Brain MRI of a patient with multiple sclerosis who developed natalizumab-associated PML

Brain MRI (A-B and D-H, axial; C, sagittal) shows development of PML in a 36-year-old patient with multiple sclerosis treated with natalizumab.
(A) One year prior to presentation, FLAIR image shows foci of subcortical and deep white matter demyelination consistent with multiple sclerosis (arrowheads).
(B-H) While being treated with natalizumab one year later, FLAIR images demonstrate a white matter lesion in the subcortical white matter of left pre-central gyrus consistent with PML (arrows). Typical periventricular lesions consistent with multiple sclerosis are also present (arrowheads).