A Low-intensity Rim on T2-weighted Brainstem Imaging is a Normal Finding and a Mimicker of Superficial Siderosis

Purpose: In healthy participants, T2 superficial localized low intensity (T2-SLL) similar to superficial siderosis has been observed in the brainstem. This study aimed to determine the incidence and causes of T2-SLL. Methods: To determine the incidence, T2-weighted imaging (T2WI) was performed on 114 patients (68 males; mean age: 59.1 years) using a 3.0-T magnetic resonance (MR) scanner and visually assessed by two radiologists. T2-SLL 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-SLL, an experimental MR imaging (MRI) was performed on a healthy male volunteer. To evaluate the chemical shift effect, the bandwidth and encoding direction were modified. To assess the magnetic susceptibility effect, T2*WI was performed using varying echo times (TEs). Results: A moderate inter-rater score agreement (κ=0.556) was observed. T2-SLL 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-SLL were observed across the varying bandwidths and encoding directions. However, the superficial low signal for T2WI thickened as the TE lengthened, similar to blood vessels, suggesting a magnetic susceptibility effect. Conclusion: T2-SLL can be regarded as a normal structure that may be associated with blood vessels.