Global Connectivity estimation: Methods

Data acquisition Diffusion-weighted data were acquired with an optimized method based on echo planar imaging, implemented on a General Electric 1.5 T Signa Horizon scanner with a standard quadrature head-coil and maximum gradient strength of 22 $mT m^{-1.}$

The diffusion weighting followed an optimized scheme [21] where the diffusion weightings were isotropically distributed along 54 directions. With the diffusion parameters d and D equal to 34 and 40ms respectively, the b-value was 1150 $smm^{-2}$, the optimum for white matter DTI measurements. 6 diffusion-weighted volumes were acquired with b-value 300 $smm^{-2}$, and 6 volumes were acquired with no diffusion weighting. Each volume covered the whole brain with 60 slices of 2.3 mm slice thickness, field of view $220 \times 220 mm^2$. An imaging matrix of 96 x 96 was used, giving isotropic voxels of $2.3 \times 2.3 \times 2.3 mm^3$ and the images were reconstructed on a $128\times128$ matrix, giving a final resolution of $1.7\times 1.7\times 2.3 mm^3$. An optimized cardiac gating scheme [21] was used to minimize artifacts arising from cerebrospinal fluid pulsatile flow. The total scan time for the DTI protocol was approximately (depending on heart rate) 20 minutes.

The high resolution T1 weighted scan was obtained with a 3D inversion recovery prepared spoiled gradient echo (IR-SPGR). Parameters for the acquisition were: FOV = $310\times155$; matrix size = $256\times128$; in-plane resolution = $1.2\times1.2 mm^2$; 156 slices of 1.2mm slice thickness; inversion time = 450ms; repetition time = 2s; echo time = 53ms.