How do you interpret CT (Computed Tomography) chest imaging?

How to Interpret CT Chest Imaging

Systematic Approach to CT Chest Interpretation

Use a standardized window/level technique to evaluate different anatomical compartments sequentially, starting with lung windows (width 1500 HU, level -600 HU) followed by mediastinal windows (width 350 HU, level 40 HU). 1

Technical Protocol Requirements

• High-resolution CT (HRCT) with thin-section images is essential for evaluating lung parenchyma, particularly when diffuse lung disease is suspected 2
• Volumetric CT data acquisition allows multiplanar thin-section reconstructions, which aids in evaluating disease distribution 2
• Include both inspiratory and expiratory images when evaluating for air trapping or small airways disease 2
• Add prone images to distinguish dependent atelectasis from true parenchymal abnormality 2

Step-by-Step Interpretation Algorithm

1. Lung Parenchyma Evaluation (Lung Windows)

• Scan systematically from apex to base, examining each lobe sequentially 3
• Look for the signet ring sign (bronchus-to-artery ratio >1:1), which confirms bronchiectasis with >90% sensitivity and specificity 4
• Identify ground-glass opacities, consolidation, nodules, masses, or interstitial patterns 2
• Assess distribution: upper vs. lower lobe, central vs. peripheral, symmetric vs. asymmetric 2
• CT is far more sensitive than chest radiography for detecting small pulmonary nodules and early parenchymal changes, with chest radiographs missing up to 72% of abnormalities detected on CT 2

2. Airways Assessment

• Evaluate trachea and main bronchi for narrowing, deviation, or masses 3
• Assess bronchial wall thickening and bronchiectasis 2
• Look for mucus plugging or endobronchial lesions 3

3. Mediastinum Evaluation (Mediastinal Windows)

• Lymph nodes <1.0 cm are normal; 1.0-1.5 cm are suspicious; >1.5 cm have high probability of malignancy 5
• Evaluate all mediastinal compartments: prevascular, visceral, paravertebral 2
• Assess for masses, noting location, attenuation (fat, fluid, soft tissue, calcium), and enhancement pattern 2
• Pre- and post-contrast imaging distinguishes enhancing cellular components from cystic/necrotic areas 2

4. Pleura and Chest Wall

• CT provides superior visualization of the entire pleura compared to radiography, including posterior recesses and mediastinal surfaces 6
• Identify pleural thickening (nodular vs. smooth), effusions, or pneumothorax 2, 6
• Assess for chest wall invasion by evaluating tissue plane transgression and osseous erosion 2

5. Cardiovascular Structures

• Evaluate great vessels for aneurysm, dissection, or mass effect 3
• IV contrast improves detection of mediastinal and hilar adenopathy by distinguishing nodes from vessels 2
• Assess pericardium for effusion or thickening 2

6. Bones and Soft Tissues

• Examine ribs, sternum, and thoracic spine for fractures or lytic/blastic lesions 2
• Evaluate chest wall soft tissues for masses or edema 3

Key Diagnostic Patterns

For Diffuse Lung Disease

• HRCT pattern characterization is especially important for diagnosing usual interstitial pneumonia and other interstitial lung diseases 2
• CT findings often permit either a limited differential or confident single diagnosis 2
• Expiratory CT reflects airflow limitation and correlates with dyspnea severity 2

For Mediastinal Masses

• CT can demonstrate and distinguish calcium, macroscopic fat, and water-attenuation fluid, permitting noninvasive diagnosis of many mature teratomas 2
• Assess for invasion across tissue planes, which indicates higher probability of incomplete surgical resection 2

For Pulmonary Nodules

• Nodules with Hounsfield numbers >175 HU can be presumed calcified and benign 5
• Evaluate size, margins, attenuation, and enhancement characteristics 5

Common Pitfalls to Avoid

• Do not rely on chest radiography alone—it may appear normal in early bronchiectasis or diffuse lung disease 2, 4
• Chest radiography has only 28% sensitivity for detecting pulmonary metastases compared to CT 2
• Do not mistake benign hyperattenuating thymic cysts for thymomas on CT; MRI provides superior soft tissue characterization 2
• Node size criteria alone should not deny surgery—consider location and presence of infection 5
• Physical examination findings like crackles do not correlate reliably with CT-diagnosed bronchiectasis 4

When to Use IV Contrast

• Use IV contrast to evaluate mediastinal masses, lymphadenopathy, and vascular structures 2
• Contrast helps distinguish enhancing tumor from necrosis or cyst 2
• For screening pulmonary metastases in high-risk patients, non-contrast CT is adequate 2
• Avoid contrast if evaluating for pulmonary embolism requires dedicated CT pulmonary angiography protocol 2