Transcript Slide 1
DTI Basics – Water Diffusion Einstein on Brownian Motion 1905 five important papers Why MRI : Detection of Acute Stroke “Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center Conventional T2 WI DW-EPI Sodium ion pumps fail, water goes in cells and can not diffuse. Tumor T2 (bright water) T1 + Gadolinium DWI (x direction) (T2 (bright water)+(diffusion)) Why MRI (more recently): Fiber Tracking 1st level of complexity Diffusion Weighted Image X direction Higher diffusion in X direction lower signal Artifact or Abnormality David Porter - November 2000 Pulse Sequence: Gradient-Echo Diffusion Weighting in X direction Excitation RF 90o G Gx G - diffusion gradients Gy Gz EPI (T2) Image Acquisition Gradient Coils y Time RF Gx x - Gy Gz NO DIFFUISION protons (Hydrogen) with DIFFUISION signal loss T2 + diffusion T2 Time RF Gx Gy Gz (gradient strength) - T2 DIFFUSION MAPS (used to remove spin density, T1, T2, TR, and TE effects) S=S0e(-bDxx) Ln(S) = Ln(S0) – bDxx or Dxx = (Ln(S0) – Ln(S))/b Dxx Most Important image S0 (T2* EPI) weak S (T2* EPI + weak Diffusion in X direction) 2nd Level of complexity DWI : 3 Direction single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Higher diffusion lower signal Dxx Dzz Dyy courtesy of Dr Sorensen, MGH, Boston David Porter - November 2000 A Little More Detail Z Diffusion-Weighting X Diffusion-Weighting y PE FE x SS z GFE x GPE y GSS z RF Y DiffusionWeighting Apparent Diffusion Coefficient ADC (AKA TRACE) used in clinical stroke, tumor, etc ADC = (Dxx + Dyy + Dzz)/3 Orientation independent No directional information (ie direction of greatest diffusion) 3rd level of complexity Diffusion Tensor Imaging Basics Measures water diffusion in at least 6 directions – we use 12 Echo-planar imaging (fast acquisition) Collecting small voxels (1.8x1.8x3mm), scanning takes about 10 minutes Higher diffusion lower signal • Useful for following white matter tracts Higher diffusion lower signal Isotropic Anisotropic Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455 DTI ellipsoid measure 6 directions to describe z x no diffusion y Ellipsoid Image Information available through DTI Tract Pierpaoli and Basser, Toward a Quantitative Assessment of Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996) Tract tracing problems (fiber crossing) Tractography Superior view color fiber maps Lateral view color fiber maps Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf. axial sag MRISC cor Diffusion Tensor Imaging data for cortical spinal tract on right side blue = superior – inferior fibers green = anterior – posterior fibers red = right – left fibers Note tumor is darker mass on left side of axial slice But what is a diffusion tensor? no diffusion It is a mathematical description of the ellipsoid. xy -xy y-z xz Dxx Dxy Dxz Dyx Dyy Dyz Dzx Dzy Dzz z y-z z x y -xz What is diffusion “Tensor” (D)? difussion gradient direction vector y S=S0exp(-bD) lab reference frame Dxx Dxy Dxz Dyx Dyy Dyz Dzx Dzy Dzz ellipsoid reference frame = (mathematical manipulation) Dx’x’ 0 0 0 Dy’y’ 0 0 0 Dz’z’ Calculate FA (fractional anisotropy) Fiber track FA (fractional anisotropy) Information available through DTI Dy’y’ Dx’x’ Dz’z’ FA = ((Dx’x’-Dav)2 + (Dy’y’-Dav)2 + (Dz’z’-Dav)2)0.5 s av2)0.5 (Dx’x’2+Dy’y’2+Dz’z’ FA = 0.9 FA = 0 FA (fractional anisotropy) MAP FA + color (largest diffusion direction) red = right – left green = anterior – posterior blue = superior - inferior probably no use Tractography Superior view color fiber maps Lateral view color fiber maps Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf. Signal loss : by intra-voxel phase dispersion At the echo time TE, NMR signal is decayed by, - T2 decay (spin-spin diffusion) - diffusive motion G For any set of diff. gradient pulses G S(TE, bij ) Soe TE / T2 e bij Dij bij Dij ( / )Gi DijG j 90 180 b ( / )G 2 echo TE DTI Scalar Parameters Trace: Magnitude of diffusion in a voxel. – Increases in damaged white matter Fractional Anisotropy (FA): Measure of directionallyrestricted diffusion. – Decreases in damaged white matter Rosenbloom M, et al. (July 2004). NIAA pubs; http://www.niaaa.nih.gov/publications/arh27-2/146-152.htm The Diffusion Tensor, D Diffusion is not equal in all directions (anisotropic). Use this to probe brain structure! Represent the diffusion pattern at each point in the brain using an ellipsoid. Diffusion Vector (Colour) Map The three magnitudes of the diffusion ellipsoid can be shown using three colours (RGB). – Red = Left – Right – Green = Ant. – Pos. – Blue = Sup. – Inf. Map of major directions of water movement in the brain. Fibre Tractography In principle, the locations of major white matter fibre tracts in the brain can be mapped using the information in the colour map, by “following the arrows.” Hindered Diffusion (diffusion ellipsoid) without hindrance WILSON with hindrance Information available through DTI – Orientation of λ1 • Useful for following white matter tracts Information available through DTI -- Aσ 1 s ( D) As D 2 av • Related to the shape of the ellipsoid • Independent of Dav (normalized) • Zero for a sphere, positive for other shapes • Sensitive to myelination and cortical development Normal Adult Brain (As) Diffusion Tensor Imaging (As maps) Higher diffusion lower signal CELL FREELY DIFFUSING WATER IN EXTRA-CELLULAR SPACE EXTRA-CELLULAR SPACE Tissue Sample A Freely Diffusing Water = Dark Larger D Tissue Sample B Restricted Diffusion = Bright Smaller D K-space view of the spin echo imaging Ky 1 2 3 . . . . . . . n Kx Diff. Grad. along different axis X Diffusion-Weighting PE FE SS GFE GPE GSS RF Y DiffusionWeighting Z Diffusion-Weighting DTI (Diffusion Tensor Imaging) single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Higher diffusion lower signal courtesy of Dr Sorensen, MGH, Boston David Porter - November 2000 Detection of Acute Stroke “Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center Conventional T2 WI DW-EPI Sodium ion pumps fail, water goes in cells and can not diffuse. Tumor T2 (bright water) T1 + Gadolinium DWI (x direction) (T2 (bright water)+diffusion) Do X, Y , and Z at the same time Excitation RF 90o G Gx G - Image Gy Gz Acquisition The Diffusion Tensor, D Diffusion is not equal in all directions (anisotropic). Use this to probe brain structure! Diffusion ellipsoid for each voxel z x y TE / T2 bij Dij S(TE, bij ) Soe e bij Dij ( / )Gi DijG j b ( / )G 2 DTI Scalar Parameters Trace: The magnitude of diffusion in a voxel. Fractional Anisotropy (FA): The extent to which diffusion is directionally restricted.