Tim trio 3t mr system

All MRI procedures were performed at the Research Imaging Institute, University of Texas Health Science Center at San Antonio using the same Siemens Tim Trio 3T MR system (Erlangen, Germany) equipped with a multichannel head coil using a protocol described elsewhere (Wijtenburg et al., 2010). Imaging sessions were conducted approximately 1‐year apart (±30 days) for the youth cohort and 24–48 hr apart for the adult cohort. A single‐shot, echo‐planar, single refocusing spin‐echo sequence was used to acquire diffusion‐weighted data with a spatial resolution of 1.7 × 1.7 × 3.0 mm. The sequence parameters were as follows: TE/TR = 83/7000 ms, FOV = 200 mm, two diffusion‐weighting values, b = 0 and 700 s/mm², and five b₀ (nondiffusion‐weighted) images, 64 isotropically distributed diffusion‐weighted directions, and axial slice orientation with 50 slices and no gaps. The number of directions, b₀ images, and the magnitude of the b values were calculated using an optimization technique that maximizes the contrast to noise ratio based on the average diffusivity of the cerebral white matter and the T₂ relaxation times (Jones, Horsfield, & Simmons, 2007). High‐resolution T1‐weighted data were also collected using an optimized protocol described previously (Kochunov et al., 1999).