Brain: Patterns, Compartments, and Vascular Logic
Core concepts
Build every brain question around three axes: location (supra- vs infratentorial, intra- vs extra-axial), number/timing (single vs multiple, acute vs chronic), and signal behavior (T1, T2/FLAIR, DWI/ADC, enhancement, susceptibility). Know the compartments: extra-axial spaces (epidural, subdural, subarachnoid) and intra-axial white/gray matter. Master the vascular territories (ACA, MCA, PCA, basilar perforators, watershed zones) and the classic stroke evolution timeline on DWI/ADC. For white matter disease, separate demyelination (periventricular/perivenular, Dawson fingers, open-ring enhancement), small-vessel ischemia (confluent, periventricular halo), and PRES (posterior, vasogenic edema). For masses, learn the mnemonic-style discriminators: ring-enhancing lesion in a ring-enhancer vs. tumefactive demyelination vs. abscess (restricted diffusion centrally).
Traps
Questions love to test ADC vs. DWI mismatches (T2 shine-through), hemorrhage age (T1 hyperintensity of methemoglobin, susceptibility of hemosiderin), and midline vs. off-midline location for pediatric tumors. A common trap is confusing vasogenic (finger-like, spares cortex) with cytotoxic (involves cortex, restricts diffusion) edema. Another is calling a chronic microhemorrhage an acute bleed.
Worked rule of thumb
For any ring-enhancing brain lesion, ask: does the center restrict diffusion? If yes, favor abscess; if the ring is incomplete (open toward cortex) with peripheral restricted diffusion, favor demyelination; if thick irregular with surrounding vasogenic edema, favor neoplasm. Use location plus patient immune status to finalize.