Diffusion Tensor Imaging Protocol

After obtaining informed consent in accordance with our institutional guidelines, we scanned five healthy right-handed male volunteers aged between 24 and 32 y (mean = 29.4, S.D. = 3.4). Imaging was performed on an Achieva 3T Philips scanner using a diffusion weighted single-shot EPI sequence with a TR of 4,200 ms and a TE of 89 ms. The maximum diffusion gradient intensity was 80 mT/m, the gradient duration δ was 32.5 ms and the diffusion time Δ was 43.5, yielding a maximal b-value of 9,000 s/mm². Q-space was sampled over 129 points located inside a hemispherical area of a cubic lattice, by varying the diffusion gradient intensity and direction such that q = aq_x + bq_y + cq_z, (where a, b, and c are integers such that
≤ 4; q_x, q_y, and q_z denote the unit diffusion sensitizing gradient vectors in the three respective coordinate directions; and q = γδg_, where γ is the gyromagnetic ratio and g is the gradient strength (mT/m). The axial field of view was set to 224 by 224 mm and the acquisition matrix was 112 by 112, yielding an in-plane resolution of 2 × 2 mm. Parallel imaging was used with our eight-channel head coil with a reduction factor of 3. 36 contiguous slices of 3-mm thickness were acquired in two blocks resulting in an acquisition time of 18 minutes. In addition, a high resolution T1-weighted gradient echo sequence was acquired in a matrix of 512 × 512 × 128 voxels of isotropic 1-mm resolution.
Data reconstruction was performed according to a DSI protocol [26 ,27 (link),51 (link)]. In every brain position, the diffusion probability density function (PDF) was reconstructed by taking the discrete 3D Fourier transform of the signal modulus symmetric around the center of q-space. The signal was pre-multiplied by a Hanning window before Fourier transformation in order to ensure smooth attenuation of the signal at high |q| values. The 3D PDF was normalized by dividing by its integral at every voxel. The orientation distribution function (ODF) φ was derived directly from the PDF by taking a radial summation of the 3D PDF p(r):
where ρ is the radius and u is a unit direction vector. The integral was evaluated as a discrete sum over the range ρ ∈ [0,5]. The ODF is defined on a discrete sphere and captures the diffusion “intensity” in every direction. It was evaluated for a set of vectors u_i that are the vertices of a tessellated sphere with mean nearest-neighbor separation approximately 10°. The result was a diffusion map composed of ODFs at every location in the brain. The ODFs were represented as deformed spheres with the radius proportional to φ(u).