Diffusion Tensor Imaging I: The basics. Jennifer Campbell

Transcription

1 Diffusion Tensor Imaging I: The basics Jennifer Campbell

2 Diffusion Tensor Imaging I: The basics Jennifer Campbell

3 Diffusion Imaging

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

5 T1 & T2 relaxation in CNS at 3 T T 1 relaxation T 2 relaxation WM CSF 0.6 GM 0.6 Mz CSF Mxy GM WM Time (s) 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

6 Diffusion MRI measures Brownian motion of water molecules Path of diffusing water molecule Water displacement distribution

7 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

8 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

9 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

10 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

11 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

12 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

13 Diffusion MRI Tractography

14 Diffusion MRI Tractography

15 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

16 The diffusion tensor

17 The diffusion tensor e 1

18 The diffusion tensor e 1

19 The diffusion tensor e 1

20 The diffusion tensor e 1

21 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

22 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 l 3 2 )

23 Diffusion tensor imaging (DTI) cylindrical symmetry

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

25 Diffusion tensor imaging (DTI) normal white matter

26 Diffusion tensor imaging (DTI) myelin degeneration: radial diffusivity increases FA decreases

27 Diffusion tensor imaging (DTI) normal white matter

28 Diffusion tensor imaging (DTI) axonal fragmentation: axial diffusivity decreases FA decreases

29 Diffusion tensor imaging (DTI) normal white matter

30 Diffusion tensor imaging (DTI) decreased axon density: radial and axial diffusivity increase FA decreases

31 Diffusion tensor imaging (DTI) normal tissue

32 Diffusion tensor imaging (DTI) cellular swelling: MD decreases

33 Diffusion tensor imaging (DTI) curvature: FA lower than for single direction

34 Diffusion tensor imaging (DTI) splay: FA lower than for single direction

35 Diffusion tensor imaging (DTI) crossing fibers: FA lower than for single direction

36 Diffusion tensor imaging (DTI) degeneration of one fiber population in crossing case: FA increases

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

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

39 Measuring diffusion with MRI: diffusion weighted images (DWIs) large magnetic field gradients sensitize the image to diffusion in the direction of the gradient

40 Diffusion weighted MRI sequence TE echo G G b value indicates the magnitude of the diffusion weighting: δ Δ b = g 2 G 2 d 2 (D -d / 3)

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

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

43 Acquisition parameters for diffusion weighted images (DWIs) at least one image with no diffusion encoding (b=0 s/mm 2 ) series of DWIs: encoding directions; minimum 6 for tensor full brain coverage: minutes per 100 directions b value: 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

44 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