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Idiopathic normal pressure hydrocephalus (iNPH) is a common and treatable neurological disorder in older adults with ventriculomegaly due to disturbed cerebrospinal fluid (CSF) dynamics. The pathophysiology of iNPH is still incompletely understood, and there is growing evidence that neuroinflammation is partly responsible. Ventricular size is radiologically assessed with linear measures such as the Evans index (EI), but these measures are insensitive to clinically relevant changes in ventricular volume, so postoperative monitoring remains challenging. Shunt treatment is frequently complicated by overdrainage, underdrainage and suspected malfunction. The overall aim of this thesis was to develop and evaluate a clinically feasible quantitative tool for monitoring shunt treatment and apply it in a clinical cohort of iNPH patients with a special focus on the following: 1. Ventricular volumetry based on 3D quantitative MRI (qMRI). 2. Very early postoperative ventricular change in relation to initial valve setting and in vivo MRI resistance of a modern adjustable shunt valve. 3. Long-term relationships between volumetric change, clinical response and overdrainage 4. Inflammatory and neurodegenerative biomarker profiles in iNPH patients across plasma, lumbar CSF and ventricular CSF. The LiNPH study, a prospective controlled clinical longitudinal cohort study with a randomized controlled trial component, provided the data for the four papers in this thesis. Paper I was a methodological reliability study (45 qMRI examinations from iNPH patients and controls) comparing manual, automated and reviewed ventricular segmentations. Paper II was a randomized, double-blinded clinical study (n=45) with qMRI volumetry preoperatively and ~36 hours post-operatively that compared Certas Plus shunt valve settings 4 (open shunt) versus 8 (virtually closed) and evaluated the in vivo MRI stability of the valve. Paper III was a prospective cohort study (50 enrolled; follow-up at 3, 12 and 36 months) relating ventricular volumetry to clinical outcomes, valve adjustments and complications. Paper IV utilized CSF and plasma samples collected preoperatively from iNPH patients (n=56) and neurologically healthy controls (n=19) to profile 71 inflammatory cytokines, chemokines, growth factors and classical neurodegenerative markers, including paired plasma, lumbar CSF and ventricular CSF, via multiplex immunoassays and multivariate modelling. qMRI-based ventricular volumetry demonstrated excellent intra- and interrater reliability (intraclass correlation coefficient 0.999--1.000), whereas EI failed to reflect several clinically important volume differences. Early after shunting, ventricular volumes decreased significantly in both randomized groups, but the reduction was greater at setting 4 than at setting 8 (mean 16±9 mL vs 5±5 mL), whereas linear measures were equal except for THC (tight high convexity, the amount of CSF in the parasagittal convexity sulci). There were no inadvertent valve adjustments after 156 MRI examinations were performed. Over three years, the mean ventricular volume decreased from 134±35 mL preoperatively to 97±31 mL at 36 months, whereas gait and the iNPH scale improved markedly early and remained improved at the group level. Compared with nonresponders, responders presented greater reductions, and patients with overdrainage symptoms (orthostatic headaches and more) presented greater reductions than asymptomatic patients did. Biomarker analyses revealed a robust iNPH inflammatory profile compared with that of the controls and strong compartment concentration separation. In conclusion, qMRI-based ventricular volumetry is a sensitive, reliable and clinically feasible method for longitudinal iNPH follow-up, providing clinically valuable information on shunt effects, valve adjustments, suspicion of malfunction and overdrainage risk. The Codman Certas Plus valve is MRI resistant in vivo. There is a clear inflammatory component in iNPH pathophysiology, and there are substantial differences in cytokine and neurodegenerative biomarker concentrations among ventricular CSF, lumbar CSF and plasma, which are relevant to future study designs.