Doug Ballon, Jon Dyke, Katherine Kolbert.
• Sackler Institute
• BJ Casey, Matt Davidson, Katie Thomas.

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Outline

• Background
• Diffusion
• Restricted Diffusion and Anisotropy
• Methods
• Data Acquisition
• Display of Diffusion Tensor Data
• Fiber Tracking
• Problems and Limitations
• Examples

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Diffusion
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Diffusion Equation
r Displacement (mm) D Diffusion
constant (mm2/s) t Time (mm)
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Distance Scales
Question What distance do protons travel during
an EPI readout time?
Assume Diffusion constant 10-3 mm2/s Time
100 ms 0.1s
The root mean square (RMS) distance is 0.02mm
20µm
Such an experiment is sensitive to changes in
diffusion caused by structures on this scale or
smaller
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Diffusion Imaging of Leukemia
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Diffusion Imaging of Leukemia
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Spin Echo
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Spin Echo
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Spin Echo
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Data Acquisition Spin Echo
Echo
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Restricted Diffusion
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Diffusion Ellipsoid in White Matter
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Anisotropy
Isotropic Having the same properties in all
directions
Anisotropic Not isotropic having different
properties in different directions
Websters Dictionary
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Data Acquisition Spin Echo
Echo
Linear combination of gradients - measure
component of diffusion in any direction
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Diffusion Tensor Imaging

• Tensor is a mathematical model of the directional
  anisotropy of diffusion
• Represented by a 3x3 symmetric matrix ? 6 degrees
  of freedom
• Fit experimental data to the tensor model
• From the tensor, we can calculate
• Direction of greatest diffusion
• Degree of anisotropy
• Diffusion constant in any direction

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Calculated Quantities
Various definitions
T2-Weighted Image
Average Diffusion
Degree of Anisotropy
Diffusion along X
Diffusion along Y
Diffusion along Z
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1. (Approximately) Isotropic Diffusion
How a blob of ink would spread out
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2. Anisotropic Diffusion
How a blob of ink would spread out
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Vector Plot
In-plane
Through-plane
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Direction of Greatest Diffusion



X-component
Y-component
Z-component
Anisotropy
Color (Hue) Direction of highest
diffusion Brightness Degree of anisotropy

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Diffusion Tensor Colour Map
Left-Right
Anterior-Posterior
Superior-Inferior
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DTI Color Map
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Diffusion Tensor 3D Colour Map
Left-Right
Anterior-Posterior
Superior-Inferior
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How Many Measurements?
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Which Directions?
Isotropic resolution diffusion tensor imaging
with whole brain acquisition in a clinically
acceptable time D.K. Jones, S.C.R. Williams, D.
Gasston, M.A. Horsfield, A. Simmons, R.
Howard Human Brain Mapping 15, 216-230 (2002)
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Fiber Tracking Discrete Case
Direction of Greatest diffusion
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Fiber Tracking Discrete Case
Direction of Greatest diffusion
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Fiber Tracking Continuous Case
Direction of Greatest diffusion
Mori et al, 1999
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Fiber Tracking Where to Start

• Everywhere Seed points distributed evenly
  throughout volume

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DTI Tractography
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Fiber Tracking Where to Start

• Within a plane All fibers within or crossing a
  selected plane are tracked

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Fiber Tracking Corpus Callosum
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Fiber Tracking Corpus Callosum
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Fiber Tracking Where to Start

• Within a small volume

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Fiber Tracking - CST
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Human Neuroanatomy Carpenter Sutin 1981
Upper Extremity
Trunk
Lower Extremity
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Human Neuroanatomy Carpenter Sutin 1983
Upper Extremity
Trunk
Lower Extremity
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Fiber Tracking - CST
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Fiber Tracking - CST
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Combining DTI and fMRI
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fMRI Feet Movement
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fMRI Finger Tapping
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fMRI Tongue Movement
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Results fMRI Feet, Fingers, Tongue
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Images of Mind, Posner and Raichie, 1999
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Fiber Tracking - CST
Subject 1
Subject 2
Subject 3
Subject 4
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Crossing Fibers
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DTI Tracking below SLF
Tongue
Feet
Fingers
Upper
Trunk
Lower
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DTI Tractography Clinical Example
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DTI Tractography Clinical Example
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Limitations of DTI/Fiber Tracking

• Partial volume
• A single voxel may contain fibers running in
  multiple directions average anisotropy measured
• Tensor may not be a good representation
• Need to distinguish kissing and crossing

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More Pretty Pictures

• Isotropic resolution diffusion tensor imaging
  with whole brain acquisition in a clinically
  acceptable time
• D.K. Jones, S.C.R. Williams, D. Gasston, M.A.
  Horsfield, A. Simmons, R. Howard
• Human Brain Mapping 15, 216-230 (2002)

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Conclusions, the Future

• DTI provides the only non-invasive method to
  study organization white matter fibers. Previous
  studies have been limited to animal models and
  stroke patients
• Current limitations on DTI and Fiber Tracking
• Partial volume effects
• SNR
• Acquisition time/physiological noise
• Advances
• High field, faster gradients, more efficient
  coils, motion detection/correction, new pulse
  sequences (eg. 3D, spiral)
• Higher SNR can be traded for smaller voxels,
  reducing partial volume effects
• Beyond the tensor model HARD imaging, q-space
  imaging
• New tracking algorithms

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DTI Tracking below SLF
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DTI Tracking below SLF
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References

• High-resolution isotropic 3D diffusion tensor
  imaging of the human brain.
• X. Golay, H. Jiang, P.C.M. van Zijl, S. Mori
• Magn. Res. Med. 47, 837-843 (2002)
• White matter mapping using diffusion tensor MRI
• C.R. Tench, P.S. Morgan, M. Wilson, L.D.
  Blumhardt
• Magn. Res. Med. 47, 967-972 (2002)
• Three-dimensional tracking of axonal projections
  in the brain by magnetic resonance imaging
• S. Mori, B.J. Creain, V.P. Chacko, P.C.M. van
  Zijl
• Ann. Neurol. 45, 265-269 (1999)
• Diffusion tensor imaging Concepts and
  applications
• D. Le Bihan et al
• J. Magn. Res. Imaging 13, 534-546 (2001)
• In vivo three dimensional reconstruction of rat
  brain axonal projections by diffusion tensor
  imaging
• R. Xue, P.C.M. van Zijl, B.J. Cain, M.
  Solaiyappan, S.Mori
• Magn. Res. Med. 42 1123-1127 (1999)
• A direct demonstration of both structure and
  function in the visual system combining
  diffusion tensor imaging with functional magnetic
  resonance imaging
• D.J. Werring, C.A. Clark, G.J.M. Parker, D.H.
  Miller, A.J. Thompson, G.J. Barker
• NeuroImage 9, 352-361 (1999)
• Orientation-independent diffusion imaging without
  tensor diagonalization anisotropy definitions
  based on the physical attributes of the diffusion
  ellipsoid

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References

• Imaging cortical association tracts in the human
  brain using diffusion-tensor based axonal
  tracking
• S. Mori et al
• Magn. Res. Med. 47, 215-223 (2002)
• Isotropic resolution diffusion tensor imaging
  with whole brain acquisition in a clinically
  acceptable time
• D.K. Jones, S.C.R. Williams, D. Gasston, M.A.
  Horsfield, A. Simmons, R. Howard
• Human Brain Mapping 15, 216-230 (2002)
• Diffusion tensor imaging and axonal tracking in
  the human brainstem
• B. Stietjes et al
• NeuroImage 14 723-735 (2001)
• Tracking neuronal fiber pathways in the living
  human brain
• T.E. Conturo et al
• Proc. Natl. Acad. Sci. 96 10422-10427 (1999)
• The future for diffusion tensor imaging in
  neuropsychiatry
• K.H. Taber et al
• J. Neuropsychiatry Clin. Neurosci. 14 1-5 (2002)
• Tensorlines Advection-diffusion based
  propogation through diffusion tensor fields
• D. Weinstein, G. Kindlmann, E. Lundberg

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The Diffusion Tensor
where
Identical if
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How Many Measurements?
7 degrees of freedom S0, Dxx, Dyy, Dzz, Dxy,
Dxz, Dyz
Need at least 7 directions but more is
better! 30 slices x 32 directions 960 images
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Corresponding Tensor
mm2/s
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Eigenvalues and Eigenvectors of the Diffusion
Tensor
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Corresponding Tensor
mm2/s
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Eigenvalues and Eigenvectors of the Diffusion
Tensor