mr dti non invasive imaging of neuroanatomy of white matter guido gerig l.
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MR-DTI: Non-invasive imaging of neuroanatomy of white matter Guido Gerig. Acknowledgments. Contributors: Martin Styner Susumu Mori Andy Alexander Gordon Kindlmann Randy Gollub National Alliance for Medical Image Computing (NIH U54EB005149). Use of these slides.

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acknowledgments
Acknowledgments

Contributors:

  • Martin Styner
  • Susumu Mori
  • Andy Alexander
  • Gordon Kindlmann
  • Randy Gollub
  • National Alliance for Medical Image Computing (NIH U54EB005149)
use of these slides
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.
slide4

T1w

T2w

slide6

5D

6Mo

14Mo

networking and brain connectivity
Networking and Brain Connectivity

Major Fiber Tracts extracted from DT MRI

UNC Computer Science: Network wire cabinets

diffusion tensor imaging dt mri reveals white matter structure

Gray matter

DT MRI

White matter

Diffusion Tensor Imaging (DT MRI) reveals White Matter Structure

Courtesy of Susumu Mori, JHU

white matter structure

White matter

White Matter Structure
  • Goal: Measure properties associated with the direction of white matter Fibers

White Matter

Fibers

example corticospinal tract
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

diffusion

t1

t2

t3

Diffusion
  • Random ‘Walk’ of Water Molecules

DT-MRI A. Alexander

diffusion12
Diffusion
  • Diffusion: Brownian motion of one material through another
  • Anisotropy: diffusion rate depends on direction

newspaper

Kleenex

Gordon Kindlimann

biological restricted diffusion
Biological Restricted Diffusion
  • Sextra >> Sintra
  • Diffusion influenced by mean free path
    • Tortuosity

DT-MRI A. Alexander

biological restricted diffusion14
Biological Restricted Diffusion
  • Cellular degeneration (necrosis)- Diffusion increases

DT-MRI A. Alexander

aniostropic restricted diffusion
Aniostropic Restricted Diffusion
  • Diffusion has angular dependence

DT-MRI A. Alexander

slide18

Main diffusion direction

Nodes of Ranvier

Myelin sheet

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
slide19

(An)isotropic diffusion

Isotropic diffusion

Free diffusion

Probability Distribution

Anisotropic diffusion

Restricted diffusion

Probability Distribution

Courtesy of Susumu Mori, John Hopkins University Medical School

slide20

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

slide21

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

slide22

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 > Dy

magnetic resonance imaging mri
Magnetic Resonance Imaging (MRI)
  • Larmor Frequency
  • Magnetic Field Gradient, G
dw mri ii
DW-MRI II

Attenuation!

slide26

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.

slide27

Equation for the diffusion attenuation

G



S



2

2

2

ln

G

D

= - bD





S

3

0

Signal Intensity

D

b-value

slide28

Measuring D for a Given Direction: Simplified model of two b values

(b=0 and b=nnnn)

intercept = S0

b-value

1000

0

slope = D

ln(S)

slide29

DWI and ADC

1 G/cm 6 G/cm 10 G/cm 13 G/cm

Signal Intensity

b-value

slide30

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

slide31

Apparent Diffusion Coefficient (ADC) Map with Different Measurement Direction

Gradient direction

X

Y

Z

Only the diffusion along a gradient direction can be

measured

Courtesy of Susumu Mori, John Hopkins University Medical School

diffusion weighted images
Diffusion Weighted Images

T2W Reference

So (b ~ 0 sec/mm2)

12 DW encoding directions

Si (b=912 sec/mm2)

DT-MRI Alexander

Courtesy JE Lee

slide33

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), >256 dirs
    • Others: Van Wedeen (MIT), Frank (UCSD)

Modified from DavidTuch, MGH

dwi three coordinate systems

Image:

“IJK”

x

Gradients:

g1 = (1,0,1)

g2 = (1,-1,0)

Dxx, Dxy …

slow=K

y

medium=J

z

fast=I

World:

e.g. “RAS”

superior

anterior

right

DWI: Three Coordinate Systems

“Image

Orientation”

“Measurement

Frame”

measured apparent diffusivities
Measured Apparent Diffusivities

12 encoding directions

DT-MRI Alexander

Courtesy JE Lee

slide36

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).

simplification and assumption
Simplification and assumption

Orientational Diffusion Fct

Diffusion ellipsoid

Courtesy of Susumu Mori, John Hopkins University Medical School

the diffusion tensor
The Diffusion Tensor

DT-MRI Alexander

Courtesy JE Lee

dwi summary mri
DWI summary: MRI

newspaper

Kleenex

  • 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
dwi crash course model
DWI crash course: Model

Single Tensor Model (Basser 1994)

Ai

Dxz

Dxx

Dxy

Tensor

estimation

D

A0

Dyy

Dyz

Dzz

gi

anisotropy color coded orientation
Anisotropy & Color-coded Orientation

Isotropic GM

Anisotropic WM

Courtesy of Susumu Mori, John Hopkins University Medical School

dti tensor visualization
DTI Tensor Visualization

Color: FA value

ITK: DTIFiberTubeSpatialObject & SpatialObjectViewers (Julien Jomier)