Application of volumetric MR spectroscopic imaging for localization of neocortical epilepsy

Abstract

Purpose: The aim of this study was to evaluate volumetric proton magnetic resonance spectroscopic imaging (MRSI) for localization of epileptogenic foci in neocortical epilepsy.

Methods: Twenty-five subjects reporting seizures considered to be of neocortical origin were recruited to take part in a 3-T MR study that included high-resolution structural MRI and a whole-brain MRSI acquisition. Using a fully automated MRSI processing protocol, maps for signal intensity normalized N-acetylaspartate (NAA), creatine, and choline were created, together with the relative volume fraction of grey-matter, white-matter, and CSF within each MRSI voxel. Analyses were performed using visual observation of the metabolite and metabolite ratio maps; voxel-based calculation of differences in these metabolite maps relative to normal controls; comparison of average grey-matter and white-matter metabolite values over each lobar volume; and examination of relative left-right asymmetry factors by brain region.

Results: Data from 14 subjects were suitable for inclusion in the analysis. Eight subjects had MRI-visible pathologies that were associated with decreases in NAA/creatine, which extended beyond the volume indicated by the MRI. Five subjects demonstrated no significant metabolic alterations using any of the analysis methods, and one subject had no findings on MRI or MRSI.

Conclusions: This proof of principle study supports previous evidence that alterations of MR-detected brain metabolites can be detected in tissue areas affected by neocortical seizure activity, while additionally demonstrating advantages of the volumetric MRSI approach.

Figures

Figure 1

MRI (a) and NAA/Cre image (b) for subject 4, and example spectra (c) from the proximity of a cortical lesion in the occipital gyrus. The spectrum overlying the cortical lesion is indicated by the arrow.

Figure 2

Results for subject 8 at two axial slices, with the NAA/Cre images (a) and (c), and the corresponding T1-weighted MRI (b) and (d) generated by summing five 1-mm slices corresponding to the thickness of the MRSI slice. Spectra from both slices and selected from the right and left regions, as indicated by the arrows on the NAA/Cre images, are shown in (e) and (f).

Figure 3

Results for subject 12, showing (a) the NAA/Cre images at 7 axial slices and (b) the T2-weighted MRI corresponding to the center of the metabolite image slice. In (c) is shown the z-score image for NAA/Cre, which is shown in a color scale with black representing no change from normal values and running from −3σ to +3σ over the blue and red colors. The z-score image is shown in the standard spatial reference frame, and the T2 MRI is again shown in (d) following affine registration to this frame. In (e) are shown spectra from the numbered locations indicated in (b).

Figure 4

Results for subject 17 that exhibits a clearly defined left parietal lesion seen on the T2-weighted MRI (a) and by decreased NAA/Cre (b). Spectra selected from contralateral anatomical locations indicated by the triangle (subject right) and circle (subject left) symbols are shown in (c) for the left, middle, and rightmost slices shown.

Figure 5

The z-score map corresponding to the NAA/Cre image shown in Fig. 4b, shown as a color overlay thresholded to highlight voxels with NAA/Cre reduced by an amount between −1σ and −3σ, and superimposed on the T1-weighted MRI.

Figure 6

The NAA/Cre image (a) at four axial slices and the corresponding T1-weighted MRIs at 5 mm slice thickness (b) for subject 22. The arrows on the leftmost images indicate the location of a lesion and contours derived from the MRI are superimposed on the corresponding slices in (a) to illustrate the slight differences in localization between the features seen in the metabolic and structural images.

Figure 7

NAA/Cre images (a) and T1-weighted MRI (b) for subject 24. Interpretation in this portion of the occipital lobe must account for normal variations of all metabolite values in the cerebellum and periventricular regions.

Figure 8

Z-score map for (a) NAA, and (b) NAA/Cre for the same subject as shown in Fig. 7a. The z-score is shown as a color overlay on the T1-weighted MRI and windowed for values greater than 2σ and limited to a maximum value of 3σ. Both z-score results show good localization to the occipital-temporal region.

Figure 9

Spectra summed over 9 voxels selected from a region of mild hyperintensity seen on the T2-weighted MRI (a) and a contralateral region (c), as indicated by the arrows on the T2 MRI (b), from subject 24.