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AJNR - American Journal of Neuroradiology

3D fiber tracking of the fornix (green) and cingulum (orange) overlaid on an anatomical, T1-weighted 3D–magnetization-preparation rapid gradient echo (MPRAGE) volume. The anatomy of these 2 fiber bundles can be thought as 2 nested semicircles. The tracts shown were selected by brute force for display purposes only. The labeled portions of the tracts (indicated by yellow lines for the cingulum and black lines for the fornix) were analyzed subsequently by using judicious tract selection regions, as shown in Figs 2 and 3.

The fornix was selected in 3 distinct portions, because this enhances the selection accuracy. For each portion, tract-selection regions were manually drawn on either FA or principal diffusivity color maps. To be selected, the tracts had to go through the middle tract-selection region (B, F, and J, green boxes) and through either one of the extreme regions (A or C; E or G; I or K, for the crus, body, and columns, respectively). The FA maps presented are derived from CSF-suppressed DTI. The same procedure was repeated for the standard DTI datasets by using identical tract selection regions. The resulting FLAIR DTI-derived tracts of a healthy subject are presented in 3D views overlaid on anatomic 3D-MPRAGE axial and sagittal sections (D, H, and L). Subsequent analyses of all segments were performed only between the 2 extreme tract selection regions. A, anterior; P, posterior; ac, anterior commissure; fc, fusion of the crura; sCCi, inferior border of the splenium of the corpus callosum. Panel E is 0.5 cm posterior to the vertical AC line. Panel K is parallel to the inferior border of the body of the fornix.

Three portions of the cingulum—namely, the descending, superior, and anterior portions—were selected after fiber tracking of the entire volume in both standard and FLAIR DTI datasets. For each portion, tract-selection regions were manually drawn on either FA or principal diffusivity color maps. To be selected, the tracts had to go through the middle tract-selection region (B, F, and J, orange boxes) and through either one of the extreme regions (A or C; E or G; I or K, for the descending, superior, and anterior portions, respectively). The FA maps presented are derived from CSF-suppressed DTI. The same procedure was repeated for the standard DTI datasets by using identical tract selection regions. The resulting FLAIR DTI-derived tracts of a healthy subject are presented in 3D views overlaid on anatomic 3D-MPRAGE axial and sagittal sections (D, H, and L). Subsequent analyses of all segments were performed only between the 2 extreme tract selection regions. A, anterior; P, posterior; ac, anterior commissure; bf, body of fornix; gCCp, posterior border of the genu of the corpus callosum; sCCa, anterior border of the callosal splenium; gCCi, inferior border of the callosal genu.

Because of CSF signal intensity contamination, the crus of the fornix is difficult to depict with fiber tracking derived from standard DTI datasets. Tissue voxels belonging to the top portion of the crus of the fornix (yellow arrow) suffer partial volume averaging and their diffusion anisotropy drops below the fiber tracking FA threshold (0.3) to values that range from 0.15 to 0.25. When CSF suppression is performed, the same region contains higher anisotropy values (0.30–0.40) and its voxels are included by the tracking algorithm, which results in a complete depiction of the structure. The axial nondiffusion-weighted images (b = 0 s/mm²) show the bright CSF signal intensity in standard DTI, which is suppressed with FLAIR DTI. The portions of the fornix are overlaid on T1-weighted, anatomical 3D-MPRAGE axial and sagittal sections.