Low-intensity rim on T2-weighted brainstem imaging: a universally observed structure exhibiting a negative magnetic susceptibility effect

PURPOSE: In healthy participants, T2 physiological rim (T2-PR) similar to superficial siderosis has been observed in the brainstem. This study aimed to determine the incidence and causes of T2-PR. MATERIALS AND METHODS: To determine the incidence, T2-weighted imaging (T2WI) was performed on 106 patients (65 males; mean age: 62.5 years) using a 3.0-T scanner and visually assessed by two radiologists. T2-PR presence in 22 brain areas was evaluated using the following system: 0 for absence, 1 for < 50% surface, 2 for ≥ 50% but not the entirety, and 3 for the entirety. After assessing inter-rater agreement, the scores were averaged. To investigate the causes of T2-PR, an experimental MR imaging (MRI) was performed on a healthy volunteer. To evaluate the chemical shift effect, the bandwidth and encoding direction were modified. To assess the magnetic susceptibility effect, χ-separation analysis and T2WI/T2*WI was performed using varying echo times (TEs). RESULTS: A substantial inter-rater score agreement (κ = 0.790) was observed. T2-PR was identified in all participants and was most frequently observed on the frontal and lateral sides of the midbrain and pons, with the highest occurrence on the frontal of the upper pons (median 2.0; interquartile range 2.0-3.0). In the experimental MRI, no differences in T2-PR were observed across the varying bandwidths and encoding directions. The low signal for T2WI/T2*WI thickened as the TE lengthened. Moreover, χ-separation analysis revealed diamagnetic susceptibility properties in the region where T2-PR was observed. CONCLUSION: T2-PR can be observed in the midbrain and pons. Both these areas exhibit the diamagnetic susceptible properties.