Figure 1. Serial MRI images. At baseline, axial proton-density spin-echo images showed hyperintense lesions in cortical and juxtacortical areas of the frontal lobes (A1). Abnormal hyperintensity of the middle cerebellar peduncles was also observed on axial turbo-spin-echo (TSE) T2-weighted images (A2), consistent with progressive multifocal leukoencephalopathy (PML). After administration of gadolinium, a subtle enhancement of these lesions on axial T1-weighted magnetization transfer (MT) images was observed (A3 and A4). After 6 weeks, progression of the PML lesions on axial double inversion recovery (DIR) images (B1) and TSE T2-weighted images (B2) was observed. In addition, Gd+ contrast enhancement became more conspicuous on axial T1-weighted MT images (B3 and B4). In the cerebral MRI performed after 15 weeks, the hyperintense lesions on the frontal lobes and middle cerebral peduncles had enlarged, as shown on axial DIR images (C1) and on TSE T2-weighted images (C2). Worsening of these findings was more subtle in the frontal lobes (C4) than in the cerebellum (C3) by axial post-contrast T1-weighted MT images. Cerebral-MRI performed after 31 weeks showed improvement in PML lesions as shown on DIR images (D1 and D2). MRI findings suggested neurodegenerative phenomena as we observed hyperintensity of pontocerebellar fibers (arrow) on axial TSE T2-weighted images (D3) and atrophy at previous PML lesions in the cerebral hemispheres. These findings were more conspicuous in the right precentral gyrus (arrow) on T1 inversion recovery image (D4).