What is the best imaging test for pleural effusion?

Best Imaging for Pleural Effusion

Thoracic ultrasound (TUS) is the best initial imaging modality for pleural effusion, serving as the gold standard for detecting and characterizing pleural fluid with 93-96% sensitivity and specificity, while chest radiography (PA and lateral views) remains appropriate as the first-line screening test in stable patients. 1

Initial Imaging Strategy

Chest Radiography as First-Line Screening

• PA and lateral chest radiographs should be obtained first in stable patients, detecting >75 mL of fluid on lateral view and >175 mL on frontal view 1
• Chest X-ray has 83.9% sensitivity when using CT as reference standard, but only 60% specificity for detecting complicated parapneumonic effusions 2
• The lateral view is more sensitive than frontal view alone, as single-view radiographs may miss effusions, particularly with coexistent lower lobe consolidation 2

Thoracic Ultrasound as the Diagnostic Gold Standard

• Ultrasound detects >20 mL of pleural fluid and should be used for all diagnostic aspirations, as it is more accurate than chest radiography for estimating fluid volume 3, 1
• Point-of-care ultrasound has superior diagnostic accuracy compared to chest radiography (94.54% vs. 67.68% sensitivity and 97.88% vs. 85.30% specificity) 4
• Ultrasound-guided pleural aspiration yields fluid in 97% of cases after unsuccessful thoracentesis or in loculated effusions 3
• TUS provides critical information on effusion size, character, and signs of malignancy (nodularity of diaphragm and parietal pleura) that streamline the diagnostic pathway 3

When to Proceed Directly to Advanced Imaging

CT with Contrast Enhancement

• If thoracentesis is unsafe based on ultrasound evaluation, CT chest with IV contrast should be obtained as the next step 3
• When malignancy is suspected, CT should include chest, abdomen, and pelvis; for non-malignant cases, CT thorax with pleural contrast (venous phase) is sufficient 3
• CT scans must be performed with contrast enhancement, acquired 60 seconds after IV contrast bolus to optimize pleural visualization 3, 1
• Contrast-enhanced CT should be performed before full drainage of fluid, as pleural abnormalities are better visualized with fluid present 3

Specific CT Indications

• Loculated effusions requiring difficult drainage: CT delineates size and position of loculations with characteristic lenticular shape 3
• Suspected malignancy: Multiple pleural nodules and nodular pleural thickening (>1 cm parietal thickening, circumferential thickening, mediastinal involvement) have 88-100% specificity for malignant disease 3, 5
• Differentiating pleural from parenchymal disease: CT is superior to plain radiographs for this distinction 3

Algorithmic Approach

Step 1: Clinical Assessment

• Obtain PA and lateral chest radiographs in stable patients as initial screening 1, 2
• Document occupational history (asbestos exposure) and complete medication history 1

Step 2: Ultrasound Evaluation

• Perform thoracic ultrasound on all patients before any pleural procedure to answer: "Is it safe to perform diagnostic aspiration?" 3
• Ultrasound characterizes effusion size, identifies septations (better visualized than on CT), and detects signs of malignancy 3
• Always use ultrasound guidance for thoracentesis to reduce complications 1

Step 3: Decision Point

• If safe to aspirate: Proceed with ultrasound-guided thoracentesis 3, 1
• If unsafe to aspirate or malignancy suspected: Obtain CT chest with IV contrast 3
• If effusion too small to sample: Watchful waiting with interval CT scans is appropriate 3

Critical Pitfalls to Avoid

• Never proceed with thoracentesis without ultrasound guidance, as this significantly increases complication risk 1
• Do not use CT angiography for pleural evaluation, as contrast timing (<60 seconds) is too early for adequate pleural enhancement 1
• Do not rely on supine chest radiographs alone, as they underestimate fluid volume and show only hazy opacity with preserved vascular shadows 3
• Do not perform CT after complete drainage, as this reduces visualization of pleural abnormalities 3

Role of MRI

• MRI does not provide better imaging than CT scanning and has a limited evolving role 3