Post on 29-May-2020
Introduction to Diffusion Tensor Imaging
November 2010
Jennifer Campbell
Introduction to Diffusion Tensor Imaging
• Molecular diffusion• Diffusion weighting in MRI• The diffusion tensor in use• Problems and solutions
Introduction to Diffusion Tensor Imaging
• Molecular diffusion• Diffusion weighting in MRI• The diffusion tensor in use• Problems and solutions
Diffusion MRI measures Brownian motion of water molecules
Path of diffusing water molecule
Water displacement distribution
Diffusion MRI measures Brownian motion of water molecules
−−•
−
=d
T
d
drrDrr
DrrP
τπττ
4)()(exp
)4(1),|( 0
10
30 dDr τ6; 2 >=<
Diffusion MRI measures Brownian motion of water molecules
Restricted diffusion: Apparent Diffusion Coefficient (ADC)
Diffusion MRI measures Brownian motion of water molecules
Restricted diffusion: Apparent Diffusion Coefficient (ADC)
Diffusion MRI measures Brownian motion of water molecules
−−•
⋅
−
=d
T
d
drrrrrrP
τπττ
4)((ADC))(exp
)4(ADC1),|( 0
10
30
Tissue structures determine which directions of motion are most probable
White matter fibre bundle: oriented
structure
Water molecules prefer to travel parallel to fibre
direction
Tissue structures determine which directions of motion are most probable
−−•
−
=d
T
d
drrrrrrP
τπττ
4)()(exp
)4(||1),|( 0
10
30
DD
The diffusion tensor
λ1
λ3λ2
e1
Introduction to Diffusion Tensor Imaging
• Molecular diffusion• Diffusion weighting in MRI• The diffusion tensor in use• Problems and solutions
Diffusion weighted MR sequenceTE
G
90° 180° echo
G
Stejskal-Tanner diffusion weighted sequence: pulsed-gradient spin-echo (PGSE)
)3/(222 δδγ −∆= Gb
∆δ
bDoeSbS −=)(
bDoeSbS −=)(
b
)(bS
Diffusion weighted MR sequence
B0B0 + 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 B0B0
Diffusion weighted images (DWIs)
• series of DWIs: 3-6-60-100+ encoding directions• Full brain coverage: 15-20 minutes per 100 directions • b value: 1000-3000 s/mm2
• Voxel size: roughly 2mm x 2mm x 2mm isotropic voxels typically used
Acquisition parameters for diffusion weighted images (DWIs)
Sequence modifications: trace weighted sequence
GGx
RF
90° 180°
G
GyG G
GzG G
Computing the diffusion tensor
Computing the diffusion tensor
λ1
λ3λ2
e1
Computing the diffusion tensor
FA: anisotropy
index
trace of diffusion tensor
RGB plot: principal
eigenvector (e1) direction, scaled
by FA
Maps obtainable from diffusion tensor
)()()()(
23FA 2
32
31
23
22
21
2 λλλλλλλλλ
++−+−+−
=
Diffusion tensor imaging (DTI)
λ1
λ2∼λ3
cylindrical symmetry
Diffusion tensor imaging (DTI)
λ⊥, radial
λ||, axial
cylindrical symmetry
Introduction to Diffusion Tensor Imaging
• Molecular diffusion• Diffusion weighting in MRI• The diffusion tensor in use• Problems and solutions
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
myelin degeneration:•radial diffusivity ↑•FA ↓
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
axonal fragmentation:•axial diffusivity ↓•FA ↓
Diffusion tensor imaging (DTI)
normal white matter
Diffusion tensor imaging (DTI)
decreased axon density:•radial diffusivity ↑•FA ↓
Diffusion tensor imaging (DTI)
crossing fibres:•FA lower than for single direction
Diffusion tensor imaging (DTI)
degeneration of one fibre population in crossing case:•FA ↑
Diffusion tensor imaging (DTI)
curvature:•FA lower than for single direction
Diffusion tensor imaging (DTI)
splay:•FA lower than for single direction
normal tissue
Diffusion tensor imaging (DTI)
cellular swelling:•MD ↓
Diffusion tensor imaging (DTI)
Fibre tracking using diffusion MRI data
Diffusion MRI data
Water displacement profile
Fibre directions
Fibre tracts
Tractography
What does tractography tell us?-is A connected to B?-how are A and B connected?-how confident are we in this
connection?
Applications of tractography:-visualization in 3D: localization,
education-segmentation (both white matter and
grey matter)-investigating clinically relevant
differences between pathways in different populations-neuroanatomical questions
Tractography
Streamline integration methods:• “fibre assignment using continuous tracking”
(FACT)• Euler• RK4
Tract delineation techniques:• tract delineating ROIs (as many as desired): drawn
manually, by protocol, or based on other data• exclusion masks, stopping criteria S. Mori et al, AN 45:265-269, 1999
Tractography: warnings
Diffusion MRI tractography results include many false positives and false negatives: priors are essential.– Tractography is good for segmentation of parts of
pathways, but segmenting the entire pathway can be challenging.
– Characterizing unknown new anatomy is much more difficult than studying known anatomy.
– Tractography does not distinguish between afferent and efferent pathways, or between mono- and multisynaptic connections.
Tractography incorporating uncertainty
MamayyezSiahkal et al. 2009
Applications• basic human neuroanatomy• structure and function• understanding disease• understanding therapies• nonhuman primates; other animal
studies
• stroke• MS• cancer• trauma• dyslexia• epilepsy• schizophrenia
Applications
• ALS• dementia• CJD• surgical planning• surgical outcome• blindsight• depression
• neuroanatomy• development• aging
Introduction to Diffusion Tensor Imaging
• Molecular diffusion• Diffusion weighting in MRI• The diffusion tensor in use• Problems and solutions
Diffusion weighted signal intensity: high value in directions perpendicular to fibres
Diffusion weighted signal profile
Probability of water displacement orientation distribution function (ODF)
Diffusion tensor model
High Angular Resolution Diffusion Imaging (HARDI)
q-ball imaging
QBI DTI
High Angular Resolution Diffusion Imaging (HARDI)
Why use high angular resolution diffusion MRI?• Reduce false positive and false negative tractography results due to
crossings• In voxels where fibres cross, single fibre approaches can yield:
– ambiguous fibre direction– incorrect fibre direction– only one fibre direction
(that of fascicle with largest volume fraction) • Examples of such regions:
– basis pontis, subcortical white matter, superior longitudinal fasciculus, acoustic radiations, projection from subgenual white matter to amygdala, optic chiasm, caudate nucleus, corpus callosum, cortical spinal tract, cingulate bundle.
Crossing fibre reconstruction approaches
• multi-tensor approaches (Alexander et al., Parker et al., others)
• multi ball and stick (Behrens et al.)
• Composite hindered and restricted model of diffusion (CHARMED) (Assaf et al.)
• diffusion spectrum imaging (DSI) (Wedeen et al.)
• q-ball imaging (QBI) (Tuch et al.)
• spherical deconvolution (Tournier et al., Anderson, others)
• other variants
DTI vs. HARDI :Tractography in phantom
single fibre multi-fibreCampbell et al. ISBI 2006.
Tractography: false negatives
Lateral connections?
Signal to noise ratio (SNR); spatial resolution
(b=3000 s/mm2)(b=1000 s/mm2)
Jumping from tract system to tract system
Effects of limited spatial resolution
Jumping from tract system to tract system
Effects of limited spatial resolution
Heidemann et al. 2010.
Signal to noise ratio (SNR); spatial resolution
Higher field strength (7T)
• diffusion MRI acquisitions generally use fast imaging techniques which can create artifacts such as magnetic susceptibility-induced distortion
• these artifacts confound registration with standard structural scans
Magnetic susceptibility induced distortion
Magnetic susceptibility induced distortion
Heidemann et al. 2010.
segmented readout
Eddy-current induced distortion
• eddy currents are induced by the rapidly changing magnetic field gradients required for diffusion sensitization
• these eddy currents induce magnetic fields which distort the image.
artifacts: shifting, sheering, stretching, compression, signal loss
TE
G G
90° 180° echo
G
180°
G
Twice refocused balanced echo (TRBE)
Eddy-current induced distortion
Suggested review articles• Susumu Mori et al. Principles of diffusion tensor imaging and its
application to basic neuroscience research. Neuron 51, 527-539, 2006.
• Basser et al. Diffusion-tensor MRI: theory, experimental design and data analysis – a technical review. NMR in Biomedicine 15, 456-467, 2002.
• Mori et al. Fiber tracking: principles and strategies - a technical review. NMR in Biomedicine 15, 468-480, 2002.
• Johansen-Berg et al. Just pretty pictures? What diffusion tractography can add in clinical neuroscience. Curr Opin Neurol 19, 379-385, 2006.
Software packages
• mincdiffusion (BIC)• mincDTI (Andrew Janke - BIC)• FSL (FMRIB, Oxford)• DTI Studio (Johns Hopkins)• Camino (Manchester/London)• CINCH (Stanford)• Mrtrix (Brain Research Institute, Melbourne)• more …
Acknowledgements
G. Bruce PikeKaleem SiddiqiIlana LeppertPeter SavadjievParya Mamayyez Siahkal Steven FreyIves LevesqueLuis Concha
Nikola StikovClarisse MarkEva Alonso OrtizHalleh GhaderiMike FerreiraSilvain BeriaultChristine Tardif