MRI Features of PML
| Features | |
|---|---|
| Location | Subcortical white matter U-fibers |
| Predilection for frontal lobe > occipital lobe > parietal lobe | |
| Size | Small, <3 cm when asymptomatic |
| In symptomatic patients, lesions are often large (>3 cm) | |
| Morphology | Punctate lesions often identified in asymptomatic PML |
| Symptomatic PML typically demonstrates more confluent and diffuse lesions | |
| Microcysts | |
| No associated edema or mass effect | |
| T1WI and T2WI | FLAIR hyperintensity with corresponding T1 hypointense lesions |
| T1 hypointesity is less pronounced in asymptomatic PML | |
| DWI | Hyperintensity on DWI |
| T1 postcontrast | 30% will demonstrate linear or punctate enhancing lesions; if nodular or rim enhancement is present, active MS plaque or IRIS should be considered |
| SWI | Low signal intensity in U-fibers |
| Differential diagnosis | MS plaque |
| Acute disseminated encephalomyelitis | |
| Tumefactive lesion | |
| Ischemic changes | |
| Posterior reversible encephalopathy syndrome |