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Chest X-Ray (CXR) - A Complete Guide
Standard Views
A chest radiograph is the most widely used and cost-effective initial imaging test for thoracic disease. Two standard projections are obtained:
- PA (posteroanterior) - patient stands erect, X-ray beam passes back-to-front. This is the standard view.
- Lateral - taken alongside the PA; better shows posterior lung bases, retrocardiac region, anterior mediastinum, and thoracic spine.
- AP (anteroposterior) - portable/bedside view, used in ICU or immobile patients. Magnifies mediastinal structures and is less sensitive for pneumothorax and effusions.
Supplementary views: lordotic (lung apices), expiratory (small pneumothorax), lateral decubitus (mobile vs. loculated effusion).
- Fishman's Pulmonary Diseases and Disorders, p.465
Normal PA Chest Radiograph
Figure: Normal PA radiograph - Fishman's Pulmonary Diseases
Normal Lateral Chest Radiograph
Figure: Normal lateral radiograph - Fishman's Pulmonary Diseases
Cardiac Anatomy on Lateral CXR (with overlay)
Figure: Lateral CXR with cardiac chambers overlaid - red = right-sided structures; blue = left-sided + pulmonary vasculature - Goldman-Cecil Medicine
Bronchial Anatomy Visible on CXR
Figure: Right upper, middle, and lower lobe bronchi labeled on PA CXR - Fishman's Pulmonary Diseases
What to Look at on a Chest X-Ray - Systematic Approach
A good approach uses the mnemonic ABCDE (or similar structured scan):
| Zone | What to Assess |
|---|---|
| Airway | Trachea - midline? Carina angle (<70°)? |
| Bones | Ribs, clavicles, scapulae, spine - fractures, lytic lesions |
| Cardiac | Cardiothoracic ratio (<0.5 on PA), shape, chamber borders |
| Diaphragm | Right higher than left by ~1.5 cm, costophrenic angles sharp |
| Everything else | Lung fields, hila, mediastinum, soft tissues |
Cardiac Silhouette
The cardiothoracic (CT) ratio = maximum transverse cardiac diameter / internal thoracic diameter at the level of the right hemidiaphragm. Normal is <0.5 on PA film.
Right heart border (from top to bottom):
- Superior vena cava (SVC)
- Right atrium (RA)
Left heart border (from top to bottom):
- Aortic knuckle (aortic arch)
- Pulmonary artery (PA)
- Left atrial appendage
- Left ventricle (LV) - forms most of the left border
Left ventricular dilation: concave mid-left border, downward-pointing apex below the diaphragm. On lateral film, LV extension >2 cm posterior to the inferior vena cava = LV enlargement.
- Goldman-Cecil Medicine, p.444
Key CXR Abnormalities
1. Pleural Effusion
- Small: blunted posterior then lateral costophrenic angles (requires ~200-500 mL)
- Classic sign: homogeneous basal opacity with a concave upper border (meniscus sign) higher laterally
- Large: dense hemithoracic opacity + contralateral mediastinal shift
- Absent mediastinal shift with a large effusion suggests lung collapse or pleural malignancy (mesothelioma)
- ~1000 mL required to reach level of the 4th anterior rib
- Grainger & Allison's Diagnostic Radiology, p.61
2. Cardiac Failure
| Feature | Type |
|---|---|
| Cardiomegaly, central/basilar opacities, peribronchial cuffing, vascular congestion | Cardiogenic pulmonary edema |
| Normal heart size, peripheral distribution, air bronchograms (80%) | ARDS/non-cardiogenic |
| Cephalization of pulmonary vessels, Kerley B lines, pleural effusion | Elevated wedge pressure (>18 mmHg) |
- Goldman-Cecil Medicine
3. Pneumothorax
- Visible lung edge with absent lung markings peripheral to it
- Erect PA most sensitive; expiratory film may help in small cases
- Tension pneumothorax: tracheal deviation away from affected side
4. Consolidation
- Air space opacity, often with air bronchograms
- Lobar distribution suggests bacterial pneumonia; bilateral perihilar "bat-wing" pattern suggests pulmonary edema
5. Collapse (Atelectasis)
- Tracheal and mediastinal shift toward the affected side
- Elevation of hemidiaphragm, crowding of ribs
6. Trauma Findings
- Rib fractures 1-2 (first/second) → suspect aortic injury
- Lower rib fractures → liver/spleen injury
- Supine AP: hemothorax appears as homogeneous haze; pneumothorax appears more radiolucent
- Up to 300 mL blood can pool behind the diaphragm and not be visible
- Bailey & Love's Surgery, p.895
Technical Adequacy - Before Reading Any CXR
Always check:
- Rotation - medial clavicular heads equidistant from spinous processes
- Inspiration - 5-6 anterior ribs or 9-10 posterior ribs should be visible above the diaphragm
- Penetration/Exposure - vertebral bodies just visible behind cardiac shadow
Advantages of CXR vs Advanced Imaging
| Feature | CXR | CT |
|---|---|---|
| Cost | Low | High |
| Radiation | Low | Higher |
| Availability | Universal, portable | Limited/fixed |
| Detail | Good overview | Superior (3D reconstruction) |
| Small pneumothorax | Lower sensitivity | High sensitivity |
| Mediastinal detail | Limited | Excellent |
The CXR remains the cornerstone of initial imaging for most thoracic conditions. CT is required when CXR findings are ambiguous or more precise characterization of disease is needed.
Sources: Fishman's Pulmonary Diseases & Disorders; Goldman-Cecil Medicine; Grainger & Allison's Diagnostic Radiology; Bailey & Love's Surgery
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