Introduction
A liver abscess is a localized collection of pus within the liver, most often due to bacterial (pyogenic) or parasitic (amoebic) infection. Radiology plays a crucial role in diagnosis, guiding drainage, and differentiating from other hepatic lesions.
Ultrasound (USG) is the initial investigation of choice, while CT/MRI provide better characterization and are invaluable for treatment planning.
Ultrasound Findings
- Pyogenic abscess:
- Hypoechoic to complex cystic lesion.
- May have internal echoes, septations, or debris.
- Ill-defined margins in acute cases.
- Amoebic abscess:
- Typically single, large, round, hypoechoic lesion in the right lobe.
- “Anchovy paste” aspirate is classic.
- Color Doppler: Peripheral hyperemia, absent central vascularity.
CT Imaging Features (Gold Standard for Diagnosis)
- Pyogenic abscess:
- Hypodense lesion with peripheral rim enhancement (“double target sign”).
- May contain gas (air-fluid level).
- Multiloculated appearance in chronic cases.
- Amoebic abscess:
- Solitary, round/oval hypodense lesion, often subcapsular.
- Smooth margins, minimal peripheral enhancement.
- Complications: Rupture into peritoneum, pleura, or pericardium.
MRI Findings
- T1-weighted: Hypointense lesion.
- T2-weighted: Hyperintense with hypointense rim.
- Post-contrast: Peripheral rim enhancement with central non-enhancing cavity.
- Diffusion: Restricted diffusion in pus-filled cavity (helps differentiate from cyst or necrotic tumor).
Differential Diagnosis
- Necrotic/metastatic tumor (irregular enhancing margins, solid components).
- Hepatic cyst (no rim enhancement, no diffusion restriction).
- Hydatid cyst (daughter cysts, calcified wall, internal membranes).
📊 Pyogenic vs Amoebic Liver Abscess – Imaging Comparison
| Feature | Pyogenic Liver Abscess | Amoebic Liver Abscess |
|---|---|---|
| Number | Often multiple | Usually single |
| Location | Any lobe, commonly right | Predominantly right lobe, subcapsular |
| Ultrasound | Complex, hypoechoic/cystic lesion with septa, internal echoes | Hypoechoic, round/oval, homogenous with smooth margins |
| CT Findings | Hypodense lesion with peripheral rim enhancement (“double target sign”), may contain gas | Hypodense, smooth-walled, minimal rim enhancement, usually no gas |
| MRI T1 | Hypointense | Hypointense |
| MRI T2 | Hyperintense with hypointense rim | Hyperintense, relatively uniform |
| Enhancement | Thick, irregular rim; multiloculated in chronic cases | Thin, smooth rim; minimal enhancement |
| Classic Sign | “Double target sign” | Large solitary lesion with smooth margin (“anchovy paste” aspirate) |
| Complications | Gas formation, rupture into peritoneum/pleura | Rupture less common, but can occur into pleura/pericardium |
Role of Radiology
- Early diagnosis and differentiation of abscess type.
- Guiding percutaneous aspiration/drainage.
- Assessing complications like rupture or sepsis.
- Follow-up to monitor resolution.
Teaching Points
- Liver abscess = hypoechoic/complex cystic lesion on USG, hypodense with rim enhancement on CT.
- Double target sign = classic for pyogenic abscess.
- Amoebic abscess → usually solitary, right lobe, smooth walls, minimal enhancement.
- Radiology guides diagnosis + intervention.
Conclusion
Radiology provides the most accurate and non-invasive method to diagnose liver abscesses. Recognizing imaging features on USG, CT, and MRI not only helps in distinguishing pyogenic from amoebic abscesses but also assists in timely intervention and management.
