Word encoding activation

Fig. 3. Patients with left and right mesial temporal lobe epilepsy (MTLE) were studied with fMRI during encoding of various stimuli. The figure presents statistical maps of group-level activation during word encoding for the two groups. Patients with right MTLE activate the left hippocampal region during language encoding, similar to healthy control subjects (not shown). Patients with left-sided MTLE, however, have activation in the right hippocampal region during word encoding consistent with reorganization of material-specific verbal encoding processes to the contralateral MTL. (See Color Plate 3 following p. 112.)

Left MTLE Right MTLE

Fig. 3. Patients with left and right mesial temporal lobe epilepsy (MTLE) were studied with fMRI during encoding of various stimuli. The figure presents statistical maps of group-level activation during word encoding for the two groups. Patients with right MTLE activate the left hippocampal region during language encoding, similar to healthy control subjects (not shown). Patients with left-sided MTLE, however, have activation in the right hippocampal region during word encoding consistent with reorganization of material-specific verbal encoding processes to the contralateral MTL. (See Color Plate 3 following p. 112.)

this is predictive of postoperative outcome. Moreover, these studies appear to be particularly challenging to administer, analyze, and interpret and will require significant modification before they can become part of a routine preoperative evaluation. For example, as part of a protocol attempting to use fMRI comprehensively to test several neurologic functions, Deblaere et al. (78) were able to demonstrate anterior hippocampal activation in only four of nine subjects.

Diffusion Tensor Imaging

Injury to the white matter tracts connecting the motor, somatosensory, visual, and language cortices causes significant neurological deficits similar to injuries involving the corresponding cortex. To achieve the goal of maximal safe resection while preserving neurologic function, knowledge of white matter tract location and the relationship to a lesion is as important as defining the lesion's relationship with eloquent cortical areas. However, conventional structural MRI does not provide information on the location of white matter tracts and their relationship to the lesion.

The trajectory and location of white matter tracts can be detected by means of anisotropic DTI. This imaging method is based on measuring the diffusion of water protons in the cerebral white matter. The cerebral white matter consists of a network of tightly packed fiber tracts with different orientations. Owing to the cell membranes and myelin sheaths, water proton diffusion is facilitated parallel to the fibers and restricted perpendicular to them. It follows that by imaging water proton diffusion, one can obtain an image of these structures depicting both their orientation and possible alteration by pathologic processes (107). Theoretically, three types of lesion/fiber tract interactions may occur: fiber tract displacement, fiber tract infiltration, and fiber tract destruction. In our experience with DTI, all three scenarios have been encountered, isolated or in combination (Fig. 4; see Color Plate 4 following p. 112). Each of these is likely to be associated

Fig. 4. Diffusion tensor imaging of white matter tracts in a patient with a low-grade glioma demonstrates disruption of white matter tracts in the supplementary motor area by the tumor (arrowheads). Left panel shows a 2D representation displaying tensors. Right panel presents a 3D rendering showing tumor in green, functional activation in yellow and tractography of the corticospinal tract in red. (Courtesy of Dr. Ian-Florin Talos, Brigham and Women's Hospital, Department of Radiology. See Color Plate 4 following p. 112.)

Fig. 4. Diffusion tensor imaging of white matter tracts in a patient with a low-grade glioma demonstrates disruption of white matter tracts in the supplementary motor area by the tumor (arrowheads). Left panel shows a 2D representation displaying tensors. Right panel presents a 3D rendering showing tumor in green, functional activation in yellow and tractography of the corticospinal tract in red. (Courtesy of Dr. Ian-Florin Talos, Brigham and Women's Hospital, Department of Radiology. See Color Plate 4 following p. 112.)

with different consequences if fiber tracts are disrupted during surgery. Recent work has demonstrated the ability to trace the corticospinal tract and its relation to the brain tumor (108). By combining DTI with fMRI it is possible to demonstrate both a particular functional area and its connections to other areas.

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