What are the key zones and steps to interpret a normal chest x-ray (Chest Radiograph)?

How to Read a Normal Chest X-ray: Zones and Systematic Approach

Technical Requirements

Both posteroanterior (PA) and lateral views are essential for complete chest radiograph interpretation, as the lateral view reveals approximately 15% of lung tissue hidden on the PA film alone. 1, 2

• PA and lateral views are preferred over portable imaging 1
• Technique should be optimized to include both anterior and posterior costophrenic angles 1
• Always compare with prior films when available to confirm findings 2

Lung Zones for Systematic Evaluation

The lungs should be divided into three primary zones—upper, middle, and lower—with evaluation at five standardized anatomical levels to ensure complete coverage. 1

Standard Zone Division:

• Upper zone: Apex to the level of the aortic arch 1
• Middle zone: Aortic arch to the inferior pulmonary veins 1
• Lower zone: Inferior pulmonary veins to the diaphragm 1

Each zone can be further subdivided into segments representing approximately 10% of lung parenchyma, with half-segments corresponding to 5% and quarter-segments to 2.5% 1

Systematic Interpretation Approach

Follow the same systematic approach every time to avoid missing abnormalities, evaluating structures in a consistent order. 3, 2

1. Soft Tissues and Chest Wall 3

• Examine subcutaneous tissues for emphysema or masses
• Assess breast shadows for symmetry
• Evaluate for chest wall masses or deformities

2. Bony Structures 3

• Ribs: Look for fractures, lytic or blastic lesions
• Clavicles: Assess for fractures or erosions
• Spine: Evaluate vertebral bodies and alignment
• Shoulder girdle: Check for abnormalities

3. Pleura 3

• Examine costophrenic angles (should be sharp) 1
• Look for pleural thickening, calcifications, or effusions
• Assess fissures for displacement

4. Mediastinum 3, 4

• Width: Normal mediastinum should not exceed specific measurements (widened mediastinum has only 64% sensitivity for aortic disease) 5
• Contours: Evaluate cardiac silhouette and great vessels
• Position: Should be midline; deviation suggests volume loss or mass effect

5. Cardiac Silhouette 6, 4

A normal cardiomediastinal silhouette indicates the heart is not enlarged, major blood vessels appear normal in size and position, and there is no pericardial fluid. 6

• Cardiothoracic ratio should be assessed (heart width to chest width)
• Right heart border: Formed by right atrium 4
• Left heart border: Formed by left ventricle and left atrial appendage 4
• Aortic knob should be visible and appropriately sized 4

6. Pulmonary Vasculature and Hila 1, 3

The main pulmonary artery should measure ≤35 mm from midline to left lateral border on PA view. 1

• Right descending pulmonary artery: Should be ≤16 mm in men, ≤15 mm in women at the hilum 1
• Left descending pulmonary artery: Should be ≤18 mm on lateral view 1
• Hilar structures should be symmetric and well-defined
• Vascular markings should taper toward periphery

7. Lung Parenchyma 3, 2

Evaluate each zone systematically, comparing right to left sides for symmetry. 2

• Assess lung density and transparency
• Look for nodules, masses, or infiltrates
• Examine for interstitial patterns or consolidation
• Check for air bronchograms or cavitation

8. Diaphragm 3

• Right hemidiaphragm typically 1-2 cm higher than left
• Contours should be smooth and well-defined
• Look for free air under diaphragm

9. Below the Diaphragm 3

• Examine visible upper abdomen for masses or free air
• Assess gastric bubble position

Key Normal Findings

A normal chest X-ray demonstrates no acute cardiopulmonary process, meaning no pneumonia, heart failure, pulmonary edema, pneumothorax, pleural effusion, or pulmonary embolism. 6

• Clear, symmetric lung fields with normal vascular markings 6
• Sharp costophrenic angles bilaterally 1
• Normal cardiomediastinal silhouette 6
• No focal consolidations or masses 6
• Appropriate position of mediastinal structures 4

Critical Pitfalls to Avoid

A completely normal chest X-ray does not exclude significant pathology—chest radiography has limited sensitivity (64%) for detecting thoracic aortic disease and performs poorly in detecting mild pulmonary hypertension. 1, 5

• Hidden areas: Apices, behind the heart, behind the diaphragm, and the retrocardiac space require careful attention 2
• Lateral view is mandatory: 15% of lung tissue is hidden on PA view alone 2
• Early disease may be invisible: Very early heart or lung disease may not cause visible changes 6
• Always read the film yourself first before reviewing the radiologist's report to develop interpretive skills 2
• Persistent symptoms warrant further imaging: If clinical suspicion remains high despite normal chest X-ray, proceed to CT imaging 1, 6

When to Proceed to Advanced Imaging

Cross-sectional imaging with CT is required when chest radiography shows equivocal findings or when clinical suspicion remains high despite a normal radiograph. 1

• Suspected pulmonary hypertension with normal or equivocal chest X-ray 1
• Evaluation of mediastinal masses or lymphadenopathy 1
• Assessment of pleural disease extent 1
• Characterization of pulmonary nodules or masses 1
• Persistent unexplained symptoms despite normal radiograph 1, 6