Intracerebral abscess is a focal, suppurative infection of the brain parenchyma caused by bacteria, fungi, or parasites. Early and accurate imaging is critical to differentiate it from tumors or other ring-enhancing lesions.
📌 CT Imaging
- Non-contrast CT
- Early: ill-defined hypodensity (cerebritis stage).
- Mature abscess: hypodense core with hyperdense rim and surrounding vasogenic edema.
- Contrast CT
- Smooth, thin, ring-enhancement.
- Mass effect with midline shift may be present.
- Limitation: CT cannot reliably differentiate abscess from necrotic tumors.
📌 MRI Imaging
1. Cerebritis Stage (1–3 days)
- T2/FLAIR: ill-defined hyperintensity.
- Minimal or no enhancement.
2. Capsule Formation (after ~1 week)
- T1WI: Hypointense center, iso/hyperintense capsule.
- T2WI/FLAIR: Hyperintense center, hypointense capsule (due to collagen + paramagnetic effect), surrounding edema.
- Post-contrast: Smooth, thin, uniform ring enhancement.
3. Advanced Techniques
- Diffusion-Weighted Imaging (DWI):
- Restricted diffusion in pus (bright on DWI, low ADC).
- Helps differentiate abscess (restricted diffusion) vs necrotic tumor (no restriction).
- MR Spectroscopy (MRS):
- Elevated lactate, amino acids, acetate, succinate.
- Absence of tumor metabolites (choline, NAA).
- SWI: Can detect hemorrhage or calcification if present.
🔎 Differentials for Ring-Enhancing Lesions
- High-grade glioma
- Metastasis (necrotic)
- Resolving hematoma
- Demyelinating lesions (e.g., tumefactive MS)
- Tuberculoma
👉 Clue: Restricted diffusion on DWI is the hallmark of abscess.
📌 Teaching Pearls
- CT is fast and first-line in emergencies.
- MRI with DWI is gold standard for diagnosis.
- Abscess capsule is T2 hypointense, unlike most tumors.
- Always evaluate for source: sinusitis, otitis, dental infection, congenital heart disease.
🧾 Abscess vs Necrotic Tumor on Imaging
| Feature | Intracerebral Abscess | Necrotic Tumor (e.g., GBM, Metastasis) |
|---|---|---|
| CT (non-contrast) | Hypodense center, smooth margin | Hypodense necrotic center, irregular margin |
| CT (contrast) | Thin, smooth, uniform ring enhancement | Thick, irregular, nodular ring enhancement |
| T1 MRI | Hypointense center, iso/hyperintense capsule | Hypointense necrotic core, variable rim |
| T2/FLAIR | Hyperintense core, T2 hypointense capsule, marked edema | Hyperintense necrotic core, rim usually hyperintense, edema variable |
| DWI / ADC | Restricted diffusion (bright DWI, low ADC) | No restriction (necrotic fluid → low DWI, high ADC) |
| MR Spectroscopy | Amino acids, acetate, succinate, lactate | Elevated choline, reduced NAA, lipid/lactate peaks |
| Enhancement pattern | Smooth thin capsule | Irregular, nodular, thick rim |
| Edema | Prominent vasogenic edema, mass effect | May be present, but variable |
| Clinical setting | Infection (sinusitis, otitis, congenital heart disease, immunosuppression) | Malignancy (primary GBM, or metastatic disease) |
📌 Teaching Pearl
- Restricted diffusion = abscess.
- Irregular, nodular rim = tumor.
