OBJECTIVE: Few studies have explored the morphology of massa intermedia (MI). The aim of the present study was to characterize it using magnetic resonance imaging (MRI).
METHODS: A total of 205 patients were enrolled in this study. Following initial examinations with conventional MRI sequences, thin-slice coronal and sagittal T2-weighted imaging was performed. For MI localization, the third ventricle was arbitrarily divided into nine areas on the midsagittal image.
RESULTS: MI was identified in 93% of the total patients-89% in male and 91% in female patients. Among them, 68% showed a single, styloid-shaped MI with variable thickness and cross sectional configuration, followed by broad and double MIs that were found in 18% and 10% patients, respectively. In the anteroposterior dimension, 99% of the MIs were identified in the middle third area, followed by the posterior third area. In the supero-inferior dimension, 95% of the MIs were identified in the middle third area, followed by the upper third area. With a significant difference, a broad MI was more frequently found in women than in men.
CONCLUSIONS: MIs are commonly located in the middle third of the third ventricle as a single commissure with high morphological variability. Compared to men, women may have a well-developed, broader MI.

The role of massa intermedia (MI) is poorly understood in humans. Recent studies suggest its presence may play a role in normal human neurocognitive function while prior studies have shown the absence of MI correlated with psychiatric disorders. There is growing evidence that MI is likely a midline white matter conduit, responsible for interhemispheric connectivity, similar to other midline commissures. MI presence was identified in an unrelated sample using the Human Connectome Project database. MI structural connectivity maps were created and gray matter target regions were identified using probabilistic tractography of the whole brain. Probabilistic tractography revealed an extensive network of connections between MI and limbic, frontal and temporal lobes as well as insula and pericalcarine cortices. Women compared to men had stronger connectivity via their MI. The presented results support the role of MI as a midline commissure with strong connectivity to the amygdala, hippocampus, and entorhinal cortex.