Diffusion Tensor Imaging I: The basics...Diffusion tensor imaging (DTI) crossing fibers: •FA lower than for single direction . Diffusion tensor imaging (DTI) degeneration of one

Diffusion Tensor Imaging I:

The basics

Jennifer Campbell

Diffusion Tensor Imaging I:

The basics

Jennifer Campbell

Diffusion Imaging

T1W PDW T2W

MRI: many different sources of contrast

Perfusion DW BOLD

0 0.6 0.8 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (s)

T2 relaxation

0 1 2 3 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (s)

T1 relaxation

GM: T1 = 1300 ms T2 = 100 ms r = 0.8

WM: T1 = 800 ms T2 = 80 ms r = 0.7

CSF: T1 = 5500 ms T2 = 2200 ms r = 1.0

WM

GM

GM

WM

CSF

CSF

0.2 0.4

Mz Mxy

T1 & T2 relaxation in CNS at 3 T

Diffusion MRI measures Brownian motion

of water molecules

Path of diffusing water

molecule Water displacement

distribution


of water molecules

P(r | r0,t d) =1

D(4pt d)3· exp

(r - r0)T D-1(r - r0)

4t d

æ

èçö

ø÷;< r2 >= 6Dt d


of water molecules

• Hindered and restricted diffusion: Apparent Diffusion

Coefficient (ADC)

• Diffusion is “hindered” outside cells and “restricted”

inside cells


of water molecules

• Hindered and restricted diffusion: Apparent Diffusion

Coefficient (ADC)

• Diffusion is “hindered” outside cells and “restricted”

inside cells


of water molecules

P(r | r0,t d) =1

ADC ×(4pt d)3· exp

(r - r0)T (ADC)-1(r - r0)

4t d

æ

èçö

ø÷

Tissue structures determine which

directions of motion are most probable

White matter fiber

bundle: oriented

structure

Water molecules prefer to

travel parallel to fiber

direction



White matter fiber

bundle: oriented

structure

Causes of anisotropic diffusion

in white matter:

• myelin

• axon membranes

• neurofilaments

Diffusion MRI Tractography

Diffusion MRI Tractography



P(r | r0,t d) =1

|D | (4pt d)3· exp

(r - r0)TD-1(r - r0)

4t d

æ

èçö

ø÷

The diffusion tensor


1

3

2

e1


1

3

2

e1


1

3

2

e1


1

3

2

e1

Maps obtainable from the diffusion tensor

RGB plot: principal

eigenvector (e1)

direction, scaled by FA

trace of diffusion tensor

(mean diffusivity)

anisotropy

index: fractional

anisotropy (FA)

anisotropy

index: fractional

anisotropy (FA)

trace of diffusion tensor

(mean diffusivity)

RGB plot: principal

eigenvector (e1)

direction, scaled by FA

FA =3

2

(l1 - l)2 + (l 2 - l)2 + (l 3 - l)2

(l12 + l2

2 + l 32 )

Maps obtainable from the diffusion tensor

Diffusion tensor imaging (DTI)

1

2 3

cylindrical symmetry


λ||, axial

cylindrical symmetry

λ , radial

T


normal white matter


myelin degeneration:

•radial diffusivity increases

•FA decreases


normal white matter


axonal fragmentation:

•axial diffusivity decreases

•FA decreases


normal white matter


decreased axon density:

•radial and axial diffusivity increase

•FA decreases

normal tissue


cellular swelling:

•MD decreases



curvature:

•FA lower than for single direction


splay:



crossing fibers:



degeneration of one fiber population in crossing case:

•FA increases

B0 B0 + G B0

G

180° echo

B0, 180o

G

B0 + G

Gradients, phase, and motion

G

180° echo

G

Gradients, phase, and motion

B0 + G B0, 180o B0 + G B0 B0

Measuring diffusion with MRI:

diffusion weighted images (DWIs) • large magnetic field gradients sensitize the image to

diffusion in the direction of the gradient

Diffusion weighted MRI sequence

TE

G

90° 180° echo

G

Δ

δ b = g 2G2d 2(D -d / 3)

• b value indicates

the magnitude of

the diffusion

weighting:

Computing the diffusion tensor

Signal decrease with b value:

b matrix used in 3D tensor

calculation:

Computing the diffusion tensor

Signal decrease with b value:

b matrix used in 3D tensor

calculation:

• at least one image with no diffusion encoding (b=0 s/mm2)

• series of DWIs: 3-6-30-60-100+ encoding directions; minimum 6 for tensor

• full brain coverage: 10-20 minutes per 100 directions

• b value: 1000-3000 s/mm2; b~1000 s/mm2 best for tensor

• voxel size: roughly 2mm x 2mm x 2mm isotropic voxels typically used

• adjust for population being studied, e.g. infants, patients

Acquisition parameters for

diffusion weighted images (DWIs)

• stroke

• MS

• cancer

• trauma

• dyslexia

• epilepsy

• schizophrenia

• drug effects

Applications of diffusion MRI

• ALS

• dementia

• CJD

• cerebral palsy

• blindsight

• depression

• therapy outcome

• meditation

• neuroanatomy

• development

• aging

• surgical planning

• surgical outcome

• plasticity

• phenotype

characterization

Documents

Diffusion Tensor Imaging I: The basics...Diffusion tensor imaging (DTI) crossing fibers: •FA lower than for single direction . Diffusion tensor imaging (DTI) degeneration of one