Is CT Adequate for Diagnosing Normal Pressure Hydrocephalus?
CT scan of the brain can be used as an alternative imaging modality for NPH when MRI is contraindicated or unavailable, but it is not the preferred diagnostic tool because it has lower sensitivity for detecting key NPH features and may miss important findings that influence treatment decisions. 1, 2, 3
Why MRI is Preferred Over CT
The American College of Radiology recommends MRI without IV contrast as the first-line imaging modality for diagnosing NPH due to its superior ability to detect characteristic features that CT cannot visualize. 1, 2, 3
Critical Features MRI Detects That CT Cannot:
- Cerebral aqueduct flow void - This finding is associated with good response to shunt surgery and is completely invisible on CT imaging 3
- Periventricular white matter changes - MRI has significantly higher sensitivity for detecting these changes compared to CT 3
- Small obstructing lesions - CT may miss these entirely, potentially leading to misdiagnosis 3
- Interstitial edema (transependymal CSF flow) - MRI is more sensitive to this finding than CT 4
- Hyperdynamic CSF flow - This marker of shunt-responsive NPH is better visualized on MRI 4
What CT Can Detect
When MRI is not available, CT head without IV contrast can identify several important NPH features 1, 2, 3:
- Ventriculomegaly (Evans index >0.3)
- Narrowed posterior callosal angle
- Effaced sulci along high convexities
- Widened sylvian fissures
- Dilatation of temporal horns 5
Clinical Implications of Using CT Instead of MRI
The limitation of CT becomes particularly problematic because approximately 75% of NPH patients severe enough to require treatment also have another comorbid neurodegenerative disorder 2, 6. The superior soft-tissue characterization of MRI is essential for distinguishing NPH from other conditions and identifying patients who will respond to shunt surgery 4.
Common Pitfall to Avoid:
- Relying solely on CT findings without MRI confirmation when MRI is possible can lead to misdiagnosis and inappropriate treatment decisions 1. Since NPH is potentially reversible with ventriculoperitoneal shunt placement, but 80% of cases remain unrecognized and untreated, using the most sensitive imaging modality is critical 6.
Algorithmic Approach to Imaging Selection
If MRI is available and not contraindicated → Use MRI without IV contrast 1, 2, 3
If MRI is contraindicated or unavailable → Use CT head without IV contrast, but recognize its limitations in detecting cerebral aqueduct flow void and periventricular white matter changes 1, 2, 3
If CT shows ventricular enlargement without typical NPH features → Consider additional diagnostic procedures such as CSF tap-test or prolonged lumbar drainage to simulate shunt effect 7, 5