How do I interpret a computed tomography (CT) scan of the chest?

How to Interpret a CT Chest Scan

Adopt a systematic, algorithmic approach to CT chest interpretation that begins with technical assessment, proceeds through anatomic compartments, and categorizes findings by pattern recognition—this structured method substantially enhances diagnostic accuracy across the wide spectrum of thoracic pathology. 1, 2

Step 1: Technical Assessment and Image Quality

Before interpreting any findings, verify the technical adequacy of the study:

• Confirm appropriate protocol selection: High-resolution CT (HRCT) with thin-section images (≤1.5 mm, typically 1.0 mm) is essential for diffuse lung disease evaluation 3, 4
• Check for contrast administration: Determine whether IV contrast was given, as this affects your ability to evaluate vascular structures, mediastinal masses, and pleural disease 3
• Review the scanogram (scout image): This preliminary radiograph provides overview of skeletal structures, soft tissues, and gross abnormalities 2
• Assess image quality: Verify adequate inspiration, absence of motion artifact, and appropriate window settings 1

Step 2: Systematic Anatomic Review

Scroll through the entire study once without interruption to establish an overview of disease presence and extent before detailed analysis. 2 Then systematically evaluate each anatomic compartment:

Mediastinum

• Vascular structures: Assess the aorta, pulmonary arteries (including segmental P3 branches with artery-to-bronchus ratio >1:1 suggesting pulmonary hypertension), and superior/inferior vena cava 5
• Mediastinal masses: Localize to anterior/middle/posterior compartment; characterize tissue density (fat, fluid, soft tissue, calcium) 3
• Lymph nodes: Evaluate size, distribution, and enhancement pattern (contrast-enhanced studies superior for this assessment) 3

Airways

• Trachea and main bronchi: Measure diameter, assess for stenosis (CT with virtual bronchoscopy has 93% sensitivity and accuracy for central airway tumors), evaluate wall thickening 3
• Segmental and subsegmental bronchi: Look for bronchiectasis, mucus plugging, or endobronchial lesions 3

Lung Parenchyma - Pattern Recognition Approach

Categorize predominant pattern into one of three groups, as this determines your differential diagnosis pathway: 2

Pattern A: Predominantly Reticular Disease

• Assess distribution (upper vs. lower lobe, peripheral vs. central, subpleural vs. random) 3
• Identify specific features: honeycombing, traction bronchiectasis, ground-glass opacity 3
• Critical for usual interstitial pneumonia diagnosis: HRCT pattern characterization is especially important and sufficient for confident diagnosis in appropriate clinical context 3

Pattern B: Predominantly Nodular Disease

• Determine nodule size, attenuation (solid vs. subsolid), and distribution 4
• For solid nodules <6 mm in low-risk patients: No routine follow-up required 4
• For solid nodules >8 mm: Consider 3-month CT, PET/CT, or tissue sampling regardless of risk 4
• Avoid FDG-PET/CT for nodules <8 mm due to limited spatial resolution 4

Pattern C: Altered Lung Density

• Decreased density: Emphysema, air trapping (expiratory images helpful) 3
• Increased density: Ground-glass opacity, consolidation, mosaic attenuation 3, 5
• Mosaic pattern at segmental level: May indicate chronic thromboembolic disease when corresponding to vascular territories 5

Pleura and Chest Wall

• Pleural effusions: Characterize as simple vs. complex (septations best seen on contrast-enhanced CT or ultrasound) 3
• Pleural thickening: Contrast-enhanced CT slightly better at distinguishing benign from malignant 3
• Rib fractures: Dedicated rib series more sensitive than standard chest CT; consider cough-induced fractures (82.4% of post-tussive chest pain, typically 10th rib) 3, 6
• Chest wall masses: MRI superior to CT for assessing invasion across tissue planes and neurovascular involvement 3

Step 3: Utilize Multiplanar Reconstructions

Always review coronal and sagittal reformations—volumetric CT data acquisition facilitates multiplanar thin-section reconstructions that aid evaluation of disease distribution and anatomic relationships. 3

• Coronal images: Optimal for craniocaudal distribution of disease, diaphragmatic assessment 3
• Sagittal images: Superior for anterior-posterior relationships, spine evaluation 3
• 3D volume-rendered images: Useful for airway stenosis planning and vascular anatomy 3, 5

Step 4: Integrate Clinical Context

CT findings and patterns are often sufficient for limited differential or confident single diagnosis, but must be integrated with clinical history, particularly for diffuse lung disease. 3

• Multidisciplinary discussion incorporating HRCT interpretation improves diagnostic accuracy and confidence 3
• Consider patient risk factors: smoking history, occupational exposures, immunosuppression 4
• Correlate with symptoms: dyspnea, chest pain, hemoptysis 3

Critical Pitfalls to Avoid

• Don't rely on chest radiography for nodule follow-up <1 cm: Most are not visible on plain films 4
• Don't order partial thoracic scans for nodule surveillance: May miss important findings 4
• Don't assume artery-to-bronchus ratio ≤1:1 excludes pulmonary hypertension: Particularly in underlying lung disease 5
• Don't use standard chest CT protocols for diffuse lung disease: HRCT with thin sections is essential 3
• Don't interpret FDG-PET/CT avidity in prevascular mediastinum as definitively malignant: Normal and hyperplastic thymus, benign thymic cysts can be FDG-avid 3

When Contrast Is and Isn't Needed

IV contrast is NOT required for: 4

• Detecting or characterizing pulmonary nodules
• Assessing nodule growth or stability
• Evaluating diffuse lung disease patterns
• Following lung nodules of any size

IV contrast IS beneficial for: 3

• Mediastinal mass characterization and vascular invasion assessment
• Distinguishing benign from malignant pleural thickening
• Evaluating complex pleural effusions with septations
• Cardiac and great vessel evaluation
• Detecting active hemorrhage in trauma (arterial and venous phases) 3