Age-Associated Variation in Brainstem Anteroposterior Dimensions on 3-Tesla MRI: A Single-Center Study of an Imaging-Normal Clinical Cohort

Introduction: Brainstem morphometry on magnetic resonance imaging (MRI) provides quantitative information on age-related structural variation.Although previous studies have reported brainstem dimensions in different populations, the relationship between age and anteroposterior (AP) brainstem dimensions remains incompletely characterized in imaging-normal clinical cohorts.This study evaluated age-and sex-related variation in the AP diameters of the midbrain, pons, and medulla in adults undergoing brain MRI.Methods: This retrospective cross-sectional study included 140 adults (70 males and 70 females) aged ≥18 years who underwent brain MRI for non-brainstem-related clinical indications and demonstrated no brainstem abnormalities.MRI examinations were performed using a 3.0-Tesla Philips Ingenia MRI system (Philips Medical Systems Nederland B.V., Best, The Netherlands).AP diameters of the midbrain, pons, and medulla were measured on midsagittal T1-weighted images using electronic calipers.Differences between sexes were assessed using independent-samples t-tests, while differences across five age categories (18-30, 31-40, 41-50, 51-60, and >60 years) were assessed using one-way analysis of variance with post-hoc testing.Age was also analyzed as a continuous variable using regression models adjusted for sex.Statistical analyses were performed using IBM SPSS Statistics for Windows, Version 25 (Released 2017; IBM Corp., Armonk, New York).Results: The mean AP diameters of the midbrain, pons, and medulla were 16.84 ± 1.50 mm, 22.06 ± 1.45 mm, and 12.74 ± 1.06 mm, respectively.No statistically significant sex differences were observed for the midbrain (t(138) = -1.96,p = 0.052), pons (t(138) = 0.15, p = 0.885), or medulla (t(138) = 1.33, p = 0.185), although a marginal trend was observed for the midbrain (p = 0.052).Midbrain AP diameter differed across age categories (F(4,135) = 7.29, p < 0.001, η² = 0.178), with the smallest mean diameter in participants aged >60 years.Post-hoc analysis showed greater midbrain diameters in the 18-30, 31-40, and 41-50-year groups than in the >60-year group.Pons diameter showed an overall age-group difference (F(4,135) = 2.73, p = 0.032, η² = 0.075), but no individual pairwise comparison was statistically significant.Medulla diameter did not differ significantly across age groups (F(4,135) = 0.41, p = 0.803, η² = 0.012).In sex-adjusted continuous-age regression, midbrain AP diameter showed an inverse association with age (β = -0.036mm/year, t(137) = -5.23,95% CI: -0.050 to -0.023, p < 0.001), whereas associations were not statistically significant for the pons (β = -0.002mm/year, t(137) = -0.27,95% CI: -0.016 to 0.013, p = 0.793) or medulla (β = -0.005mm/year, t(137) = -0.98,95% CI: -0.015 to 0.005, p = 0.333).No statistically significant age-by-sex interactions were identified.Conclusions: In this single-center study of an imaging-normal clinical cohort, midbrain AP diameter demonstrated an age-associated reduction, whereas the pons and medulla showed no significant linear age association.No statistically significant sex differences or age-by-sex interactions were identified.These findings provide exploratory baseline measurements of brainstem AP dimensions in an imaging-normal clinical cohort and support further investigation using larger multicenter cohorts, standardized MRI protocols, reproducibility assessment, and three-dimensional or automated morphometric approaches.