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

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Slide 2 Acknowledgments Contributors: Martin Styner Susumu Mori Andy Alexander Gordon Kindlmann Randy Gollub National Alliance for Medical Image Computing (NIH U54EB005149)

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Slide 3 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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Slide 5

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Slide 6 5D6Mo14Mo

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Slide 7 Networking and Brain Connectivity Major Fiber Tracts extracted from DT MRI UNC Computer Science: Network wire cabinets

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Slide 8 Diffusion Tensor Imaging (DT MRI) reveals White Matter Structure Gray matter White matter Courtesy of Susumu Mori, JHU DT MRI

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Slide 9 White Matter Structure directionGoal: Measure properties associated with the direction of white matter Fibers White matter White Matter Fibers

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Slide 10 Example: Corticospinal Tract Tractography: Coronal view 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

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Slide 11 Diffusion Random Walk of Water Molecules DT-MRI A. Alexander

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Slide 12 Diffusion Diffusion: Brownian motion of one material through another Anisotropy: diffusion rate depends on direction Gordon Kindlimann Kleenex newspaper

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Slide 13 Biological Restricted Diffusion S extra >> S intra Diffusion influenced by mean free path –Tortuosity DT-MRI A. Alexander

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Slide 14 Biological Restricted Diffusion Cellular degeneration (necrosis) - Diffusion increases DT-MRI A. Alexander

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Slide 15 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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Slide 17

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Slide 18 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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Slide 19 (An)isotropic diffusion Free diffusion Restricted diffusion Isotropic diffusion Anisotropic diffusion Courtesy of Susumu Mori, John Hopkins University Medical School Probability Distribution

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Slide 20 DWI (indirectly) senses the structure of the tissue by measuring water molecule displacement along a chosen direction. r r' y diffusion coefficient in the y direction (= D y ) Start End

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Slide 21 r r' If the path of the water molecule is affected by restrictions such as cellular material, the measured diffusion coefficient is reduced extracellular space intracellular space

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Slide 22 If the tissue structures are oriented, the path of the water molecule (and the measured diffusion coefficients) will reflect this. r r' y diffusion coefficient in the y direction (= D y ) x diffusion coefficient in the x direction(= D x ) D x > D y

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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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Slide 26 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 (S 0 ) 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 b-value Signal Intensity D ln S S 0 2 G D= - bD

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Slide 28 Measuring D for a Given Direction: Simplified model of two b values (b=0 and b=nnnn) b-value ln(S) slope = D intercept = S

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DWI and ADC 1 G/cm 6 G/cm 10 G/cm 13 G/cm b-value Signal Intensity

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Slide 30 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. Increasing b-value Images courtesy: Susumu Mori (JHU)

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Slide 31 Apparent Diffusion Coefficient (ADC) Map with Different Measurement Direction YXZ Only the diffusion along a gradient direction can be measured Courtesy of Susumu Mori, John Hopkins University Medical School Gradient direction

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ASNR 2003 –Washington,DC DT-MRI Alexander Diffusion Weighted Images 12 DW encoding directions S i (b=912 sec/mm 2 ) T2W Reference S o (b ~ 0 sec/mm 2 ) Courtesy JE Lee

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Slide 33 Measurement along Multiple Directions Modified from DavidTuch, MGH 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)

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Slide 34 DWI: Three Coordinate Systems right anterior superior World: e.g. RAS fast=I medium=J slow=K Image: IJK Gradients: g 1 = (1,0,1) g 2 = (1,-1,0) … D xx, D xy … x y z Image Orientation Measurement Frame

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

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Slide 36 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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Slide 37 Simplification and assumption Diffusion ellipsoid Courtesy of Susumu Mori, John Hopkins University Medical School Orientational Diffusion Fct

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ASNR 2003 –Washington,DC DT-MRI Alexander The Diffusion Tensor Courtesy JE Lee

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National Alliance for Medical Image Computing DWI summary: MRI 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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National Alliance for Medical Image Computing DWI crash course: Model Single Tensor Model (Basser 1994) A0A0 gigi AiAi D Dxx Dxy Dxz Dyy Dyz Dzz Tensor estimation

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Slide 41 Anisotropy & Color-coded Orientation Isotropic GM Anisotropic WM Courtesy of Susumu Mori, John Hopkins University Medical School

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Slide 42 DTI Tensor Visualization Color: FA value ITK: DTIFiberTubeSpatialObject & SpatialObjectViewers (Julien Jomier)

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Here comes Ross Whitaker

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