How to Interpret a Chest X-Ray (CXR)

Step 1: Before You Interpret - Check Technical Quality (RIP)

Always note technique limitations before interpreting - these introduce disclaimers that protect both the patient and clinician.

Step 2: PA vs AP vs Portable

• PA (Posteroanterior) - standard view; beam travels back-to-front; patient stands upright, takes a deep breath. Most accurate for cardiac size.
• AP (Anteroposterior) - portable/bedside; beam travels front-to-back. Cardiomegaly definitions differ - AP views magnify the heart, so don't apply PA cardiac size criteria.
• Lateral view - essential in ambulatory adults. Reveals retrocardiac and retrosternal (anterior clear space) lesions that the PA view misses.

Step 3: Systematic Review - ABCDE Approach

A - Airway

• Is the trachea midline? Deviation suggests tension pneumothorax, large effusion (pushes trachea away), or fibrosis/collapse (pulls trachea toward).
• Check the carina - normally the angle of bifurcation is <70°. Widening (>70°) suggests left atrial enlargement.
• Are the main bronchi visible and symmetric?

B - Breathing / Lungs

• In 70% of normals, left hilum is higher than the right. The right hilum is never normally higher than the left.
• Bilateral hilar enlargement = sarcoid, lymphoma, TB, primary pulmonary hypertension.
• Unilateral enlargement = malignancy, TB, infection.

• Vascular markings normally stop 3-5 mm short of the chest wall.
• Cephalization of flow (enlarged upper lobe vessels) = early CHF.
• Silhouette sign: when a lesion shares the same density as an adjacent border (heart, aorta, diaphragm), that border is "erased." This localizes the lesion:
  • Right heart border erased → right middle lobe pathology
  • Left heart border erased → lingula pathology
  • Diaphragm border erased → lower lobe pathology

• Nodule = 5-30 mm diameter
• Mass = >30 mm diameter
• Small lesions 2-10 mm are usually benign calcified granulomas; follow with serial imaging

• Pneumothorax: absent vascular markings extending to the inner thoracic wall (no lung markings reaching the rib edges)
• Pleural effusion: blunting of costophrenic angles; the meniscus sign. Causes include infection, malignancy, CHF, renal failure, hypoalbuminemia, pancreatitis, subphrenic abscess.
• Untreated chronic effusions can loculate and form adhesions.

C - Cardiac Silhouette

• Cardiothoracic (CT) ratio on PA view: cardiac transverse diameter / thoracic diameter at the same level. Normal = <0.5 (50%).
• If >0.5 on a true PA view = cardiomegaly. (Note: AP views make the heart appear larger - do not use the same cutoff.)
• A "thin heart" in COPD (hyperinflation flattens and elongates it) is not pathological.

• Enlarged pulmonary artery segment = extra hump on left cardiac border → suggests pulmonary hypertension
• Pneumopericardium = black line around the heart border
• On the lateral view: left ventricular enlargement casts a shadow >2 cm posterior to the inferior vena cava shadow

• Right heart border = right atrium
• Left heart border = left ventricle (superior portion = left atrial appendage)

D - Diaphragm

• Right diaphragm is normally higher than left by 2-20 mm (liver pushes it up; gastric bubble is under the left).
• Abnormal elevation of a hemidiaphragm: atelectasis, phrenic nerve palsy, effusion, post-lobectomy.
• Air under the diaphragm (free intraperitoneal air) = surgical emergency until proven otherwise (perforated viscus).
• Check costophrenic angles - should be sharp. Blunting = ~200-300 mL of pleural fluid.

E - Everything Else (Bones, Soft Tissues, Mediastinum, Devices)

• Normal width = <8 cm in adults (measured at aortic knob level). If >25% of thoracic diameter at the carina = widened.
• Widened mediastinum: aortic dissection, pericardial tamponade, lymphoma, thymoma, germ cell tumor, thyroid mass.
• "Thin" mediastinum has no clinical significance.
• Note: children <5 years normally have a wide-looking mediastinum and large thymic shadow.

• Ribs - fractures, metastatic lesions, notching (coarctation of aorta causes rib notching on the inferior rib margins, 3rd-8th ribs bilaterally)
• Vertebrae - compression fractures, lytic/sclerotic lesions
• Clavicles and shoulder joints

• Subcutaneous emphysema
• Breast shadows (can mimic lung opacification)
• Foreign bodies, implanted devices

• ETT tip: should be 3-5 cm above the carina
• Central venous catheter: tip at the SVC-RA junction
• NGT: below the diaphragm, midline
• Chest drains: in the pleural space

Step 4: The Lateral View

1. Spine - disc spaces, vertebral body density. Normally the vertebrae should appear progressively darker (more lucent) as you go inferiorly. If they get whiter toward the base = posterior lung pathology (e.g., lower lobe consolidation).
2. Diaphragms - both should be visible separately. Loss of posterior diaphragm contour = lower lobe consolidation.
3. Anterior clear space (retrosternal) - should be clear. Fullness = anterior mediastinal mass (thymoma, teratoma, thyroid, terrible lymphoma - "4 T's").
4. Retrocardiac space - should be clear. Opacification = left lower lobe consolidation or effusion.

Step 5: Top 10 "Normal" Benchmarks to Memorize

1. Integrate history and physical - always correlate clinically
2. Clavicular heads equidistant from spinous processes (no rotation)
3. At least 9 posterior ribs visible on good inspiration
4. Intervertebral spaces NOT visible below the diaphragm
5. Before age 5, a large thymus can widen the mediastinum normally
6. Adults: mediastinum ≤ 8 cm wide
7. Left hilum higher than right (normal in 70%)
8. Right diaphragm higher than left
9. Lesions >10 mm may benefit from a second opinion or CT
10. "Chance favors the prepared mind" - experience + pattern recognition builds skill

Common CXR Patterns and Their Diagnoses

Summary Workflow

1. Clinical context (symptoms, old films)
2. RIP - check Rotation, Inspiration, Penetration
3. PA + Lateral views both assessed
4. ABCDE: Airway → Breathing → Cardiac → Diaphragm → Everything else
5. Ping-pong comparison: left vs right lung fields
6. Apply silhouette sign to localize lesions
7. Final interpretation: normal / probably insignificant / questionable / abnormal
8. Plan: CT, repeat film, consultation, or immediate action