Elsevier

NeuroImage

Volume 33, Issue 1, 15 October 2006, Pages 154-160
NeuroImage

Quantitative analysis along the pyramidal tract by length-normalized parameterization based on diffusion tensor tractography: Application to patients with relapsing neuromyelitis optica

https://doi.org/10.1016/j.neuroimage.2006.03.055Get rights and content

Abstract

In this study, we introduced a length-normalized parameterization method to establish anatomical correspondence of white matter fiber tracts across subjects and applied this method to investigate the presence of abnormal diffusion along the pyramidal tract (PYT) of relapsing neuromyelitis optica (RNMO) patients without visible brain lesions. In this approach, the part of the PYT between the lowest slice of the cerebral peduncle and the uppermost slice of the lateral ventricle was reconstructed to establish the anatomical correspondence across subjects using diffusion tensor tractography. Then it was parameterized by normalizing its length and dividing equally the normalized length into a certain number of segments, so that the comparability of each segment across subjects along the PYT was established. Tract-specific diffusion indices, including directionally averaged diffusivity (Dav), fractional anisotropy (FA), primary diffusivity (λ1) and transverse diffusivity (λ23), were obtained from each segment. Thus, the distribution maps of these indices along the PYT were obtained. The distribution maps of Dav, FA, and λ23 of RNMO patients were significantly different from those of healthy controls, especially in the lower part of the PYT. The differences may be caused by secondary degeneration to lesions in the spinal cord. In conclusion, a length-normalized parameterization method is proposed to establish anatomical correspondence for the PYT. Compared with existed methods, a major merit of our method is to provide comparability across subjects along the PYT on the basis of diffusion tensor tractography and to make it possible for the quantitative analysis along the fiber tract. This method can also be used to quantitatively analyze other white matter fiber tracts between two definite anatomic landmarks in many neurological or psychiatric diseases.

Introduction

Diffusion tensor tractography is a non-invasive imaging method with the potential to visualize in vivo brain white matter fiber tracts such as the corpus callosum, cingulum and pyramidal tract (PYT), in three-dimensional (3D) space. The streamline method, a popular tractography algorithm, exploits each voxel's principal diffusion direction to follow the trajectories of a fiber tract in 3D space from voxel to voxel (Basser et al., 2000, Conturo et al., 1999, Mori et al., 1999). Tractography is capable of visualizing the shape, location, and topology of a fiber tract and has been applied in a wide range of clinical and scientific studies (Bammer et al., 2003, Berman et al., 2004, Hoon et al., 2002, Lee et al., 2005, Nguyen et al., 2005, Witwer et al., 2002, Zhang et al., 2004).

Compared with tract visualization, tract-based quantitative measurements can provide more information about microstructural changes in brain tissue. Some authors considered a fiber tract as a 3D region of interest (ROI) and then analyzed its diffusion indices (Berman et al., 2005, Glenn et al., 2003, Lin et al., 2005, Pagani et al., 2005, Partridge et al., 2005). This method provides global information for the tract, but it fails to show the regional information along the fiber tract. Thus, Fillard et al. (2003) developed a method for quantitative measurements along fiber tracts. However, because of the lack of anatomical correspondence across subjects, the reliability of this method is weakened during population analysis.

Therefore, in this study, we introduced a length-normalized parameterization method to establish anatomical correspondence of white matter fiber tracts and applied this method to investigate the presence of abnormal diffusion along the PYT between the lowest slice of the cerebral peduncle and the uppermost slice of the lateral ventricle in relapsing neuromyelitis optica (RNMO) patients without visible brain lesions. For this purpose, we followed several steps. First, to establish the anatomical correspondence of PYT across subjects, we selected the lowest slice of the cerebral peduncle as the origin and the uppermost slice of the lateral ventricle as the terminus. Then the PYT between these two anatomical landmarks for each subject was reconstructed on the basis of diffusion tensor tractography. Finally, the part of the PYT was parameterized by normalizing its length and dividing equally the normalized length into a certain number of segments. In this way, the comparability of each segment across subjects along the PYT was established. Tract-specific diffusion indices, including directionally averaged diffusivity (Dav), fractional anisotropy (FA), primary diffusivity (λ1), and transverse diffusivity (λ23), were obtained from each segment. Thus, the distribution maps of these diffusion indices along the PYT were obtained. And then the tract-based statistical analysis was carried out.

Section snippets

Subjects

In this study, sixteen patients (15 females and 1 male) with a relapsing course satisfied the proposed diagnostic criteria of NMO (Wingerchuk et al., 1999). They satisfied all absolute criteria and at least one major supportive criterion or two minor supportive criteria. All these RNMO patients had normal brain MRI scans. Their mean age was 35.4 ± 11.8 years (range, 19.0–55.0 years), and the mean duration of the disease was 4.7 ± 2.4 years (range, 1.0–8.0 years). Their mean Expanded Disability

Results

Friedman test, with the group (patient versus control) as an inter-subject factor and the side (left versus right) as an intrasubject factor, was carried out for each segment. There were no significant differences by side effect or interaction (group-by-side) effect for all segments and for all these four diffusion indices. Therefore, the values derived from the left and right PYT were averaged to obtain the FA, Dav, λ1, and λ23 of the corresponding segment for each subject. Fig. 2 shows the

Discussion

In this study, a length-normalized parameterization method was proposed to establish anatomical correspondence of white matter fiber tracts and was applied to investigate the presence of abnormal diffusion along the PYT of RNMO patients without visible brain lesions. The statistical results indicated that RNMO patients had abnormal diffusion along the PYT, especially in the lower part of it. Our results were obtained without correction for multiple comparisons. With strict Bonferroni

Conclusion

In conclusion, a length-normalized parameterization method was proposed to establish anatomical correspondence for white matter fiber tracts. The main merit of our method is to provide for comparability across subjects along the PYT on the basis of diffusion tensor tractography and to make quantitative analysis along the fiber tract possible. Our method has been applied to investigate abnormal diffusion along the PYT of RNMO patients without visible brain lesions. The proposed method may also

Acknowledgments

The authors are highly grateful to the anonymous reviewers for their significant and constructive remarks and suggestions, which are greatly helpful in improving the paper. The authors also thank Drs. Edmund F. and Rhoda E. Perozzi of Beijing University of Technology for extensive English language assistance. This work was partially supported by the Natural Science Foundation of China, Grant Nos.30570509, 30425004, 60121302 and 30470519, and the National Key Basic Research and Development

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