Pyogenic Brain Abscess — Ring Enhancement with DWI Restriction

A 47-year-old man with poorly controlled diabetes (HbA1c 10.8%) presents with a 10-day history of progressive headache, low-grade fever, and a new seizure. He had a dental extraction for periapical abscess 3 weeks ago. MRI brain with gadolinium shows a 2.5 cm round lesion in the right parietal lobe with a thin smooth ring of enhancement and a central non-enhancing cavity. The region marked **B** in the diagram (central cavity with restricted diffusion) demonstrates markedly bright signal on diffusion-weighted imaging and dark signal on the ADC map. Which of the following best explains the restricted diffusion within the structure marked **B**?