MRI Sequences for PML Evaluation
Order a brain MRI with FLAIR, T2-weighted, T1-weighted pre- and post-contrast, and diffusion-weighted imaging (DWI) sequences to evaluate for PML. 1
Essential MRI Protocol
The following sequences should be included in your imaging protocol:
Primary Sequences (Must Include)
FLAIR (Fluid-Attenuated Inversion Recovery): This is the preferred and most sensitive sequence for PML diagnosis due to the characteristic subcortical location of PML lesions 1. FLAIR best demonstrates the hyperintense signal changes in subcortical white matter and U-fiber involvement that characterize PML.
T2-weighted imaging: Shows diffuse hyperintensity with irregular signal intensity within lesions, often with a punctate microcystic appearance 1. Small punctate T2 lesions may be visible near the main lesion.
T1-weighted imaging (pre- and post-gadolinium contrast): Essential for detecting enhancement patterns. While less than half of PML cases show enhancement at presentation, when present it typically appears as patchy or punctate, with possible rim enhancement at the leading edge of larger lesions 1. Contrast enhancement is critical for differentiating PML from other conditions and for identifying immune reconstitution inflammatory syndrome (IRIS).
Diffusion-weighted imaging (DWI): Acute PML lesions appear hyperintense on DWI 1. This sequence is particularly valuable for detecting new PML lesions within confluent areas of chronic white matter disease and correlates with disease activity 2, 3, 4. The extent of abnormal diffusion correlates with speed of clinical progression 4.
Key Imaging Features to Identify
When reviewing the MRI, look for these characteristic PML findings:
- Location: Subcortical rather than periventricular, affecting U-fibers and extending into the gyrus, with cortical gray matter involvement in 50% of cases 1
- Size and distribution: Generally large lesions (>3 cm) in unifocal, multifocal, or widespread distribution 1
- Borders: Irregular shape with ill-defined borders toward white matter but sharp borders toward cortical gray matter 1
- Mass effect: Typically absent even in large lesions (unless inflammatory response is present) 1
- Progressive expansion: Lesion volume increases continuously to contiguous and non-contiguous regions confined to white matter tracts 1
Critical Diagnostic Pitfalls
None of the MRI features are pathognomonic for PML 1. The absence of contrast enhancement does not exclude PML—in fact, lack of enhancement should raise suspicion for PML rather than CNS lymphoma or other inflammatory conditions 5.
Early detection at the asymptomatic stage via MRI surveillance is associated with substantially better outcomes in terms of survival and functional parameters 2. Serial MRI examinations are essential if clinical suspicion remains high, as lesions evolve over time and may show different characteristics at follow-up 3.
Advanced sequences like diffusion tensor imaging (DTI) and MR spectroscopy can provide additional information but are not required for initial diagnosis 6, 7.