Effect of voxel position on single-voxel MR spectroscopy findings

Single-voxel MR spectroscopy is a widely used tool for evaluating brain tumors. Although extensive data are available on the MR spectral appearance of tumors, less is known about the effect of voxel position on the accuracy of single-voxel MR spectroscopy findings. The purpose of this study was to test the hypothesis that the accuracy of single-voxel MR spectroscopy in the categorization of lesions as either tumor or not tumor is dependent on voxel position.

Fifty single-voxel MR spectra acquired with a fully automated stimulated-echo spectroscopy sequence were reviewed retrospectively in 43 patients with new or previously treated intra-axial brain tumors. Spectra were analyzed for the presence of choline, creatine, N-acetylaspartate (NAA), and lipid/lactate. Choline/creatine and NAA/creatine peak area ratios were assessed qualitatively. Lesions were grouped into one of three categories on the basis of spectral pattern: tumor, not tumor, or indeterminate. Results of MR spectroscopy were compared with the final histopathologic diagnosis.

Histologic confirmation was obtained in 19 patients; MR spectra were interpretable in 17 of those. MR spectra correctly categorized nine of 17 lesions (six tumor, three nontumor). All eight misdiagnosed lesions were tumors. When the MR spectroscopy voxel included the enhancing edge of the lesion, the spectra correctly categorized seven of eight lesions (four of five tumors and all three cases of radiation necrosis). When the MR spectroscopy voxel was positioned centrally within the lesion, the spectra correctly reflected histologic outcome in two of nine lesions (all tumors).

The reliability of single-voxel MR spectroscopy findings is dependent on voxel position. Spectra obtained from voxels at the enhancing edge of a tumor more accurately reflect lesion histopathology than do spectra obtained from the lesion center, even if the centrally placed voxels contain solidly enhancing tissue.