MRI Brain for Normal Pressure Hydrocephalus: IV Contrast is NOT Needed
IV contrast should not be used when ordering an MRI brain to evaluate for normal pressure hydrocephalus (NPH). MRI brain without IV contrast is the appropriate imaging study for this indication 1.
Guideline-Based Recommendation
The American College of Radiology (ACR) Appropriateness Criteria explicitly state that for patients with suspected idiopathic normal pressure hydrocephalus, MRI head without IV contrast is usually appropriate for initial imaging 1. The guidelines emphasize that the diagnostic findings for NPH "are optimally visualized on a noncontrast MRI; MRI with IV contrast is not needed" 1.
Key Diagnostic Features Visible Without Contrast
The essential imaging features required to diagnose probable NPH are all visible on non-contrast MRI sequences 1:
- Ventricular enlargement (Evans index >0.3, calculated as maximal width of frontal horns divided by maximal width of inner skull) 1
- Enlargement of temporal horns 1
- Callosal angle <90 degrees 1
- Evidence of altered brain water content (transependymal CSF flow) 1, 2
- Aqueductal or fourth ventricle flow void on MRI 1
- Absence of macroscopic CSF flow obstruction 1
When Contrast Might Be Considered
The only scenario where IV contrast could have limited utility is when infectious or inflammatory conditions are among the differential diagnoses being considered 1. However, this represents a different clinical question beyond straightforward NPH evaluation and would suggest an alternative or additional diagnosis is being pursued.
Advanced MRI Techniques for NPH
For NPH evaluation, specialized non-contrast sequences provide additional diagnostic value 3, 4:
- Phase-contrast MRI can measure CSF flow velocity and stroke volume through the aqueduct of Sylvius, with elevated aqueductal CSF stroke volume predicting excellent shunt responsiveness 3, 4
- Cine MRI can demonstrate increased ventricular flow rate, supporting the diagnosis 1
- 3-D T2-weighted sequences optimize visualization of CSF spaces 4
Clinical Pitfall to Avoid
Do not order MRI with contrast reflexively for NPH evaluation, as this adds unnecessary cost, time, and potential contrast-related risks without improving diagnostic accuracy for this specific condition 1. The structural and flow-related abnormalities diagnostic of NPH are all detectable on non-contrast sequences 1, 2, 4.