Signa hdxt system

MRI scans were performed using a 1.5-Tesla GE Signa HDxt system (General Electric, Milwaukee, WI, USA) with a maximum gradient amplitude of 33 mTm^-1 and a proprietary head coil. Patients were neutrally positioned in the head coil using an alignment marker at the nasal bridge to standardise head position. Foam pads and a Velcro strap across the forehead were used to minimise head movement during the scan.
The imaging protocol lasted approximately 45 minutes and included the following scan sequences which all provided whole head coverage: I. axial Fluid Attenuated Inversion Recovery (FLAIR): repetition time (TR) = 9000ms, echo time (TE) = 130ms, inversion time (TI) = 2200ms, 28 slices of 5mm thickness without slice gap, field of view (FOV) = 240x240mm², matrix = 256x192; II. coronal T1-weighted spoiled gradient recalled echo (SPGR): TR = 11.5ms, TE = 5ms, 176 slices without slice gap, FOV = 240x240mm², matrix = 256x192, flip angle = 18°, providing 1.1mm³ isotropic voxels; and III. axial single shot spin echo planar diffusion-weighted imaging: TR = 15600ms, TE = 93.4ms, 55 slices without slice gap with isotropic voxels of 2.5mm³, FOV = 240x240 mm², matrix = 96x96, 8 non-diffusion-weighted images (b = 0 smm^-2) followed by diffusion-weighted volumes with diffusion gradients applied (b = 1000 smm^-2) in 25 non-collinear directions and the negative of these.

Among adults, we obtained 842 T1-weighted magnetic resonance imaging (MRI) scans with two 3T General Electric platforms: 411 scans were obtained using a GE 3T Signa HDxt scanner with an eight-channel head, and 431 scans using a GE 3T Discovery 750 scanner with a 32-channel head coil. Among adolescents, 85 T1-weighted MRI scans were obtained using the same scanners: 49 scans were acquired using the GE Signa HDxt system and 36 scans were acquired using the GE Discovery 750 system. The acquisition sequences used are described in online Supplementary material.
In order to obtain the TGMV, all MRI scans were processed using FreeSurfer v6.0 (Fischl, 2012 (link)). TGMV was calculated as the sum of the cerebral cortical volume, the subcortical grey matter and the cerebellum grey matter (https://surfer.nmr.mgh.harvard.edu/fswiki/MorphometryStats). Quality inspection and editing was performed by trained research assistants following standard FreeSurfer procedures (McCarthy et al., 2015 (link)).

Subjects were scanned at 3 T using T2-weighted, axial acquisition sequences with a scan time of 5 min and 8 s. MRI data was acquired on either a Siemens TrioTim system or GE Medical Systems SignaHDxt system. The resolution ranged from 0.49 mm × 0.49 mm × 2.0 mm to 0.94 mm × 0.94 mm × 3.0 mm, TR (repetition time) ranged from 3000 to 4500 ms, TE (echo time) ranged from 11 to 105.98 ms, flip angle was either 90° or 150°, and matrix size ranged from 204 × 256 × 48 voxels to 512 × 512 × 84 voxels.