Hi, can you please tell us how to interpret chest X-ray? Uh, how to interpret, how to interpret, uh, anteroposterior, posterior, anterior, uh, chest X-ray? What are the points to keep in mind and what are the abnormalities in the, uh, chest X-ray?

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How to interpret a chest X-ray: a practical, systematic method

First, identify the projection. This changes how you judge the heart, mediastinum, and lungs.

PA vs AP chest X-ray

View	Meaning	Typical setting	Main implication
PA	Posteroanterior: X-ray beam travels from back to front	Standing outpatient film	Standard, most reliable view for heart size
AP	Anteroposterior: beam travels from front to back	Portable bedside, ICU, trauma, very unwell patient	Heart and mediastinum can look falsely enlarged because they are farther from the detector
Lateral	Side view	Often taken with a PA film	Helps localize a lesion and assess retrosternal/retrocardiac areas

A PA plus lateral study is usually the standard ambulatory examination. AP films are commonly portable and can be taken erect, semi-erect, or supine. Do not call cardiomegaly confidently from a portable AP film without considering magnification and poor inspiration. The SAEM chest radiograph guide explains these projection-related limitations.

Step 1: Confirm the basics

Before interpreting anatomy, check:

Patient name, age, date, and comparison with prior X-rays
Right/left marker
Projection: PA, AP, lateral
Position: erect, supine, semi-erect
Presence and position of lines, tubes, devices, surgical clips

Always interpret the image with the clinical history in mind, but do not stop once you see one abnormality. A systematic review helps prevent missing additional disease. Bailey and Love's Short Practice of Surgery, p. 217-241 also emphasizes a structured assessment of chest radiographs.

Step 2: Assess image quality using RIPE

R: Rotation

Check whether the medial ends of both clavicles are equally distant from the thoracic spinous processes.

No rotation: clavicles are symmetric around the spine.
Rotation: can falsely make the trachea, mediastinum, or heart look shifted or enlarged.

I: Inspiration

Count posterior ribs. On a good inspiratory PA film, you should see about 9 to 10 posterior ribs above the diaphragm, or 5 to 6 anterior ribs.

Poor inspiration: crowded lung markings, raised diaphragms, basal haze, apparent heart enlargement.
This may mimic lower-lobe pneumonia, pulmonary edema, or cardiomegaly.

P: Projection

Determine PA versus AP.

Clues for AP:

Scapulae project over the lungs.
Clavicles appear more horizontal.
Heart may appear enlarged.
Often labelled “portable” or “AP erect/supine.”

E: Exposure or penetration

Look through the heart:

Adequate penetration: thoracic vertebrae are faintly visible behind the heart.
Underpenetrated film: too white, may hide lower-lobe disease and falsely suggest edema.
Overpenetrated film: too dark, may obscure subtle infiltrates or a small pneumothorax.

Also check that the film includes the apices, both costophrenic angles, and lateral chest walls.

Step 3: Use a fixed ABCDE approach

A: Airway

Check:

Trachea: central, deviated, narrowed?
Carina and main bronchi
Endotracheal tube position, if present

Tracheal deviation

Toward one side: volume loss, such as upper-lobe collapse or post-pneumonectomy.
Away from one side: large pleural effusion, tension pneumothorax, large mass.

A small degree of tracheal displacement may be normal due to the aortic arch.

B: Breathing and lungs

Inspect both lungs systematically, comparing right and left from apex to base.

Look for:

Opacity: “whiter than it should be”

Possible causes:

Consolidation, for example pneumonia
Atelectasis or collapse
Pulmonary edema
Pleural effusion
Mass
Fibrosis
Hemorrhage

Ask:

Is it focal or diffuse?
Is it unilateral or bilateral?
Is there volume loss?
Does it obscure a normal border?

Air bronchograms within an opacity favor alveolar consolidation, commonly pneumonia.

Increased lucency: “blacker than it should be”

Possible causes:

Pneumothorax
Hyperinflation in COPD/asthma
Bullae
Reduced pulmonary blood flow
Technical overexposure

For pneumothorax, seek:

A visible visceral pleural line
Absence of lung markings beyond that line
In a supine patient, an abnormally deep and lucent costophrenic angle can be a clue

Interstitial patterns

Reticular: fine linear network, often fibrosis or interstitial edema
Nodular: metastases, infection, pneumoconiosis, inflammatory disease
Kerley B lines: short peripheral basal septal lines, suggest interstitial pulmonary edema

Peripheral basal reticulation and honeycombing suggest advanced fibrotic lung disease. Murray & Nadel's Textbook of Respiratory Medicine, p. 2021 describes the characteristic peripheral and basal distribution in idiopathic pulmonary fibrosis.

C: Cardiac silhouette and mediastinum

Check:

Heart size and shape
Mediastinal width
Aortic contour and aortic knuckle
Hila
Pulmonary vascularity

Heart size

On a well-inspired PA film, the cardiothoracic ratio should generally be less than 50%.

Do not apply this rigidly to an AP portable film because AP magnification and low inspiration can produce apparent enlargement.

Hila

Compare both sides. Look for:

Enlargement
Mass
Prominent pulmonary arteries
Lymphadenopathy
Change in hilar position

Widened mediastinum

May reflect AP technique, rotation, aortic disease, mediastinal mass, lymphadenopathy, hemorrhage, or thyroid enlargement. In trauma with an appropriate mechanism, it needs urgent clinical correlation and often CT.

D: Diaphragm and pleura

Check:

Both hemidiaphragms
Costophrenic angles
Cardiac borders
Pleural margins
Free subdiaphragmatic air

Normal

Right hemidiaphragm is often slightly higher than the left.
Costophrenic angles should be sharp.

Pleural effusion

Look for:

Blunting of the costophrenic angle
Meniscus sign
Large effusion causing mediastinal shift away from the affected side

Pneumothorax

Look for a pleural line with no peripheral vascular markings. In tension pneumothorax, there may be mediastinal shift away, diaphragm depression, and severe clinical deterioration. This is an emergency.

Free air under diaphragm

A crescent of gas under the diaphragm, especially on the right, suggests possible perforated abdominal viscus and requires urgent assessment.

E: Everything else

Review:

Bones: ribs, clavicles, scapulae, vertebrae, shoulders
Soft tissues: subcutaneous emphysema, breast shadows, chest-wall masses
Upper abdomen: gastric bubble, free air, enlarged organs
Hidden areas:
- Lung apices
- Behind the heart
- Below the diaphragms
- Hila
- Costophrenic angles

F: Foreign bodies, lines, and tubes

If present, check:

Endotracheal tube: tip usually a few cm above the carina
Nasogastric tube: should pass below the diaphragm into the stomach
Central venous catheter: tip position and complications
Chest drain: side holes should be inside the thorax
Pacemaker, prosthetic valves, sternal wires

A trauma-film review should specifically assess lines and tubes, airways, focal or diffuse opacities, mediastinal position, bones, and surgical emphysema. Bailey and Love's Short Practice of Surgery, p. 217-241.

High-yield abnormal X-ray patterns

Finding	Common possibilities
Lobar opacity with air bronchograms	Pneumonia
Wedge/linear opacity with volume loss and elevated hemidiaphragm	Atelectasis/collapse
Bilateral perihilar “bat-wing” opacities, cardiomegaly, effusions	Cardiogenic pulmonary edema
Blunted costophrenic angle	Pleural effusion
Pleural line with absent peripheral markings	Pneumothorax
Hyperinflated lungs, flattened diaphragms	COPD/emphysema
Bilateral diffuse reticular or reticulonodular pattern	Interstitial lung disease, edema, infection
Solitary pulmonary nodule or mass	Requires comparison imaging and usually CT assessment
Enlarged cardiac silhouette on PA film	Cardiomegaly or pericardial effusion
Loss of right heart border	Right middle-lobe process
Loss of left heart border	Lingular process
Loss of hemidiaphragm silhouette	Lower-lobe process

This “loss of a normal border” is the silhouette sign, which can help localize consolidation. The Radiology Assistant review gives useful examples of evaluating interfaces between lung, heart, diaphragm, and mediastinum.

A concise reporting template

Study: AP portable chest radiograph, semi-erect.
Quality: Mild rotation, low lung volumes, adequate penetration.
Findings: Cardiomediastinal silhouette is mildly enlarged, noting AP technique. Patchy right lower-zone air-space opacity. No pleural effusion or pneumothorax.
Impression: Right lower-zone air-space opacity, concerning for pneumonia in the appropriate clinical setting.

Important safety points

A chest X-ray can be normal early in pneumonia, pulmonary embolism, some cancers, and other serious disease.
AP portable and supine films are less reliable for heart size, small pleural effusions, and pneumothorax.
Escalate urgently for signs of tension pneumothorax, a large pleural effusion with respiratory compromise, severe pulmonary edema, malpositioned airway devices, or suspected free intraperitoneal air.
Final interpretation should be made by a qualified clinician or radiologist alongside the patient’s symptoms, examination, oxygenation, and prior imaging.

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