What type of radiographic workup is recommended for a patient with empyema thoracis (empyema of the thoracic cavity)?

Radiographic Workup for Empyema Thoracis

Begin with posteroanterior (PA) and lateral chest radiographs as the initial imaging study, followed by chest ultrasound to confirm the effusion and guide thoracentesis, then proceed to contrast-enhanced CT chest if the diagnosis remains unclear or complications are suspected. 1, 2

Initial Imaging: Chest Radiography

• Obtain PA and lateral chest radiographs first in all patients with suspected empyema, as consensus guidelines endorse this as the initial imaging modality 1
• PA and lateral views achieve 83.9% sensitivity for detecting parapneumonic effusions, significantly superior to single-view AP radiographs which only achieve 67.3% sensitivity 1, 3
• Be aware that chest radiography has modest specificity (only 60%) for detecting complicated parapneumonic effusions requiring thoracentesis 1
• Most missed effusions occur when lower lobe consolidation is present, masking the fluid collection 1, 3

Second-Line Imaging: Chest Ultrasound

Ultrasound is the preferred modality after initial radiography and should be performed in all cases to guide management. 1, 2

• Ultrasound differentiates free from loculated pleural fluid, determines fluid echogenicity, demonstrates pleural thickening, and guides thoracentesis or chest drain placement 1, 2, 3
• Specific ultrasound findings associated with empyema include septations, increased echogenicity, pleural thickening, and microbubbles 1
• Ultrasound demonstrates superior specificity (90%) compared to CT for diagnosing complicated parapneumonic effusion, though sensitivity is slightly lower at 69.2% 1
• Fibrinous septations are better visualized on ultrasound than CT scanning, making it the gold standard for characterizing pleural fluid collections 2, 4

Advanced Imaging: CT Chest with IV Contrast

Obtain contrast-enhanced CT chest when the diagnosis remains uncertain, when ultrasound suggests complex loculated disease, or when patients fail to improve after 5-7 days of appropriate treatment. 1, 2, 3

Key CT Indications:

• Delineating loculated pleural fluid collections 1, 2, 3
• Detecting airway or parenchymal lung abnormalities 2
• Differentiating empyema from lung abscess in diagnostically difficult cases 1, 5
• Evaluating patients who fail initial chest tube drainage 1

Critical CT Findings:

• The "split pleura" sign (enhancing pleural tissue visible on both visceral and parietal surfaces) is highly suggestive of empyema 1, 3, 6
• Pleural enhancement has 84% sensitivity and 83% specificity for empyema 3
• Pleural thickening shows 68% sensitivity and 87% specificity 3
• Loculation demonstrates 52% sensitivity and 89% specificity 3
• Extrapleural fat proliferation has 53% sensitivity and 91% specificity 3

Technical Considerations:

• Acquire images 60 seconds after IV contrast bolus to optimize pleural visualization 1, 3, 4
• Standard CTA protocols with earlier contrast timing (<60 seconds) do not allow sufficient time for pleural enhancement 1
• CT correctly diagnoses 100% of empyema cases when properly performed 5

Imaging Algorithm Summary:

1. Start with PA and lateral chest radiographs to detect pleural effusion and pulmonary infiltrates 1, 2
2. Proceed immediately to chest ultrasound to confirm fluid, assess for loculations, and guide diagnostic thoracentesis 1, 2, 3
3. Obtain contrast-enhanced CT chest if diagnosis unclear, complex loculations present, or patient fails to improve with initial drainage 1, 2, 3

Critical Pitfalls to Avoid:

• Do not rely on single-view AP radiographs, as they miss nearly one-third of parapneumonic effusions 1
• Do not skip ultrasound before attempting thoracentesis, as it significantly improves safety and diagnostic yield 1, 2
• Do not order CT without IV contrast for suspected empyema, as pleural enhancement is a key diagnostic feature requiring contrast administration 1, 3
• Do not assume small effusions (<2.5 cm) require thoracentesis, as many resolve with antibiotics alone 3, 4
• Perform diagnostic thoracentesis in all patients with suspected pleural infection to obtain fluid for pH, glucose, LDH, Gram stain, and culture 1, 2

Role of MRI:

• MRI has no established role in the initial workup of empyema in adults 1
• Limited pediatric data suggest MRI may be noninferior to CT, but practical limitations (longer acquisition time, cost, availability) restrict its use 1