Incorporating functional MR imaging into diffusion tensor tractography in the preoperative assessment of the corticospinal tract in patients with brain tumors

Abstract

Background and purpose: Our goal was to improve the preoperative assessment of the corticospinal tract (CST) in patients with brain tumors. We investigated whether the integration of functional MR imaging (fMRI) data and diffusion tensor (DT) tractography can be used to evaluate the spatial relationship between the hand and foot fibers of the CST and tumor borders.

Materials and methods: We imaged 10 subjects: 1 healthy volunteer and 9 patients. Imaging consisted of a 3D T1-weighted sequence, a gradient-echo echo-planar imaging (EPI) sequence for fMRI, and a diffusion-weighted EPI sequence for DT tractography. DT tractography was initiated from a seed region of interest in the white matter area subjacent to the maximal fMRI activity in the precentral cortex. The target region of interest was placed in the cerebral peduncle.

Results: In the healthy volunteer, we successfully tracked hand, foot, and lip fibers bilaterally by using fMRI-based DT tractography. In all patients, we could track the hand fibers of the CST bilaterally. In 4 patients who also performed foot tapping, we could clearly distinguish hand and foot fibers. We were able to depict the displacement of hand and foot fibers by tumor and the course of fibers through areas of altered signal intensity.

Conclusion: Incorporating fMRI into DT tractography in the preoperative assessment of patients with brain tumors may provide additional information on the course of important white matter tracts and their relationship to the tumor. Only this approach allows a distinction between the CST components, while visualization of the CST is improved when fiber tracking is hampered by tumor (infiltration) or perifocal edema.

Fig 1.

Orthogonal axial/coronal (A) and axial (B) projection of T1-weighted MR images with fiber tracts depicted in color in the healthy volunteer (violet and yellow, lip fibers; green and red, hand fibers; pale blue and dark blue, foot fibers) The hand and foot fibers of the corticospinal and the lip fibers of the corticobulbar tract can be clearly distinguished and visualized (A and B). The different fMRI activation areas used to choose the seed regions of interest are shown in color (A). The course of the fibers through the corona radiata follows the known somatotopic distribution (B). C, The results are shown from conventional DT tractography (in orange) as well as from fMRI-based fiber tracking with region-of-interest placement in the PMA of the lip of the right hemisphere (in green), projected on a coronal T1-weighted image of the healthy volunteer. fMRI activation (shown in yellow-orange) is visible in the PMA of the lip and supplementary motor area in both images. Clearly, the lip fibers are only visualized by using the fMRI-based fiber tracking approach and not with the conventional fiber tracking approach.

Fig 2.

Axial T1-weighted MR image of a patient with a brain tumor with fibers of the corticospinal tract projected in color. Results from conventional fiber tracking based on anatomic landmarks (in orange) show dispersion and diminished visualization of the CST fibers in the hemisphere affected by the lesion (arrowheads), compared with the unaffected hemisphere. DT tractography based on seed region-of-interest selection in the PMA of the hand as established with fMRI shows, in blue, the CST fibers of the hand in the affected hemisphere, allowing a clear demarcation of the fibers of interest within the CST.

Fig 3.

Orthogonal axial/sagittal projection of a T1-weighted MR image with overprojection of the hand PMA and the hand fibers of the CST. In this patient, the left PMA, as shown by fMRI (in orange-yellow), was displaced by tumor, as was the tracked CST (in green).

Fig 4.

Axial T1-weighted MR image with fiber tracts projected in color (red and green for the right and left hemispheres, respectively). In this patient, hand fibers ran through an area of altered signal intensity, due to edema or tumor infiltration.

Fig 5.

Axial T1-weighted MR images with foot fibers projected in color. Fibers pass through an area of altered signal intensity on T1-weighted images. Varying the FA thresholds for fiber tracking in this patient had a considerable influence on the fibers depicted (A–C, FA thresholds used are shown in each image).