The lateral chest X‑ray

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 25 October 2026 · Level: Intermediate

The lateral chest radiograph is taken less often than it used to be, and many clinicians feel less confident reading it than the PA film. That is a pity, because it shows parts of the chest the frontal film hides: behind the heart, behind the sternum, below the domes of the diaphragm and in front of the spine. Raoof and colleagues estimate that around 15% of the lung is hidden on the PA view. The lateral film also lets you place a lesion in three dimensions. This page explains the normal anatomy of the lateral view and a systematic way to read it, based on the approach Feigin described in 2010.

1. When a lateral is taken, and how

A lateral is not a routine addition to every chest film. Gaber and colleagues list its main uses as: localising a lesion seen on the frontal film, clarifying lobar collapse or consolidation, investigating a shadow behind the sternum or the heart, and confirming fluid trapped in a fissure (a "pseudotumour"). The reason for the lateral should be on the request.

The side of interest is placed nearest the detector, because that side is less magnified. If no side is specified, a left lateral (left side against the detector) is usually taken. The side further from the detector is magnified more, which is useful when you need to tell left from right (see section 6).

Always read the lateral together with the frontal film. A finding on one view should be checked on the other, and the pair together tell you where an abnormality lies. See X-ray projections and how to read a chest X-ray.

2. A system for the lateral film

Feigin's systematic approach has four steps:

1. Overview: an initial look at the whole film.
2. Airway and hila: follow the trachea down and identify the major hilar structures.
3. The areas where the image should darken: the retrosternal space, the retrocardiac space and the lower thoracic spine. These areas have no visible structural edges, so any increased density there is easy to miss unless you look for it.
4. The periphery and upper abdomen: the sternum, spine and ribs, both hemidiaphragms, the posterior costophrenic recesses and the area below the diaphragm.

3. Trachea and the retrotracheal triangle

Follow the air column of the trachea down towards the hila. Behind the trachea is the retrotracheal space, also called Raider's triangle. It is a lucent triangle bounded in front by the posterior tracheal (tracheo-oesophageal) stripe, behind by the thoracic vertebral bodies, below by the aortic arch and above by the thoracic inlet. Its size and shape vary a lot with body habitus and lung volume, so only a definite opacity in it should raise concern. Conditions described in this space include an aberrant right subclavian artery.

4. The hila and the inferior hilar window

The lateral film can help you decide whether a "big hilum" on the PA view is just vessels or a mass or nodes.

• Left pulmonary artery: arches over the left main bronchus and sits higher than the right.
• Right pulmonary artery: passes in front of the right main bronchus and appears as a rounded or oval density anterior to the airway.
• Both arteries and veins can be identified on a normal lateral, and they should not be mistaken for lymph nodes. Round opacities in places where vessels are not normally seen strongly suggest enlarged nodes.

The inferior hilar window is a normally avascular area below the right pulmonary artery and vein shadow and in front of the descending left pulmonary artery. In a study by Park, Webb and Klein (1991), a soft-tissue opacity here was a good marker of a hilar mass or lymphadenopathy: lateral films were 88% sensitive and 94% specific against CT for masses or nodes larger than 10 mm. The lateral film was poor at telling which side the abnormality was on (correct in only 32%), so the PA film or CT is needed for that. See mediastinum and hila.

5. The clear spaces and the spine sign

Retrosternal clear space

The retrosternal space lies directly behind the sternum, in front of the ascending aorta, and extends down to where the right ventricle meets the sternum. It contains aerated lung and should be lucent, about as dark as the retrocardiac space.

• Opacified: consider an anterior mediastinal mass (the "4 Ts": thymoma, teratoma and other germ cell tumours, thyroid mass, and lymphoma), an upper lobe mass, or enlargement of the right ventricle, which fills the space from below.
• Enlarged: seen with hyperinflation in emphysema. See COPD and airways disease.
• Fine lines: reticular lines in this space (Kerley D lines) can occur with interstitial oedema or pulmonary fibrosis and should prompt further assessment.

Retrocardiac space

The retrocardiac space lies between the back of the heart and the spine. It should be dark. Increased density here suggests lower lobe consolidation or collapse, an effusion or a mass. On the lateral view the left atrium forms the upper part of the posterior heart border and the left ventricle the lower part, so enlargement of either changes the shape of this space. The inferior vena cava is normally seen entering the right atrium at the lower posterior heart border.

The spine sign

On a normal lateral, the thoracic vertebral bodies get progressively darker from top to bottom, because the upper spine is surrounded by more soft tissue and the lower spine by more aerated lung. If the lower vertebrae stay white, or become whiter, something of soft-tissue density is lying over them at the lung base: lower lobe consolidation, collapse, an effusion or a mass. This is the spine sign. It is particularly useful for left lower lobe disease hidden behind the heart on the PA film. See silhouette sign and lobar collapse.

6. Telling the hemidiaphragms apart

Both hemidiaphragms should be visible, each outlined by lung above and abdominal soft tissue below. Telling them apart:

• The right hemidiaphragm can normally be followed all the way forward to the anterior chest wall.
• The left hemidiaphragm is usually lost anteriorly where the heart sits on it (the silhouette sign at work).
• The gastric air bubble sits under the left hemidiaphragm.
• The hemidiaphragm that meets the larger, more magnified ribs belongs to the side further from the detector.

If only one hemidiaphragm is visible, the outline of the other has been lost, which suggests fluid or consolidation above it. On a lateral, a posterior soft-tissue lesion can also make the back of the diaphragm disappear.

7. The fissures

Fissures are seen only where the X-ray beam runs along them, so on the lateral they appear as thin lines:

• Oblique (major) fissures: both run from high posteriorly, at about the level of T4 to T5, down through the hilum to the diaphragm anteriorly. The left is usually steeper. They separate the lower lobes behind from the upper lobe (and on the right, the middle lobe) in front.
• Horizontal (minor) fissure: on the right only, running forward from the hilum and separating the right upper lobe from the middle lobe. There is no horizontal fissure on the left.

Fissures are a powerful tool for lobar collapse and consolidation. In collapse, the fissure moves towards the collapsing lobe. In consolidation without volume loss, the opacity stops sharply at a fissure that stays in its normal position. Fluid trapped in a fissure appears as a lens-shaped (biconvex) density along its line, which explains a "pseudotumour" on the frontal film. Normal positions are also covered in normal chest X-ray anatomy.

8. Pitfalls

• Reading the lateral alone. Always correlate with the frontal view, and with CT if one exists.
• Calling the cardiac incisura or breast soft tissue disease. Check the PA film before suggesting middle lobe or lingular pathology.
• Mistaking vessels for nodes. Know where the pulmonary arteries lie, and look for a mass where vessels should not be (the inferior hilar window).
• Assuming laterality. The lateral film is good at showing that an abnormality is present, but not always at showing which side it is on.
• Osteophytes over the spine: spondylotic osteophytes overlying the vertebrae can add density. Correlate with the PA film before calling a spine sign.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The three dark areas (retrosternal, retrocardiac, lower spine), the spine sign, and how to tell the hemidiaphragms apart.
Nurse	Know why a lateral is requested and that a small effusion or a hidden lower lobe pneumonia may show only on the lateral.
Junior doctor	Use the lateral to localise and confirm: lower lobe disease, small effusions, fissural fluid. Read it alongside the PA film and request CT when needed.
Radiographer	Side of interest against the detector (left lateral if unspecified), a true lateral without rotation, and full coverage including the posterior costophrenic recesses.
Advanced / FRCR	The inferior hilar window, Raider's triangle, fissure displacement in lobar collapse and the four Ts of the anterior mediastinum.

Practise on chest X-rays → Unfamiliar term? See the glossary.

References

1. Feigin DS. Lateral chest radiograph: a systematic approach. Acad Radiol 2010;17(12):1560–6. doi:10.1016/j.acra.2010.07.004
2. Raoof S, Feigin D, Sung A, et al. Interpretation of plain chest roentgenogram. Chest 2012;141(2):545–58. doi:10.1378/chest.10-1302
3. Gaber KA, McGavin CR, Wells IP. Lateral chest X-ray for physicians. J R Soc Med 2005;98(7):310–2. pmc.ncbi.nlm.nih.gov
4. Green DB, Legasto AC, Drexler IR, Gruden JF. Pulmonary fibrosis on the lateral chest radiograph: Kerley D lines revisited. Insights Imaging 2017;8(5):483–9. pmc.ncbi.nlm.nih.gov
5. Park CK, Webb WR, Klein JS. Inferior hilar window. Radiology 1991;178(1):163–8. doi:10.1148/radiology.178.1.1984297
6. Blackmore CC, Black WC, Dallas RV, Crow HC. Pleural fluid volume estimation: a chest radiograph prediction rule. Acad Radiol 1996;3(2):103–9. doi:10.1016/s1076-6332(05)80373-3
7. The Radiology Assistant. Chest X-ray: basic interpretation. radiologyassistant.nl
8. Radiopaedia. Retrotracheal space (Raider triangle). radiopaedia.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.