Diffusion Tensor Imaging I: The basics. Jennifer Campbell

Diffusion Tensor Imaging I: The basics Jennifer Campbell

Diffusion Imaging

MRI: many different sources of contrast T1W T2W PDW Perfusion BOLD DW

T1 & T2 relaxation in CNS at 3 T T 1 relaxation T 2 relaxation 1 1 0.9 0.9 0.8 0.7 WM 0.8 0.7 CSF 0.6 GM 0.6 Mz 0.5 0.4 CSF Mxy 0.5 0.4 0.3 0.3 GM 0.2 0.2 0.1 0.1 WM 0 0 1 2 3 4 Time (s) 0 0 0.2 0.4 0.6 0.8 Time (s) WM: T 1 = 800 ms T 2 = 80 ms r = 0.7 GM: T 1 = 1300 ms T 2 = 100 ms r = 0.8 CSF: T 1 = 5500 ms T 2 = 2200 ms r = 1.0

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 æ (r - r0) T D -1 (r - r0) ö exp 3 D(4pt d) è ç 4t d 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 P(r r0,t d) = 1 æ (r - r0) T (ADC) -1 (r - r0) ö exp 3 ADC (4pt d) è ç 4t d 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 Causes of anisotropic diffusion in white matter: myelin axon membranes neurofilaments White matter fiber bundle: oriented structure

Diffusion MRI Tractography

Tissue structures determine which directions of motion are most probable P(r r0,t d) = 1 æ (r - r0) T D -1 (r - r0) ö exp 3 D (4pt d) è ç 4t d ø

The diffusion tensor

The diffusion tensor 1 3 2 e 1

Maps obtainable from the diffusion tensor anisotropy index: fractional anisotropy (FA) trace of diffusion tensor (mean diffusivity) RGB plot: principal eigenvector (e 1 ) direction, scaled by FA

Maps obtainable from the diffusion tensor anisotropy index: fractional anisotropy (FA) trace of diffusion tensor (mean diffusivity) RGB plot: principal eigenvector (e 1 ) direction, scaled by FA FA = 3 2 (l1 - l) 2 + (l 2 - l) 2 + (l 3 - l) 2 (l1 2 + l 2 2 + l 3 2 )

Diffusion tensor imaging (DTI) 1 2 3 cylindrical symmetry

Diffusion tensor imaging (DTI) λ, axial λ, radial T cylindrical symmetry

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

Diffusion tensor imaging (DTI) normal tissue

Diffusion tensor imaging (DTI) cellular swelling: MD decreases

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

Gradients, phase, and motion 180 echo G G B 0 + G B 0, B180 o 0 B 0 + G B 0

Gradients, phase, and motion 180 echo G G B 0 + G B 0, 180 o B 0 + G B 0 B 0

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 90 180 echo G G b value indicates the magnitude of the diffusion weighting: δ Δ b = g 2 G 2 d 2 (D -d / 3)

Computing the diffusion tensor Signal decrease with b value: b matrix used in 3D tensor calculation:

Acquisition parameters for diffusion weighted images (DWIs) at least one image with no diffusion encoding (b=0 s/mm 2 ) 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/mm 2 ; b~1000 s/mm 2 best for tensor voxel size: roughly 2mm x 2mm x 2mm isotropic voxels typically used adjust for population being studied, e.g. infants, patients

Applications of diffusion MRI stroke MS cancer trauma dyslexia epilepsy schizophrenia drug effects ALS dementia CJD cerebral palsy blindsight depression therapy outcome meditation neuroanatomy development aging surgical planning surgical outcome plasticity phenotype characterization