Slide 1

DTI group (Pitt) Instructor: Kevin Chan Kaitlyn Litcofsky & Toshiki Tazoe 7/12/2012 Slide 2 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 Slide 3 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 Slide 4 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 Slide 5 Effect of b-values on DWI: free diffusion? b=500b=1000b=1500b=2000b=2500 Mean DWI (50 directions) b=0 300 0 Water phantom (NiS04.6H20/NaCL) Signal, DWIln S/S0ADC b-value (s/mm 2 ) (mm 2 /s) SNR 1 Slide 6 Effect of b-values on DWI: free diffusion? b=500b=1000b=1500b=2000b=2500 Mean DWI (50 directions) b=0 300 0 b-value (s/mm 2 ) (mm 2 /s) DWI ln(S/S 0 ) ADC WM GM Slide 7 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 ) Slide 8 x y z 11 22 33 Fractional anisotropy: FAAxial diffusivity: // Radial diffusivity: 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% Slide 9 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 Slide 10 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 Slide 11 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 Slide 12 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 Slide 13 fMRI data as DTI seed regions Upper visual field stimulation Lower visual field stimulation b = 1000 s/mm 2 b = 2500 s/mm 2 Slide 14 fMRI data as DTI seed regions b=1000b=2500 b=1000b=2500 b=1000b=2500 b=1000b=2500 # of voxels Mean FA n=8 Slide 15 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 Slide 16 Thank you! Dr. Kevin Chan Dr. Seong-Gi Kim Dr. Bill Eddy Tomika Cohen Rebecca Clark MNTP program