Diffusion Tensor Imaging by Wim Van Hecke Louise Emsell & Stefan Sunaert

Author:Wim Van Hecke, Louise Emsell & Stefan Sunaert
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Higher Order Model Tractography

The diffusion tensor m odel assumes a single-fibre bundle traversing through a voxel. It has however been estimated that up to 70 % of all white matter voxels contain contributions of at least two bundles [13]. Fortunately, despite this model error, it is possible to successfully reconstruct the core structure of major bundles, which is sufficient for preclinical research studies investigating groups of subjects. Note however, this important limitation means that DTI tractography is not the most appropriate method for presurgical planning (see Chaps. 15 and 21).

As stated in Chap. 21, a large number of higher order models for complex fibre configurations have been proposed. Estimating higher order models requires higher quality data with a greater number of diffusion gradient directions and higher b-values compared to classical DTI (see Chap. 6). This type of acquisition is commonly referred to as high angular resolution diffusion imaging or HARDI . A higher b-value of b > 1500 s/mm2 and >60 gradient directions are preferred, compared to the typical b = 1000 s/mm2 and 30 directions for DTI. This increases the scan time beyond 8 min per average. Previously this has proved an obstacle for clinical research; however, it is now possible to obtain good quality HARDI data in under 15 min.

Figure 11.7 presents an example of tractography using CSD in the pons of a patient with an embryonic defect in axonal guidance [38]. The patient showed symptoms of reduced coordination of muscle movements (ataxia), eye movements (nystagmus), sensory deafness and episodes of hypothermia. Common interhemispherical connections were diminished in this patient (data not shown); instead an additional transversal bundle was observed, connected to an abnormal projection out of the brain stem.

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