Radiologia Para Neumonias

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European Journal of Radiology 51 (2004) 102113

Radiology of bacterial pneumonia

Jos Vilar, Maria Luisa Domingo, Cristina Soto, Jonathan Cogollos

Radiology Department, Hospital Universitario Doctor Peset, Valencia, Spain

Received 23 February 2004; received in revised form 26 February 2004; accepted 1 March 2004

Abstract

Bacterial pneumonia is commonly encountered in clinical practice. Radiology plays a prominent role in the evaluation of pneumonia. Chest

radiography is the most commonly used imaging tool in pneumonias due to its availability and excellent cost benefit ratio. CT should beused in unresolved cases or when complications of pneumonia are suspected. The main applications of radiology in pneumonia are oriented

to detection, characterisation and follow-up, especially regarding complications. The classical classification of pneumonias into lobar and

bronchial pneumonia has been abandoned for a more clinical classification. Thus, bacterial pneumonias are typified into three main groups:

Community acquired pneumonia (CAD), Aspiration pneumonia and Nosocomial pneumonia (NP).The usual pattern of CAD is that of the

previously called lobar pneumonia; an air-space consolidation limited to one lobe or segment. Nevertheless, the radiographic patterns of CAD

may be variableand are oftenrelated to the causative agent. Aspiration pneumonia generally involves the lower lobeswith bilateral multicentric

opacities. Nosocomial Pneumonia (NP) occurs in hospitalised patients. Theimportance of NP is related to its high mortality and, thus, the need

to obtaina prompt diagnosis. Therole of imaging in NP is limited butdecisive. Themost valuableinformation is when thechestradiographsare

negative and rule out pneumonia. The radiographic patterns of NP are very variable, most commonly showing diffuse multifocal involvement

and pleural effusion. Imaging plays also an important role in the detection and evaluation of complications of bacterial pneumonias. In many

of these cases, especially in hospitalised patients, chest CT must be obtained in order to better depict these associate findings.

2004 Elsevier Ireland Ltd. All rights reserved.

Keywords: Pneumonia; Bacterial pneumonia; Pulmonary CT; Nosocomial pneumonia

1. Introduction

Bacterial pneumonias account for a large percentage of

all pneumonias. They have been classified into three main

groups: lobar pneumonia, bronchopneumonia and acute in-

terstitial pneumonia [1]. Lobar pneumonias are characterised

by confluent areas of focal airspace disease, usually limited

to one lobe or segment. Bronchopneumonia has a multi-

focal distribution with nodules that tend to join producing

air-space consolidations affecting one or more lobes. Acute

interstitial pneumonias are produced by involvement of thebronchial and bronchiolar wall, and of the pulmonary inter-

stitium, and are most commonly caused by viral organisms

and Mycoplasma pneumoniae.

This classic morphologic classification is of limited use-

fulness because the radiographic pattern often cannot be

used to predict the causative organism. The appearance of

new infective organisms, the increasing age of the popula-

tion and the wide use of antibiotics have changed the pat-

Corresponding author.

E-mail address: vilar [email protected] (J. Vilar).

terns of this disease [2]. This is why most authors prefer

to classify pneumonias from the perspective of the mecha-

nism of origin. Thus, we will refer to three main groups of

pneumonias: community acquired pneumonia (CAP), noso-

comial pneumonia (NP) and aspiration pneumonia.

Streptococcus pneumoniae is the most common cause of

CAP while Gram-negative bacteria and Staphylococcus au-

reus are more often responsible for hospital acquired pneu-

monia [2]. Aspiration pneumonias are usually produced by

micro-organisms that colonize the oropharynx which include

Gram-positive cocci, Gram-negative rods, and rarely, anaer-

obic bacteria.

This article will review the most common and some un-

usual radiographic presentations of bacterial pneumonia in

inmunocompetent patients.

2. Imaging pneumonia

In patients with suspected pneumonia, imaging plays a

major role in the detection, characterisation and follow-up

of the disease.

0720-048X/$ see front matter 2004 Elsevier Ireland Ltd. All rights reserved.

doi:10.1016/j.ejrad.2004.03.010


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J. Vilar et al. / European Journal of Radiology 51 (2004) 102 113 103

2.1. Detection

The basic and most diffused imaging tool to diagnose

pneumonia remains the chest radiograph. Indeed pulmonary

infections are the most common reason for obtaining an

emergency chest film. Pneumonia may present with a

wide spectrum of symptoms and often the initial clinicalmanifestations are clear. Although the chest radiograph is

Fig. 1. Additional value of CT: CAP (a) chest radiograph: there is a paratracheal opacity in the right upper lobe. (b) CT of the same patient shows

clearly the opacity due an air-space consolidation.

often regarded as the reference standard for the diagnosis

of community-acquired pneumonia, its reliability is lim-

ited by significant interobserver variability in radiographic

interpretation [3].

Other techniques like computed tomography (CT) can be

useful, showing some infiltrates not visualised in the chest

radiographs (Fig. 1) and can assure the existence of cavita-tion or other complications, [4] but the use of CT is only


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104 J. Vilar et al. / European Journal of Radiology 51 (2004) 102113

recommended in cases uncertain to the chest film, complica-

tions of pneumonia or suspicion of an underlying additional

lesion such as bronchogenic carcinoma.

Magnetic resonance imaging (MRI) can demonstrate pul-

monary consolidations. It can be used as an alternative to CT

in patients who should not be exposed to ionising radiation.

2.2. Characterisation

Is imaging reliable for distinguishing the infective or-

ganism? Tew et al. [5] reviewed 31 patients with bacte-

rial and non-bacterial pneumonias. The diagnostic accuracy

was 67% for bacterial pneumonia and 65% for non-bacterial

pneumonia. The authors concluded that radiology alone was

unable to distinguish bacterial from non-bacterial pneumo-

nias. In a review of 114 cases of pneumonia, Reittner et al.

concluded that CT is also unable to differentiate the aeti-

ology of various types of pneumonia except Pneumocystis

carinii [6]. The characterisation of some NP may be quite

difficult, especially in patients with assisted ventilation whenother pulmonary conditions may coincide [7]. Despite these

limitations, imaging may be of great help in detecting the as-

sociated findings. A study by Albaum et al. [3] showed that

the chest radiograph reliability for detecting pleural fluid and

multiple infiltrates was good. This is important since both

findings are related to a worse prognosis.

2.3. Follow-up

Most pnemonias will resolve in 1 or 2 weeks. Slow reso-

lution can occur when there are certain associated conditions

such as chronic obstructive pulmonary disease, alcoholism,diabetes and immune-deficiency. Otherwise, if the pneumo-

nia does not resolve, an underlying pathology should be sus-

pected, especially bronchogenic carcinoma. In these cases,

as mentioned previously, CT is recommended [8,9].

3. Community acquired pneumonia (CAP)

The aetiology of CAP varies widely according to the

different reviews published. It is highly influenced by the

geographic area, the population studied and the diagnos-

tic methods used [10]. The most common bacterial agents

responsible for CAP are S. pneumoniae, M. pneumoniae,

Chlamydia pneumoniae and Legionella pneumophila. S.

aureus may complicate a viral pneumonia. CAP may be

caused by Gram-negative organisms in elderly patients,

alcoholics, patients with cardiopulmonary disease and due

to the widespread use of broad-spectrum antibiotics [1].

The incidence of these organisms varies according to the

different authors. Thus, in a study by Lim et al. [11], the

most common agent producing CAP was S. pneumoniae

(48%) followed by virus (19%), C. pneumoniae (13%),

Haemophilus influenzae (20%) and M. pneumoniae (3%),

while another publication [2] reported S. pneumoniae

Fig. 2. Community acquired pneumonia (Streptococcus pneumoniae) (a)

and (b): PA and lateral chest films show consolidation in the lateral

segment of the middle lobe, abutting the major and minor fissures.


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Fig. 3. PA chest radiograph shows an alveolar consolidation involving the right and left lower lobes in a patient infected by Streptococcus pneumoniae.

(920%), M. pneumoniae (1337%) and C. pneumoniae

(17%) as most common agents.

The usual imaging finding in CAP coincides with the clas-

sic presentation of lobar pneumonia: an airspace consolida-

tion in one segment or lobe, limited by the pleural surfaces

(Fig. 2). CT may additionally show ground glass attenua-

tion, centrilobular nodules, bronchial wall thickening and

centrilobular branching structures [4] (Fig. 1b).

Fig. 4. Mycoplasma pneumonia: chest radiograph. There is a diffuse peripheral and bilateral interstitial involvement.

3.1. Pneumococcal pneumonia

S. pneumoniae is the most frequent micro-organism caus-

ing CAP [2,11]. The usual presentation is a lobar pneumo-

nia involving one segment or lobe. Nowadays, the use of

antibiotics has changed the appearance of Pneumoccoccal

pneumonia, and it may appear as patchy confluent areas that

may be multilobar or bilateral (Fig. 3). Kantor [12] found


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Fig. 5. (a) Legionella pneumonia: chest radiograph of a patient with fever, dyspnea and myalgias. There is a smooth bilateral perihilar consolidation. (b)

Chest radiograph obtained 48 h later, notice the rapid extension of the consolidation. (c) and (d) On CT, the consolidations are multiple and bilateral.


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Fig. 5. (Continued).


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that the patterns of lobar pneumonia and bronchopneumo-

nia were equally frequent in Pneumococcal pneumonia. An-

other common finding in Pneumococcal pneumonia is the

presence of small pleural effusions that are usually reactive.

3.2. Mycoplasma pneumonia

The incidence of Mycoplasma infection is variable ac-

cording to different series and may be influenced by epi-

demics. Every 48 years, the incidence may reach up to

50%. This is a pneumonia of children, adolescents and

adults below 40 years of age [13]. Mycoplasma pneumo-

nia has variable radiographic appearances. In 1975, Putnan

et al. [14] identified two main clinical and radiographic

groups: one group had unilateral or bilateral air-space dis-

ease with a lobar or segmental distribution, while the other

with a longer duration of symptoms, had a diffuse bilateral

reticulo-nodular pattern (Fig. 4). A review of 31 cases of M.

pneumoniae in outpatients revealed no predominant radio-

graphic pattern (interstitial or alveolar) with more frequentinvolvement of the lung bases [15].

3.3. Chlamydia pneumonia

The radiographic appearance ofC. pneumoniae is similar

to that of M. pneumoniae, most commonly as a localised

area of consolidation which may be patchy or homogeneous.

Chlamydia and Mycoplasma often coexist [1].

3.4. Legionella pneumonia

Legionnella pneumophila is responsible for Legionnellapneumonia or Legionnaires disease. These infections are

acquired by breathing droplets of contaminated water. The

disease may be sporadic or may occur in outbreaks, most

frequently in places where the population is exposed to air

conditioning towers, water distribution systems and humid-

ifiers colonised by the germ [16]. The clinical features of

Legionella pneumonia are typical, consisting in diarrhoea,

headache, myalgias, dyspnea and cough. The radiographic

findings are often those of segmental peripheral consolida-

tions that spread rapidly producing opacification of one or

more lobes (Fig. 5). They become bilateral in half of the

cases [17].

3.5. Unusual patterns of CAP

3.5.1. Round pneumonia (Fig. 6)

It was described in children but occasionally it may hap-

pen in adults. In the presence of a pulmonary nodule, round

pneumonia should be suspected especially if no previous

films are available, a rapid growth is observed or there are

signs of infection [18]. A variant of this could be the cases

described in screening for lung cancer where some small

pulmonary nodules detected will disappear after the antibi-

otic treatment [19].

Fig. 6. Round pneumonia: a consolidation is seen in the right lower lobe

lung of this adult patient. Streptococcus pneumoniae was obtained in the

sputum cultures.

3.5.2. Bilateral or multilobar pneumonia

CAP can be diffuse and bilateral in patients with underly-

ing chronic obstructive pulmonary disease due to the distor-

tion and destruction of the pulmonary parenchyma (Fig. 7).

Some of these cases will present as a linear pattern that could

be confused with other aetiologies.

4. Aspiration pneumonia

Aspiration is the inhalation of orofaringeal or gastric con-

tents into the larynx and lower respiratory tract. If the in-halation is of regurgitated sterile gastric contents, aspiration

pneumonitis is caused; and if it is of colonised oropharingeal

material, aspiration pneumonia occurs [20].

Factors that predispose to aspiration pneumonitis are

those that produce disturbance of consciousness such as

drug abuse, seizures, massive cerebrovascular accident, or

the use of anaesthesia. Aspiration pneumonia is conditioned

by neurologic disphagia, anatomic abnormalities of the up-

per aerodigestive tract, gastroesophageal reflux in elderly

persons, or poor oral care.

The radiographic appearance of aspiration pneumonia and

pneumonitis is variable [21] but the most common pattern

is that of bilateral and multicentric opacities, particularlyin the right lung, with a perihilar and basal distribution

(Fig. 8).

5. Nosocomial pneumonias

Nosocomial pneumonia or hospital acquired pneumonia

is defined as a pneumonia occurring 48 h after hospital ad-

mission, excluding any infection that is incubating at the

time of hospital admission, and also a pneumonia which

occurs within 48 h after discharge from the hospital [22].


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Fig. 7. (a) Chest radiograph of a patient with bullous emphysema. (b) The same patient with pneumonia in the left upper lobe. An airfluid level (arrows)

within the bullae mimics cavitation. (c) CT of this area showing the fluid filled bulla.

Fig. 8. Aspiration pneumonia: chest radiograph of a patient in a comatose

condition due to drug abuse. Bilateral lower lobe consolidations.

According to the literature, the incidence of NP is vari-

able, probably because the groups of patients studied differ

and the diagnostic criteria vary. These variations depend

greatly on the type of hospitalisation and wards (surgical or

medical).

Risk factors involved in NP are the previous condition of

the patient, age, severity of the underlying disease, the length

of hospitalisation and the instrumentation used in invasive

techniques. The most common micro-organisms responsible

for NP are aerobic Gram-negative bacilli (Enterobacteriae,

E. coli, Pseudomona aeruginosa), and some Gram-positive

cocci such as S. aureus and S. pneumoniae. Anaerobic or-

ganisms are less common. Quite often, multiple different

germs are found [23].

In patients hospitalised in Intensive Care Units, these

pneumonias are more frequent, and the mortality is very

high (1050%). Mechanical ventilation constitutes a great

risk factor for NP since it can facilitate the growth and


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Fig. 9. Ventilator assisted pneumonia: chest radiograph of a patient obtained after 5 days of mechanical ventilation. There is a right perihilar consolidation.

Acinetobacter was obtained from bronchoaspirate cultures.

dissemination of germs and the cough mechanism is re-

duced. This has been denominated as ventilator associated

pneumonia (VAP). Nevertheless, NP in the Intensive Care

Units may also occur in non-ventilated patients. Thus NP

has been classified in two groups: ventilator associated

pneumonia and pneumonia in non-ventilated patients [24].The incidence and mortality of the former is much higher

Fig. 10. Nosocomial pneumonia: chest radiograph shows patchy and peripheral areas of consolidation in a hospitalised non-ventilated patient under a

long-term treatment with steroids. The responsible organism was Pseudomona aeruginosa.

than that of NP in non-ventilated patients, and they also

differ in their treatment. Micro-organisms responsible for

VAP vary according to the duration of mechanical venti-

lation: VAP occurring in the first 5 days of ventilation is

usually due to S. pneumoniae, H. influenzae or Moxarella

catarrhalis and uncommonly by anaerobes, while VAP oc-curring after 5 days (Fig. 9) of ventilation is most commonly


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produced by Pseudomonas aeruginosa, Acitenobacter or

Enterobacter spp., or methicillin-resistant S. aureus [25].

The radiographic pattern of NP may be quite variable

These pneumonias are most commonly bilateral with diffuse

or multiple foci of consolidation not limited to one lobe [7].

They may frequently associate pleural effusion (Fig. 10).

The role of portable chest films in cases of suspected NPis limited, since the presence of focal alveolar consolida-

tions is quite frequent in these patients, and often caused

by atelectasis, pulmonary infarction, oedema or acute res-

piratory distress syndrome (ARDS). The radiographic signs

of NP are non-specific. A study by Wunderink et al. found

that the only reliable sign of pneumonia was the pres-

ence of air bronchograms, except in patients with ARDS

[26]. Atelectasis may solve rapidly, especially after vigor-

ous physiotherapy. In patients with ARDS, the diagnosis

of pneumonia becomes very difficult [27,28]. Generally,

ARDS is bilateral, symmetric and more evident in depen-

dent areas [29].The presence of focal areas of consolidation

favours the diagnosis of pneumonia but asymmetry may alsooccur in ARDS [29]. Additionally, the agreement between

Fig. 11. Hospital acquired pneumonia: pulmonary gangrene produced by

Klebsiella pneumoniae in a hospitalised patient. Notice sloughed lung

tissue due to extensive necrosis in a large cavity with an airfluid level.

readers in this pathology is very low, and other factors such

as the technique used to obtain the chest radiograph and the

ventilator settings may influence the results [30].

In summary, the role of radiology in NP is limited but

decisive. Delay in treating pneumonia may be fatal and

treating with antibiotics other entities (pulmonary infarction,

oedema) may also have negative results. In hospitalized pa-tients, the chest radiographs are most helpful when they are

normal and rule out pneumonia [7]. CT may be of great help

in some cases when the chest films are inconclusive espe-

cially in patients with ARDS.

6. Complications

All pneumonias, CAP and nosocomial may complicate.

Complications are more common in inmunodepressed pa-

tients and in nosocomial pneumonias.

Fig. 12. (a) Chest radiograph of a 12 months old child, with a consolidation

in left lower lobe. (b) Chest radiograph obtained 4 weeks later. A cystic

space has developed in the area of previous pneumonia, corresponding to

a pneumatocele (arrows).


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Cavitation suggests bacterial disease rather than viral or

Mycoplasma infection. S. aureus, Gram-negative, anaerobic

bacteria are the most common agents.

Pulmonary gangrene is a rare but interesting form of

cavitation that produces sloughed lung within a large cav-

ity secondary to thrombosis of the pulmonary vessels

[17]. S. pneumoniae and Klebsiella are the most com-mon agents responsible for cavitation in inmunocompetent

patients and Aspergillus in the inmunocompromised host

(Fig. 11).

Fig. 13. Loefflers pneumonia: (a) the chest radiograph shows an opacity

in the left upper lobe. (b) Lateral chest radiograph showing posterior

displacement of the major fissure due to abundant exudate by Klebsiella

pneumoniae.

Pneumatocele [1] is an air cystic space that may develop as

a complication of acute staphylococcal infection in children

(Fig. 12).

Care needs to be taken to avoid misdiagnosing cavitation

and pneumatocele formation when the focal lucencies within

the consolidation are due to underlying emphysema (Fig. 7).

6.1. Pleural effusion and empyema

Parapneumonic effusions complicate the course of

2060% of patients hospitalised with bacterial pneumo-

nia. Pleural effusion in CAP is less frequent and usually

reactive. Most of these effusions follow an uncomplicated

course and resolve with antibiotic therapy of the underlying

pneumonia. In 510% cases, they become complicated and

progress to empyema [31].

6.2. Lobar enlargement

This sign was well described by Felson et al. in 1949 and

initially attributed to Klebsiella pneumonia (Friedlanders

pneumonia) [32]. Swellling of a lobe occurs when there is

an extensive exudative process. Other infectious processes

such as tuberculosis and pneumococci can also demonstrate

lobar enlargement (Fig. 13).

7. Conclusions

Pneumonias can be classified in three main groups: com-

munity acquired pneumonia, nosocomial pneumonia and

aspiration pneumonia. The role of the radiologist is to bedecisive in their diagnosis and follow-up. The chest radio-

graph remains a basic tool for this purpose. CT is used as a

complement to plain films and especially in the evaluation

of complications or unfavourable resolution of a pulmonary

infiltrate. The role of radiology in the intensive care unit

patient is more limited since there is a great overlap of

pathologies that can have similar radiographic signs. Close

follow-up of these patients and adequate clinical correla-

tion is mandatory. CT in these cases can add significant

information when portable films are inconclusive.

References

[1] Bhalla M, McLoud TC. Pulmonary infections in the normal host.

In: McLoud TC, editor. Thoracic radiology, the requisites. Mosby,

USA; 1998.

[2] American Thoracic Society. Guidelines for the management of adults

with community-acquired pneumonia. Am J Respir Crit Care Med

2001;163(7):173054.

[3] Albaum MN, Hill LC, Murphy M. Interobserver reliability of chest

radiograph in community-acquired pneumonia. Chest 1996;110:343.

[4] Tanaka N, Matsumoto T, Kuramitsu T, et al. High resolution CT

findings in community-acquired pneumonia. J Comput Assist Tomogr

1996;20:6008.


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