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DTI group (Pitt) Instructor: Kevin Chan Kaitlyn Litcofsky & Toshiki Tazoe 7/12/2012

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Aims 1.Understand basic principles of MRI 2.Examine factors affecting DWI 1.b-values 2.Gradient direction 3.Examine effect of b-values on DTI 4.Integrate fMRI and DTI

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TE: 146 ms TE: 73 ms TE: 18 ms TR: 500 msTR: 6000 ms MR signal S = M 0 ・ (1 - e -TR/T1 ) ・ (e -TE/T2 ) Subject 1

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MR signal and diffusion S = M 0 ・ e -bD b = diffusion gradient D = diffusion coefficient = S 0 ・ e -bD ・ (1 - e -TR/T1 ) ・ (e -TE/T2 ) Gaussian distribution of diffusion b-value Signal S = S 0 ・ e -bD -bD = ln(S/S 0 )D = ln(S/S 0 ) / -b If DW signal comes from free diffusion, gradient magnetic pulse would decay DW signal mono-exponentially with b-value Diffusivity across b-value decreases linearly Diffusion coefficient across b-value is constant ln(S/S 0 ) D b-value

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Effect of b-values on DWI: free diffusion? b=500b=1000b=1500b=2000b=2500 Mean DWI (50 directions) b= Water phantom (NiS04.6H20/NaCL) Signal, DWIln S/S0ADC b-value (s/mm 2 ) (mm 2 /s) SNR ≒ 1

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Effect of b-values on DWI: free diffusion? b=500b=1000b=1500b=2000b=2500 Mean DWI (50 directions) b= b-value (s/mm 2 ) (mm 2 /s) DWI ln(S/S 0 ) ADC WM GM

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Slow diffusion Fast diffusion RL x y DW signal at diffusion gradient (0.79, 0.61, 0.06) Effect of varied b-values on DWI: gradient direction R_Optic_Radiation (Fast)L_Optic_Radiation (Slow) b value (s/mm 2 )

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x y z λ1λ1 λ2λ2 λ3λ3 Fractional anisotropy: FAAxial diffusivity: λ // Radial diffusivity: λ ⊥ Effect of varied b-values on DTI WM GM b-value (s/mm 2 ) (mm 2 /s) 10% 17% 38% 25% 30% 21% Mean diffusivity b-value (s/mm 2 ) (mm 2 /s) 20% 37%

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Effect of varied b-values on DTI Voxel-based method, 8 subjects – Tract-based Spatial Statistics – FA at b = 1000 s/mm 2 and b = 2500 s/mm 2 P = 0.05 P ≒ 0.00

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DTI tractography Inputs – Principal vector – FA Tractography: FACT method – DTIStudio – Fiber Assignment by Continuous Tracking (FACT) approach Start/Stop tracking threshold: FA = 0.2 Turn threshold: 70 degrees

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DTI tractography by manual ROI b=1000b=2500b=500b=1500b=2000 Number of voxels passed through b value (s/mm 2 ) Slice 31 Slice 0 Corticospinal tract

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fMRI data as DTI seed regions Compare tractography of posterior visual pathways for upper and lower field visual stimulation at b=1000 s/mm 2 and b=2500 s/mm 2 1.fMRI vision hemifield task – Block design Rest-Upper-Rest-Lower 12 s blocks, 6 repetitions – TR = 2000 ms – TE = 26 ms – 8 subjects 2.fMRI analysis – FSL FEAT 3. Create masks for DTI from fMRI activation maps Rest Upper field stimulation Lower field stimulation

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fMRI data as DTI seed regions Upper visual field stimulation Lower visual field stimulation b = 1000 s/mm 2 b = 2500 s/mm 2

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fMRI data as DTI seed regions b=1000b=2500 b=1000b=2500 b=1000b=2500 b=1000b=2500 # of voxels Mean FA n=8

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1.Diffusion is not free / Gaussian-distributed in the brain – b-values and direction of gradient affects DWI – b-values affect DTI metrics Caution has to be taken when interpreting brain DWI/DTI metrics at different b-values 2.Lower b-values (at ~1000 s/mm 2 or 1/ADC) may be more beneficial for evaluating DTI metrics given the higher SNR and potentially smaller errors in estimation (Jones & Basser, 2004) 3.Higher b-values (e.g., 2500 s/mm 2 ) may be more beneficial for tractography given higher number of voxels traced, likely as a result of greater sensitivity in detecting smaller fibers (Rane, Nair & Duong, 2010) Conclusions

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Thank you! Dr. Kevin Chan Dr. Seong-Gi Kim Dr. Bill Eddy Tomika Cohen Rebecca Clark MNTP program

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