@imshahm: One ring on the MRI. Four diseases wear it. The ring's edge names the lesion. Smooth and thin with bright diffusion is abscess, never biopsy pus. Thick, irregular, crossing the midline like a butterfly is glioblastoma. Many small rings at the gray-white junction are metastasis. The open ring broken toward the cortex is demyelination, not a tumor. Which lesion, and what must you never do? Answer below. #USMLE #Neurology #Radiology #MedStudent