chest X-ray
Summary
Chest X-ray (CXR) is a plain radiograph of the thorax used to evaluate the lungs, heart, mediastinum, and bony structures. It is a first-line imaging modality for cardiopulmonary complaints, trauma, and routine screening. Standard views include posteroanterior (PA), lateral, and anteroposterior (AP, often portable/supine).
Detail
A chest X-ray uses ionizing radiation to produce a 2D image based on differential tissue density: air appears black (radiolucent), bone/metal appear white (radiopaque), and soft tissue/fluid appear intermediate gray. Systematic interpretation typically follows the ABCDE approach: Airway (tracheal position, deviation), Bones (fractures, lytic/sclerotic lesions), Cardiac silhouette (cardiothoracic ratio <0.5 on PA view suggests normal heart size), Diaphragm (elevation, free air under diaphragm indicating perforation), and Effusions/fields (pulmonary infiltrates, effusions, pneumothorax).
Clinical uses include evaluation of pneumonia (consolidation, air bronchograms), heart failure (cardiomegaly, cephalization, Kerley B lines, pleural effusions, pulmonary edema pattern), pneumothorax (visceral pleural line, absence of lung markings peripherally), pleural effusion (blunted costophrenic angle, meniscus sign), COPD (hyperinflation, flattened diaphragms, increased retrosternal air space), interstitial lung disease (reticular/nodular patterns), malignancy (mass lesions, hilar adenopathy), and line/tube placement confirmation (ET tube, central line, NG tube).
PA view is preferred over AP because it minimizes cardiac magnification (heart is closer to the film) and provides better visualization of the mediastinum; AP portable films (common in ICU/bedridden patients) exaggerate heart size and are used when PA is not feasible. Lateral views help localize lesions and evaluate retrosternal and retrocardiac spaces.
Limitations: CXR has lower sensitivity/specificity compared to CT for many conditions (e.g., pulmonary embolism, small nodules, early interstitial disease) and is a 2D representation of 3D structures, leading to superimposition artifacts. It remains valuable due to low cost, low radiation dose (~0.1 mSv), and rapid acquisition, making it ideal for initial screening and follow-up.
Key boards associations: silhouette sign (loss of a normal border indicates adjacent pathology, e.g., right heart border loss with right middle lobe pneumonia), air-fluid levels (abscess), egg-on-a-string (TGA), boot-shaped heart (tetralogy of Fallot), water-bottle heart (pericardial effusion), and widened mediastinum (aortic dissection, though CT angiography is confirmatory).
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- Felson's Principles of Chest Roentgenology
- UWorld Step 2 CK
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