Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Tensor MR Imaging & White Matter Tracking Elias R. Melhem, M.D., Ph.D. Professor of Radiology and Neurosurgery Chief, Division of Neuroradiology Hospital of the University of Pennsylvania Overview q Definitions: – Diffusion Tensor Imaging (DTI) q DTI – Data Acquisition – Data Processing (Tractography) q Applications q Advances Definitions 1
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Principle Tissue differences in the diffusivity of water molecules (slow incoherent motion) can be brought out by adding strong motion-sensitizing gradients to the MR sequence Ln(S/S 0 ) = -bD diffusion coefficient dependent on sensitizing gradients Pulsed Diffusion Gradients 90 ο 180 ο echo RF G G Pulsed Gradients SS / FE / PE δ δ ∆ b = γ 2 G 2 δ 2 ( ∆ - δ /3) The Effect of Bipolar Gradients on the M xy of Tissues with Random Motion (Brownian) SI SI X SI G dephasing G time Pulsed Gradients rephasing 2
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete The Effect of Bipolar Gradients on the M xy of Tissues with Reduced Random Motion T2 - decay SI SI SI SI G time dephasing G Pulsed Gradients rephasing Isotropic vs. Anisotropic Diffusion Isotropic Anisotropic Diffusion Anisotropy: Orientation of Fiber Tracts in White Matter b = 560 b = 560 Direction of diffusion-sensitizing gradients 3
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Tensor Imaging Diffusion Tensor q In anisotropic media (WM) diffusion can be characterized by a Tensor q When the Tensor is determined for every voxel: Diffusion Tensor Imaging Basser et al. 1994 J Magn Reson Diffusion Tensor ( Mxx Mxy Mxz ) Myx Myy Myz Mzx Mzy Mzz Diagonalize λ xx ( 0 0 ) λ yy 0 0 λ zz 0 0 4
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Tensor q Eigenvalues: shape of the ellipsoid q Trace ( λ xx + λ yy + λ zz): volume of the ellipsoid q Euler angles: direction of the ellipsoid Diffusion Ellipsoid Map Data Acquisition 5
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Data Acquisition DTI: 2D multi-shot or single-shot SE-EPI – Cardiac-gated – Navigator-echo-corrected – TR/TE:5000 – 6000 / 92 – Number of Averages: ~ 4 – FOV:24 x 24 cm 2 - Slice thickness: 3mm (no gap) – Matrix:64 x 64 (zero-filled 128) – Diffusion: 6 to 40 directions – Two b-values (0 and 800 s/mm 2 ) D-WI Using 6 Different Directions x y z xy xz yz i b=0 Data Processing Step 1: DTI images realigned using the AIR program Step 2: Elements of the Tensor obtained with multi-variant linear square fitting Step 3: Eigenvalues maps determined by diagnolizing the Tensor Step 4: Principal eigenvectors used for vector maps Step 5: Fractional anisotropy (FA) and Trace calculation Step 6: Tracking Algorithm and Color Maps 6
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Tensor Imaging: Eigen Maps λ xx λ yy λ zz Diffusion Tensor MR Imaging Σ ( λ i – D) 2 = Trace FA ( λ i ) 2 Σ Fractional Anisotropy & Vector Maps 7
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete White Matter Tractography B A * * C Thresholds: FA > 0.6 Angle < 40 o White Matter Tractography: DTI corticospinal tract cingulum corpus callosum superior longitudinal fasciculus inferior longtudinal fasciculus uncinate fasciculus optic radiation anterior inferior occipitofrontal thalamic radiation fasciculus Brain Stem Tractography: DTI 8
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Applications Stroke Transient Left Sided Weakness:: Relationship of Infarct to Corticospinal Tract 9
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete White Matter Diseases q Biophysical basis for Diffusion Anisotropy in WM q Biophysical basis for Reduced Diffusivity in Demyelinating Diseases Adrenoleukodystrophy (ALD) q X-linked disorder that affects brain white matter q Most severe phenotype: Cerebral Form q Cerebral form has 3 distinct pathologic: I- Vasogenic edema and active demyelination II- Inflammatory reaction and disruption of BBB III- Axonal destruction and cavitation Diffusion Anisotropy: ALD FA T2-W ADC Itoh et al. Neurology 10
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Diffusion Anisotropy: ALD FA Average ADCi (SD) FA (SD) NAWM 0.66 (0.12) 0.52 (0.16) Area I 0.84 (0.16) 0.31 (0.12) Area II 1.36 (0.19) 0.22 (0.07) Area III 1.73 (0.15) 0.13 (0.05) Itoh et al. ISMRM 2000 DTI and MR Spectroscopic Imaging Eichler et al. Radiology in-press ALD: MR Tractography Using DTI Mori et al. ISMRM 2000 1200 1 Fractional Anisotropy ( ) 0.9 3 ) ( ) 1000 0.8 0.7 800 genu ALD Normal 0.6 Volume (mm 600 0.5 0.4 400 Volume 0.3 0.2 200 Splenium 0.1 0 0 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 Anterior Slice position ( colonal ) Posterior 11
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Decreased ADC?? q Cytotoxic Edema + Demyelination q Irreversible axonal damage q Predictor of Activity q Predictor of Progression Acute Demyelination: Multiple Sclerosis Chronic Demyelination: Multiple Sclerosis 12
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Acute Disseminated Encephalomyelitis T2 D-WI ADC Central Pontine Myelinolysis T2-W SE D-WI ADC Asphyxia in Premature Infants 13
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Periventricular Leukomalacia q Selective vulnerability of the periventricular white matter in the premature neonate to ischemic , infectious , or metabolic insults q Volume of lateral ventricles is strongly related to severity of spastic cerebral palsy Melhem et al. Radiology Periventricular WM Restricted Diffusion Acute Follow up ADC MPRAGE Periventricular Leukomalacia Normal PVL 14
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Periventricular Leukomalacia Normal PVL Hoon et al. Neurology Developmental Brain Anomalies Holoprosencephaly q Spectrum of complex congenital brain and facial malformations resulting from defective cleavage of the developing forebrain during the first five weeks of embryonic development q HPE is divided into four major categories Alobar Semilobar Lobar Middle hemispheric variant 15
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Holoprosencephaly CPST MLF SCP MCP ICP Alobar Absent Fused Present Present Present (n=2) Semilobar Absent Present Present Present Present (n=5) Lobar Present Present Present Present Present (n=2) Melhem et al. Radiology Alobar Holoprosencephaly Semilobar Holoprosencephaly Normal 16
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Syntelencephaly FA FA Focal Cortical Dysplasia Band Heterotopia 17
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Tumors MR Tractography in Brain Tumors MR Tractography in Brain Tumors Tumor B more infiltrative than Tumor A Mori et al. MRM 2002 18
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Amytrophic Lateral Sclerosis (ALS) ALS Disease of Lower and often Upper Motor Neurons q Pre-clinical markers of disease q Markers of disease progression q Ability to identify and quantify upper motor neuron involvement ALS Control ALS with UMN involvement 19
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Advances q Faster Acquisitions – Parallel Imaging (SENSE-SMASH) q Number and Orientation of Diffusion Gradients q Greater SNR – High field Scanner – Better receiver coils q Improvement in Tracking Methods q Spatial Normalization Faster Acquisitions DTI with SENSE 1.8 minutes per average! 256x256x60/ SENSE: 2 20
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Number of Direction of Diffusion Gradients 30 Diffusion Directions � Jones 30 � : 2 averages / direction; 256x256x40 / SENSE: 2 4 minutes per average! 3D Isotropic DTI 21
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete High Resolution Isotropic 3D DTI Vector Map Anisotropy Map FA Voxel: 0.8 mm 3 Fiber Tracking Fiber Tracking: Diffusion Spectrum Technique kissing fibers A B C A B C Higher Spatial Resolution intertwining fibers A A B B Higher Spatial Resolution D D C C Wiegell et al. Radiology 2000 22
Click Here & Upgrade Expanded Features PDF Unlimited Pages Documents Complete Connectivity of Corticospianl Tracts Using Probabilistic Approach Callosal Connectivity Using Probabilistic Approach Spatial Normalization of WM Tracts 23
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