Regular articleWhite matter fiber tracking in patients with space-occupying lesions of the brain: a new technique for neurosurgical planning?
Introduction
A significant proportion of brain tumors and space-occupying lesions originate within and propagate throughout the white matter of the brain. Magnetic resonance imaging (MRI) of the brain is the method of choice for characterization and determination of lesion anatomical location. Conventional MRI techniques typically used for radiological assessment and localization include T2-weighted, contrast-enhanced T1-weighted and FLAIR imaging (Gillespie and Jackson, 2000). This imaging information is then used to determine the appropriate course of therapy and is often used for neurosurgical planning if lesion resection is required.
A major disadvantage of current MRI methods is that they provide little information about the integrity and location of white matter tracts in the brain. Functional MRI or direct stimulation of the cortex can be used to localize important cortical areas for neurosurgical planning Mueller et al., 1996, Roux et al., 1999. However, these techniques provide limited information concerning the status of the white matter structures, particularly those in deep white matter. Knowledge of the structural integrity and location of white matter tracts is crucial, however, as damage to clinically eloquent pathways during surgery can have devastating consequences for the patient. Informed assessment of the structural integrity and orientation of white matter pathways that may be involved or affected by a lesion may facilitate their exclusion from the resection volume leading to improved outcomes.
Diffusion tensor imaging (DTI) is a magnetic resonance technique that is sensitive to the diffusion of water in brain tissue (Basser et al., 1994). Specifically, spatial characteristics of the diffusion displacements reveal the anisotropy and orientation of white matter tracts in the brain (Pierpaoli et al., 1996). This orientation information can then be used to delineate white matter pathways of the brain by employing so-called fiber tracking algorithms which compare local tensor field orientations from voxel to voxel Basser et al., 2000, Mori et al., 1999, Jones et al., 1999, Poupon et al., 2000, Conturo et al., 1999.
Fiber tracking using DTI has generated a great deal of interest because it offers the only method for tracking the white matter pathways of the brain in vivo. The technique has hitherto been largely restricted to studies of the healthy human brain and has provided demonstrations of white matter tract structures that compare favorably with long established atlases of brain anatomy (Catani et al., 2002).
Previous investigations in patients with space-occupying lesions have employed diffusion-weighted imaging to highlight cortico-spinal tract trajectory (Coenen et al., 2001), DTI to demonstrate local abnormalities in tract orientation Wieshmann et al., 2000, Witwer et al., 2002, and more recently fiber tracking to illustrate displacement of cortico-spinal tract, superior longitudinal fasciculus, and corona radiata Mori et al., 2002, Holodny et al., 2001.
In this study we have applied in vivo fiber tracking to investigate white matter tract orientation in four patients with space-occupying lesions of the brain. In so doing we have set out to investigate the potential of fiber tracking for neurosurgical planning. Distortion of frontal white matter, corpus callosum, and cortico-spinal tracts is demonstrated. It is also shown that coherent fiber tracking pathways can be produced within the lesion volume indicated on T2-weighted MRI, suggesting that diffusion tensor fiber tracking may also serve as a technique for detecting intact axonal pathways within the lesion volume which could potentially be resected in surgery. Current limitations of the technique and suggested directions for further developments are discussed.
Section snippets
MRI data acquisition
Preoperative imaging was performed on a 1.5-T General Electric Signa MRI system with a maximum field gradient strength of 22 mT m−1. A quadrature head coil was used for transmission and reception of the NMR signal. DTI data were obtained using a single-shot echo planar imaging (EPI) sequence with diffusion sensitizing gradients positioned either side of the 180° refocusing pulse. The degree of diffusion weighting (b value) was set to 1000 s mm−2. Following an acquisition without diffusion
Patient 1
The fiber tracking results indicated disruption of the superior longitudinal fasciculus (SLF) by the suspected glioma. Tracking of the SLF was generated from a single seed ROI defined from a coronal FA map. The FA threshold was set to 0.01, angular threshold to 18°, and vector step length to 0.7 mm. The tracking results are illustrated in Fig. 1. The tracking indicates that the SLF is displaced superiorly and is disrupted in the tumor region. Tracking also revealed that disruption of the SLF
Discussion
This study demonstrates that fiber tracking techniques can be used to reveal the trajectories of white matter pathways in patients with space-occupying lesions. The results described here complement those of Mori et al. (2002), who used the FACT fiber tracking algorithm (Mori et al., 1999) in two patients with anaplastic astrocytoma. In one of the cases studied, displacement of the corona radiata and SLF was demonstrated.
In the present study fiber tracking indicated a disruption and
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