A 32-year-old man with cyanotic congenital heart disease (uncorrected) presents with a 2-week history of progressive headache, low-grade fever, and focal left-sided weakness. MRI brain with contrast shows a ring-enhancing lesion in the right MCA territory with surrounding vasogenic edema and mass effect. Diffusion-weighted imaging (DWI) shows the lesion core marked **A** is hyperintense with restricted diffusion (low ADC). Which of the following best explains the imaging findings at the structure marked **A**?

Explanation

## Why "Viscous purulent material with high cellularity and debris causing restricted diffusion" is right The structure marked **A** — the ring-enhancing lesion core with restricted diffusion on DWI/ADC — is pathognomonic for a brain abscess. The restricted diffusion (hyperintense DWI, low ADC) is caused by the viscous, purulent material within the abscess cavity, which contains high cellularity, inflammatory debris, and bacterial products. This restricts the free movement of water molecules, producing the characteristic DWI signal. The clinical context (cyanotic CHD with R→L shunt bypassing the pulmonary filter, hematogenous seeding to gray-white junction in MCA territory, subacute presentation with headache and fever) confirms brain abscess. Harrison 21e Ch 137 emphasizes that DWI with restricted diffusion is the KEY DISCRIMINATOR that distinguishes abscess from other ring-enhancing lesions. ## Why each distractor is wrong - **Liquefied necrotic center with facilitated diffusion due to cell death and lysis**: This describes the necrotic core of a glioblastoma or metastasis, which shows LOW signal on DWI and HIGH ADC (facilitated diffusion). The question explicitly states the core marked **A** has RESTRICTED diffusion, ruling out this option. This is the classic teaching point: abscess = restricted DWI; tumor necrosis = facilitated DWI. - **Solid tumor matrix with central hemorrhage and hemosiderin deposition**: Glioblastomas and other malignancies may show hemorrhage and hemosiderin, but they do not produce the restricted diffusion pattern seen in abscess. Hemorrhage would show signal dropout on gradient echo sequences, not the hyperintense DWI with low ADC characteristic of pus. - **Demyelinating plaque with inflammatory infiltrate and myelin breakdown products**: Demyelinating lesions (MS, ADEM) may show ring enhancement and edema, but they do not produce the marked restricted diffusion of an abscess core. Demyelinating plaques typically show facilitated or normal diffusion, not the viscous pus-related restriction seen here. **High-Yield:** Brain abscess core = restricted DWI (pus); tumor necrosis = facilitated DWI (liquefied). Cyanotic CHD + hematogenous seeding = multiple abscesses at gray-white junction. [cite:Harrison 21e Ch 137]