Post on 29-May-2020
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
Diffusion MRI measures Brownian motion
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
Diffusion MRI measures Brownian motion
of water molecules
• Hindered and restricted diffusion: Apparent Diffusion
Coefficient (ADC)
• Diffusion is “hindered” outside cells and “restricted”
inside cells
Diffusion MRI measures Brownian motion
of water molecules
• Hindered and restricted diffusion: Apparent Diffusion
Coefficient (ADC)
• Diffusion is “hindered” outside cells and “restricted”
inside cells
Diffusion MRI measures Brownian motion
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
Tissue structures determine which
directions of motion are most probable
White matter fiber
bundle: oriented
structure
Causes of anisotropic diffusion
in white matter:
• myelin
• axon membranes
• neurofilaments
Diffusion MRI Tractography
Diffusion MRI Tractography
Tissue structures determine which
directions of motion are most probable
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
The diffusion tensor
1
3
2
e1
The diffusion tensor
1
3
2
e1
The diffusion tensor
1
3
2
e1
The diffusion tensor
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
Diffusion tensor imaging (DTI)
λ||, axial
cylindrical symmetry
λ , radial
T
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
myelin degeneration:
•radial diffusivity increases
•FA decreases
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
axonal fragmentation:
•axial diffusivity decreases
•FA decreases
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
decreased axon density:
•radial and axial diffusivity increase
•FA decreases
normal tissue
Diffusion tensor imaging (DTI)
cellular swelling:
•MD decreases
Diffusion tensor imaging (DTI)
Diffusion tensor imaging (DTI)
curvature:
•FA lower than for single direction
Diffusion tensor imaging (DTI)
splay:
•FA lower than for single direction
Diffusion tensor imaging (DTI)
crossing fibers:
•FA lower than for single direction
Diffusion tensor imaging (DTI)
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