Objective: To define the extent of neuronal injury and loss in thalamic gray matter in patients with relapsing-remitting (RR) MS and to characterize how these neuronal pathologic changes are related to disease duration. Methods: The authors studied 14 patients with RRMS (Expanded Disability Status Scale score, mean 3.25, range 2.0 to 6.0) and 14 (8 men, 6 women) age-matched healthy controls. Structural MR and MRS studies were performed in a single scanning session using a 3T MR system. Results:N-acetylaspartate (NAA) concentrations (a measure of the apparent neuronal density) were decreased approximately 11% in the thalami of the patients with RRMS relative to controls (p < 0.05). The patients with RRMS also had an almost 25% lower mean normalized thalamic volume than controls (p < 0.005). Decreases in thalamic NAA concentration correlated strongly with thalamic volume loss for individual patients (r = 0.85, p < 0.01). Both the NAA concentration (r = −0.48, p = 0.044) and normalized thalamic volume (r = −0.60, p = 0.01) were correlated inversely with disease duration. There was a trend for a correlation between the thalamic NAA/creatine (Cr) ratio and the NAA/Cr in the frontal normal-appearing white matter (r = 0.56, p < 0.08). Conclusions: The reduction of both NAA concentration and thalamic volume suggests that a neurodegenerative component may contribute to the pathology of MS even in the earlier RR stage. The trend toward a relationship between thalamic NAA/Cr and distant normal-appearing white matter changes implies that there may be a common mechanism for the white matter axonal loss and thalamic neuronal injury.

The RF B₁ distribution was studied, theoretically and experimentally, in phantoms and in the head of volunteers using a 3 T MRI system equipped with a birdcage coil. Agreement between numerical simulation and experiment demonstrates that B₁ distortion at high field can be explained with 3D full‐Maxwell calculations. It was found that the B₁ distribution in the transverse plane is strongly dependent on the dielectric properties of the sample. We show that this is a consequence of RF penetration effects combined with RF standing wave effects. In contrast, along the birdcage coil z‐axis the B₁ distribution is determined mainly by the coil geometry. In the transverse plane, the region of B₁ uniformity (within 10% of the maximum) was 15 cm with oil, 6 cm with distilled water, 11 cm with saline, and 10 cm in the head. Along z the B₁ uniformity was 9 cm with phantoms and 7 cm in the head. Magn Reson Med 46:379–385, 2001.