MR-DTI: Non-invasive imaging of neuroanatomy of white matter Guido Gerig - PowerPoint PPT Presentation

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MR-DTI: Non-invasive imaging of neuroanatomy of white matter Guido Gerig

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Title: MR-DTI: Non-invasive imaging of neuroanatomy of white matter Guido Gerig


1
MR-DTINon-invasive imaging of neuroanatomy of
white matter Guido Gerig
2
Acknowledgments
  • Contributors
  • Martin Styner
  • Susumu Mori
  • Andy Alexander
  • Gordon Kindlmann
  • Randy Gollub
  • National Alliance for Medical Image Computing
    (NIH U54EB005149)

3
Use of these slides
  • Slides were borrowed from various researchers,
    and we are working on getting permissions for
    distribution.
  • Slides can be used for own purposes.
  • Please do not distribute these slides.
  • Please do no put slides into public download
    space.

4
T1w
T2w
5
(No Transcript)
6
5D
6Mo
14Mo
7
Networking and Brain Connectivity
Major Fiber Tracts extracted from DT MRI
UNC Computer Science Network wire cabinets
8
Diffusion Tensor Imaging (DT MRI) reveals White
Matter Structure
Courtesy of Susumu Mori, JHU
9
White Matter Structure
  • Goal Measure properties associated with the
    direction of white matter Fibers

White Matter Fibers
10
Example Corticospinal Tract
Source Duke NeuroAnatomy Web Resources
(Christine Hulette)
B Superior longitudinal fasciculus C Superior
occipitofrontal fasciculus D Cingulum E
Inferior longitudinal fasciculus F Inferior
occipitofrontal fasciculus
Tractography Coronal view
11
Diffusion
  • Random Walk of Water Molecules

DT-MRI A. Alexander
12
Diffusion
  • Diffusion Brownian motion of one material
    through another
  • Anisotropy diffusion rate depends on direction

newspaper
Kleenex
Gordon Kindlimann
13
Biological Restricted Diffusion
  • Sextra gtgt Sintra
  • Diffusion influenced by mean free path
  • Tortuosity

DT-MRI A. Alexander
14
Biological Restricted Diffusion
  • Cellular degeneration (necrosis)- Diffusion
    increases

DT-MRI A. Alexander
15
Aniostropic Restricted Diffusion
  • Diffusion has angular dependence

DT-MRI A. Alexander
16
How can we measure diffusion without perturbing
the system?
17
(No Transcript)
18
Diffusion and white matter
  • Diffusion MRI measures diffusion of mainly water
    molecules
  • Isotropic medium ? molecules move with Brownian
    motion.
  • In biological tissues diffusion is often
    anisotropic
  • In white matter Local structure
  • Insulating myelin sheet, low probability to cross
    into axon
  • Dense axon bundles exhibits strongly directional
    local structure
  • Diffusion along fiber bundle is main diffusion
    direction

19
(An)isotropic diffusion
Free diffusion
Isotropic diffusion
Probability Distribution
Anisotropic diffusion
Restricted diffusion
Probability Distribution
Courtesy of Susumu Mori, John Hopkins University
Medical School
20
DWI (indirectly) senses the structure of the
tissue by measuring water molecule displacement
along a chosen direction.
r'
End
diffusion coefficient in the y direction ( Dy)
?y
r
Start
21
If the path of the water molecule is affected by
restrictions such as cellular material, the
measured diffusion coefficient is reduced
intracellular space
r
r'
extracellular space
22
If the tissue structures are oriented, the path
of the water molecule (and the measured diffusion
coefficients) will reflect this.
r'
diffusion coefficient in the y direction ( Dy)
?y
r
diffusion coefficient in the x direction( Dx)
?x
Dx gt Dy
23
Magnetic Resonance Imaging (MRI)
  • Larmor Frequency
  • Magnetic Field Gradient, G

24
Diffusion Weighted (DW) MRI
  • Accumulated Phase

25
DW-MRI II
Attenuation!
26
The pixel signal intensity, S, is related to the
b-value and the diffusion coefficient, D,
through
This equation (Steyskal Tanner Equation) has two
unknowns, the signal intensity for b 0 (S0) and
D. Therefore, at least 2 measurements must be
made, each at a different b-value to calculate D.
27
Equation for the diffusion attenuation
?
G
?
S
?
??
??
2
2
2
ln
?
?
?
G
?
?
?
D
- bD
??
??
S
3
0
Signal Intensity
D
b-value
28
Measuring D for a Given Direction Simplified
model of two b values (b0 and bnnnn)
intercept S0
b-value
1000
0
slope D
ln(S)
29
DWI and ADC
1 G/cm 6 G/cm 10 G/cm 13 G/cm
Signal Intensity
b-value
30
The b-value is the contrast knob in a diffusion
experiment and is varied in magnitude and in a
specified number of directions. Increasing the
b-value increases the contrast between slow and
fast diffusing water molecules.
Images courtesy Susumu Mori (JHU)
Increasing b-value
31
Apparent Diffusion Coefficient (ADC) Map with
Different Measurement Direction
Gradient direction
Y
X
Z
Only the diffusion along a gradient direction can
be measured
Courtesy of Susumu Mori, John Hopkins University
Medical School
32
Diffusion Weighted Images
T2W Reference So (b 0 sec/mm2)
12 DW encoding directions Si (b912 sec/mm2)
Courtesy JE Lee
33
Measurement along Multiple Directions
  • Diffusion MRI measures along single gradient
    directions
  • Diffusion Weighted Images (DWI)
  • In principle Arbitrary gradient directions
  • 6 different directions ? Tensor
  • 12/24 directions ? stability
  • Diffusion Tensor Imaging (DTI)
  • High angular acquisition
  • Sampling of orientation diffusion
  • Higher order representations (fiber crossings)
  • Qball (D. Tuch, MGH), gt256 dirs
  • Others Van Wedeen (MIT), Frank (UCSD)

Modified from DavidTuch, MGH
34
DWI Three Coordinate Systems
Image Orientation
Measurement Frame
35
Measured Apparent Diffusivities
12 encoding directions
Courtesy JE Lee
36
What is Diffusion Weighted Imaging?
In Conventional MRI, image contrast
reflects the local relaxation (T1, T2)
environment of the water molecules. In
Diffusion-Weighted Imaging (DWI),
image contrast reflects the physical structure of
the Tissue (via the local diffusion
distribution).
37
Simplification and assumption
Orientational Diffusion Fct
Diffusion ellipsoid
Courtesy of Susumu Mori, John Hopkins University
Medical School
38
The Diffusion Tensor
Courtesy JE Lee
39
DWI summary MRI
newspaper
  • Diffusion Brownian motion of one material
    through another
  • Anisotropy diffusion rate depends on direction
  • Magnetic gradients create spatial planar waves of
    proton phase
  • Destructive interference measures diffusion along
    gradient direction only

Kleenex
40
DWI crash course Model
  • Single Tensor Model (Basser 1994)

Ai
Dxx
Dxz
Dxy
Tensor estimation
D
A0
Dyy
Dyz
Dzz
gi
41
Anisotropy Color-coded Orientation
Isotropic GM
Anisotropic WM
Courtesy of Susumu Mori, John Hopkins University
Medical School
42
DTI Tensor Visualization
Color FA value ITK DTIFiberTubeSpatialObject
SpatialObjectViewers (Julien Jomier)
43
Here comes Ross Whitaker
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