We carried out a literature review of classic signs in thoracic radiology,
using 19 real examples from the archive at Centro Hospitalar de São João,
identifying the respective clinical context and the matching imaging findings.
Signet ring sign
One of the most common and basic signs of bronchiectasis in chest CT imaging is the “signet ring” sign (Fig. 1): a ring of soft-tissue attenuation (bronchial wall) surrounding a round lucent area (bronchial lumen with air in cross-section),
with an adjacent smaller nodular structure,
with soft-tissue attenuation (pulmonary artery branch).
This is typical of bronchiectasis,
particularly cylindrical-shaped ones.
Whenever the lumen of a bronchus exceeds by more than half the lumen of the adjacent pulmonary artery (normal bronchovascular ratio is 1:1 or less),
a “signet ring” sign is seen and bronchiectasis are most likely diagnosed.
Visualization of this sign should promptly precipitate searching for other signs of bronchiectasis,
like thickened bronchial walls,
lack of bronchial tapering and visible bronchioli within 1 cm of the pleura.
Fig. 1: Bronchiectasis in the left inferior lobe, seen in cross-section (inside red circle; magnified inside red rectangle), where three “signet ring” signs can be identified, representing different bronchioli with bronchovascular ratio >1:1, indicating the presence of cylindrical-shaped bronchiectasis.
Another classical sign of bronchiectasis – the “tram-track” sign (Fig. 2) – is seen when dilated bronchi are seen on a longitudinal plane,
where parallel linear opacities (bronchial thickened walls) surround the air-containing lumen.
This sign is seen in cylindrical-shaped bronchiectasis,
in cases of bronchial obstruction,
infection or congenital defects like pulmonary sequestration.
Fig. 2: Bilateral bronchiectasis shown by bronchi with thickened walls and no decreasing calibre throughout its course (red arrows, bilaterally), resembling “tram-tracks”.
When multiple branching linear opacities lead to small adjacent centrilobular soft-tissue nodules,
stemming from a proximal structure,
a “tree-in-bud” pattern is present (Fig. 3).
It represents small airway mucus plugging,
and bronchial wall thickening and inflammation.
It is a non-specific finding in small airway disease,
with many causes,
including infection (bacterial,
fungal or viral),
aspiration of foreign bodies,
toxic fume inhalation,
and immunologic and connective tissue disorders.
Fig. 3: Peripheral linear opacities in the right upper lobe associated with adjacent centrilobular micronodules (inside red circle; magnified inside red square), forming a “tree-in-bud” pattern. This case was due to bacterial infection, having resolved after antibiotic therapy.
Air bronchogram sign
In normal healthy patients,
bronchi are not visible structures on CT scans,
due to their air content.
in the setting of parenchymal consolidation,
their course may be discernible if their lumen remains patent.
In this case,
normal air is still flowing due to proximal airway patency,
whereas the parenchyma itself is consolidated,
thus producing the air bronchogram sign (Fig. 4),
also seen in chest X-ray.
Although mostly encountered in cases of pneumonia and pulmonary edema,
other causes are acute respiratory distress syndrome (ARDS),
post-radiation fibrosis and sarcoidosis.
Fig. 4: Large pneumonia consolidation of the right inferior lobe due to bacterial infection. Inside the consolidated lung parenchyma air-filled patent bronchi can be observed (red arrows) forming the air bronchogram sign.
CT angiogram sign
Somewhat analogous to the air bronchogram sign,
the CT angiogram sign (Fig. 5) is seen in contrast-enhanced CT scans,
where enhanced pulmonary vessels are patent within consolidated pulmonary parenchyma.
Although not as frequent as the air bronchogram sign,
it is classically identified in cases of pneumonia,
but also in other entities such as lung adenocarcinoma,
lymphoma and metastatic cancer of gastrointestinal origin.
Fig. 5: Contrast-enhanced CT scan showing a large consolidation in the right lung, due to pneumonia (left: axial view: right: coronal view). Patent contrast-enhanced blood vessels can be seen within the consolidation (red arrows) – CT angiogram sign.
Mucus impaction inside bronchial structures conveys an image of lobular branching opacities,
originating from the hilum and directed peripherally,
surrounded by aerated lung,
resembling gloved fingers (Fig. 6).
seen both in X-ray films and CT scans,
can develop both in obstructive and non-obstructive lung diseases,
causing bronchiectasis with mucoid impaction within them,
being more frequent in the upper lobes.
Any obstructive lesion can present this sign,
so it can be found in cases of hamartomas,
Non-obstructive causes might include allergic bronchopulmonary aspergillosis (ABPA),
mostly due to allergic reaction in asthmatic patients,
and cystic fibrosis,
where there is mucociliary dysfunction with thick mucous production.
Fig. 6: Bronchiectasis on the posterior segment of the right upper lobe, showing an opaque lumen (inside red circle; magnified inside red rectangle) due to mucoid impaction, resembling fingers inside a glove – “finger-in-glove” sign.
Although it is not an actual sign,
ground-glass opacification (GGO) identification is important due to being present in many classical signs of thoracic CT imaging.
It is defined as a hazy and slightly hyper-attenuated area of lung parenchyma,
where the bronchovascular architecture is preserved (Fig. 7).
partial airway filling,
alveoli collapse or even normal expiration might cause GGO.
Common causes include pulmonary edema or haemorrhage,
acute respiratory distress syndrome (ARDS),
viral or bacterial pneumonia,
hypersensitivity pneumonia and other diffuse interstitial lung diseases.
Fig. 7: Several areas of hazy and discreet hyper-attenuation (inside red dotted perimeters) – ground-glass opacities – in a patient with hypersensitivity pneumonia.
Whenever a nodule,
mass or consolidation is surrounded by a ground-glass opacity,
it resembles a halo,
hence being called the halo sign (Fig. 8).
A multitude of diseases may present this finding,
emphasizing its non-specific characteristic.
Its appearance is mostly related to pulmonary haemorrhage,
but it can also be seen in various infectious diseases,
inflammatory conditions and neoplasms.
In immunocompromised patients presenting neutropenia,
recognition of the halo sign carries a reserved prognosis,
since it suggests angioinvasive fungal infection with high mortality rate.
Apart from invasive aspergillosis (usually in early stages), other causes include bacterial infections – tuberculosis,
viral infections such as cytomegalovirus and herpes simplex virus,
cryptogenic organizing pneumonia (COP),
metastatic tumour and adenocarcinoma in situ.
Fig. 8: A mass is seen on the left upper lobe (red dotted perimeter), surrounded by hazy ground-glass opacification halo (red arrows) – halo sign. The mass was due to an adenocarcinoma in situ lesion.
Reversed halo / atoll sign
A focal ground-glass opacification surrounded by a crescent or ring-shaped parenchymal consolidation is called the reversed halo or atoll sign (Fig. 9).
The surrounding air-space consolidation has to cover more than 75% of the ground-glass opacity and be at least 2 mm in thickness.
Although initially thought as highly specific of cryptogenic organizing pneumonia,
it has also been reported in cases of pulmonary fungal infections such as paracoccidioidomycosis and lymphomatoid granulomatosis.
the central opacity represents an area of alveolar septal inflammation and cellular debris,
and the opacified ring represents an area or organizing pneumonia.
Fig. 9: A ground-glass opacity surrounded by a thick circular consolidation is seen at the lingula (red arrows) – reversed halo / atoll sign – in a patient with cryptogenic organizing pneumonia.
Comet tail sign
Subpleural masses can cause distortion of surrounding bronchovascular architecture,
with appearance of curvilinear opacities (resembling a comet tail) which extend from the mass to the ipsilateral hilum (Fig. 10).
This phenomenon is characteristically associated with pulmonary round atelectasis (Blesovsky syndrome),
with findings of pleural thickening or effusion being also frequent.
It is mostly found in the posterior aspect of lower lobes,
although it can occur at any pulmonary location.
The affected lobes usually present volume loss.
The main causative agent is asbestosis with pleural disease,
but less common pathologies such as tuberculosis,
non-specific pleuritis or congestive heart failure are also possible.
Fig. 10: Bilateral pleural effusion associated with two posterior subpleural masses, one on each inferior lobe. Several curvilinear opacities extending from both hila to the masses can be seen – “comet-tail” sign – indicating the presence of round atelectasis.
Bulging fissure sign
A classic sign of pulmonary consolidation caused by Klebsiella pneumoniae in the right upper lobe,
also called Friedländer’s pneumonia,
the bulging fissure sign (Fig. 11) represents the downward expansion (bulging) of the horizontal or oblique right fissure,
seen both in plain X-ray films and CT scans,
by large inflammatory exudates caused by the bacterial infection.
Other bacterial agents which might show a similar sign include tuberculosis,
lung abscess and adenocarcinoma in situ.
with standardised prophylactic implementation of antibiotic therapy,
this sign is relatively less frequent in developed countries.
Fig. 11: Large consolidation of the right inferior lobe, with expansion of the oblique fissure anteriorly – bulging fissure sign. This is a particular case, since this sign is more frequent with consolidation of the right upper lobe and downward expansion of the horizontal fissure.
Air crescent sign
As the name says,
this sign results from air accumulation,
in a crescent shape,
surrounding any mass lodged inside a pulmonary cavity (Fig. 12).
Invasive pulmonary aspergillosis,
particularly in immunocompromised and neutropenic patients,
is the main etiology,
although it can be seen in other entities which present cavities,
such as neoplasms,
lung abscesses and infections.
Other less frequent causes are pulmonary gangrene,
Rasmussen aneurysm in a tuberculous cavity,
and hydatid cyst with bronchial involvement.
The air crescent sign usually heralds clinical and laboratorial improvement,
correlating with recovery of immune activity in invasive pulmonary aspergillosis,
with leukocyte activity increase on necrotic tissue,
leaving air in its place.
This sign is often confused with the Monod sign,
where a previous pulmonary cavity (old tuberculosis,
neoplasm) is colonized by fungal agents like Aspergillus fumigatus,
with formation of a fungus ball within it (aspergilloma/mycetoma),
surrounded by air.
fungus balls are mobile inside cavities,
moving towards dependent locations,
so CT acquisition with ventral decubitus should be performed whenever this suspicion is raised.
Fig. 12: Large cavity on the right lung apex (axial view: left; coronal view: right). Inside the cavity there is a necrotic lung mass, due to angioinvasive aspergillosis, surrounded by air in a crescent-shape (red arrows) – air crescent sign – indicating reabsorption of necrotic tissue by leucocyte activity.
Feeding vessel sign
On contrast-enhanced CT scans,
identification of an artery leading to a nodule or mass implies a haematogenous origin of the lesion and constitutes the feeding vessel sign (Fig. 13).
More than 70% of septic embolism cases present this sign,
although it might be found in haemorrhagic nodules,
pulmonary vasculitis or pulmonary infarction.
Fig. 13: Mass in the posterior segment of the right upper lobe (axial view: left; coronal view: right), showing irregular shape and an enhanced vessel leading to the center of the mass (red arrows) – feeding vessel sign.
Lung parenchymal destruction with fibrotic tissue and the formation of resulting multiple cystic airspaces with thick fibrous walls can be found at late stages of many lung diseases,
receiving the term “honeycomb” pattern (Fig. 14).
Cystic changes vary in size from a few millimetres to several centimetres and their usual distribution is subpleural.
This pattern is a hallmark of pulmonary fibrosis and is used as criterion for usual interstitial pneumonia.
Fig. 14: Fibrotic and cystic parenchymal changes in both upper lobes (inside red perimeters), more pronounced on the left upper lobe – “honeycomb” pattern. This was a case of idiopathic pulmonary fibrosis.
Crazy paving sign
Originally found among patients with pulmonary alveolar proteinosis (PAP),
the “crazy paving” sign combines the visualization of thickened interlobular septa intercalated with diffuse ground-glass opacities,
as well as intralobular reticular thickening,
resembling irregularly shaped paving stones (Fig. 15).
Since PAP is a rare condition,
other more common causes include adult respiratory distress syndrome,
Pneumocystis jirovecii pneumonia and bacterial pneumonias.
Other rare causes are alveolar sarcoidosis,
non-specific interstitial pneumonia and pulmonary haemorrhage syndromes.
Fig. 15: Large areas of ground-glass opacification with intralobular reticular thickening, resembling an irregular paving stone pattern (inside red perimeter) – “crazy paving” sign – in a patient with pulmonary alveolar proteinosis (PAP).
Described in sarcoidosis,
this sign shows mass-like regions in lung parenchyma composed of clusters of smaller nodules,
surrounded by satellite nodules,
resembling a cluster galaxy (Fig. 16).
This nodular pattern is more common in patients with more than 50 years of age at presentation.
Fig. 16: Sarcoidosis affecting both lungs, forming clusters of nodules surrounded by smaller satellite nodules (red dotted perimeters) – “galaxy” sign.
Typical of hypersensitivity pneumonia,
this sign refers to simultaneous visualization of three different patterns: ground-glass opacification (GGO) and patchy mosaic attenuation with focal lucencies and normal lung parenchyma (Fig. 17).
It represents processes of infiltrative (GGO) nature,
as well as obstructive phenomena mixed with unaffected parenchymal segments (mosaic attenuation).
It is highly specific of hypersensitivity pneumonia,
although it has been reported in cases of sarcoidosis and atypical infections with bronchiolitis.
Fig. 17: Case of hypersensitivity pneumonia where both lungs present normal parenchyma (green arrows), focal lucencies (yellow areas) and ground-glass opacification (red arrows) – “headcheese” sign (resembling the cold-cut charcuterie with the same name).
Polo mint sign
On contrast-enhanced CT scans,
visualization a blood vessel in cross-section with a central filling defect and thin rim of contrast around it represents the “polo mint” sign (Fig. 18).
It is highly suggestive of pulmonary thromboembolism with a central thrombus in the visualized vessel.
other signs of thromboembolism should be sought.
Although initially described for pulmonary thrombus,
this sign can be seen in any vessel,
such as the superior vena cava or the portal vein.
Fig. 18: Contrast-enhanced CT scan showing pulmonary thromboembolism on the left inferior lobe, with a central thrombus surrounded by a rim of contrast (red arrows) – “polo mint” sign (resembling a British breath mint with the same name). A large central thromboembolism can also be seen in the right main bronchus.
Split pleura sign
Bacterial pneumonia can spread into pleural space,
causing an empyema.
Fluid accumulation in pleural space leads to spreading of the parietal and visceral pleural layers (Fig. 19).
The separated layers become thickened,
showing increased enhancement,
thus producing the split pleura sign,
seen in contrast-enhanced CT scans.
Less common causes are mesothelioma,
talc pleurodesis and post-lobectomy status.
Fig. 19: Presence of an empyema on the left lung, with a liquid component spreading the visceral and parietal pleural layers, which appear thickened and enhanced (red arrows) – split pleura sign. Some air component is also seen within the empyema.