Normal chest X-ray anatomy

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Beginner

You cannot recognise an abnormal chest film until you know the normal one in detail. Almost every chest sign is a departure from a normal outline: a border that has vanished, a hilum that has moved, a fissure that has shifted, a stripe that has widened. This page walks through the structures you see on a normal PA and lateral chest radiograph, which part of the anatomy forms each outline, and the normal relationships and measurements you will use every day. It pairs with how to read a chest X-ray.

1. The cardiac and mediastinal outlines

On a PA film the central soft-tissue shadow is the heart and mediastinum. Its edges are visible only where they touch aerated lung, which is why disease in adjacent lung erases them (the silhouette sign).

Outline	Formed by	Lost when
Right upper mediastinal border	Superior vena cava (and, in older patients, an unfolded ascending aorta)	Right upper lobe (anterior segment) disease
Right heart border	Right atrium	Right middle lobe (medial segment) disease, pectus excavatum
Aortic knob (aortic arch)	The arch seen end-on at the top left	Left upper lobe (apicoposterior) or posterior mediastinal disease
Aortopulmonary (AP) window	A concave notch between the knob and the pulmonary trunk	Becomes straight or convex with lymphadenopathy
Pulmonary trunk	The bulge below the AP window	Enlarged in pulmonary arterial hypertension
Left heart border	Mainly the left ventricle, with the left atrial appendage just below the pulmonary trunk	Lingula (left upper lobe) disease
Descending aorta	A vertical line behind the heart, left of the spine	Left lower lobe disease

The right ventricle does not form a border on the PA film: it lies anteriorly and is best judged on the lateral view. The left atrium lies posteriorly, below the carina; when it enlarges it may produce a double density through the right heart border and splay the carina.

2. Trachea, carina and stripes

• Trachea: a midline lucent column. A slight deviation to the right at the level of the aortic knob is normal.
• Carina: where the trachea divides, usually around T4–T6 depending on inspiration. The angle between the main bronchi is roughly 60–75° in many sources, though normal ranges vary widely; widening can suggest left atrial enlargement or subcarinal nodes.
• Main bronchi: the right is shorter, wider and more vertical than the left. This is why inhaled material and a too-deep endotracheal tube tend to go right. See ETT position.
• Right paratracheal stripe: the thin line of soft tissue between the air in the trachea and the right lung. Normally about ≤4 mm wide; widening (some trauma sources use more than 5 mm) suggests lymphadenopathy, haematoma or fluid.
• Paraspinal lines: thin lines alongside the thoracic spine. Focal widening, particularly on the left, may indicate haematoma (after trauma), a mass or infection.
• Azygo-oesophageal recess: the interface between the right lower lobe and the mediastinum behind the heart; distortion can indicate subcarinal masses or nodes.

3. The hila

Each hilum is made mostly of the pulmonary arteries and upper-lobe veins, with the bronchi; normal lymph nodes are too small to contribute. Assess each hilum for:

• Position: the left hilum is higher than the right in about 97% of people, by up to roughly 1.5 cm, or the two are level. The right hilum should not be higher than the left. If it is, suspect right upper lobe volume loss pulling it up, or left lower lobe volume loss pulling the left down.
• Size: the right lower-lobe (interlobar) artery is normally about ≤16 mm wide in men and about ≤15 mm in women [VARIES].
• Density: both hila should be similar in density. A hilum that is denser than the other suggests a mass or nodes, even if it is not enlarged.
• Shape: each hilum is roughly concave laterally, where upper and lower vessels meet. A lumpy, lobulated or convex outline suggests nodes.

4. Lungs, zones, lobes and fissures

On the frontal film, each lung is described in zones: upper (above the anterior second rib), middle (second to fourth anterior ribs) and lower (below the fourth). Zones are descriptive regions of the image, not anatomy. Lobes overlap on a frontal film: the lower lobes extend high up posteriorly, and the upper lobes extend low anteriorly. So "right lower zone shadowing" could be in the right lower lobe or the right middle lobe.

Lung	Lobes	Fissures
Right	Upper, middle, lower	Horizontal (minor) fissure, separating upper from middle, at roughly the level of the right 4th anterior rib on the PA film. Oblique (major) fissure, separating the lower lobe from the others.
Left	Upper (including the lingula), lower	Oblique fissure only. There is no horizontal fissure on the left.

The horizontal fissure is often visible as a fine line on the PA film, but not always. The oblique fissures run obliquely from about T4/T5 posteriorly to the anterior diaphragm and are seen on the lateral film, not usually on the PA. Displacement of a fissure is the most reliable sign of lobar collapse.

Normal lung vessels branch and taper from the hila outwards and are visible to within a couple of centimetres of the chest wall. On an erect film the lower-zone vessels are larger than those in the upper zones, because of gravity. Equalisation or reversal of this pattern (upper lobe diversion) is an early sign of raised pulmonary venous pressure.

5. Diaphragm, pleura and bones

• Hemidiaphragms: smooth domes. The right is usually higher than the left, by up to about 1.5–2 cm, because of the liver. A left hemidiaphragm higher than the right is unusual and worth explaining (volume loss, phrenic nerve palsy, a subpulmonic effusion or distended bowel).
• Costophrenic angles: sharp and acute. Blunting suggests fluid or pleural thickening. The cardiophrenic angles may contain fat pads, which can blur the heart edge in obese patients.
• Gastric bubble: under the left hemidiaphragm, normally within about 2 cm of it. A larger gap suggests a subpulmonic effusion or a mass.
• Pleura: the normal pleura is invisible except where it forms fissures. A visible pleural line at the edge of the lung means a pneumothorax until proven otherwise.
• Bones and soft tissues: ribs (posterior ribs are more horizontal, anterior ribs slope down and fade into cartilage), clavicles, scapulae, thoracic spine, proximal humeri, and the breast shadows.

6. The lateral film

The lateral view adds depth. Learn these normal features:

• Retrosternal clear space: a lucent area behind the sternum, in front of the ascending aorta. It fills in with anterior mediastinal masses, right ventricular enlargement or lymphadenopathy, and enlarges in emphysema (more than about 2.5 cm is often used as a sign of hyperinflation [VARIES]).
• Retrocardiac space: lucent area behind the heart and in front of the spine.
• The spine: the vertebral bodies should become darker as you move down. If they look whiter lower down (the spine sign), suspect lower-lobe consolidation, collapse or effusion.
• Fissures: both oblique fissures run from upper posterior to lower anterior; the horizontal fissure runs forward from the right hilum.
• Hemidiaphragms: the right is usually seen along its whole length; the anterior part of the left is lost where the heart sits on it. The gastric bubble lies under the left.
• Posterior costophrenic recesses: the lowest part of the pleural space, where small effusions collect first.

7. Normal variants and age changes

• Azygos lobe: a curved fissure in the right upper zone with a teardrop-shaped azygos vein at its base. Harmless.
• Unfolded aorta in older adults: a prominent knob and a widened upper mediastinum to the right. Common and usually harmless, but compare with old films.
• Thymus in infants and young children: can widen the superior mediastinum (the "sail" or "wave" sign). See neonatal chest X-ray.
• Breast tissue, nipples and pectoral muscles alter lung density. See artefacts and mimics.
• Costal cartilage calcification increases with age and can mimic nodules.
• Pericardial fat pads at the cardiac apex can make the heart look larger.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Which structure forms each border, the hilar relationship, the fissures, and zones versus lobes.
Nurse	Know what a central trachea and sharp costophrenic angles look like, so a change is obvious.
Junior doctor	Use the borders to localise disease, and check the AP window, paratracheal stripe and hila on every film.
Radiographer	Positioning that keeps normal anatomy symmetrical, and recognising the normal lateral.
Advanced / FRCR	Mediastinal lines and stripes, the azygo-oesophageal recess, and lateral film anatomy in collapse and mediastinal masses.

Practise normal versus abnormal →

References

1. Radiology Masterclass. Chest X-ray anatomy. radiologymasterclass.co.uk
2. Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com
3. Radiopaedia. Right paratracheal stripe. radiopaedia.org
4. Radiopaedia. Cardiothoracic ratio. radiopaedia.org
5. Felson B. Chest Roentgenology. Philadelphia: WB Saunders; 1973.
6. Hansell DM, et al. Fleischner Society: glossary of terms for thoracic imaging. Radiology 2008;246:697–722. pubs.rsna.org

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.