The silhouette sign

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

The silhouette sign is the loss of a normal border on a radiograph when something of the same density lies directly against it. Described and popularised by Benjamin Felson, it is the single most useful localising tool on the frontal chest film. If the right heart border disappears behind an opacity, that opacity is touching the heart, so it is in the right middle lobe, not the lower lobe behind it. Knowing which structure forms each border lets you place disease in a lobe or compartment from one PA film.

1. The principle

A plain film shows an edge only where two adjacent structures have different densities (see how X-ray images are made). The heart, aorta and diaphragm are visible because each is soft-tissue density lying against air-filled lung. When the adjacent lung fills with fluid, pus, cells or collapses, it becomes soft-tissue density too, and the boundary vanishes.

Two conditions must be met:

1. The abnormality must be of soft-tissue (water) density, such as consolidation, collapse, fluid or a mass.
2. It must be in direct anatomical contact with the border. Lying in front of or behind it on the same film is not enough; the X-ray beam still crosses an air–tissue interface at the border itself.

The sign works in reverse for air: an air bronchogram appears when air-filled bronchi lie inside airless lung, a different but related idea.

2. Which border, which lobe

Lost border	Structure forming it	Location of the abnormality
Right heart border	Right atrium	Right middle lobe (medial segment)
Left heart border	Left ventricle	Lingula (left upper lobe)
Right hemidiaphragm	Diaphragm dome	Right lower lobe (or right pleural fluid)
Left hemidiaphragm	Diaphragm dome	Left lower lobe (or left pleural fluid)
Aortic knob	Aortic arch	Left upper lobe (apicoposterior segment) or posterior mediastinum
Right upper mediastinal / paratracheal border	Superior vena cava	Right upper lobe (anterior segment)
Descending aorta	Descending thoracic aorta	Left lower lobe

Sources: Felson; Radiopaedia; Radiology Masterclass. See normal chest X-ray anatomy for the borders themselves.

3. A preserved border is also a clue

If there is an opacity overlying the heart but the heart border remains crisp, the abnormality is not touching the heart. It must lie posteriorly, usually in a lower lobe. The same applies to the aortic knob and the diaphragm.

• Right basal opacity, right heart border sharp, right hemidiaphragm lost: right lower lobe.
• Left basal opacity behind the heart, left heart border sharp, descending aorta and medial left hemidiaphragm lost: left lower lobe (classically left lower lobe collapse with the sail sign).
• Upper zone opacity with a sharp aortic knob: probably anterior, in the upper lobe in front of the arch, or in the anterior mediastinum.

4. Related localising signs

Felson and others described a family of signs that use the same physics to place a lesion in three dimensions:

Sign	What you see	What it means
Cervicothoracic sign	The upper edge of a mediastinal mass is visible above the clavicles	The mass lies posteriorly, where it is still surrounded by lung. A mass whose edge fades at the clavicle lies anteriorly, where it meets the soft tissues of the neck.
Thoracoabdominal sign	The lower edge of a mass is visible below the dome of the diaphragm	The mass is posterior (retrocrural), outlined by lung in the posterior costophrenic recess.
Hilum overlay sign	Hilar vessels are visible through an apparent hilar mass	The mass is not in the hilum; it lies in front of or behind it (for example in the anterior mediastinum).
Hilum convergence sign	Vessels converge into the enlarged hilum	The enlargement is the pulmonary artery, not a mass.
Extrapleural sign	A lesion with obtuse angles to the chest wall and tapering margins	The lesion is pleural or in the chest wall. Lesions in the lung make acute angles with the chest wall.

See mediastinum and hila for mediastinal compartments and masses.

5. Confirming on the lateral film

If a lateral view is available, use it to check your localisation:

• Right middle lobe disease lies anteriorly, between the horizontal fissure and the lower part of the oblique fissure, often as a wedge pointing to the hilum.
• Lower lobe disease lies posteriorly, behind the oblique fissure, and often makes the lower spine whiter (the spine sign).
• Lingular disease lies anteriorly, overlapping the heart.

Worked example

A febrile patient has a PA film with increased opacity in the left mid and lower zones. The left heart border is indistinct, but the left hemidiaphragm and the descending aorta are both clearly seen through the heart. The opacity is therefore touching the left ventricle but not the diaphragm or the aorta: it lies anteriorly, in the lingula. If instead the heart border were sharp and the descending aorta and medial hemidiaphragm were lost, the disease would be in the left lower lobe. If everything were lost and there were a meniscus, you would add a pleural effusion to the description. Working through each border in turn, rather than glancing at the overall whiteness, is what makes the sign reliable.

Even without a lateral, the silhouette sign plus fissure position usually gets you to the right lobe. CT is the definitive test when the answer matters, for example when a mass is suspected.

6. Pitfalls and mimics

• Pectus excavatum blurs the right heart border without any lung disease, mimicking right middle lobe consolidation. Look for the characteristic rib shape and a leftward-displaced heart. See artefacts and mimics.
• Pericardial fat pads and obesity can make the cardiac apex and left heart border indistinct.
• Under-penetration hides the descending aorta and the left hemidiaphragm behind the heart. Check exposure first; see RIPE.
• Rotation changes which parts of the heart and mediastinum form the borders.
• Pleural fluid also obliterates the diaphragm. A meniscus and a blunted costophrenic angle point to effusion rather than lower-lobe consolidation; often both are present. See pleural effusion.
• The sign does not tell you what the opacity is. Consolidation, collapse, fluid and tumour all produce it. Use volume (preserved in consolidation, lost in collapse), air bronchograms and the clinical picture to decide. See consolidation and lobar collapse.
• A sharp border does not exclude disease in the same lobe. Small or peripheral areas of consolidation may not touch the border at all.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Memorise the border-to-lobe table, and use it to turn "zone" into "lobe" when you present a film.
Nurse	Understand why a report may say "right middle lobe" from a single frontal film.
Junior doctor	Localise consolidation and collapse confidently, and recognise when volume loss changes the plan.
Radiographer	Adequate penetration and minimal rotation, so retrocardiac borders can be assessed.
Advanced / FRCR	Cervicothoracic, thoracoabdominal, hilum overlay and extrapleural signs in mediastinal and chest wall masses.

Practise localising disease →

References

1. Felson B. Chest Roentgenology. Philadelphia: WB Saunders; 1973.
2. Radiopaedia. Silhouette sign. radiopaedia.org
3. Radiology Masterclass. Chest X-ray anatomy. radiologymasterclass.co.uk
4. Radiology Masterclass. Lung collapse. radiologymasterclass.co.uk
5. Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com

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.