[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Chest](https://durgical.com/en/radiology/areas/chest/) › The silhouette sign: which lobe is it?

# The silhouette sign: which lobe is it?

- Sign

On an X-ray, two things of the same density that touch lose the edge between them. Which edge is lost tells you where the disease is.

Updated Nov 12, 2026

## Key points

- Same density touching = no edge: the lost border localises the lesion.
- Right heart border = right middle lobe; left heart border = lingula.
- Hemidiaphragm lost = lower lobe; heart border kept with an opacity = lower lobe.
- Check rotation, inspiration and pectus before trusting the sign.

## What it is

An X-ray shows an edge only where two densities meet, such as the soft-tissue heart against the air-filled lung. When the lung next to a border fills with fluid, pus, blood or tissue, it becomes the same density as the heart, aorta or diaphragm and that border disappears. This loss of a normal outline is the silhouette sign. Because each border touches a known part of the lung, the lost border points to the lobe involved, and a border that stays sharp next to an opacity means the opacity lies in front of or behind that structure, not against it.

## What it looks like

- Right heart border lost: right middle lobe (medial segment).
- Left heart border lost: lingula of the left upper lobe.
- Right or left hemidiaphragm lost: the lower lobe on that side.
- Aortic knob lost: left upper lobe (apicoposterior segment); descending aorta lost: left lower lobe.
- Ascending aorta or right upper mediastinal border lost: right upper lobe (anterior segment).
- An opacity over the heart with a sharp heart border is in the lower lobe, behind the heart.

## Why it happens

- An edge on a radiograph is the interface between two different densities; there is no line where density stays the same.
- Airless lung (consolidation, collapse, mass) has the density of soft tissue and water, the same as the heart, vessels and diaphragm.
- The sign works only when the opacity is in direct contact with the structure; contact, not overlap, is what removes the edge.

| Border lost | Lobe or segment in contact |
| --- | --- |
| Right heart border | Right middle lobe |
| Right hemidiaphragm | Right lower lobe |
| Ascending aorta, right upper mediastinum | Right upper lobe (anterior segment) |
| Left heart border | Lingula (left upper lobe) |
| Aortic knob | Left upper lobe (apicoposterior segment) |
| Left hemidiaphragm, descending aorta | Left lower lobe |

## What it suggests

- Consolidation (pneumonia) and collapse are the commonest causes; collapse also shows signs of volume loss.
- A mass, a pleural effusion or a pericardial fat pad against a border can also remove it.
- Pectus excavatum often blurs the right heart border without lung disease.
- Poor inspiration, rotation and an AP or supine film can blur borders falsely; check film quality first.
- Above the clavicles, a mass with a sharp upper edge is posterior (it is surrounded by lung); an anterior mass fades out at the clavicles (cervicothoracic sign).

## Next step

- Check film quality (rotation, inspiration, projection) before calling a lost border.
- Look at the lateral view when available: it shows directly whether the opacity is in the middle lobe, lingula or a lower lobe.
- Compare with previous films to see whether the change is new.
- Opacity that does not clear, or signs of volume loss in an adult smoker: CT of the chest to look for an obstructing lesion.
- If the pattern does not fit the clinical picture, ask the radiologist.

## Test yourself

Think of the answer, then tap the question to see it.

## A patient has an opacity that removes the right heart border on a PA film. Which lobe is involved?

The right middle lobe, whose medial segment touches the right heart border.

## An opacity projects over the left heart but the left heart border stays sharp. Where is it?

Behind the heart, in the left lower lobe: it overlaps the heart without touching its border, so the edge is kept.

## Why does pneumonia next to the heart remove the heart border?

Airless lung has soft-tissue density like the heart; with no density difference at the interface, no edge is drawn.

## Which normal variant often blurs the right heart border without any lung disease?

Pectus excavatum (a sunken sternum), which displaces the heart and changes the soft tissues in front of the right heart border.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/the-silhouette-sign-which-lobe-is-it/)

## Connections

## Related 5

- [The five densities on an X-ray](https://durgical.com/en/radiology/the-five-densities-on-an-x-ray/)
- [Consolidation and air bronchograms](https://durgical.com/en/radiology/consolidation-and-air-bronchograms/)
- [Lobar collapse (atelectasis) on the chest X-ray](https://durgical.com/en/radiology/lobar-collapse-on-the-chest-x-ray/)
- [Reading a chest X-ray step by step](https://durgical.com/en/radiology/reading-a-chest-x-ray-step-by-step/)
- [Is this chest X-ray good enough? Projection, rotation, inspiration and exposure](https://durgical.com/en/radiology/is-this-chest-x-ray-good-enough-projection-rotation-inspiration-and-exposure/)

[← All of radiology](https://durgical.com/en/radiology/)

Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: [Chest and Mediastinal Imaging](https://www.ncbi.nlm.nih.gov/books/NBK559145/) (StatPearls (NCBI Bookshelf), 2026); [Atelectasis](https://www.ncbi.nlm.nih.gov/books/NBK545316/) (StatPearls (NCBI Bookshelf), 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/radiology/the-silhouette-sign-which-lobe-is-it/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/the-silhouette-sign-which-lobe-is-it/