Prefrontal N-acetylaspartate and poststroke recovery: a longitudinal proton spectroscopy study.

BACKGROUND AND PURPOSE: Functional imaging studies suggest that poststroke recovery is related to the reorganization in both contralesional and ipsilesional prefrontal cortex. Little is known, however, about how longitudinal metabolic changes in prefrontal regions relate to the improvement after stroke. We sought to determine whether poststroke recovery is associated with changes in N-acetylaspartate/creatine (NAA/Cr) ratio within contralesional prefrontal regions. MATERIALS AND METHODS: Twenty-seven patients with a first ischemic stroke located outside the frontal lobes were included. Proton MR spectroscopy ((1)H-MRS) was performed on a 1.5T scanner. Point-resolved spectroscopy sequence (PRESS) was used. NAA/Cr was measured both in ipsilesional and contralesional prefrontal regions in early (14 +/- 6 days after stroke) and chronic phases of the disease (110 +/- 30 days after). Patients' neurologic status was assessed using Scandinavian Stroke Scale (SSS) at discharge from the stroke unit and during second (1)H-MRS examination. RESULTS: Subjects showing increased contralesional NAA/Cr from first to follow-up examination improved significantly more on the SSS than patients not showing this increase. Analysis was performed while correcting for change in NAA/Cr levels in the ipsilesional hemisphere. For the whole group, the change in contralesional NAA/Cr was significantly correlated to the change in SSS scores (r = 0.40, P = .03). Change in the ipsilesional NAA/Cr measures did not correlate with the change in SSS scores. CONCLUSION: Poststroke recovery was related to the increase in contralesional prefrontal NAA/Cr. This association may reflect recovery mechanisms involving the nonaffected hemisphere. Further assessment of these regions may provide information about mechanisms contributing to neurologic improvement.

MRI disturbances caused by dental materials.

Dental materials are common sources of artifacts during brain MRI studies. The aim of the present study was to estimate the degree of image disturbance caused by dental materials during an MRI examination, and the risk of ferromagnetic materials being dislocated in the magnetic field. MR sequences where dental materials containing metals produced the smallest artifacts was also calculated. Thirty-five dental material samples containing metals were examined using the 1.5 T MR system. They consisted of the most commonly used dental restorations and root-canal instruments left in the oral cavity for iatrogenic reasons. Additionally, cubes (10×5×5 mm) and balls (10 mm diameter) were cast from commonly used dental alloys. No dislocation of the examined samples in the gelatin medium was observed. All the materials were found to produce artifacts characterised by similar images. The study showed that the presence of distant artifacts, besides those located in the area immediately surrounding the object, depends on the shape of the given material. The size of artifacts generated by the most commonly used fixed restorations was graded. The sequences in which artifacts were least pronounced were selected. No dislocation of samples in the 1.5 T field was observed during the examination. However they produced artifacts; the typical image of an artifact - a signal void zone surrounded by a hyperintensive rim. Alloys consisting of Ag-Pd, dental gold and amalgam distorted the MR image to the least degree. In the case of a routine spin echo (SE) sequence examination the smallest artifacts were generated in the FSE T2 and PD sequences.