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**MR-DTI: Non-invasive imaging of neuroanatomy of white matter Guido Gerig**

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**Acknowledgments Contributors: Martin Styner Susumu Mori Andy Alexander**

Gordon Kindlmann Randy Gollub National Alliance for Medical Image Computing (NIH U54EB005149)

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Use of these slides Slides were borrowed from various researchers, and we are working on getting permissions for distribution. Slides can be used for own purposes. Please do not distribute these slides. Please do no put slides into public download space.

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T1w T2w

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5D 6Mo 14Mo

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**Networking and Brain Connectivity**

Major Fiber Tracts extracted from DT MRI UNC Computer Science: Network wire cabinets

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**Diffusion Tensor Imaging (DT MRI) reveals White Matter Structure**

Gray matter White matter Courtesy of Susumu Mori, JHU

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**White Matter Structure**

Goal: Measure properties associated with the direction of white matter Fibers White Matter Fibers White matter

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**Example: Corticospinal Tract**

Source: Duke NeuroAnatomy Web Resources (Christine Hulette) B: Superior longitudinal fasciculus C: Superior occipitofrontal fasciculus D: Cingulum E: Inferior longitudinal fasciculus F: Inferior occipitofrontal fasciculus Tractography: Coronal view

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**Diffusion Random ‘Walk’ of Water Molecules t1 t2 t3**

DT-MRI A. Alexander

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**Diffusion Diffusion: Brownian motion of one material through another**

Anisotropy: diffusion rate depends on direction Kleenex newspaper Gordon Kindlimann

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**Biological Restricted Diffusion**

Sextra >> Sintra Diffusion influenced by mean free path Tortuosity DT-MRI A. Alexander

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**Biological Restricted Diffusion**

Cellular degeneration (necrosis) - Diffusion increases DT-MRI A. Alexander

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**Aniostropic Restricted Diffusion**

Diffusion has angular dependence DT-MRI A. Alexander

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**How can we measure diffusion without perturbing the system?**

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**Diffusion and white matter**

Diffusion MRI measures diffusion of mainly water molecules Isotropic medium → molecules move with Brownian motion. In biological tissues diffusion is often anisotropic In white matter: “Local structure” Insulating myelin sheet, low probability to cross into axon Dense axon bundles exhibits strongly directional local structure Diffusion along fiber bundle is main diffusion direction Myelin sheet Nodes of Ranvier Main diffusion direction

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**(An)isotropic diffusion**

Free diffusion Isotropic diffusion Probability Distribution Restricted diffusion Anisotropic diffusion Probability Distribution Courtesy of Susumu Mori, John Hopkins University Medical School

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DWI (indirectly) senses the structure of the tissue by measuring water molecule displacement along a chosen direction. r' End diffusion coefficient in the y direction (= Dy) y r Start

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If the path of the water molecule is affected by restrictions such as cellular material, the measured diffusion coefficient is reduced intracellular space r r' extracellular space

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If the tissue structures are oriented, the path of the water molecule (and the measured diffusion coefficients) will reflect this. r' diffusion coefficient in the y direction (= Dy) y r diffusion coefficient in the x direction(= Dx) x Dx > Dy

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**Magnetic Resonance Imaging (MRI)**

Larmor Frequency Magnetic Field Gradient, G

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**Diffusion Weighted (DW) MRI**

Accumulated Phase

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DW-MRI II Attenuation!

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**The pixel signal intensity, S, is related to the **

b-value and the diffusion coefficient, D, through: This equation (Steyskal Tanner Equation) has two unknowns, the signal intensity for b = 0 (S0) and D. Therefore, at least 2 measurements must be made, each at a different b-value to calculate D.

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**Equation for the diffusion attenuation**

G S ln 2 G 2 2 D = - bD S 3 Signal Intensity D b-value

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**Measuring D for a Given Direction: Simplified model of two b values**

(b=0 and b=nnnn) intercept = S0 b-value 1000 slope = D ln(S)

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DWI and ADC 1 G/cm G/cm 10 G/cm G/cm Signal Intensity b-value

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The b-value is the contrast “knob” in a diffusion experiment and is varied in magnitude and in a specified number of directions. Increasing the b-value increases the contrast between slow and fast diffusing water molecules. Images courtesy: Susumu Mori (JHU) Increasing b-value

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**Apparent Diffusion Coefficient (ADC) Map with Different Measurement Direction**

Gradient direction X Y Z Only the diffusion along a gradient direction can be measured Courtesy of Susumu Mori, John Hopkins University Medical School

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**Diffusion Weighted Images**

T2W Reference So (b ~ 0 sec/mm2) 12 DW encoding directions Si (b=912 sec/mm2) ASNR 2003 –Washington,DC DT-MRI Alexander Courtesy JE Lee

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**Measurement along Multiple Directions**

Diffusion MRI measures along single gradient directions Diffusion Weighted Images (DWI) In principle: Arbitrary gradient directions 6 different directions → Tensor 12/24 directions → stability Diffusion Tensor Imaging (DTI) High angular acquisition Sampling of orientation diffusion Higher order representations (fiber crossings) Qball (D. Tuch, MGH), >256 dirs Others: Van Wedeen (MIT), Frank (UCSD) Modified from DavidTuch, MGH

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**DWI: Three Coordinate Systems**

fast=I medium=J slow=K Image: “IJK” Gradients: g1 = (1,0,1) g2 = (1,-1,0) … Dxx, Dxy … x y z right anterior superior World: e.g. “RAS” “Image Orientation” “Measurement Frame”

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**Measured Apparent Diffusivities**

12 encoding directions ASNR 2003 –Washington,DC DT-MRI Alexander Courtesy JE Lee

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**What is “Diffusion – Weighted” Imaging?**

In “Conventional” MRI, image contrast reflects the local relaxation (T1, T2) environment of the water molecules. In “Diffusion-Weighted” Imaging (DWI), image contrast reflects the physical structure of the Tissue (via the local diffusion distribution).

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**Simplification and assumption**

Orientational Diffusion Fct Diffusion ellipsoid Courtesy of Susumu Mori, John Hopkins University Medical School

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**The Diffusion Tensor Courtesy JE Lee ASNR 2003 –Washington,DC**

DT-MRI Alexander Courtesy JE Lee

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**DWI summary: MRI newspaper Kleenex**

Diffusion: Brownian motion of one material through another Anisotropy: diffusion rate depends on direction Magnetic gradients create spatial planar waves of proton phase Destructive interference measures diffusion along gradient direction only Kleenex newspaper

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**DWI crash course: Model**

Single Tensor Model (Basser 1994) Ai Tensor estimation Dxx Dxy Dxz D A0 Dyy Dyz Dzz gi

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**Anisotropy & Color-coded Orientation**

Isotropic GM Anisotropic WM Courtesy of Susumu Mori, John Hopkins University Medical School

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**DTI Tensor Visualization**

Color: FA value ITK: DTIFiberTubeSpatialObject & SpatialObjectViewers (Julien Jomier)

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**Here comes Ross Whitaker**

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