Figure 1
From: Quantitative evaluation of intraorbital optic nerve in optic atrophy using diffusion tensor imaging
Overview of diffusion tensor imaging (DWI) analysis. (A) Preprocessing of raw DWI data. A set of 50 images was acquired from each participant, including 48 sampling directions and two b0 images of anteroposterior (AP) and posteroanterior (PA) phase-encoding directions (i). One corrected b0 image was generated, and the entire diffusion data was applied by estimating artifacts from two pairs of b0 images (ii). Finally, a set of 49 images was co-registered to the T1 image (iii). (B) Diffusion tensor model. To represent the optic nerve in the axial (i), sagittal (ii), and coronal (iii) views on the FA image, diffusion directionality was color-coded with green representing anterior–posterior directions, red representing left–right directions, and blue representing superior-inferior directions for a single subject using FSL eyes. A close-up view of 3 × 3 voxels of the optic nerve behind the eye globe in the axial (iv), three voxels in the sagittal (v), and three voxels in the coronal views (vi). The diffusion tensor D was estimated from a set of DWIs. The three orthotropic axes of a diffusion ellipsoid can be determined by the decomposition of the tensor into eigenvectors and eigenvalues (vii). (C) Tensor-derived diffusion measures. Cropped image of the fractional anisotropy (FA) (i), mean diffusivity (MD) (ii), axial diffusivity (AD) (iii), and radial diffusivity (RD) (iv). FA values are bounded between zero (a perfect sphere) and one (an infinitely long cigar shape). MD is the mean of the eigenvalues of the diffusion tensor. AD indicates an eigenvalue of λ1, which is diffusivity along the principal axes, while RD is diffusivity along non-principal axes by averaging the eigenvalues of λ2 and λ3. (D) Optic nerve fiber tractography. Two spherical regions of interest (ROIs) are placed posterior to the globe of the eye and in the orbital apex at the center of the annulus of Zinn to identify the optic nerve pathway. Placement of the two ROIs overlaid on T1 (i) and FA image (ii). Visualization for cleaned optic nerve tractography (iii) and a central fiber in the optic nerve passing through the ROI (iv).
