NS191B - All About DTI

NS191B - All About DTI - All About DTI All NS191B DTI...

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Unformatted text preview: All About DTI All NS191B DTI Basics – Water Diffusion DTI (DTI – Diffusion Tensor Imaging) Einstein on Brownian Motion 1905 five important papers K-space view of the spin echo imaging Ky 1 2 3 . . . . . . . n Kx Why USE DTI 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 Imaging DW-EPI Sodium ion pumps fail - water goes in cells and can not diffuse – DW image gets bright (note – much later cells burst and stroke area gets very dark) Why USE DWI MRI? Tumors T2 (bright water) T2 (bright water) DWI (x direction) (T2 (bright water)+(diffusion)) Contrast (T1 + Gadolinium) Pulse Sequence: Gradient-Echo Pulse Diffusion Weighting in X direction Excitation RF 90o G Gx - G EPI (T2) Image Acquisition Gy diffusion gradients Gz Do X, Y , and Z at the same time Excitation RF 90o G Gx - G Image Gy Acquisition Gz Diff. Grad. along different axis X Diffusion-Weighting Diffusion-Weighting Y DiffusionWeighting Z Diffusion-Weighting PE FE SS A Little More Detail Z Diffusion-Weighting X Diffusion-Weighting Diffusion-Weighting y Y DiffusionWeighting PE FE SS z x x y z Example: Diffusion Weighted Image X direction Example: Higher diffusion in X direction lower signal Artifact or Abnormality T2 + diffusion T2 RF Gx Gy Gz Excite Sequence Time - T2 Image Regular T2 image Measure diffusion (gradient strength) single­shot EPI diffusion­weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Measuring Diffusion in other directions (examples) Higher diffusion lower signal Dzz Dxx Dyy How can we track white matter How fibers using DTI? fibers Measures Measures water diffusion in at least 6 directions directions Echo-planar imaging (fast acquisition) Collecting small voxels (1.8 x 1.8 x 3mm), Collecting scanning takes about 10 minutes scanning Higher diffusion lower signal water Diffusion ellipsoid Diffusion ellipsoid White matter fibers • Useful for following white matter tracts in healthy brain Higher diffusion lower signal White matter fibers Isotropic Anisotropic Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455 The Diffusion Tensor, D The Diffusion Diffusion is not equal in all directions (anisotropic). (anisotropic). Use this to probe brain structure! Represent the diffusion pattern at each point in Represent the brain using an ellipsoid. the DTI Ellipsoid measure 6 directions to describe z no diffusion y x Ellipsoid represents magnitude of diffusion in all directions by distance from center of ellipsoid to its surface. DTI Scalar Parameters DTI Trace: Magnitude of Magnitude diffusion in a voxel. diffusion Increases Increases in damaged white matter matter Fractional Anisotropy (FA): Measure of directionallyMeasure restricted diffusion. Decreases Decreases in damaged white matter matter Rosenbloom M, et al. (July 2004). NIAA pubs; http://www.niaaa.nih.gov/publications/arh27-2/146-152.htm Apparent Diffusion Coefficient ADC (AKA TRACE) used in clinical stroke, tumor, etc ADC = (Dxx + Dyy + Dzz)/3 Orientation independent X Diffusion-Weighting Y Diffusion Weighting Z Diffusion-Weighting No directional information (i.e. direction of greatest diffusion) Information available through DTI – Orientation of λ1 • Useful for following white matter tracts 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 σ (D 2+D 2+Dav2)0.5 x’x’ y’y’ z’z’ FA = 0.9 FA = 0 Diffusion Tensor Imaging (FA) Diffusion Normal Adult Brain (Aσ maps) FA + color FA (largest diffusion direction) (largest red = right – left green = anterior – posterior blue = superior - inferior axial cor sag 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 Why DTI MRI (more recently): Fiber Tracking Fiber Tractography Fiber In In principle, the locations of major white matter fiber tracts in the brain can be mapped using the information in the Dinformation matrix matrix decomposition map, by “following the arrows.” arrows.” Ellipsoid Image Information available through DTI Tract Pierpaoli and Basser, Toward a Quantitative Assessment of Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996) Tractography 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. ...
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This note was uploaded on 01/14/2010 for the course NS 191B taught by Professor Vanhorn during the Spring '09 term at UCLA.

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