Disproportionately elevated sulcal index (DESI): An automatically driven index representing disproportionate subarachnoid space enlargement in brain MRI scans.

INTRODUCTION: Idiopathic normal pressure hydrocephalus (iNPH) is frequently underdiagnosed due to non-specific symptoms and the risks of invasive testing. While disproportionately enlarged subarachnoid space hydrocephalus (DESH) is a hallmark imaging feature, manual assessment is subjective and qualitative. We developed the Disproportionately Elevated Sulcal Index (DESI), a fully automated deep learning-based volumetric biomarker, to objectively quantify these morphological changes.

METHODS: We trained a U-Net model with an EfficientNet-B0 encoder on T1-weighted MRI scans from the Baltimore Longitudinal Study of Aging and Johns Hopkins Clinic (n = 1248) to segment Sylvian fissures and superior sulcal spaces. DESI was defined as the volumetric ratio of the Sylvian fissure to superior sulci within an AC-PC aligned wedge. The model was externally validated on the multi-site PENS trial dataset (n = 94), comparing NPH patients against healthy controls and participants with Alzheimer's disease and vascular dementia.

RESULTS: In external validation, DESI demonstrated high diagnostic accuracy. The index distinguished NPH patients with DESH features from non-DESH NPH cases with an Area Under the Curve (AUC) of 0.97. When differentiating NPH with DESH from a pooled group of healthy controls and neurodegenerative mimics, DESI achieved an AUC of 0.99 (sensitivity 98%, specificity 100%). In a broad comparison of all NPH cases versus all non-NPH groups, DESI maintained an AUC of 0.94.

CONCLUSIONS: DESI provides a robust, fully automated quantification of sulcal disproportion that effectively differentiates iNPH from normal aging and neurodegenerative mimics. This continuous, non-invasive metric offers a scalable tool for accurate iNPH screening and patient stratification in clinical settings.