Evaluating requirements for spatial resolution of fMRI for neurosurgical planning

Abstract

The unambiguous localization of eloquent functional areas is necessary to decrease the neurological morbidity of neurosurgical procedures. We explored the minimum spatial resolution requirements for functional magnetic resonance imaging (fMRI) data acquisition when brain mapping is used in neurosurgical planning and navigation. Using a 1.5 Tesla clinical MRI scanner, eight patients with brain tumors underwent fMRI scans using spatial resolution of approximately 4 x 4 x 4 mm(3) to map the eloquent motor and language areas during the performance of cognitive/sensorimotor tasks. The fMRI results were then used intra-operatively in an open MRI system to delineate eloquent areas. Retrospectively, activation patterns were visually inspected by a neurosurgeon to determine qualitatively whether ambiguity with respect to the activation boundaries, due to low spatial resolution, could be of potential significance for surgical guidance. A significant degree of ambiguity in both the extent and shape of activation was judged to be present in data from six of the eight patients. Analysis of fMRI data at multiple resolutions from a normal volunteer showed that at 3 mm isotropic resolution, eloquent areas were better localized within the gray matter although there was still some potential for ambiguity caused by activations appearing to cross a sulcus. The data acquired with 2-mm isotropic voxels significantly enhanced the spatial localization of activation to within the gray matter. Thus, isotropic spatial resolution on the order of 2 x 2 x 2 mm(3), which is much higher than the resolutions used in typical fMRI examinations, may be needed for the unambiguous identification of cortical activation with respect to tumors and important anatomical landmarks.

Figure 1

Spatial resolutionsoused in recent fMRI investigations for neurosurgical planning. All studies used square pixel elements in the in‐plane direction. Data plotted reflect effective spatial resolutions, including the effect of spatial smoothing and slice gap. The dotted line indicates isotropic voxels.

Figure 2

A: A coronal section m sixthe first set of EPI images acquired inxthe fMRI experiment. B: The EPI image was thresholded to delineate the CSF space and overlaid on the T1‐weighted anatomical image. C: Overlay of the segmented CSF‐space obtained f sixthe T1‐weighted anatomical images.

Figure 3

A: The functional map f sixSubject 1 (33/F) in axial view (with overlay threshold P < 10⁻⁶). The tumor boundary is delineated with a green line overlaid on the images. Activation was observed adjacent to the border of a tumor in the precentral gyrus (see arrow). B: Intra‐operative view seen by the surgical team. The location of the cortical stimulator is shown by placement of a 3‐D rendered wand in the image. Eloquent motor areas are shown inxred. C: The functional map f sixSubject 2 (34/F) in axial view. Activation is observed at the boundary between two adjacent gyri, i.e., inferior aspect of STG and the superior aspect of MTG (arrow). D: Intra‐operative view seen by the surgical team. Intra‐operative cortical stimulation identified speech/language areas in the right posterior superior temporal lobe as shown.

Figure 4

Results m sixmotor fMRI study of noa frosubject. A: Subject's coronal anatomy with region‐of‐interest (see box) in motor‐related functional area. B: Delineation of the central sulcus. (C) Low, (D) intermediate, and (E) high spatial resolution functional maps obtained f sixdata according to the protocols described in Table II. F: Functional map obtained f sixhigh spatial resolution data that were spatially smoothed to the equivalent level of the low‐resolution data.

Figure 5

Results m sixlanguage fMRI study of noa frosubject. A: Subject's sagittal anatomy with region‐of‐interest (see box) at the border of primary auditory areas and inferior parietal lobe. B: Delineation of lateral fissure. (C) Low, (D) intermediate, and (E) high spatial resolution functional maps obtained f sixdata according to the protocols described in Table II. F: Functional map obtained f sixhigh spatial resolution data that was spatially smoothed to the equivalent level of the low‐resolution data.