What you see is not always what you get—MRI-based ganglionic eminence volumetry challenges subjective assessment in CNS anomalies

OBJECTIVES: To compare prenatal imaging-based subjective assessment of the ganglionic eminence (GE) on fetal MRI with three-dimensional volumetric analysis in patients with structural central nervous system anomalies. MATERIALS AND METHODS: This retrospective study investigated 17 fetuses (undergoing 20 fetal MRIs, mean gestational age 26.3 weeks, SD 3.3, range 21.7-33.4 weeks) with enlarged GE based on subjective assessment of fetal neuroimaging experts and concurrent structural brain anomalies. Three-dimensional volumetry of super-resolution MRI was performed and compared to age-matched neurotypical controls (94 fetuses, 100 MRIs, mean age 27.2 weeks, SD 3.6, range 21.7-34.0 weeks). RESULTS: Among 20 MR examinations, in only 5 cases (25%) GE hyperplasia was confirmed, while 12 (60%) were found to show normal GE volumes. In 3 (15%), GE volumes were smaller compared to controls. Most patients (80%) were found to have increased total brain volume, while only 30% had an increased intracranial volume-ventriculomegaly (75%) seemingly being the most common underlying cause. Brain parenchyma volume was enlarged in only 20%. No correlation was found between GE volumes and volumes of 10 other substructures of the fetal head in pathological cases. CONCLUSION: The study suggests the unreliability of expert visual assessment of GE size, despite excellent examination conditions, emphasizing the need for three-dimensional volumetric GE measurements. Specifically in patients with structural brain anomalies, quantitative fetal neuroimaging serves as an emerging tool to identify abnormalities in GE size and complement current diagnostic techniques by objectifying subjective impressions. KEY POINTS: Question To evaluate the reliability of expert visual assessment of ganglionic eminence size in fetal MRI compared to super-resolution brain volumetry. Findings Expert visual assessment and 2D fetal brain biometry were insufficient to provide correct volumetric quantification of the ganglionic eminence in the investigated patient collective. Clinical Relevance This study highlights the value of volumetric quantification of the ganglionic eminence in fetal MRI to provide an objective, reproducible and valid quantitative tool for assessment of the fetal brain, specifically in the presence of CNS anomalies.