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

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

Guido Gerig

Slide 2

Acknowledgments

Contributors:

• Martin Styner • Susumu Mori• Andy Alexander• Gordon Kindlmann• Randy Gollub

• National Alliance for Medical Image Computing (NIH U54EB005149)

Slide 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.

T1w T2w

Slide 5

Slide 6

5D 6Mo 14Mo

Slide 7

Networking and Brain Connectivity

Major Fiber Tracts extracted from DT MRI

UNC Computer Science: Network wire cabinets

Slide 8Diffusion Tensor Imaging (DT MRI) reveals

White Matter Structure

Gray matter

White matter

Courtesy of Susumu Mori, JHU

DT MRI

Slide 9

White Matter Structure

• Goal: Measure properties associated with the directiondirection of white matter Fibers

White matter

Wh

ite M

atte

rF

ibe

rs

Slide 10

Example: Corticospinal Tract

Tractography: Coronal viewTractography: Coronal view

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

Slide 11

Diffusion

• Random ‘Walk’ of Water Molecules

1 2 3

x2 6D

DT-MRI A. Alexander

Slide 12

Diffusion• Diffusion: Brownian motion of one material

through another

• Anisotropy: diffusion rate depends on direction

Gordon Kindlimann

Kleenex newspaper

Slide 13

Biological Restricted Diffusion

• Sextra >> Sintra

• Diffusion influenced by mean free path– Tortuosity

DT-MRI A. Alexander

Slide 14

Biological Restricted Diffusion

• Cellular degeneration (necrosis)- Diffusion increases

DT-MRI A. Alexander

Slide 15

Aniostropic Restricted Diffusion

• Diffusion has angular dependence

DT-MRI A. Alexander

How can we measure diffusion without perturbing the system?

Slide 17

Slide 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

Myelin sheet Nodes of Ranvier

Main diffusion direction

Slide 19

(An)isotropic diffusion

Free diffusion

Restricted diffusion

Isotropic diffusion

Anisotropic diffusion

Courtesy of Susumu Mori, John Hopkins University Medical School

Probability Distribution

Probability Distribution

Slide 20

DWI (indirectly) senses the structure of the tissue by measuring water molecule

displacement along a chosen direction.

r

r'

y

diffusioncoefficientin the y direction(= Dy)

Start

End

Slide 21

r

r'

If the path of the water molecule is affected by restrictions such as cellular material, the measured diffusion coefficient is reduced

extracellularspace

intracellularspace

Slide 22

If the tissue structures are oriented, the path of the water molecule (and the measured diffusion coefficients) will reflect this.

r

r'

y

diffusioncoefficientin the y direction(= Dy)

x diffusion coefficientin the x direction(= Dx)

Dx > Dy

Magnetic Resonance Imaging (MRI)

• Larmor Frequency

• Magnetic Field Gradient, G

Diffusion Weighted (DW) MRI

• Accumulated Phase

)t(G

DW-MRI II

x

N

ox Attenuation!

Slide 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.

DbeSS

0

Equation for the diffusion attenuation

GG

b-value

Sig

nal I

nten

sity

D

lnSS0

2G2 2

3

D= - bD

Slide 28

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

(b=0 and b=nnnn)

b-value

ln(S)

slope = D

intercept = S0

0 1000

DWI and ADC

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

b-value

Sig

nal I

nten

sity

Slide 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.

Increasing b-value

Images courtesy: Susumu Mori (JHU)

Slide 31

Apparent Diffusion Coefficient (ADC) Map with Different Measurement Direction

YX ZOnly the diffusion along a gradient direction can be measured


Gradient direction

ASNR 2003 –Washington,DC DT-MRI Alexander

Diffusion Weighted Images

12 DW encoding directionsSi (b=912 sec/mm2)

T2W ReferenceSo (b ~ 0 sec/mm2)

)ˆˆexp( itioi ggbSS D

)3/(222 Gb

Courtesy JE Lee

Slide 33

Measurement along Multiple Directions

Modified from DavidTuch, MGH

• 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)

Slide 34

DWI: Three Coordinate Systems

rightanterior

supe

riorWorld:

e.g. “RAS”

fast=I

med

ium

=J

slow=KImage:“IJK” Gradients:

g1 = (1,0,1)g2 = (1,-1,0)… Dxx, Dxy …

x

y

z

“ImageOrientation”

“MeasurementFrame”


Measured Apparent Diffusivities

12 encoding directions

b

SSD ioi

lnln

Courtesy JE Lee

Slide 36

What is “Diffusion – Weighted” Imaging?

In “Conventional” MRI, image contrast reflectsthe local relaxation (T1, T2) environment of the water molecules.

In “Diffusion-Weighted” Imaging (DWI), imagecontrast reflects the physical structure of the Tissue (via the local diffusion distribution).

Slide 37

Simplification and assumption

Diffusion ellipsoidCourtesy of Susumu Mori, John Hopkins University Medical School

Orientational Diffusion Fct


The Diffusion Tensor

zzzyzx

yzyyyx

xzxyxx

DDD

DDD

DDD

D

0,, zzyyxx DDD zyyzzxxzyxxy DDDDDD ;;

Courtesy JE Lee

National Alliance for Medical Image Computing http://na-mic.org

DWI summary: MRI

• 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 newspaper

National Alliance for Medical Image Computing http://na-mic.org

DWI crash course: Model

Single Tensor Model (Basser 1994)

A0

gi

Ai

D

Dxx Dxy Dxz

Dyy Dyz

Dzz

Tensorestimation

Slide 41

Anisotropy & Color-coded Orientation

Isotropic GM

Anisotropic WM


Slide 42

DTI Tensor Visualization

Color: FA valueITK: DTIFiberTubeSpatialObject & SpatialObjectViewers (Julien Jomier)

Here comes Ross Whitaker

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