Ventricular volumetry and free-water corrected diffusion tensor imaging of the anterior thalamic radiation in idiopathic normal pressure hydrocephalus

Background and purpose The pathophysiology of idiopathic normal pressure hydrocephalus (iNPH) has not been completely clarified. We investigated the brain structure in iNPH using automatic ventricular volumetry, single-tensor diffusion tensor imaging (DTI) and bi-tensor free-water (FW) imaging analyses while focusing on cognitive impairments before and after lumboperitoneal shunt surgery. Materials and methods This retrospective study included 12 iNPH patients with structural magnetic resonance imaging (MRI) and diffusion MRI (dMRI) on a 3T-MRI scanner who underwent neuropsychological assessments before and after shunting and 8 healthy controls. Ventricular volumetry was conducted on structural MRI datasets using FreeSurfer. Ventricular volume was compared pre- and postoperatively. Correlation analyses were performed between ventricular volume or volume change and neuropsychological scores or score change. Tract-based spatial statistics were performed using dMRI datasets for group analyses between iNPH and controls and between pre- and post-surgery iNPH patients and for correlation analyses using neuropsychological scores. Tract-specific analyses were performed in the anterior thalamic radiation (ATR), followed by comparison and correlation analyses. Results The third ventricular volume was significantly decreased after shunting; its volume reduction negatively correlated with a neuropsychological improvement. Compared with controls, iNPH patients had lower fractional anisotropy and higher axial, radial, and mean diffusivities, and FW in the periventricular white matter including ATR, resulting in no difference in FW-corrected indices. Single-tensor DTI indices partially correlated with neuropsychological improvements, while FW-corrected indices had no correlations. Conclusion Third ventricle enlargement is possibly linked to cognitive impairment and FW imaging possibly provides better white matter characterization in iNPH.