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Methods for Medical Imaging– Prof. G. Baselli 2012 Diffusion weighted MRI and DTI tractography Maria Giulia Preti

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Presentation on theme: "Methods for Medical Imaging– Prof. G. Baselli 2012 Diffusion weighted MRI and DTI tractography Maria Giulia Preti"— Presentation transcript:

1 Methods for Medical Imaging– Prof. G. Baselli 2012 Diffusion weighted MRI and DTI tractography Maria Giulia Preti

2 MRI contrasts Contrast between two tissues A and B  C AB = abs (I A – I B ) / I REF NB: MRI offers several contrast types, they dipend on weighing (T1, T2, T2*, Proton Density, Diffusion, etc.) Definition: T1 T2 Diffusion weighted imaging, DWI  Normally, acquisition sequences are designed to enhance a specific diffusion weight (e.g., T1, T2, DWI) Liquor White Matter, WM Gray Matter, GM

3 The amount of motion of water molecules diffusing within tissues is observed Molecular Diffusion MOLECULAR DIFFUSION: caotic motion of molecules, due to their thermal agitation (Brownian motion) Definition (Einstein, 1905) free diffusion = equal displacement probability in all directions ISOTROPIC DIFFUSION D = DIFFUSION COEFFICIENT (mass, viscosity, temperature) Diffusion Weighted Imaging (DWI)

4 Isotropic Diffusion Distribution of displacements Gaussian r = displacement of molecules from time t1 to time t2 Meand squard displacement in 1D In 3D: Δ = diffusion time (t2–t1)

5 Diffusion in biological tissues  In tissues, water diffusion finds barriers: it is hindered  The apparent diffusion coefficient (ADC) is lower and depends on microscopic structure Higly hindered Less hindered ISOTROPIC NON ANISOTROPIC  3D description by the Diffusion Tensor (DT)

6 Rephasing Dephasing Rephasing Dephasing Rephasing Dephasing Slice selection GzGz GyGy GxGx Phase Encoding Frequency Encoding Signal TE 90° 180° Diffusion weighted spin-echo EPI Addition of a bipolar gradient pulse Δ δ G

7 y Diffusion weighing by bipolar gradient pulse G position dependent dephasing y -G Dephasing Rephasing

8 The final phase shift of spins requires displacemnt Phase t=0 Phase t=Δ Position t=Δ Position t=0 x1=x2 (NO DIFFUSION)NO Dephase, NO signal attenuation G gradient pulse amplitude δ= duration of gradient pulse Δ = Δt between the two pulses = diffusion time γ = gyromagnetic ratio Dephasing Δ δ G Rephasing Diffusion weighing by bipolar gradient pulse

9 Rephasing Dephasing 90° 180° B0B0 Diffusion weighing by bipolar gradient pulse

10 Dephasing 90° 180° B0B0 Diffusion weighing: low diffusion

11 Rephasing Dephasing 90° 180° B0B0 Diffusion weighing: high diffusion

12 DWI Contrast DWI: MORE DIFFUSIONE  LESS SIGNAL (DARKER) b-value  DIFFUSION WEIGHING INDEX Liquor  >diffusion 

13 Stejkal andTanner’s equation Diffusion weighing in the gradient direction b=0  imge weithed byT2 only b≠0  weighted by T2 and by diffusion  DWI ADC estimate by log ratio of T2 and DWI: Signal attenuation: DWI G gradient pulse amplitude δ= duration of gradient pulse Δ = Δt between the two pulses = diffusion time γ = gyromagnetic ratio

14 Apparent Diffusion Coefficient (ADC) ADC Map  Image of diffusion voxel by voxel. A refernce (S0) and a DWI are necessary (or a low b and a high b DWI) b=0 S 0 b=1200 sec/mm² S Peri-tumoral edema area has the same intensity than other tissues ADC=-1/b ln(S/S 0 ) ADC map Edema area is enhanced

15 Diffusion Tensor Imaging (DTI) Orderly oriented structures: Preferential diffusion parallel to fibers, hindered or even restricted in the orthogonal directions. NOTE: DTI model does not distinguish restricted diff. (not Gaussian) WHITE MATTER (WM) IN THE CNS Exploration in the 3D space Description in each voxel by a 3x3 symmetric matrix: DIFFUSION TENSOR DTI NON ISOTROPIC DIFFUSION in a preferred direction along fibers

16 Calcolo del tensore di diffusione Diffusion Tensor (DT) symmetry  6 independent components  each scan requires ad at least 6 DWI acquisitions along maximally distant directions + 1 reference image (b=0) Often, more directions are acquired: 12 and more Minimal set acquisition and gradient components Least squares solution of a system of Stejkal andTanner eq. Z X Y i,j = x,y,z B ij = ( ɣ δ) 2 (Δ - δ /3) G i G j

17 1. PRINCIPAL DIFFUSION DIRECTION: eigenvector (e1) of the largest eignevalue The DT of each voxel provides the eigenvalues and eigenvectors 2. MEAN DIFFUSIVITY: diffusion averaged over all directons 3. FRACTIONAL ANISOTROPY: measure of ordered directionality Diffusion Tensor Imaging (DTI) e1e1 e2e2 e3e3 fiber Tensor eigen- vectors oriented parallel (e1) and orthogonally (e2, e3) to fibers scanner reference system Isotropic Non Isotropic

18 Reconstruction of fibers following the principal direction voxel through voxel 2 STOPPING RULES: o Minimum AF o Maximum bending angle from voxel to voxel Start from: seed points [ ROI of seed points ]  ASSUMPTION  Principal direction = average fiber orientation AF < threshold X angle > threshold X Diffusion Tensor Tractography (DTT)

19 ILF ARCUATE UNCINATE CINGULATE CORPUS CALLOSUM IFOF WHOLE BRAIN Tractography: reconstructed bundles or fascicles

20 Positioning of ROI for seed points ROI of seed points - ideally: anatomical region crossed by all fascicle fibers and not crossed by other fascicles. Locate usual on the FA map  good contrast of fbers. FA Example: 3 ROIs for identifying 3 portions of corpus callosum (CC) (genu- body-splenium)  ROIs on the central sagittal plane  CC extends from ROIs to the emispheres

21  Afferent and efferent fibers not distinguish  One single principal direction per voxel, no distinction of fibers with different directions (see below)  Partial volume effects (e.g. GM); particularly severe the effect of free water (isotropic) in edema  FA drop  fiber reconstruction stops  Low resolution for SNR and acquisition time DT Tractorgraphy limitations In case of mixed directions the principal directions is actually the average direction “kissing”, “crossing” and “diverging” fibers.


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