Kaitlyn A. Litcofsky 1 , Toshiki Tazoe 2 , Kevin C. Chan 3

1. z x y Assessing the effects of diffusion MR parameters on diffusion weighted imaging, diffusion tensor imaging and diffusion tensor tractography x y Kaitlyn A. Litcofsky1, Toshiki Tazoe2, Kevin C. Chan3 λ1 1Department of Psychology, Pennsylvania State University, 2Systems Neuroscience Institute, University of Pittsburgh, 3Center for the Neural Basis of Cognition, University of Pittsburgh, Carnegie Mellon University λ2 • Effect of b-value on DWI: • Restricted diffusion Aims fMRI data as DTI seed regions Examine effect of b-value and gradient direction on DWI Examine effect of b-value on DTI Integrate fMRI and DTI λ3 Rest Mean DWI (50 directions) fMRI task: Visual hemifield stimulation 300 • MR signal and diffusion Lower field stimulation • Block design • Rest-Upper-Rest-Lower • 12s blocks, 6 repetitions • TR: 2000 ms, TE: 26 ms • 8 healthy subjects • S = M0 • ・ (1 - e -TR/T1) • ・ (e -TE/T2) • ・e-bD • b = diffusion gradient • D = diffusion coefficient Upper field stimulation b=0 b=500 b=1000 b=1500 b=2000 b=2500 0 • Gaussian distribution of diffusion • If DW signal comes from free diffusion, gradient magnetic pulse would decay DW signal mono-exponentiallywith b-value (mm2/s) DWI ADC ln(S/S0) Tractography based on fMRI seed regions S = S0・ e-bD -bD= ln(S/S0) • D = ln(S/S0) / -b WM Upper visual field stimulation Lower visual field stimulation GM • Signal ln(S/S0) D b-value (s/mm2) b-value (s/mm2) b-value (s/mm2) • Effect of gradient direction on DWI • b-value • b-value • b-value R_Optic_Radiation (Fast) L_Optic_Radiation (Slow) DW signal at diffusion gradient (0.79, 0.61, 0.06) • DWI acquisition parameters b = 1000 s/mm2 b = 2500 s/mm2 R L • Spin echo EPI sequence; Siemens 3T Allegra • Gradient directions: 50; TE: 112 ms; TR: 5400 ms • b-values: 0, 500, 1000, 1500, 2000, 2500 s/mm2 # of voxels Mean FA n=8 * * Fast diffusion Slow diffusion • Effect of b-value on DWI: • Free diffusion b-value (s/mm2) • Effect of b-value on DTI Water phantom (NiS04.6H20/NaCL) Mean DWI (50 directions) Axial diffusivity: λ// • Fractional anisotropy: FA b=1000 b=2500 b=1000 b=2500 b=1000 b=2500 b=1000 b=2500 300 (mm2/s) 38% 10% Conclusions b=0 b=500 b=1000 b=1500 b=2000 b=2500 0 25% 17% • Changing b values may affect DWI and 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, MagResMed2004) • 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, NMRBiomed, 2010) SNR≒ 1 SNR≒ 1 Signal, DWI ln S/S0 ADC b-value (s/mm2) b-value (s/mm2) (mm2/s) • Radial diffusivity: λ⊥ • Mean diffusivity: MD (mm2/s) (mm2/s) 20% 21% 37% 30% b-value (s/mm2) b-value (s/mm2) b-value (s/mm2) Water phantom shows pattern of free diffusion for b=500 s/mm2 to b=1500 s/mm2. At higher b-values, there is not sufficient signal-to-noise ratio. b-value (s/mm2) b-value (s/mm2)