Imaging of Pneumonia

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SERIES "THORACIC IMAGING"

Edited by P. A. Gevenois, A. Bankier and Y. Sibille

Number 5 in this Series

Imaging of pneumonia: trends and algorithms

T. Franquet

Imaging of pneumonia: trends and algorithms. T. Franquet. #ERS Journals Ltd 2001.ABSTRACT: Pneumonia is one of the major infectious diseases responsible forsignificant morbidity and mortality throughout the world. Imaging plays a crucial rolein the detection and management of patients with pneumonia.

This review article discusses the different imaging methods used in the diagnosis andmanagement of suspected pulmonary infections. The imaging examination shouldalways begin with conventional radiography. When the results of routine radiographyare inconclusive, computed tomography is mandatory. A combination of pattern

recognition with knowledge of the clinical setting is the best approach to the pulmonaryinfectious processes.

A specific pattern of involvement can suggest a likely diagnosis in many instances. Inacquired immune deficiency syndrome patients, diffuse ground-glass and interstitialinfiltrates are most commonly present in Pneumocystis cariniipneumonia whereas in thenonimmunosuppressed patients, a segmental lobar infiltrate is suggestive of a bacterialpneumonia. Round pneumonia is most often encountered in children than adults and ismost often caused by Streptococcus pneumoniae. Different combinations of paren-chymal and pleural abnormalities may be suggestive for additional diagnoses.

When an infectious pulmonary process is suspected, knowledge of the variedradiographic manifestations will narrow the differential diagnosis, helping to directadditional diagnostic measures, and serving as an ideal tool for follow-up examinations.Eur Respir J 2001; 18: 196208.

Dept of Radiology, Section of ThoracicImaging, Hospital de Sant Pau, Uni-versidad Autonoma de Barcelona, SanAntonio, Barcelona, Spain.

Correspondence: T. Franquet, Dept ofRadiology, Section of Thoracic Imag-ing, Hospital de Sant Pau, UniversidadAutonoma de Barcelona, San Antonio

Ma Claret 167, 08025 Barcelona, Spain.Fax: 93 2919282

Keywords: Chest radiographdiagnosisG-thoraxpneumoniarespiratory infections

Received: October 24 2000Accepted after revision February 92001

Despite advances in diagnosis and treatment,pulmonary infections are a major cause of morbidityand mortality in adult patients. Pneumonia is the sixthmost common cause of death in the USA and morethan 6 million cases of bacterial pneumonia occureach year in the immunocompetent population [1].The spectrum of organisms known to cause respira-tory infections is broad and constantly increasing asnew pathogens are identified and the host immuneresponse is altered by medications or other diseases orresponses. In the USA, it has been estimated thatthere are 1.1 million cases of community-acquiredpneumonia (CAP) requiring hospitalization each year,

at an estimated cost of 8 billion dollars [1]. Nosoco-mial pneumonia (NP) is the most important hospital-acquired infection because it is associated with thehighest mortality rate of nosocomial infections thatcontribute causally to death [2]. Moreover, since thebeginning of the acquired immune deficiency syn-drome (AIDS) epidemic, lungs represent an increasingsource of infections. In addition to direct patient care

costs, pneumonia is responsible for over 50 milliondays of restricted activity from work and is the sixthleading cause of death in the USA with a mortalityrate of 13.4 per 100,000 [3, 4].

Changing trends in pulmonary infections

Diagnosis of pneumonia calls for a combination ofclinical awareness, appropriate microbiological tests,and radiographical studies. Plain chest radiography isan inexpensive test that can rapidly demonstrate thepresence of pulmonary abnormalities. It represents an

important initial examination in all patients suspectedof having a pulmonary infection. In most cases theplain film findings may be diagnostic of pneumoniaand may eliminate the need for additional radio-graphic procedures.

The clinician evaluating the patient with a knownor suspected diagnosis of pulmonary infection facesa diagnostic challenge because of the majority of

Previous articles in this series: No. 1: Ghaye B, Dondelinger RF. Imaging guided thoracic interventions. Eur Respir J2001; 17: 507528. No. 2:Vansteenkiste JF, Stroobants SG. The role of positron emission tomography with 18F-fluoro-2-deoxy-D-glucose in respiratory oncology. EurRespir J 2001; 17: 802820. No. 3: Kauczor HU, Chen XJ, van Beek EJR, Schreiber WG. Pulmonary ventilation imaged by magneticresonance: at the doorstep of clinical application.Eur Respir J2001; 17: 10081023. No. 4: Hansell DM. Small airways diseases: detection andinsights with computed tomography. Eur Respir J 2001; 17: 12941313.

Eur Respir J 2001; 18: 196208Printed in UK all rights reserved

Copyright #ERS Journals Ltd 2001European Respiratory Journal

ISSN 0903-1936


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processes presenting with similar signs and symptoms,and the radiographic findings of pneumonia do notprovide a specific aetiological diagnosis. Furthermore,radiographic manifestations of a given infectiousprocess may be variable depending on the immuno-logical status of the patient as well as by pre- orcoexisting lung disease. The number of immunocom-promised patients has dramatically increased because

of three phenomena: the AIDS epidemic, advances incancer chemotherapy, and expanding organ trans-plantation. At the onset of the AIDS epidemic, in theearly and mid 1980s, there was 5080% mortality foreach episode of Pneumocystis carinii pneumonia(PCP). Since routine prophylaxis was instituted in1989, a declining incidence of PCP in the AIDSpopulation was demonstrated [5, 6]. In addition tolesser incidence, there was also a declining mortality(15%) in mild-to-moderate cases [7]. Therefore, otherinfections including bacterial pneumonia, fungalinfection, cytomegalovirus (CMV), Mycobacteriumavium complex (MAC), and tuberculosis remain asignificant cause of morbidity and mortality in these

patients [57]. Radiologists must not only documentthe location and extent of pneumonia but also assessthe evolution and course of pneumonia and detect anycomplication of the disease.

Integrating clinical and imaging findings

The most useful imaging modalities available forthe evaluation of the patient with known or suspectedpulmonary infection are chest radiography andcomputed tomography (CT). Imaging examinationsshould always be interpreted with a knowledge of howsymptomatic the patient is, the degree of dyspnoea,

the level of impairment of the carbon monoxidediffusing capacity of the lung (DL,CO), the CD4z cellcount, the presence of fever or leukocytosis, if there isa cough and whether the cough is productive, and thechronicity of symptoms [8]. Knowledge of whether thepatient has developed a CAP or NP, as well asknowledge of the immune status of the patient, can bepowerful tools in arriving at a shortlist of possiblecausative organisms [8, 9]. Clinical information cangreatly enhance the accuracy of the radiographicaldiagnosis, i.e. the AIDS patient with an acute airspaceprocess who has chills, fever, and purulent sputumprobably has pyogenic rather than a PCP. In theabsence of clinical information, radiologists cannot

reliably distinguish between pneumonia and otherpulmonary processes [10]. Unfortunately, the clinicaldata and radiographical findings often fail to leadto a definitive diagnosis of pneumonia because thereis an extensive number of noninfectious processesassociated with febrile pneumonitis i.e. drug-induced pulmonary disease, acute eosinophilic pneu-monia, bronchiolitis obliterans organizing pneumonia(BOOP), and pulmonary vasculitis that mimic pul-monary infection [11]. Distinction of localized pneu-monia from other pulmonary processes cannot bemade with certainty on radiological grounds [11, 12].Localized pulmonary disease of a lobar or segmentaldistribution can be produced not only by pneumonia

but also by pulmonary oedema and haemorrhage.Localized pulmonary oedema produced by acidgastric aspiration may result in an image identical topneumonia as well as a pulmonary infarction second-ary to thromboembolism, which may also producesimilar radiographical findings. Diagnosis is equallydifficult when pneumonia appears as a diffusepulmonary abnormality. Pulmonary oedema and the

adult respiratory distress syndrome (ARDS) are themost common conditions to be distinguished frombronchopneumonia when a generalized pulmonaryabnormality is radiographically demonstrated [1315].

Conventional chest radiography

According to American Thoracic Society guide-lines, posteroanterior (PA) (and lateral when possible)chest radiography should be obtained wheneverpneumonia is suspected in adults [16]. The role ofchest radiography has been described either as ascreening tool for the detection of new infiltrates or

for monitoring response to therapy. Other roles forchest radiography include an enhanced ability toassess the extent of disease, to detect complications(i.e. cavitation, abscess formation, pneumothorax,pleural effusion), and to detect additional or alter-native diagnoses and sometimes to guide invasivediagnostic procedures.

In most cases different abnormalities can beidentified on chest films. The more common radio-graphical findings include segmental or lobar con-solidations and interstitial lung disease. Other lesscommon radiographical findings include mediastinallymphadenopathy, pleural effusion, cavitation, andchest wall invasion. Despite that, the nonspecificity ofradiographical findings as well as the wide range ofpotential causes often lead to frustration when evaluat-ing the imaging findings of a patient with suspectedpneumonia. Pulmonary infection by PCP, typicallyseen as a diffuse homogeneous alveolar consolidation,has recently been described, in 510% of cases, withdense consolidation, nodules, miliary opacities, andpleural effusions [16]. Furthermore, equivocal or nor-mal chest radiographs are not uncommon, reportedin the range of 1039% of patients with PCP infectionand in up to 10% of patients with proven pulmonarydisease [17].

Computed tomography

CT is a useful adjunct to conventional radiographyin selected cases [10, 12, 18, 19]. There is a largeamount of literature indicating that CT is a sensitivemethod capable of imaging the lung with excellentspatial resolution, providing anatomical detail similarto that seen by gross pathological examination.Differences in tissue attenuation and parenchymalchanges caused by an acute inflammatory processcan be readily seen by CT [18, 19]. Unlike chestradiography, CT provides cross-sectional images andthe pattern and distribution of pulmonary processesare therefore, much more readily appreciated than onconventional examinations [17].

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With the advent of high-resolution CT (HRCT),a whole new lexicon of terminology to describe imag-ing findings evolved. Recognition of the secondarypulmonary lobule is essential to understand theimaging findings obtained by thin-section CT scans[18]. The findings of airspace disease, airspace (acinar)nodules, ground-glass opacities, consolidation, airbronchograms, and centrilobular or perilobular dis-

tribution are seen better by CT than by conventionalradiography [17, 18]. Airspace nodules represent thesize of the acinus (610 mm) and are centrilobular indistribution. They are best appreciated in early diseaseand best seen at the edge of the pathological processwhere consolidation is incomplete. Ground-glassopacities are defined as a localized increase in lungattenuation that allows visualization of vascular struc-tures coursing through the affected region. Groundglass is a nonspecific CT finding that may representeither alveolar or interstitial disease [10].

The CT findings of interstitial disease reflect thicken-ing by oedema, neoplasm, inflammation, or fibrosis ofthe normal interstitial structures [10, 18]. The most

common CT findings are septal thickening, bronchialwall thickening, mosaic perfusion, bronchovascularbundle thickening, interstitial nodules, and honeycomb-ing. These findings, well known from plain film studies,are more easily recognized by CT.

Although CT is not recommended for the initialevaluation of patients with pneumonia, it is a valuableadjunct to conventional radiography in patients withnonrevealing or nondiagnostic imaging findings [16].Several studies have shown that HRCT can be helpfulin the detection, differential diagnosis, and manage-ment of immunocompromised patients with pulmo-nary complications [1619].

Imaging of pneumonia in specific patient groups

Community-acquired pneumonia

CAP is a major healthcare and economic problembecause of its high morbidity and mortality rate, andbecause of its direct and indirect costs of management[1, 3]. Even in young healthy people, pneumonia hasbeen found to be the major medical cause of lostworkdays. Between 485,000 and 1 million patients arehospitalized each year in the USA for treatment ofCAP. The cost of inpatient care exceeds outpatientcare by a factor of 1520, and comprise the majority of

the estimated $8.4 billion spent annually for the careof patients with pneumonia [1, 3, 20, 21].Hospital admission rates of pneumonia episodes

vary 2251% of patients with CAP [1]. The mortality ishigher in less-developed countries, in the young andthe elderly, and varied from 10?100,000-140?100,000-1

inhabitants in three European countries [20]. Al-though it is true that the radiographical findings ofa pneumonia do not provide a specific aetiologicaldiagnosis, the differential diagnosis may be possible inCAP using radiological pattern recognition. Despitethe variability regarding the time between the onset ofclinical symptoms and the development of a radio-graphically visible infiltrate, it is well known that in

CAP the majority of pulmonary infiltrates appearwithin the time period of 12 h. In these patients,pattern recognition may help to classify groups ofpotentially underlying organisms favouring a bacterialover a viral aetiology. In CAP, diagnosis and diseasemanagement most frequently involve chest radio-graphy and generally do not require the use of otherimaging modalities [22].

The spectrum of causative organisms of CAPincludes Gram positive bacteria such as Streptococcuspneumoniae (the pneumoccocus), Haemophilus influ-enzae and Staphylococcus aureus, as well as atypicalorganisms such as Mycoplasma pneumoniae, Chlamy-dia pneumoniae, or Legionella pneumophila and viralagents such as the influenza A virus and respiratorysyncytial viruses. S. pneumoniae is by far the mostcommon cause of complete lobar consolidation[2325]. Other causative agents that produce completelobar consolidation include Klebsiella pneumoniaeand other Gram negative bacilli, L. pneumophila,H. influenzae, and occasionally M. pneumoniae [2326].

Radiographically, lobar pneumonia appears in theperiphery abutting against the pleura and spreads to-wards the core portions of the lung. Round pneumo-nia is most often encountered in children than adultsand is most often caused by S. pneumoniae (fig. 1) [27].In children, active tuberculous and fungal infectionmay also present with nodular or mass-like lesions[27]. Bacterial infections may produce multiple roundedpulmonary nodules or masses, with or without cavita-tion. This may occur from infection with Nocardia,Aspergillus, Legionella, Q fever, and M. tuberculosis[2729].

Bronchopneumonia, which is most commonlycaused by S. aureus and H. influenzae, occurs wheninfectious organisms, deposited on the epithelium ofthe bronchi, produce acute bronchial inflammationwith epithelial ulcerations and fibrinopurulent exud-ate formation. As a consequence, the inflammatoryreaction rapidly spreads through the airway wallsand spreads into the contiguous pulmonary lobules.

Fig. 1.Round pneumonia due to Streptococcus pneumoniae in a53-yr-old male. Computed tomography demonstrates a focal areaof homogeneous consolidation in the left upper lobe. Note thepresence of air-bronchogram within the consolidation. Sputumculture produced a heavy growth of S. pneumoniae. In adults, thisform of pneumonia may mimic bronchogenic carcinoma.

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Radiographically, these inflammatory aggregates causea typical patchy pattern of bronchopneumonia (fig. 2)or a homogeneous segmental consolidation that mayalso cavitate (figs. 2 and 3).

Diffuse bilateral interstitial and/or interstitial-alveolar(mixed) infiltrates are most commonly caused byviruses (fig. 4) and M. pneumoniae [30]. Up to 30% ofall pneumonias in the general population may becaused by M. pneumoniae [10]. During infection, theinitial damage is directed towards the mucosa of thebronchioles and later, the peribronchial tissue andinterlobular septa become oedematous and infiltratedwith inflammatory cells.

Hospital-acquired (nosocomial) pneumonia

An NP may be defined as one occurring afteradmission to the hospital, which was neither presentnor in a period of incubation at the time of admission[21]. NP is the leading cause of death from hospital-acquired infections and an important public healthproblem. It occurs most commonly among intensivecare unit (ICU) patients, predominately in individualsrequiring mechanical ventilation (fig. 5) [31]. Theestimated prevalence of NP within the ICU settingranges 1065%, with case fatality rates of 2055% inmost reported series [26, 31, 32]. In patients withARDS, as many as 55% have secondary pneumonia,and this complication may adversely affect survival[26].

Fig. 2.Computed tomography scan in a 35-yr-old female demon-strates multiple ill-defined subsegmental opacities in the middleand right lower lobe. Small cavities and moderate right pleuraleffussion are also appreciated. Note a focus of infection in the leftlower lobe. Cultures from a bronchoscopic specimen grewStaphylococcus aureus.

Fig. 3.Close-up view of a posteroanterior chest radiograph in a43-yr-old alcoholic male with acute cavitating pneumonia byStaphylococcus aureus. A poorly defined area of airspace consoli-dation containing a rounded radiolucency (arrowheads) is depictedin the right upper lung.

a)

b)

Fig. 4.Adenovirus pneumonia in a 28-yr-old female. a) Close-upview of a posteroanterior chest radiography demonstrates poorlydefined nodular opacities. b) Corresponding high-resolution com-puted tomography scan shows multiple poorly defined bilateralnodular opacities in a predominantly peribronchial distribution.



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The diagnosis of NP is difficult, and the criteriaused for surveillance have been based on clinicalfindings of fever, cough, and the development ofpurulent sputum in combination with a new orprogressive infiltrate on the chest radiograph. Whenpneumonia arises in the hospitalized patient, aerobicGram negative bacilli, particularly Pseudomonas

aeruginosa and Enterobacter spp. and S. aureus, arethe major causative organisms [33]. Other commoncauses of NP are H. Influenza, pneumococcus,aspiration with anaerobes, Legionella spp. and virusesin certain hosts. Respiratory syncytial virus, influenzaA and B, and parainfluenza, are responsible forw70%of nosocomial viral diseases [33]. The clinical andradiographical clues to the aetiological diagnosis of

pneumonia are shown in table 1.

Immunosuppressed host pneumonia

Patients with impaired immune function are sus-ceptible to infections by a wide range of organisms[6, 7]. In the last several decades, AIDS epidemic,advances in the treatment of cancer, organ transplan-tation, and immunossuppressive therapy has resultedin large numbers of patients who develop abnormal-ities in their immune system [3436]. Pneumonia is amajor clinical problem for immunosuppressed pati-ents and many of the bacteria causing CAP in the

healthy community are also responsible for pneumo-nia in these risk patients. Mildy impaired host immu-nity as it occurs in chronic debilitating illness, diabetesmellitus, malnutrition, alcoholism, advanced age, pro-longed corticosteroid administration and chronic obs-tructive lung disease have also been regarded aspredisposing factors of pulmonary infections [37].

Acquired immune deficiency syndrome

In AIDS patients, pulmonary complications mayresult from a number of infectious and noninfectious

Fig. 5. Hospital-acquired pneumonia in an intensive care unitpatient. Portable anteroposterior supine chest radiography showsbilateral lung consolidation. Protected bronchial brushes revealedGram positive cocci, gram positive rods, and Gram negative rodson smear. Cultures grew Staphylococcus aureus and Pseudomonasand Serratia organisms.

Table 1. Summary of clinical and radiographical clues to the aetiological diagnosis of pneumoniaRadiographical findings Clinical circumstance Organism

Segmental consolidation Community-acquired S. pneumonia, M. pneumoniaeLobar consolidation Community-acquired

DiabetesS. pneumoniae (2/3 of community-acquired pneumonias)K. pneumoniaeGram negative bacilli

Rounded pneumonia Community-acquiredAlcoholic

S. pneumoniae

Bronchopneumonia Hospital-acquired P. aeruginosa, S. aureus, streptococci, Gram negative bacilli,anaerobes, M. pneumoniae, aspiration, L. pneumophila

Interstitial pneumonia Community-acquired (winter) Virus, M. pneumoniaeCavitation/necrosis Aspiration

COPDS. aureus, Gram negative bacilli, anaerobes, actinomycosis,M. TuberculosisAspergillus

Multiple cavitary nodules Drug addict S. aureusPneumatoceles Postinfluenza S. aureusEmpyema Complication of pneumonia S. pneumoniae

S. aureusGram negative bacilliM. tuberculosis

Chest wall invasion Alcoholic ActinomycosisM. tuberculosisFungi

Lymphadenopathy M. pneumoniaeM. tuberculosis

COPD: chronic obstructive pulmonary disease; S. pneumoniae: Streptococcus pneumoniae; S. aureus: Staphylococcus aureus;M. tuberculosis: Mycobacterium tuberculosis; M. pneumoniae: Mycoplasma pneumoniae; K. pneumoniae: Klebsiella pneumoniae;P. aeruginosa: Pseudomonas aeruginosa; L. pneumophila: Legionella pneumophila. Adapted from [34].

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causes. Among the infectious pulmonary processes,major causative agents include PCP, M. tuberculosis,and MAC complex, in addition to many of the morecommon Gram positive and negative bacteria [5, 16,17]. In the past two decades, a resurgence of tuber-culosis (TB) has been seen worldwide, including a num-ber of developing countries in which the disease hadbeen on the decline for many decades. This increase

in TB is largely related to cases in AIDS patients [38,39]. Infection will depend on the patient9s immunestatus and the risk of opportunistic infections will alsochange over time [39].

Patients whohave CD4zcell countsofw200 cells?mm3

are predisposed to bronchial infections and bacterialpneumonia, whereas patients with CD4z cell countsofv200 cells?mm3 are predisposed to opportunisticinfections such as PCP [8, 39]. Most patients haveCD4z counts in the range of 5075 cells?mm3 at thetime of diagnosis of their first episode of PCP [8, 17].Therefore, it is important to interpret the radiologicalfindings in the appropriate clinical setting. By corre-lating the different radiographic patterns with pre-

senting symptoms and the CD4z cell count, theradiologist may narrow the differential diagnosis [8].Abnormal chest radiographs have been reported in upto 90% of patients showing the typical findings ofdiffuse bilateral interstitial infiltrates without a pleuraleffusion (fig. 6). As the disease progresses, alveolarinfiltrates may also develop. HRCT is the modality ofchoice to evaluate those symptomatic patients with anotherwise normal chest radiograph [17].

Bronchial invasive aspergillosis occurs most com-monly in the setting of severe neutropenia and inpatients with AIDS [4042]. Clinical manifestationsinclude acute tracheobronchitis, bronchiolitis, andbronchopneumonia. Patients with acute tracheobron-

chitis usually have normal radiological findings.Aspergillus bronchiolitis is characterized on HRCTby the presence of centrilobular nodules and branch-ing linear or nodular opacities giving an appearanceresembling a "tree-in-bud" (fig. 7) [41]. The centrilob-ular nodules have a patchy distribution in the lung

and are similar to those seen in a number of differentinfectious conditions, including endobronchial spreadof pulmonary tuberculosis, M. avium-intracellulare,viral and M. pneumonia. Aspergillus bronchopneu-monia results in predominantly peribronchial areas ofconsolidation (fig. 8) [41]. Rarely, the consolidationmay have a lobar distribution. These radiologicalmanifestations are indistinguishable from those ofbronchopneumonia caused by other organisms.

Obstructing bronchopulmonary aspergillosis (OBA)is a descriptive term for the unusual pattern of a non-invasive form of aspergillosis characterized by themassive intraluminal overgrowth of Aspergillus spp.,usually Aspergillus fumigatus, in patients with AIDS

[42]. Patients may cough up fungal casts of their bron-chi and present with severe hypoxaemia. The character-istic CT findings in OBA mimic those of allergicbronchopulmonary aspergillosis (ABPA) consisting of

Fig. 6. Posteroanterior chest radiography in a patient with acquiredimmune deficiency syndrome and a CD4z count of 50 cells?mm3.Bilateral asymetric mixed pattern (interstitial and confluent alveo-lar opacities) are clearly demonstrated. In this clinical setting,radiographical findings are considered highly diagnostic of Pneu-mocystis carinii pneumonia.

Fig. 7. A 28-yr-old patient with acute leukaemia presented withfever and a normal chest radiograph. High-resolution computedtomography scan demonstrates thickening of the bronchial andbronchiolar walls and multiple bilateral ill-defined nodular opa-cities with a "tree-in-bud" appearance. The final diagnosis wasAspergillus bronchiolitis.

Fig. 8. Posteroanterior chest radiograph reveals bilateral non-segmental consolidations in the lingula and in the right upper andlower lobes. Aspergillus fumigatus was recovered from the sputum.



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bilateral bronchial and bronchiolar dilatations, largemucoid impactions mainly in the lower lobes anddiffuse lower lobe consolidation caused by postob-structive atelectasis (fig. 9) [42].

Solid organ transplantation

Patients undergoing solid organ transplantationpresent increased susceptibility to infection whichvaries according to the time interval since transplanta-tion [35, 43, 44]. The post-transplantation timelinecan be divided into three periods: 30 days post-transplantation, 30120 days post-transplantation, and

w120 days post-transplantation [35, 43, 44]. In theimmediate postoperative period opportunistic infec-tions are usually not encountered because there is adelay between the onset of the immunosuppressivetherapy and the development of immune systemdysfunction. Suppression of the immune system ismore severe during the 14-month period after organtransplantation. During the first month after hearttransplantation, Gram negative bacterial pneumoniaare particularly frequent because of prolonged intuba-tion, pulmonary oedema, and the effects of surgery onlung mechanics [35, 36, 43, 44].

Infection rates among lung transplant recipients,occurring in up to 50% of cases, are several fold

higher than among recipients of other solid organs[35]. Both Gram negative bacteria (Enterobacter andPseudomonas) and Staphylococcus are most common,but they are not lethal as often as viral and fungalinfections [35]. CMV infection is the most commonviral pathogen encountered in the post-transplantationperiod. CMV infection typically emerges within thefirst 3 months after transplantation. Primary infection,the most serious, occurs in 50100% of seronegativerecipients who receive a graft from a seropositivedonor. As many as 40% of patients undergoing bonemarrow transplantation (BMT) develop invasive fun-gal disease [35]. Aspergillus species commonly colo-nize the airways of lung transplant recipients but only

a minority of patients develop invasive disease. Air-way invasive aspergillosis is characterized histologi-cally by the presence of Aspergillus organisms deep tothe airway basement membrane [43, 44].

Bone marrow transplantation

BMT is currently the treatment of choice for manyhaematological malignancies and severe congenital oracquired disorders of the haematopoietic or immunesystems [36]. In transplant recipients, pulmonaryinfections occur in up to 50% of patients because ofdirect lung communication with the atmosphere. Thenew onset of respiratory symptoms, or new infiltrateson chest radiography, should prompt an early anddefinitive diagnosis.

CMV is the most significant viral infection thatoccurs in organ and BMT patients. It occurs in5070% of allogeneic BMT recipients [36]. Thesepatients are at a significantly higher risk of pulmonaryinfection than autologous transplant recipients [36].CMV infection may be related to primary acquisitionor to reactivation of latent infection or re-infectionwith a different strain in a previously seropositivepatient. Approximately one-third of infected patientssubsequently develop CMV pneumonia with a medianonset time of 5060 days post-transplantation [36].CMV infection usually develops 14 months aftertransplantation. The radiographical manifestations ofthese pneumonias are nonspecific. The radiologicalfindings of CMV infection are variable consisting oflobar consolidation, diffuse and focal parenchymalhaziness, and multiple small nodules with associatedareas of ground-glass attenuation ("halo") (fig. 10)[45].

Many focal lesions are due to fungal infection,particularly due to Aspergillus species. Opportunisticfungi constitue the second most common groupof pathogens with a higher probability of causinginfection in allogeneic than in autologous transplant

Fig. 9. Bronchial obstructing aspergillosis in a 24-yr-old malewith acquired immune deficiency syndrome. Computed tomo-graphy (CT) scan shows bilateral bifurcating tubular shadowscaused by impacted mucous material within markedly dilatedbronchi. CT findings are similar to those of allergic bronchopul-monary aspergillosis.

Fig. 10. Cytomegalovirus pneumonia in a 36-yr-old female afterbone marrow transplantation. A high-resolution computed tomo-graphy scan demonstrates multiple nodular opacities with irregu-lar margins surrounded by an area of ground-glass attenuation.This halo of ground-glass attenuation is due to the haemorrhagicnature of nodules.

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recipients. The most common fungi responsible foracute lung disease in the immunocompromised patientare A. fumigatus, Candida albicans, and Histoplasmacapsulatum. Aspergillus is a ubiquitous soil fungus[40]. The histological, clinical and radiological man-ifestations of pulmonary aspergillosis are determinedby the number and virulence of the organisms and bythe patient9s immune response [40].

Angioinvasive aspergillosis occurs almost exclu-

sively in immunocompromised patients with a severeneutropenia [4042]. There has been a substantialincrease in the number of patients at risk of develop-ing invasive aspergillosis, for many reasons, includingthe development of new intensive chemotherapy regi-mens for solid tumours, difficult-to-treat lymphoma,myeloma, and resistant leukaemia as well as an in-crease in the number of solid organ transplantationand increased use of immunosuppressive regimens forother autoimmune diseases. Angioinvasive aspergil-losis is characterized histologically by invasion andocclusion of small to medium pulmonary arteries byfungal hyphae [41]. This leads to the formation ofnecrotic haemorrhagic nodules or pleural based wedge-shaped haemorrhagic infarcts. The clinical diagnosisis difficult and the mortality is high [40]. The char-acteristic CT findings consist of nodules surroundedby a halo of ground-glass attenuation (Halo sign) orpleural based wedge-shaped areas of consolidation(fig. 11) [46]. These findings correspond to haemor-rhagic infarcts. In severely neutropenic patients thehalo sign is highly suggestive of angioinvasive aspergil-losis. A similar appearance has been described in anumber of other conditions including infection by Muco-rales, Candida, herpes simplex and CMV, Wegener9sgranulomatosis, Kaposi9s sarcoma [47] and haemorrha-gic metastases.

Mild immunosupression

Mildy immunocompromised patients with chronicdebilitating illness, diabetes mellitus, malnutrition,alcoholism, advanced age, prolonged corticosteroidadministration, and chronic obstructive lung diseaseare prone to develop a distinct form of aspergillusinfection called semi-invasive or chronic necrotizing

aspergillosis, characterized histologically by the pres-ence of tissue necrosis and granulomatous inflamma-tion similar to that seen in reactivation of TB. [37].This form of aspergillus infection may be associatedwith a variety of nonspecific clinical symptoms such ascough, sputum production, and fever forw6 months.Haemoptysis has been reported in 15% of patientswith semi-invasive aspergillosis [37].

Radiological manifestations of semi-invasive asper-gillosis include unilateral or bilateral segmental areasof consolidation with or without cavitation and/oradjacent pleural thickening, and multiple nodularopacities [37]. The findings progress slowly overmonths or years. Aspergillus necrotizing bronchitis

may be seen on CT as an endobronchial mass, anobstructive pneumonitis and/or collapse, or as a hilarmass. Only a few reports have described the CTfindings of aspergillus necrotizing bronchitis involvingthe central airways; reported abnormalities includecircumferential bronchial wall thickening and bron-chial obstruction. In clinical practice, the diagnosis ofaspergillus necrotizing bronchitis is usually based onthe presence of abnormal chest radiography and bron-choscopic biopsy specimen consistent with tissue inva-sion [37]. The clinical and radiographical clues to theaetiological diagnosis of infection in the immunosup-pressed host are shown in table 2.

Interventional procedures in the patients withpneumonia

The only definitive way to reach a specific diagnosisis through demonstration of the infected organism, i.e.by examination of stained smears of sputum, pleuralfluid or other biological material, by culture of respira-tory secretions and blood, or by other interventionalprocedures. Alternatively, culture of material obtainedby transthoracic thin-needle biopsy under fluoroscopyor CT guidance could be a reliable cost-effective meansof diagnosis.

However, in most large series of pneumonia a

causative organism cannot be identified in 3345% ofpatients, even when extensive diagnostic tests areundertaken. Previously healthly patients who aremildly ill due to pneumonia are managed in anempirical fashion. However, in certain circumstances,the lack of specific organisms requires a more aggres-sive approach in order to obtain histopathological andcultural identification of the cause of the pulmonaryinfection.

There has been much debate on the diagnosticaccuracy of specimens obtained for culture with vari-ous techniques. Material obtained from the sputum ornasopharyngeal secretions have limited diagnosticvalue because of the presence of normal flora and

Fig. 11. Angioinvasive aspergillosis in a 68-yr-old male withsevere neutropenia. Magnified view of a computed tomographyscan shows a nodule in the left upper lobe surrounded by an haloof ground-glass attenuation (halo sign).



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variable results obtained for the detection of anaero-bic infection [48].

Flexible fibreoptic bronchoscopy with lung biopsy

Fibreoptic bronchoscopy with bronchoalveolarlavage utilizing a protected brush is a well-establishedtechnique in the diagnosis of pulmonary infection.Although this technique may play an important rolein the diagnosis of pulmonary infection, the yield ofbronchoalveolar lavage is variable and sometimes thediagnosis of a pulmonary infection cannot be estab-lished [49, 50]. This method has proved particularlyuseful in the diagnosis of Pneumocystis pneumonia inAIDS patients, providing an aetiological diagnosis iny95% of cases.

In the special setting of a serious pulmonary processand lack of definable cause with noninvasive methods,fibreoptic bronchoscopy in conjunction with trans-bronchial lung biopsy is indicated (fig. 12).

Transthoracic-needle aspiration

Despite the fact that reported results in the diagno-sis of pulmonary infection are variable (11.773%),percutaneous fine-needle aspiration is an alternativemethod used to identify causative pathogens in se-lected patients with pneumonia [5155]. Transthoracic-needle aspiration should be considered for patients

who have not responded to initial therapy, who mayhave nosocomial superinfection, who are immunocom-

promised, or in whom TB is suspected but has not beenconfirmed by examination of the sputum or gastriclavage. It is not clear whether use of transthoracic-needle aspiration results in a reduction in mortality andmorbidity in a cost-effective fashion, compared to a lessinvasive approach [48]. The specificity and positivepredictive value of a positive culture have been reportedto be as high as 100%, whereas the sensitivity andnegative predictive value are 61% and 34% [56].

Strategies for optimal imaging evaluation

Chest radiography should be carried out in all

patients suspected of having pulmonary infection toconfirm or exclude the presence of pulmonary abnor-malities. Although radiographical abnormalities cannever establish aetiological sources, they can be extre-mely helpful in narrowing the differential diagnosisand providing guidance for subsequent diagnosticstudies.

In patients with CAP, diagnosis and disease manage-ment most frequently rely on conventional chest filmsand usually do not require the use of further diag-nostic procedures. In the community setting,w90%of patients who develop a segmental or lobar con-solidation have either pneumococcal pneumonia or anatypical pneumonia caused by Mycoplasma or a virus.

Table 2. Summary of clinical and radiographical clues to the aetiological diagnosis of infection in the immunosuppressedhost

HRCT findings Clinical circumstance Organism

Lobar consolidation Community-acquiredAIDS

CD4zw200 cells?mm3

S. pneumoniae2 of 3 of community-acquiredpneumonias

Mild immunosuppressionDiabetesAlcoholismCOPD

Semi-invasive aspergillosis

Solid organ transplantation Gram negative bacilliStaphylococcus

Ground-glass opacity AIDSCD4z50-75 cells?mm3

Pneumocystis carinii pneumonia

Bone marrow transplant CMVBronchopneumonia Neutropenia Bronchial invasive aspergillosisInterstitial pneumonia Bone marrow transplant CMV

AIDS Pneumocystis carinii pneumoniaMultiple small nodules Bone marrow transplant CMV

AIDS CryptococcosisVaricellaHerpes

Multiple cavitary nodules Drug addict S. aureus"Halo sign" Neutropenia Angioinvasive aspergillosis"Tree-in-bud" AIDS

CD4zw200 cells?mm3

Transplantation

Bronchial infectionEndobronchial spread of

tuberculosisAspergillosis

Lymphadenopathy AIDSCD4zv50 cells?mm3

M. tuberculosis

AIDS: acquired immune deficiency syndrome; COPD: chronic obstructive pulmonary disease; S. pneumoniae:Streptococcuspneumoniae; CMV: cytomegalovirus; S. aureus: Staphylococcus aureus; M. tuberculosis: Mycobacterium tuberculosis; HRCT:high-resolution computed tomography.

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In NP infection, patchy bronchopneumonia is themost common finding and is most likely caused by oneof the Gram negative organisms, particularly Pseudo-monas or Klebsiella. In this particular setting,aspiration pneumonia is always an alternative diag-nosis and should be suspected if pneumonia is presentbilaterally in the dependent or posterior portions ofthe lungs [57]. In the ICU patients, there are fewstudies regarding the accuracy and efficacy of convent-ional chest radiography. The overall incidence of ab-normalities found on chest films in the medical ICUhas been reported to be as high as 57% in pulmonaryand unstable cardiac patients [57]. Similar results wereobtained in a study of patients in the medical ICU;43% of routine chest radiographs showed unexpectedfindings which influenced therapy [58]. Future studieson management and outcome efficay as well as overallcost are necessary to evaluate the role of the routinechest radiograph in ICU patients. Limiting the need forconventional chest radiography in the follow-up ofpulmonary infections may also reduce health costs. CTand invasive diagnostic procedures should be reservedonly for complicated cases.

Conversely, management of immunocompromisedpatients is challenging and difficult because of the

diversity of causative organisms. In this group ofpatients, thin-section CT and invasive procedures aremore often required. HRCT can be useful in patientswho have respiratory symptoms but normal results onchest films, providing further additional findings notclearly delineated by the standard chest radiograph,depicting concurrent parenchymal or pleural disease,and guiding diagnostic manoeuvres. In addition,HRCT is helpful in differentiating infectious fromnoninfectious acute parenchymal lung disease despiteits limited value in making a specific diagnosis [19].

Diagnostic information may also be obtained bymeans of bronchoalveolar lavage and transbronchial-needle aspiration. Under these circumstances, CT isextremely useful serving as a "road map" to directfibreoptic bronchoscopy toward the lesion. Algorithmfor evaluation of patients suspected of having pul-monary infection is shown in figure 13.

In conclusion, the radiologist plays an importantrole in the diagnosis and management of patients withsuspected pneumonia. Conventional chest radiog-raphy remains the first imaging procedure in theimaging work-up patients. Although computed tomo-graphy is not recommended for the initial evaluation,it is frequently appropriate in those cases with normal,

a) b)

Fig. 12. a) Close-up view of a posteroanterior chest radiography shows a rounded cavitary consolidation in the left upper lobe. b)

Material for culture was obtained through fibreoptic bronchoscopy. Cultures grew Mycobacterium tuberculosis.



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equivocal, or nonspecific radiographical findings.High-resolution computed tomography is helpful inthe differential diagnosis of infectious from noninfec-tious acute parenchymal lung disease but does notprovide the aetiological agent. Percutaneous needleaspiration using fluoroscopy and/or computed to-mography is a safe and useful diagnostic method ofobtaining specimens in immunocompromised patientswith pulmonary infection, although its impact onmorbidity and mortality remains to be proved.

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Suspected pulmonary infection

Immunocompetent host Immunosuppressed host

Chest radiograph Chest

Normal Abnormal Normal Abnormal

Stop Medical treatment HRCT Specificfindings

Nonspecific

StopFailed medical

treatmentAbnormal

Medical treatment HRCT

Follow-up by chestradiograph

HRCT

Guided BALFailed medical

treatmentSpecificfindings

Nonspecific

NonspecificGuided BAL

Medical treatment Guided BAL

Guided BALStop

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