The MR examinations were performed by using a 1.5-T magnet (Sonata; Siemens, Erlangen, Germany), identical to the one in the operating room. The following sequences were acquired: T2, FLAIR, and isotropic volumetric T1-weighted magnetization-prepared rapid acquisition gradient echo (MPRAGE) before and after the intravenous administration of paramagnetic contrast material and DT sequences. The DTI study was performed with 12 non-collinear directions (b value = 0 and 1,000 s/mm²) and echo planar sequences (TE 86 ms,TR 9,200 ms, matrix 128 × 128, FOV 240 mm, slice thickness 1.9 mm, bandwidth 1,502 Hz/Px, 60 slices, no gap, acquisition time 5 min 31 s).

Tractography post-processing was performed with a method similar to those presented by Basser et al., Mori et al., and Stieltjes et al. [2, 20, 33], using the planning software iPlan 2.6 (BrainLAB AG, Feldkirchen, Germany).

Color maps were used to define an appropriate region of interest (ROI) for the subsequent tractography procedure. The fiber-tracking technique contemplates the three-dimensional (3D) reconstruction of white matter trajectories of the CST by using a fractional anisotropy threshold of 0.17 and a processing angle above 55°. The positioning of the ROI for the fiber tracking changed according to the trajectories of the fibers to be reconstructed (posterior arm of the internal capsule for the pyramidal tract, geniculate ganglion for the optic radiation and, only in the right-handed patients with lesions on the left side, the ROI encompassed the horizontal fibers lateral to the corona radiata and medial to the cortex of the posterior part of the ventrolateral frontal lobe). Tracking was initiated in both the retrograde and orthograde directions according to the direction of the principal eigenvector in each voxel of the ROI. The reconstructed trajectories were transformed into 3D objects.

Mean data processing time of the CST was 2–3 min, while the mean data processing time of the AF was 8-10 min. Tractography results were saved in a file containing the x/y/z coordinates for each fiber. These data were imported together with the b = 0 diffusion images into the navigation software (import module for iPlan 1.0 programmed by U. Mezger [Brain Lab, Heimstetten, Germany]). After rigid registration of the b = 0 images with the anatomical volumetric package, and after having verified that there were no discrepancies between data (differences greater than 3 mm) in the region of the tumor, the white matter tracts could be displayed in standard anatomical images. Fiber margins were then segmented to allow them to be defined as objects in the navigation system and to be depicted intraoperatively. These objects were automatically enlarged by the software 2 mm in every direction. The trajectories were considered suitable for surgical planning if there were no interruptions on any of the layers at the level of the lesion. The boundaries of the tumors were defined, considering the outer rim of enhancement for grade III and IV gliomas and the T2 signal for grade II gliomas. The distance between the CST and tumor margins was calculated. The mean overall time for this data processing, which was generally performed the day prior to surgery, was 30 min. During surgery the position of the CST was used as a reference for tumor resection.

Currently we do not perform intraoperative presurgical MRI because of infrared matching of head’s patients with previously acquired volumetric exams, but for prone position.

Total acquisition time and time for sending preoperative images to the neuronavigation system for surgical planning was limited and was less than 2 min.

We perform the first intraoperative MRI after the dural opening, acquiring an intraoperative volumetric MRI for navigation and an intraoperative DTI for tractography. This step is fundamental for correcting possible brain shift.

Average total acquisition and processing time was about 15 min.

The same neuroradiologist reconstructed the white matter tracts and uploaded the new tractographic data in the neuronavigation system; these data were subsequently used for further surgery.

When the neurosurgeon was confident that most of the lesion was removed, intraoperative MRI and DTI were again performed and the same neuroradiologist reconstructed the tractographic data with the previously mentioned technique.

The new tractographic data were again uploaded into the neuronavigation system and subsequently used for further surgery and eventual intraoperative subcortical stimulation.

At the end of the procedure, we performed the last MRI to check the amount of resection and the sparing of the white matter tracts.

Pre- and intraoperative DTI were registered with automatic image fusion software (iPlan 2.6; BrainLAB AG), which was used to perform semiautomatic rigid registration. After registration, the images could be displayed side by side or in an overlay mode. The extent of shifting was considered as the maximum distance between the preoperative and intraoperative contours of the trajectories of the evaluated white matter tracts on identical/registered axial slices.

According to the direction of the shift we assigned positive or negative values in relation to the craniotomy opening. A positive value was assigned if movement was outward (toward the surface), and a negative value was assigned if movement was inward.

We measured peritumoral edema and tumor volume using the planning software noted above, and, as well, we calculated the craniotomy size and the distance of the tumor from the cortical surface.