Location of the corticospinal tract in the internal capsule at MR imaging

PURPOSE: To examine whether hyperintense foci in the posterior internal capsule (IC) on T2-weighted magnetic resonance (MR) images represent fibers of the corticospinal tract (CST). MATERIALS AND METHODS: The authors reviewed MR images of 100 control subjects and 35 patients with amyotrophic lateral sclerosis (ALS) and brain specimens from seven control subjects and five ALS patients. RESULTS: In the posterior IC, the brain specimens from control subjects demonstrated a pale area with large axons and thick myelin sheaths. In five ALS patients, the brain specimens showed degeneration of the CST in the same region. MR images demonstrated abnormal hyperintensity that represented degeneration of the CST in five of 35 ALS patients. In the same region, hyperintense foci were found in the control subjects. CONCLUSION: Normal hyperintense foci in the IC represent the fibers of the CST. The presence of large fibers with thick myelin sheaths may play an important role in these signal intensity variations.