Yes, you should order brain MRI with and without contrast for suspected PML
While FLAIR is the preferred sequence for PML diagnosis and contrast enhancement is often absent, ordering both with and without contrast is clinically appropriate because contrast can help differentiate PML from other conditions and detect inflammatory changes.
Why Both Sequences Are Valuable
Contrast Enhancement Patterns in PML
The evidence shows that less than half of PML cases demonstrate contrast enhancement at presentation, typically appearing as patchy or punctate patterns, with possible rim enhancement at the leading edge of larger lesions 1. However, the absence of contrast enhancement is itself a diagnostically useful finding that helps distinguish PML from other conditions 2.
Key Diagnostic Considerations
The lack of gadolinium enhancement can be critical for ruling out alternative diagnoses:
- In immunocompromised patients, the absence of enhancement helps differentiate PML from CNS lymphoma, which typically enhances robustly 2
- Multiple sclerosis lesions show homogeneous nodular or ring enhancement in acute phases, contrasting with PML's minimal or absent enhancement 1
- When PML is in the differential but enhancement is absent, this finding supports the diagnosis and should prompt CSF testing for JCV DNA 2
Optimal MRI Protocol for PML
Essential Sequences
FLAIR is the preferred sequence for PML diagnosis due to the subcortical location of lesions 1. Your complete protocol should include:
- FLAIR: Best for detecting subcortical white matter involvement
- T2-weighted: Shows diffuse hyperintensity with punctate microcystic appearance
- T1-weighted pre- and post-contrast: Documents hypointense lesions and detects any enhancement patterns
- Diffusion-weighted imaging (DWI): Helpful for detecting new PML lesions within confluent areas of chronic white matter disease 1
Important Caveats
Contrast enhancement may appear during immune reconstitution inflammatory syndrome (IRIS), which can develop after stopping immunosuppressive therapy or initiating antiretroviral treatment 3. Having baseline contrast imaging allows you to detect this complication, which requires different management.
Clinical Pitfalls to Avoid
- Don't rule out PML based solely on lack of enhancement - this is actually a typical finding 1, 2, 4
- Atypical presentations exist: Some cases show focal hemorrhage, faint peripheral enhancement, or deep gray matter involvement 5
- Mass effect is typically absent in PML even with large lesions, unless inflammatory response is present 1
- Hyperperfusion on arterial spin labeling (ASL) may occur in extensive cases, which can mimic lymphoma 6
Diagnostic Algorithm
When MRI shows white matter lesions in an immunocompromised patient:
- Assess enhancement pattern: Absent or minimal patchy enhancement suggests PML over lymphoma or active MS
- Evaluate U-fiber involvement: Sharp border with cortex from subcortical involvement is characteristic of PML 5
- Check for mass effect: Absence despite large lesions favors PML 1
- Proceed to CSF JCV PCR if imaging is consistent with PML, even if contrast enhancement is present 1
The contrast study provides essential information for differential diagnosis and monitoring for complications, making the "with and without" protocol the appropriate choice for suspected PML.