Aristides A. Capizzano

Proton MR spectroscopy (MRS) studies of patients with temporal lobe epilepsy (TLE) have consistently shown reduced concentration of the neuronal marker N-acetylaspartate (NAA) in the medial temporal lobe ipsilateral to the seizure focus. These findings not only replicate information available from the presence of hippocampal sclerosis (HS) on conventional MRI, but have also been detected in normal-appearing temporal lobes^1–4 and in patients with bilateral HS.⁵ Furthermore, some of these studies showed that metabolic changes in these difficult clinical cases predict surgical outcome.^4,5

Metabolic changes in TLE extend far beyond the areas of neuronal loss on histopathology, as shown by hypometabolism on interictal FDG-PET scans.⁶ Given the dependence of NAA synthesis on aerobic metabolism, we hypothesized that NAA reductions in TLE could be detected in ipsilateral cerebral cortical areas connected with the hippocampus by using a multisection MRS imaging sequence developed at the San Francisco Veterans Affairs Medical Center MRS laboratory. NAA reductions were consistently detected with high SNR spectra from the temporal operculum, insula, and lateral temporal cortex ipsilateral to the seizure focus. These findings agree with the extensive ipsilateral FDG-PET cerebral hypometabolism seen in TLE.^6,7 Follow-up studies replicated the extrahippocampal NAA reduction in TLE,^8–10 although the most recent of the latter studies using whole-brain 3D MRS coverage questioned the lateralizing value of these metabolic changes.⁹ This discrepancy with our study could result from different voxel selection approaches.

MRS can be done in the clinical setting at 1.5 or 3T using commercially available single voxel or multivoxel sequences centered at the hippocampi after manually adjusted local shimming. High-quality spectra can be obtained from the lateral temporal cortex and insula given their improved B0 homogeneity compared with the hippocampal region. However, sampling lateral cortical regions requires multisection or volumetric MRS sequences, whose availability from commercial manufacturers has been limited. Furthermore, multisection, and especially 3D MRS techniques, produce very large datasets that require time-consuming processing for quantitative interpretation, which is a limitation in the clinical arena. These

factors, together with the uncertain reimbursement of clinical MRS studies, have limited the clinical application of MRS in TLE. Notwithstanding these limitations, the authors' conviction is that MRS depicts regions of NAA reduction relevant for TLE lateralization in normal-appearing brain. MRI hardware and postprocessing software improvements, as well as increased availability of broad coverage MRS sequences, will likely result in wider clinical utilization of MRS in TLE.

References

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