Is a chest CT with contrast, without contrast, or both better for the evaluation of empyema?

Chest CT with IV Contrast is Superior for Evaluation of Empyema

Chest CT with IV contrast is the preferred imaging modality for evaluation of empyema due to its superior ability to detect pleural enhancement, which has the highest diagnostic accuracy for empyema. 1, 2

Diagnostic Accuracy of Different CT Approaches

CT with IV Contrast

• Recommended by the American Association for Thoracic Surgery consensus guidelines (class IIa recommendation) for suspected parapneumonic effusion and empyema 1
• Pleural enhancement has the highest area under curve (0.86) for diagnosing empyema and distinguishing it from simple parapneumonic effusion (0.83) 1
• The combination of parietal pleural enhancement and pleural thickening was seen in 98.7% of patients with laboratory-proven pleural infection on pleural-phase contrast-enhanced CT 1
• Technical optimization: acquiring the CT scan 60 seconds after IV contrast bolus optimizes visualization of the pleura 1

CT without IV Contrast

• Can detect 4 out of 5 chest CT findings associated with empyema, but misses the most sensitive finding (pleural enhancement) 1
• Findings visible on non-contrast CT include:
  • Pleural thickening (sensitivity 68%, specificity 87%) 1
  • Loculation (sensitivity 52%, specificity 89%) 1
  • Extrapleural fat proliferation (sensitivity 53%, specificity 91%) 1
  • Increased attenuation of extrapleural fat (sensitivity 39%, specificity 97%) 1
• Gas in the pleural space is another specific marker for complicated parapneumonic effusion (specificity 81-96%) 1

CT without and with IV Contrast

• No relevant literature supports the use of this approach for initial imaging of suspected parapneumonic effusion or empyema 1

CTA Chest with IV Contrast

• Not recommended for empyema evaluation 1, 2
• CTA employs contrast timing that is earlier than 60 seconds and does not allow sufficient time for pleural enhancement 1, 2

Key Diagnostic Features on CT with IV Contrast

• Pleural enhancement (sensitivity 84%, specificity 83%) - highest diagnostic value 1
• Pleural thickening (sensitivity 68%, specificity 87%) 1
• Loculation (sensitivity 52%, specificity 89%) 1
• Extrapleural fat proliferation (sensitivity 53%, specificity 91%) 1
• Increased attenuation of extrapleural fat (sensitivity 39%, specificity 97%) 1
• "Split pleura sign" - separation of visceral and parietal pleura 2, 3

Role of Other Imaging Modalities

Ultrasound

• Excellent for identifying internal characteristics of pleural effusions (septations, fibrin strands, complex fluid) 1, 2
• High sensitivity (94-96%) and specificity (96-100%) for differentiating lung abscess from empyema 2
• Useful for guiding interventional procedures like thoracentesis 1, 2
• Limited in evaluating the full extent of disease compared to CT 4, 3

MRI

• Limited evidence for use in adult empyema evaluation 1
• May be useful in specific cases like empyema necessitans 1
• In pediatric patients, limited data suggest MRI is noninferior to CT with IV contrast 1

Common Pitfalls to Avoid

• Using CTA protocols for suspected empyema is suboptimal as the timing does not allow for adequate pleural enhancement 1, 2
• Relying solely on non-contrast CT misses pleural enhancement, the most sensitive finding for empyema 1, 2
• Small parapneumonic effusions (<2.5 cm in anteroposterior dimension) can often be managed without thoracentesis, so accurate measurement is important 1
• CT provides diagnostic information not available from conventional radiographs in 47% of cases and more accurately delineates disease extent in an additional 34% 4, 3

Prognostic Value of CT Findings

• Certain CT findings have prognostic value for 90-day mortality in empyema:
  • Presence of bronchopleural fistula predicts higher 90-day mortality 5
  • Interlobar pleural effusion predicts lower 90-day mortality 5

In conclusion, CT chest with IV contrast provides the most comprehensive and accurate evaluation of empyema by enabling visualization of pleural enhancement, which is the most sensitive and specific finding for diagnosing empyema and distinguishing it from simple parapneumonic effusion.