Comparison of Ultrasound and MRI for Measuring Foramen Magnum Diameter in Neonates and Infants: A Preliminary Study

BACKGROUND: The foramen magnum is the largest opening at the skull base. Congenital skeletal disorders such as achondroplasia can cause foramen magnum stenosis, leading to neurological problems. In high-risk infants, repeated magnetic resonance imaging (MRI) is often used to screen for or monitor stenosis. Ultrasound (US), already used for neonatal spinal assessment, may offer a non-invasive method for evaluating the craniovertebral junction. However, to our knowledge, no studies have directly compared foramen magnum assessment between US and MRI. METHODS: We retrospectively reviewed neonates and infants admitted to the Neonatal Intensive Care Unit (NICU) who underwent MRI between December 2023 and January 2025. Foramen magnum diameter (FMD) was measured in the mid-sagittal view using a posterior cervical US approach by a single neonatologist. US measurements were compared with MRI measurements using Bland-Altman analysis. RESULTS: Thirty patients (median corrected age: 39.1 weeks, 17 males) were evaluated. There was no significant difference between FMD measured by ultrasound and MRI (p = 0.32). US measurements demonstrated excellent intra-observer reliability (ICC [1, 3] = 0.95, 95% CI: 0.92-0.98), whereas MRI measurements showed moderate inter-observer reliability (ICC [2, 1] = 0.65, 95% CI: 0.38-0.82). Bland-Altman analysis showed a mean bias of 0.12 mm (95% limits of agreement: -1.2 to +1.4 mm), indicating minimal systematic bias. CONCLUSION: Posterior cervical ultrasound provides FMD measurements in neonates and infants that closely match those obtained with conventional MRI. While US cannot replace MRI for definitive diagnosis, it may serve as a valuable, non-invasive bedside tool for initial assessment and short-term follow-up of craniovertebral junction abnormalities during early infancy.