JBR–BTR, 2005, 88: 66-71.
When pulmonary infection is sus-pected, a chest X-ray is a necessary tool in the process of diagnosis.Only when symptoms persist and/or become worse or when the medical imaging is unclear, a spiral CT or HRCT of the thorax will be taken in consideration.
There are many infections how-ever, which have no specific or even pathognomonic findings at radio-logical examinations. In these, the mechanism of spread and the tissue compartment mainly involved, can be demonstrated radiologically nar-rowing the differential diagnosis.In fact, signs of pulmonary infec-tion can be related to spreading mechanism or can be related to the microorganism itself. In the follow-ing pictorial essay we will sum-marise some of these typical radio-logical signs. Discussion
We can distinguish different mechanisms of spread, moreover depending on the infectious causal agent; typical signs can be depict-ed.Therefore, a separation can be made into signs related to “mecha-nism of spread” and signs related to “‘the microorganism itself”.
Radiological signs related to the mechanism of spread
Infection through the bronchial tree
Air-bronchogram sign
This sign is usually correlated with a lobar pneumonia and pul-monary oedema. When the patholo-gy is limited to the lung parenchyma with filling of alveoli with fluid (pus,inflammation, congestion) and when the bronchial tree is filled with air, a parenchymal consolidation with linear or branching tubular but tapering lucencies exists. These lucencies, representing bronchi or bronchioli, form the air-bron-chogram sign (1) (Fig. 1).
T ree-in-Bud sign
This sign is a finding especially visible on thin-section CT images of the lung (2, 3). The tree-in-bud sign consists of centrilobular nodules of soft tissue attenuation, which are connected to linear, branching opac-ities that have more than one branching site thus resembling a branching tree-in-bud. This sign cor-responds with bronchial dilatations,which are filled by mucus, pus or fluid (4).
These findings are usually visible in the lung periphery (Fig. 2). The term has been popularised by Im et al. to describe the CT appearance of the endobronchial spread of Myco-bacterium tuberculosis (5) b
ut it is
PICTORIAL ESSA Y
LUNG INFECTION IN RADIOLOGY:A SUMMARY OF FREQUENTL Y DEPICTED SIGNS
I.M. Van Mieghem, W.F . De Wever, J.A. Verschakelen 1
For most clinicians,the definition of pneumonia is the presence of an abnormal opacity on chest X-ray and symp-toms of respiratory infection such as cough,mucus production,fever,… The radiological signs of infection of the lower bronchial tree can be covered by other lung diseases and these signs can mimic also other lung diseases.A chest X-ray is a first chosen step in radiological imaging in patients suspected of a pulmonary infection.Only when symptoms persist and/or become worse or when the radiological imaging is unclear,a spiral CT or HRCT of the chest will be taken in consideration.
The role of medical imaging in pulmonary infection is to determine the presence,localisation and extent of the infec-tion,to detect predisposal factors,to detect complications and in the follow-up of the infection.The radiological signs are often not very typical and they have also a limited value in predictin
g the causal organism.However there are some radiological signs,which are very suggestive in predicting the causal organism or in predicting the way of spread of the infection.
Key-words:Lung infection – Thorax,radiography – Thorax,CT
.
Fig. 1.— A 38-year-old woman with a parenchymal consolidation in the left lower
lobe on axial (A) and coronal (B) CT images corresponding to a subsegmental pneu-monia. Hypodense tubular structures are lying within this consolidation correspond-ing to air-filled bronchial structures: air-bronchogram (arrow).
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LUNG INFECTION IN RADIOLOGY —VAN MIEGHEM et al.67
xposed
also common in cystic fibrosis, aspiration, diffuse panbronchiolitis,obliterative bronchiolitis, asthma and chronic airways infection (1).Infection of the lung parenchyma Bulging fissure sign
This sign is typically for a lobar pneumonia. When there is a rapid production of a voluminous inflam-matory exudate in a lobe with lobar enlargement, the extension of this exudate can be limited by the pleura or fissure. This can cause a bulging of the interlobar fissure, giving rise to the bulging fissure sign (1)(Fig.3).
Round pneumonia
Round pneumonia is most often seen in children and is mostly caused by S. pneumoniae. Early in its course, before any pleural bound -aries have been reached, radiologi-cally the pneumonia may be spheri-cal and a round consolidation, with or without air-bronchogram can be seen. In adults this form may mimic a bronchogenic carcinoma (Fig. 4).Infection through vascular spread The feeding-vessel sign
This exclusively CT sign refers to the pulmonary vessel leading to a
nodular opacity, predominantly in the periphery of the lung (6) (Fig. 5).It is not a specific sign for pul-mo
nary infection. It was initially used to diagnose haematogenous metastases; this sign was aban-doned with the advent of spiral CT in the presence of cryptogenic organising pneumonia (COP) (7). It can be seen with fungal infection or with tumoral masses.The miliary pattern
The characteristic X-ray and CT findings of a miliary pattern consist of innumerable, 1- to 3-mm nodules,scattered throughout both lungs,
Fig. 2A.— An axial sCT image of a 17-year-old boy at the level of the carina shows thickening of the wall of the central bronchial tree. There is also a centrilobular, micronodular pat-tern in the left upper lobe (arrow) and in a lesser degree in the right upper and lower lobe mimicking a tree-in-bud. T his tree-in-bud is corresponding with bronchiolitis, in this case with infec-
tious bronchiolitis.
Fig. 3.— A coronal reconstruction CT -image of a 60-year-old man with fever shows a consolidation in the right upper lobe.This consolidation has a sharp delineation at the caudal side due to the small fissure. T his fissure bulges downwards (arrow):
the bulging fissure sign suggesting a lobar pneumonia.
Fig. 4.— The frontal chest X-ray of this 6-year-old girl shows a round parenchymal consolidation in the right upper lobe with visible bulging fissure sign: round pneumonia.
Fig. 2B.— An axial HRCT image of a 40-year-old man through the lung base shows also a tree-in-bud pattern (arrow) in the periphery of both lungs. T here is also dilatation of the more cen-tral bronchi with thickening of the wall. The tree-in-bud pattern corresponds with infectious bronchiolitis.
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68JBR–BTR, 2005, 88 (2)
with a slightly predisposition of the basal lung fields.
Although the lesions are all rela-tively of the same size, the nodules in the upper lung tend to be larger than those in the lower lung.
This miliary pattern is typical for miliary tuberculosis. In 2%-6% of primary tuberculosis, massive lym-p
ho-hematogenous dissemination of tubercle bacilli presents as mil-iary tuberculosis with a miliary pat-tern (8) (Fig. 6). Miliary tuberculosis can occur in post-primary tuberculo-sis, especially in elderly or in immunocompromised patients (9,10). As the infection progresses, the miliary nodules can grow in diame-ter and even coalesce, giving a typi-cal ‘snowstorm’ image.
In the differential diagnosis of a miliary pattern we must consider also a miliary spread of lung metas-tasis.
Radiological signs related to the microorganism itself TBC
Mycobacterium tuberculosis can give two kinds of reactions: 1) pri-mary tuberculosis and 2) post-pri-mary tuberculosis. Primary tubercu-losis exists in previously unexposed patients and affects initially gravity-dependent areas of the lung. Post-
primary tuberculosis exists after
Fig. 5.— Nodular opacity in the apex of the left lower lobe corresponding with an aspergillus infection. A linear structure is passing into this nodular structure corresponding with a feed-ing vessel: the feeding vessel sign (arrow).
Fig. 6.— A frontal chest X-ray of a 85-year-old man shows a micronodular pattern, with a diffuse distribution into both lungs, corresponding with miliary tuberculosis.
Fig. 7A.— A frontal chest X-ray of a 61-year-old man shows a single round, sharp-defined in the right upper lobe correspond-ing with a tuberculoma.
Fig. 7B.— CT correlate of the same patient, nearly three years later, confirms the existence of a slowly growing lesion with rim enhancement after IV contrast administration.
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LUNG INFECTION IN RADIOLOGY —VAN MIEGHEM et al.69
reactivation of dormant bacilli or less commonly from reinfection.Mostly the apical and posterior seg-ments of the upper lobe and superi-or segments of the lower lobe are affected.
T uberculoma – rim enhancement A tuberculoma is a single round or oval nodule, 1-4cm in diameter with mostly smooth and well-defined margins and sometimes with speculated and spicular mar-gins (Fig. 7a).In 80% of the cases a tuberculo-ma is accompanied by satellite lesions. These are small discrete lesions in de neighbourhood of the principal lesion. Most of these lesions remain stable and calcify dif-fusely.
A tuberculoma is a manifestation of primary tuberculosis but is more often seen in post-primary tubercu-losis. After intravenous contrast administration there is no or little contrast enhancement but some-times rim enhancement can be detected (Fig. 7b). This rim contrast
enhancement is typical for tubercu-losis and can also be seen in enlarged mediastinal or hilar lymph nodes. This rim enhancement con-trast captation pattern is specific for tuberculosis but can also be seen in other pathologies such as infection with histoplasmosis, enlarged lymph nodes by lymphoma, or metastatic spread of a carcinoma (4,11).
Primary complex in tuberculosis infection
The primary complex is the com-bination of a Ghon lesion and Ranke complex. A Ghon lesion is a calcified or non-calcified pulmonary nodule and a Ranke complex consists of hilar lymph nodes or gran
ulo-mas (1). On CT scanning the affect-ed lymph nodes typically show the rim enhancement contrast captation pattern (Fig. 8).Aspergillus
Air crescent sign – moving ball sign The most important human pathogens are Aspergillus fumiga-tus and less common Aspergillus niger.
An aspergilloma is a mycetoma or fungus ball, lying in the depen-dent side of a pre-existing pul-monary cavity or ectatic bronchus.The fungus ball consists of a con-glomerate of intertwined fungal hyphae, admixed with mucus and cellular debris. Most commonly the pulmonary cavity is a sequel due to tuberculosis or sarcoidosis (Fig. 9).Sometimes a fungus ball can be found in a pre-existing air-filled bronchogenic cyst, a pulmonary sequestration or pneumato-coele (12).
A typical sign is the air crescent sign. When necrosis occurs in the area of infection and air gets between the wall and residual infec-tion, an air crescent sign exists.
The air crescent sign or meniscus sign is also seen with other abnor-malities such as angioinvasive aspergillosis, echinococcal cyst,tuberculosis, lung abscess, broncho-genic carcinoma, hematoma,P .carinii pneumonia infection (12).Another typical sign of an aspergilloma is the moving ball sign. The fungus ball is lying within a pre-existing cavity but is not adherent to the wall. When the patient is movi
ng, the fungus ball is always lying in the most dependent
side of the cavity (Fig. 10).
Fig. 8.— Central hypodense mediastinal nodular structure in the right paratracheal region. This structure shows peripheral rim contrast enhancement: the rim contrast captation pattern (arrow). This contrast captation pattern is suggestive for an enlarged lymph node, which can be seen in tuberculosis as in this case.
Fig. 9.— Chest X-ray (A, front view, B, profile) of a 65-year-old woman with a known history of tuberculosis with an excavated lesion in the right upper lobe. The chest X-ray shows a dense nodular mass (*) with central excavation and an air-sickle resting in the upper part of this excavation. The dense nodule on the bottom of this excava-tion corresponds with an aspergilloma; the air-sickle above the aspergilloma is the air crescent sign (arrow).
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70JBR–BTR, 2005, 88 (2)
Finger-in-glove sign
This sign, also called inverted V or Y , cluster of grapes sign is seen with ABPA (Allergic Broncho-Pulmonary Aspergillosis) or Loeffler’s like syn-drome.
Radiographically the finger in glove sign is formed by mucoid impaction within dilatated segmen-tal or subsegmental proximal bronchi, predominantly in the upper lobes (Fig. 11).Halo sign
On CT the sign is seen as an area of ground-glass attenuation around a nodule of soft tissue attenuation (Fig. 12). This ground-glass attenua-
tion represents haemorrhage and/or oedema. The nodule in pulmonary aspergillosis is caused by the inva-sion and occlusion of a small to medium-sized pulmonary artery by fungal hyphae. In time an air cres-cent sign can develop in the nodule.This sign is seen with angioinvasive aspergillosis in immunocompro-mised patients with severe neu-tropenia. A differential diagnosis for this halo sign includes vasculitis (e.g. Wegener’s granulomatosis),Kaposi’s sarcoma, and other infec-
tions such as herpes, candida,mucorales (12), and tuberculo-sis (11).
Pneumocystis carinii
In 15% of the cases the chest X-ray or chest CT is normal. T he typical presentation of Pneumocystis carinii pneumonia (PCP) consists of a dif-fuse perihilar reticular or reticulon-odular pattern that can progress to alveolar disease (13). The spread is typical symmetrically and bilateral.When the disease progresses,
Fig. 10.— Axial CT image through the apex of the right lower lobe in decubitus (A) and in procubitus (B). These images demon-strate a large cavity in the apex of the right lower lobe. A nodular mass (*) is lying in this cavity and is changing with the position of the patient: it lies always at the bottom of the cavity.
This “moving ball” sign is suggestive for an aspergilloma.
Fig. 11.— Dense tubular structures in the right middle lobe corresponding with fluid filled bronchi. These bronchi simulate a finger in glove, which is typical for an allergic broncho-
pulmonary aspergillosis. (ABPA).
Fig. 12.— A patient with angio-invasive aspergillosis shows multiple nodular consolidations, some with excavation. These nodular consolidations are surrounded by areas of ground-glass attenuation: the halo sign (arrow). In this case the halo sign is suggestive for invasive aspergillosis.
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