What is the preferred imaging modality, Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scan, for diagnosing Normal Pressure Hydrocephalus (NPH)?

MRI is the Preferred Imaging Modality for Normal Pressure Hydrocephalus

MRI without IV contrast is the optimal imaging modality for diagnosing NPH and should be used as first-line imaging whenever possible. 1 MRI provides superior soft-tissue characterization and can detect critical NPH-specific features that CT cannot visualize, directly impacting patient selection for potentially life-altering shunt surgery. 2

Why MRI is Superior to CT

MRI demonstrates cerebral aqueduct flow void, a key finding associated with good response to shunt surgery that is completely invisible on CT. 1 This flow void represents hyperdynamic CSF flow and helps identify shunt-responsive patients. 2

Additional MRI Advantages Over CT:

• Higher sensitivity for periventricular white matter changes (transependymal CSF flow), which indicate active hydrocephalus rather than simple atrophy 1, 2
• Multiplanar imaging capability allows better visualization of structural abnormalities in any plane, not just axial 2
• Detection of deep white matter ischemia that may contribute to idiopathic NPH 2
• Superior visualization of small obstructing lesions that CT may miss 1

Critical MRI Features for NPH Diagnosis

The following constellation of findings on MRI supports NPH diagnosis and predicts shunt responsiveness:

Primary Features:

• Ventriculomegaly with Evans index >0.3 (not entirely attributable to cerebral atrophy) 1, 3
• Disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern: tight high-convexity and medial subarachnoid spaces, enlarged Sylvian fissures, and ventriculomegaly 1, 4
• Narrowed posterior callosal angle (<90°, ideally 40-90°) 1, 3

Supporting Features:

• Rounded frontal horns with marked enlargement of temporal horns (not explained by hippocampal atrophy) 1, 3
• Effaced sulci along high convexities with widened Sylvian fissures 1
• Aqueductal or fourth ventricle flow void on T2-weighted sequences 1, 3
• Corpus callosum thinning and elevation 3

The DESH pattern (tight high-convexity with enlarged Sylvian fissures) is particularly important as it is included in Japanese diagnostic guidelines and has been validated in prospective studies showing 69-80% favorable outcomes after shunt surgery. 4

When to Use CT Instead

CT head without IV contrast is an acceptable alternative only when MRI is unavailable or contraindicated (pacemakers, severe claustrophobia, metallic implants). 1

CT Limitations:

• Cannot detect cerebral aqueduct flow void, missing a key predictor of shunt responsiveness 1
• Lower sensitivity for periventricular white matter changes compared to MRI 1
• May miss small obstructing lesions 1
• Limited to axial plane imaging 2

CT can still identify ventriculomegaly, narrowed callosal angle, effaced sulci, and widened Sylvian fissures, but provides less diagnostic confidence. 1

Advanced MRI Techniques

Cine MRI (phase-contrast MRI) showing increased ventricular CSF flow rate has high positive predictive value for identifying shunt-responsive NPH patients. 1 This technique quantifies CSF dynamics and can be added to standard MRI protocols when available.

Common Pitfalls to Avoid

• Do not rely on ventriculomegaly alone—this is nonspecific and occurs commonly with aging 5, 6
• Do not confuse NPH with simple cerebral atrophy—NPH shows disproportionate ventricular enlargement relative to cortical atrophy 1, 3
• Do not use cisternography as the sole basis for surgical decisions—evidence is insufficient to proceed with shunting based on DTPA cisternography findings alone 1, 7
• Recognize that approximately 30% of NPH cases have overlapping Alzheimer pathology, which may affect outcomes 5

Algorithmic Approach

1. Order MRI brain without contrast as first-line imaging for suspected NPH 1
2. Evaluate for complete DESH pattern: ventriculomegaly (Evans >0.3) + tight high-convexity sulci + enlarged Sylvian fissures 1, 4
3. Assess callosal angle (<90° supports NPH) 1, 3
4. Look for aqueductal flow void on T2-weighted sequences (predicts shunt response) 1
5. Measure temporal horn enlargement not explained by hippocampal atrophy 3
6. If MRI shows classic NPH features and clinical triad is present, proceed to CSF diversion testing (tap test or external lumbar drainage) 3
7. Consider cine MRI if available to quantify CSF flow dynamics 1
8. Use CT only if MRI is contraindicated, recognizing its limitations 1

The combination of MRI-based DESH pattern with clinical symptoms has been validated in prospective studies showing 77-89% improvement rates after shunt surgery, making MRI essential for optimal patient selection. 4