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DTI group (Pitt) Instructor: Kevin Chan

DTI group (Pitt) Instructor: Kevin Chan. Kaitlyn Litcofsky & Toshiki Tazoe 7/12/2012. Aims. Understand basic principles of MRI Examine factors affecting DWI b -values Gradient direction Examine effect of b-values on DTI Integrate fMRI and DTI. MR signal. S = M 0.

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DTI group (Pitt) Instructor: Kevin Chan

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  1. DTI group (Pitt) Instructor: Kevin Chan KaitlynLitcofsky & ToshikiTazoe 7/12/2012

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

  3. MR signal • S = M0 • ・ (1 - e -TR/T1) • ・ (e -TE/T2) Subject 1 TE: 146 ms TE: 73 ms TE: 18 ms TR: 500 ms TR: 6000 ms

  4. MR signal and diffusion • S = M0 • ・ (1 - e -TR/T1) • ・ (e -TE/T2) • ・ e-bD = S0・ e-bD • b = diffusion gradient • D = diffusion coefficient • Gaussian distribution of diffusion • If DW signal comes from free diffusion, gradient magnetic pulse would decay DW signal mono-exponentiallywith b-value • Diffusivity across b-value decreases linearly • Diffusion coefficient across b-value is constant S = S0・ e-bD -bD= ln(S/S0) • D = ln(S/S0) / -b • Signal ln(S/S0) D • b-value • b-value • b-value

  5. Effect of b-values on DWI: free diffusion? Water phantom (NiS04.6H20/NaCL) Mean DWI (50 directions) 300 SNR≒ 1 SNR≒ 1 b=0 b=500 b=1000 b=1500 b=2000 b=2500 0 Signal, DWI ln S/S0 ADC (mm2/s) (mm2/s) b-value (s/mm2) b-value (s/mm2) b-value (s/mm2)

  6. Effect of b-values on DWI: free diffusion? Mean DWI (50 directions) 300 b=0 b=500 b=1000 b=1500 b=2000 b=2500 0 (mm2/s) (mm2/s) DWI ADC ln(S/S0) b-value (s/mm2) b-value (s/mm2) b-value (s/mm2) WM GM

  7. Effect of varied b-values on DWI: gradient direction x y DW signal at diffusion gradient (0.79, 0.61, 0.06) R L R_Optic_Radiation (Fast) L_Optic_Radiation (Slow) Fast diffusion Slow diffusion b value (s/mm2)

  8. z x y Effect of varied b-values on DTI • Fractional anisotropy: FA Axial diffusivity: λ// (mm2/s) 38% 10% λ1 25% 17% λ2 b-value (s/mm2) b-value (s/mm2) • Radial diffusivity: λ⊥ • Mean diffusivity (mm2/s) (mm2/s) λ3 21% 20% WM GM 37% 30% b-value (s/mm2) b-value (s/mm2)

  9. Effect of varied b-values on DTI Voxel-based method, 8 subjects • Tract-based Spatial Statistics • FA at b = 1000 s/mm2 and b = 2500 s/mm2 P = 0.05 P ≒ 0.00

  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

  11. DTI tractography by manual ROI Corticospinal tract Number of voxels passed through Slice 31 Slice 0 b value (s/mm2) b=500 b=1000 b=1500 b=2000 b=2500

  12. fMRI data as DTI seed regions • Compare tractographyof posterior visual pathways for upper and lower field visual stimulation at b=1000 s/mm2and b=2500 s/mm2 • fMRI vision hemifield task • Block design • Rest-Upper-Rest-Lower • 12 s blocks, 6 repetitions • TR = 2000ms • TE = 26ms • 8 subjects 2. fMRI analysis • FSL FEAT 3. Create masks for DTI from fMRI activation maps Rest Upper field stimulation Lower field stimulation

  13. fMRI data as DTI seed regions b = 1000 s/mm2 Upper visual field stimulation b = 2500 s/mm2 Lower visual field stimulation

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

  15. Conclusions • 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 • Lower b-values (at ~1000 s/mm2 or 1/ADC) may be more beneficial for evaluating DTI metrics given the higher SNR and potentially smaller errors in estimation (Jones & Basser, 2004) • Higher b-values (e.g., 2500 s/mm2) 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)

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

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