Isotropic resolution diffusion tensor imaging with whole brain acquisition in a clinically acceptable time

Abstract

Our objective was to develop a diffusion tensor MR imaging pulse sequence that allows whole brain coverage with isotropic resolution within a clinically acceptable time. A single-shot, cardiac-gated MR pulse sequence, optimized for measuring the diffusion tensor in human brain, was developed to provide whole-brain coverage with isotropic (2.5 x 2.5 x 2.5 mm) spatial resolution, within a total imaging time of approximately 15 min. The diffusion tensor was computed for each voxel in the whole volume and the data processed for visualization in three orthogonal planes. Anisotropy data were further visualized using a maximum-intensity projection algorithm. Finally, reconstruction of fiber-tract trajectories i.e., "tractography" was performed. Images obtained with this pulse sequence provide clear delineation of individual white matter tracts, from the most superior cortical regions down to the cerebellum and brain stem. Because the data are acquired with isotropic resolution, they can be reformatted in any plane and the sequence can therefore be used, in general, for macroscopic neurological or psychiatric neuroimaging investigations. The 3D visualization afforded by maximum intensity projection imaging and tractography provided easy visualization of individual white matter fasciculi, which may be important sites of neuropathological degeneration or abnormal brain development. This study has shown that it is possible to obtain robust, high quality diffusion tensor MR data at 1.5 Tesla with isotropic resolution (2.5 x 2.5 x 2.5 mm) from the whole brain within a sufficiently short imaging time that it may be incorporated into clinical imaging protocols.

Figure 1

Fractional anisotropy data obtained from subject A in axial orientation. Every third slice is presented from the 60 slice data set. The data have been masked from the background.

Figure 2

Demonstration of the effect of reducing slice thickness from (A) 5 mm to (B) 2.5 mm on image sharpness for Subject A. Note that the edges of anisotropic structures are more sharply defined in (B), due to a reduction in the partial volume effect.

Figure 3

Results of reformatting the axially acquired data presented in Figure 1 into coronal plane (A) and sagittal plane (B). Note the absence of artifacts in the original slice‐selection direction (superior‐inferior), which is due to minimal slice cross‐talk resulting from a long TR.

Figure 4

Fractional anisotropy data obtained from Subject B. Note the enlarged ventricles and reduction in white matter volume when compared to the data obtained from Subject A in Figure 1.

Figure 5

Examples of simultaneous visualization of fractional anisotropy obtained from Subject A in three orthogonal planes. The benefits of visualization in this format are best appreciated at http://www.mridiffusion.com

Figure 6

Maximum intensity projection reconstruction of the fractional anisotropy data obtained from Subject A. Cine loops of this data can be viewed at http://www.mridiffusion.com

Figure 7

Two different views of white matter tract trajectories reconstructed from the DTI data obtained from Subject A. The pathways of reconstructed white matter tract trajectories are displayed using tubes of constant radius. Trajectories initiated from seedpoints placed in the cortico‐spinal tracts are shown in red, and those initiated from seedpoints placed in the body of the corpus callosum are shown in green. The three intersecting planes show the fractional anisotropy and serve as a useful frame of reference when viewing the reconstructed fiber tract trajectories.