Why is the perihilar region significant on a chest radiograph?

Why the Perihilar Region is Significant on Chest Radiography

Critical Anatomical and Diagnostic Importance

The perihilar region is significant on chest radiography because it contains the pulmonary arteries, bronchi, lymphatics, and central lung interstitium—structures whose abnormalities can indicate serious pathology including lung cancer, pulmonary hypertension, lymphadenopathy, and infectious diseases. 1, 2

Key Anatomical Structures and Their Clinical Relevance

The perihilar region encompasses several vital structures that must be systematically evaluated:

• Pulmonary vasculature: The right descending pulmonary artery diameter >15 mm in women (>16 mm in men) at the hilum indicates pulmonary hypertension with 93% sensitivity and 88% specificity 3
• Bronchial structures: The trachea and main bronchi course through this region, and deviation or narrowing suggests mass effect or mediastinal pathology 1, 2
• Lymphatic system: The peribronchovascular region contains intercommunicating prelymphatic spaces, lymphatics, and lymph nodes that enlarge with malignancy or infection 4
• Central lung interstitium: This includes collagen bundles and bronchial arteries that can be affected by diffuse lung diseases 4, 5

Detection of Malignancy

The perihilar region is a critical area for detecting lung cancer:

• Mediastinal lymphadenopathy: Enlarged hilar or mediastinal lymph nodes visible on chest radiography may indicate metastatic disease, though chest radiography is generally insensitive for detecting mediastinal lymph node involvement 6
• Perihilar linear atelectasis: Thick perihilar linear atelectasis (>5.5 mm) is a novel diagnostic sign suggesting subsegmental bronchial obstruction from primary lung cancer, with 16 of 19 patients (84%) with this finding having malignancy 7
• Mass lesions: Perihilar masses or consolidation can represent primary lung cancer, though tuberculosis and other conditions can mimic malignancy 8

Vascular Pathology Assessment

The perihilar region provides crucial information about pulmonary vascular disease:

• Pulmonary hypertension: Main pulmonary artery enlargement (>35 mm from midline to left lateral border) occurs in 96% of pulmonary hypertension cases, with chest radiography having 96.9% sensitivity and 99.1% specificity for moderate to severe disease 3
• Vascular pruning: Central pulmonary artery enlargement with rapid peripheral tapering indicates elevated pulmonary pressures 3

Systematic Evaluation Approach

The American College of Radiology recommends a structured approach to perihilar assessment:

• Trace the trachea and main bronchi for deviation or narrowing that might indicate mass effect 1, 2
• Evaluate the aortopulmonary window and hilar regions for abnormal masses or lymphadenopathy 1
• Assess mediastinal width and contour for widening suggesting aortic pathology or lymphadenopathy 1, 2
• Measure hilar vessel diameters when pulmonary hypertension is suspected 3

Common Pitfalls and Limitations

Critical considerations when evaluating the perihilar region:

• Chest radiography has limited sensitivity: It is insensitive for detecting mediastinal lymph node involvement in lung cancer and may miss early or small lesions 6
• Normal radiographs do not exclude pathology: If clinical suspicion remains high for malignancy, pulmonary hypertension, or other conditions, proceed to CT imaging 1, 3
• Rotation artifacts: Improper patient positioning can mimic or obscure true perihilar pathology including masses and lymphadenopathy 1
• Multiple disease presentations: Peribronchovascular diseases can appear as diffuse thickening, masses, ground-glass opacities, or cysts, requiring integration with clinical features 4

When to Pursue Advanced Imaging

CT chest with contrast is indicated when:

• Abnormal or equivocal chest radiograph findings require further characterization 1, 2
• Perihilar masses or lymphadenopathy are detected, as CT is the gold standard for definitive diagnosis 3, 8
• Clinical suspicion persists despite normal chest radiography, particularly for lung cancer in high-risk patients 6, 7
• Pulmonary hypertension is suspected with normal or equivocal radiography, though transthoracic echocardiography should also be performed 3