Figure 4: Contrast-enhanced T1-weighted MR (upper) and 18F-FDG PET (lower) images in PCNSL patients with atypical MR findings. MR images show a ring-like enhanced lesion in the right frontal lobe (a), multiple faint enhanced lesions in the right cerebral cortex and the corpus callosum (b), multiple small enhanced lesions in the bilateral frontal white matter and the corpus callosum (c), and no enhancing lesion in the brain (d). PET images show no 18F-FDG uptake in the lesions except for moderate 18F-FDG uptake in the right frontal cortex (arrow) in case (b).