key: cord-019089-oots4fe4
authors: Laya, Bernard F.
title: Infections
date: 2013-08-31
journal: Radiology Illustrated: Pediatric Radiology
DOI: 10.1007/978-3-642-35573-8_13
sha: 
doc_id: 19089
cord_uid: oots4fe4

Lower respiratory tract infection is a very common illness in children and is a significant cause of morbidity and mortality. Clinical signs and symptoms are nonspecific especially in infants and younger children and some even present with nonrespiratory complaints. Infectious agents causing pneumonia is not limited to viruses and bacteria, but it could also be due to Mycoplasma, Mycobacteria, fungi, protozoa, and parasites. Coinfection with two or more microbial agents can also occur. The etiologic agent of lower respiratory infection in a child is often difficult to obtain, but the patient’s age can help narrow the possible cause. Microbiological tests are important but could be difficult to obtain especially in younger children. Various medical imaging modalities not only play an important role as an aid in diagnosis but can also help during and after therapy. Imaging can also help evaluate complications to pneumonia and exclude other causes of respiratory distress including underlying developmental anomalies, foreign body, gastroesophageal reflux disease, and aspiration. In this chapter, the imaging modalities utilized in the detection of pulmonary infections will be discussed. The spectrum of typical imaging findings for various etiologic agents in both immunocompetent and immunocompromised children will be presented.

Lower respiratory tract infection is the most common cause of illness in children and is a signifi cant cause of morbidity and mortality. There are also associated complications, which should be recognized in order to make correct decisions regarding interventions or management. Clinical signs and symptoms are nonspecifi c, especially in infants and younger children. Some children with pneumonia even present with nonrespiratory symptoms including fever, malaise, decreased appetite, irritability, weakness, chest pain, and abdominal symptoms. Physical examination is also less reliable in children than adults (Donnelly 2001 ) . Microbiological tests are important but could be diffi cult to obtain especially in younger children. Various medical imaging modalities not only play an important role as an aid in diagnosis but can also help during and after therapy. Diagnosis of pneumonia calls for a combination of clinical awareness, appropriate microbiological tests, and radiological studies.

The cause of pneumonia in a child is often diffi cult to identify, but the patient's age can help narrow the possible etiologies. Viral pneumonia is rare in the neonatal period because of conferred maternal antibody protection. Group B streptococcus and Gram-negative enteric bacteria are the most common pathogen in neonates (birth to 20 days), obtained through vertical transmission from the mother during birth. From age 3 weeks to 3 months, Streptococcus pneumoniae is the most common pathogen. Viruses are the most frequent cause of community-acquired pneumonia in infants older than 4 months and in preschool-aged children, with respiratory syncytial virus (RSV) being the most common. For school-aged children (6-16 years old), the incidence of bacterial infections from Streptococcus increases, although viral disease remains the most common cause (Condon 1991 ; Ostapchuk et al. 2004 ) . Bacterial pneumonia can occur at any time in preschool and school-aged children and adolescents. Mycoplasma pneumoniae causes 30 % of lower respiratory tract infections in school-aged children (Condon 1991 ; Donnelly 2001 ) . Infectious agents causing pneumonia are not limited to viruses and bacteria, but it could also be due to Mycobacteria , fungi, protozoa, and parasites. Co-infection with two or more microbial agents can also occur.

Evaluation of suspected pulmonary infection is a very common indication for an imaging study in children. The role of imaging including chest radiographs, ultrasound, computed tomography (CT), and even magnetic resonance imaging (MRI) is to detect the presence or exclusion of pneumonia, determine its location, characterize and describe the extent of pneumonia, exclude other causes of respiratory symptoms, and show complications. It is also an important tool for image-guided interventions.

The cornerstone of imaging in children suspected of having pulmonary infection is the chest radiograph. The radiographic appearance refl ects the pathologic process occurring in the respiratory system (Bramson et al. 2005 ) . Frontal and lateral views are obtained when possible because hyperinfl ation and lymphadenopathy are more accurately evaluated on a lateral radiograph especially in younger children. Lateral decubitus views may be useful in distinguishing free fl owing pleural fl uid versus loculated fl uid collections. Chest radiographs have inherent limitations, but despite of this, there is moderate evidence to suggest that chest radiographs are suffi ciently sensitive and highly specifi c for the diagnosis of community-acquired pneumonia (Westra and Choy 2009 ). 13.5.20 Tuberculosis: Progression of Lymph Node Disease .................................................................. 460 13.5 

The use of ultrasound as an imaging tool for pulmonary infections has been increasing especially for assessment of complications. Its utility is even more important because there is no associated radiation, no sedation, no specifi c preparation, and the ultrasound machine can be transported to the patient's bedside. It can be used for planning thoracentesis, thoracotomy, and image-guided drainage procedures. Lower-frequency (3.5-7 MHz) sector transducers are initially used for overview through inter-and subcostal scanning, but higher-frequency (10-12.5 MHz) linear transducers are helpful for more detail in the near fi eld.

The development of helical and multidetector CT has revolutionized imaging evaluation of pulmonary infections. The use of intravenous contrast medium also helps optimize the assessment of pleura, mediastinum, and pulmonary parenchyma in cases of complicated pneumonia. High-resolution CT (HRCT) shows greater accuracy in characterizing diseases into interstitial, airway, and airspace processes and gives a more accurate depiction of the extent of the disease. CT has an important role when a complication is suspected, to exclude an underlying abnormality in recurrent and persistent infections, for image-guided interventions, and for the evaluation of immunocompromised children (Westra and Choy 2009 ) . Radiation-associated risks are important to consider, and thus, a clear indication for the procedure has to be present. Low radiation dose technique with 80-120 kVp, age and thoracic thickness-adjusted low milliampere-seconds, along with radiation dose modulation should be utilized.

Evaluation of lung parenchyma with conventional MR imaging has limitations because of the inherent low proton density and weak MR signal as related to the low physical density of the lung. However, lung parenchymal, pleural, and lymph node infl ammatory abnormalities can be visualized and characterized by MRI in children with pulmonary infections.

Peripheral airways disease or bronchiolitis are common terms ascribed to lower respiratory infection secondary to viruses. It commonly occurs in children less than 2 years of age, typically presenting with cough, coryza, and wheezing. RSV is the most common cause, but other viral causes include rhinovirus, parainfl uenza virus, human metapneumovirus, adenovirus, infl uenza virus, coronavirus, and human bocavirus (Eslamy and Newman 2011 ) . Following inhalation of infected aerosols, the virus migrates to small airways and alveoli resulting to bronchoconstriction and increased mucous secretion (Aherne et al. 1970 ; Swischuk and Hayden 1986 ) .

Typical chest radiographic appearances are peribronchial thickening/opacities, hyperaeration, and subsegmental atelectasis ( Fig. 13.1 ). The peribronchial infl ammation and edema manifests as increased peribronchial cuffi ng or thickening of the bronchial walls, which is usually asymmetric and radiates from the hila into the lung. Narrowed distal airway lumen due to bronchiolar wall edema and mucus results in hyperinfl ation with areas of segmental and subsegmental atelectasis (Condon 1991 ; Donnelly 2001 ) . Patchy areas of airspace consolidation have also been described in viral pneumonia. CT is rarely required in the investigation of viral lower respiratory infection, but the most common CT feature is peribronchial thickening and ground-glass attenuation without consolidation (Tanaka et al. 1996 ) .

Varicella zoster virus infection is a highly contagious but a relatively benign, self-limited disease in childhood. Varicella pneumonia is regarded as a serious manifestation in adults, but immunocompromised children are also at risk. Clinically, cough, fever, dyspnea, chest pain, and vesicular rash are generally accompanied by mild constitutional symptoms (Kim et al. 1999 ) . Radiographs of the chest initially reveal nodular infi ltrates that may progress to large segmental areas of patchy consolidation, predominantly in the bases and perihilar regions. Total clearing is virtually guaranteed, although punctate calcifi cations maybe evident within 2 years after acute illness ( Fig. 13 .2 ). Airspace disease associated with chicken pox in children occurs most often in the immunocompromised host (Blickman 1998 ) .

Certain groups of viruses have been recently reported to cause severe respiratory infection leading to respiratory failure and even death. The severe acute respiratory syndrome (SARS) caused by coronavirus A (SARS-CoV) created a scare in 2003 with over 8,000 cases reported from 29 different countries. SARS presents with a prodrome of fl u-like illness, followed by cough, dyspnea, and possibly acute respiratory distress. The initial radiographic manifestation is the presence of focal or diffuse interstitial opacities but rapidly progresses to bilateral areas of consolidation (Thibodeau and Viera 2004 ) (Fig. 13 .3 ). Another pandemic virus is the infl uenza virus A H5N1 (avian infl uenza virus), originating from Asia and spreading over many parts of the world from 2003 to 2007. More recently, infl uenza virus of swine origin, designated as infl uenza A H1N1, was fi rst reported in Mexico in 2009 and has rapidly spread globally. Symptoms range from asymptomatic infection to mild upper respiratory illness, viral syndrome, diarrhea, severe pneumonia, acute respiratory distress syndrome (ARDS), and progression to multiorgan failure. Initial chest radiographs in children with a mild and self-limited clinical course are often normal, but they may demonstrate prominent peribronchial markings with hyperinfl ation and multifocal areas of consolidation (Lee et al. 2010 ) (Fig. 13.4 ) .

The most common complication of viral pneumonia is a secondary bacterial infection. Viral infection can compromise the respiratory mucosa and render the host pulmonary respiratory system susceptible to develop superimposed bacterial pneumonia (Donnelly 1999 ) .

Postinfectious bronchiolitis obliterans (constrictive bronchiolitis or obliterative bronchiolitis) is a clinical syndrome of chronic airfl ow obstruction associated with infl ammatory changes in the small airways as response to epithelial injury associated with infections. It is particularly associated with Adenovirus , RSV, Varicella , and severe Mycoplasma infection. The chest x-ray fi ndings are often nonspecifi c and can appear normal, but the most common abnormality is hyperaeration (Yalcin et al. 2003 ). On HRCT, there is a mosaic perfusion pattern ( Fig. 13 .5 ). Perfusion is diminished in areas of parenchymal attenuation due to vasoconstriction secondary to hypoxia. Inspiratory and expiratory phases of ventilation are important in HRCT to better assess air trapping in this condition (Hansell et al. 1997 ) . Peribronchial thickening, atelectasis, bronchiectasis, and sometimes lung volume reduction can also be seen. Swyer-James is a subtype of postinfectious bronchiolitis obliterans, which is typically unilateral. It can affect one lung segment, a lobe, or the entire lung. The characteristic chest radiographic and CT fi ndings are hyperlucency due to the pulmonary hypoperfusion, reduction of vascular and hilar markings, and volume reduction of the affected lung or lobe (Daltro et al. 2011 ) (Fig. 13 .6 ).

Bacterial pneumonia occurs with the inhalation of the infectious agent into the airspaces. It is most commonly caused by S. pneumoniae , Haemophilus infl uenza type B, and Staphylococcus aureus . Staphylococcus commonly occurs in early infancy, Haemophilus most often between 6 and 12 months, and S. pneumoniae more commonly between 1 and 3 years of age. Gram-negative aerobic bacteria such as Pseudomonas aeruginosa and S. aureus are a major problem in patients with cystic fi brosis. Patients present with cough, chest pain, and high fever.

Following inhalation of the infectious agent into the airspaces, acinar exudate and edema ensues, manifesting as localized airspace consolidation with air bronchogram on chest radiographs. The typical distribution is lobar or segmental, depending on the stage of progression at the time the x-ray was obtained ( Fig. 13 .7a ) (Condon 1991 ; Donnelly 1999 ) . Round pneumonia is a spherical pneumonia, usually caused by S. pneumoniae (Rose and Ward 1973 ) (Fig. 13.7b ). It is common in children less than 8 years old, maybe due to poor development of collateral pathways of ventilation (pores of Kohn and channels of Lambert). When round pneumonia is seen in children over 8 years old, other etiologies should be considered. The CT manifestations of bacterial infection are areas of consolidation with or without air bronchogram, typically with a segmental or lobar distribution and involving the lung periphery (Tanaka et al. 1996 ) .

Parapneumonic effusions occur most commonly in bacterial pneumonia. It represents a spectrum of infl ammatory fl uid collections that ranges from transudative effusion to empyema. Parapneumonic effusions complicate pneumonia in 36-56 % of cases in pediatric patients (Kurt et al. 2006 ) , and empyema complicates an estimated 0.6 % of all childhood pneumonias (Jaffe and Balfour-Lynn 2005 ) . Pleural fl uid can usually be detected on a frontal chest radiograph, but layering of fl uid on the lateral decubitus view distinguishes a free fl owing fl uid from a loculated fl uid.

CT scan gives a better characterization of parapneumonic effusions compared to radiographs. CT fi ndings include enhancement and thickening of the parietal and visceral pleura, thickening of the extrapleural subcostal tissues, and increased attenuation of the extrapleural subcostal fat (Muller 1993 ) (Fig. 13.8 ). These CT characteristics do not accurately predict empyema and should not be used to distinguish between empyema and transudative effusions. In ultrasound, pleural fl uid can be characterized as simple effusion, complicated effusion, or fi brothorax (pleural thickening or fi brosis) ( Fig. 13.9 ) . A simple effusion appears as a clear anechoic or cloudy hypoechoic fl uid with or without swirling particles. A complicated effusion appears as a septated or multiloculated, hypoechoic fl uid with fi brinous septations, with no clear demarcation between the lung and pleural components, while a fi brothorax appears as a thickened, echogenic rind of pleural plaque (Kim et al. 2000 ) .

Suppurative lung parenchyma complications represent a spectrum of abnormalities including cavitary necrosis, lung abscess, pneumatocele, bronchopleural fi stula, and pulmonary gangrene.

Necrotizing pneumonia or cavitary necrosis is a complication of severe lobar pneumonia, characterized by massive necrosis and liquefaction of lung tissues resulting to multiple cavities rather than a solitary one. It is most commonly caused by S. pneumoniae although Aspergillus and Legionella have also been implicated in the pediatric population (Hodina et al. 2002 ) . Evidence of cavitary necrosis complicating pneumonia is often seen on CT before or in the absence of fi ndings in chest radiography. CT fi ndings include lung consolidation with decreased parenchymal enhancement, loss of lung-pleura margin, and multiple thin-walled cavities lacking an enhancing border. The adjacent visceral pleura is particularly fragile and tends to rupture, causing bronchopleural fi stula (Hoffer et al. 1999 ) (Fig. 13 .10 ). Cavitary necrosis indicates an intense and prolonged illness, but it usually resolves without surgical intervention (Donnelly and Klosterman 1997 ) .

Lung abscesses are thick-walled cavities containing purulent material resulting from pulmonary infection. An airfl uid level with reactive rim is a typical imaging appearance, as compared to necrotizing pneumonia where cavities occasionally have air-fl uid level but without rim of enhancement (Donnelly and Klosterman 1997 ) (Fig. 13.11 ). Differentiating the two is important because abscess not responding to therapy may require drainage, whereas necrotizing pneumonia does not require invasive treatment, and intervention may even be harmful resulting in complications such as bronchopleural fi stula (Hoffer et al. 1999 ) . Pneumatoceles are thinwalled cysts without septations that develop within the lung parenchyma after an acute pneumonia (Fig. 13.12 ) . It may represent a later or less severe stage of resolving or healing necrosis and is most often associated with S. aureus (Daltro et al. 2011 ) .

Bronchiectasis is the most common long-term sequelae of lung parenchymal damage from pneumonia. It is best demonstrated on high-resolution chest CT scan, and the main diagnostic features are as follows: internal diameter of the bronchus is wider than its adjacent pulmonary artery, failure of the bronchus to taper peripherally, and visualization of bronchi in the outer 1-2 cm of the lung zones (Eslamy and Newman 2011 ) (Fig. 13.13 ).

Pertussis is a highly contagious respiratory bacterial infection caused by Bordetella pertussis . It infects mainly infants and young children causing symptoms that include mild fever, runny nose, and cough, which develops into a paroxysmal cough followed by whooping (whooping cough). Pneumonia is a common complication, and untreated patients may be contagious for 3 weeks or more following onset of the cough. The spread of pertussis can be prevented by immunization.

Histopathologic examination reveals an infection dominated by necrotizing bronchiolitis, intra-alveolar hemorrhage, fi brinous edema, and angiolymphatic leukocytosis (Paddock et al. 2008 ) . Conventional radiographs reveal streaky perihilar infi ltrates with most often unilateral hilar adenopathy, a pattern sometimes called the shaggy heart appearance (Blickman 1998 ) (Fig. 13.14 ) .

M. pneumoniae is a common ubiquitous organism and treatable cause of community-acquired pneumonia, occurring primarily in children and young adults. It accounts for up to 30 % of all pneumonia in the general population, but the highest incidence is seen in children between 3 and 14 years of age. Of those infected, 50 % get tracheobronchitis, 30 % pneumonia, 10 % pharyngitis, and 10 % otitis media. Clinically, symptoms are less severe but more common than in true bacterial pneumonia (Blickman 1998 ) .

The radiographic fi ndings are nonspecifi c, have a broad spectrum of appearances, and may present with a pattern intermediate between the classic viral and bacterial pneumonia patterns (Hsieh et al. 2007 ). Some authors reported that a reticulonodular pattern or nodular opacities are typical radiographic pattern (John et al. 2001 ) , while others stress the occurrence of confl uent and patchy consolidation (Reittner et al. 2000 ) (Fig. 13.15 ). HRCT fi ndings are thickened bronchovascular bundles, ground-glass attenuation and consolidation, centrilobular nodules, and lobular distribution (Tanaka et al. 1996 ; Reittner et al. 2000 ) .

Chlamydia trachomatis is an obligate intracellular parasite. Genital chlamydial infection is recognized as the world's most common sexually transmitted disease, and the high prevalence in women of childbearing age results in exposure of neonates during childbirth. Chlamydia pneumonia is a neonatal infection acquired after passage of the fetus through the cervix and vagina. The infant typically presents at 3-6 weeks of age with respiratory symptoms and occasional pulmonary hemorrhage. C. trachomatis should be suspected in infants who are afebrile or nontoxic and have a dry cough. These patients often have a peripheral eosinophilic pleocytosis, sometimes with concomitant conjunctivitis (Ostapchuk et al. 2004 ). Most chest radiographs show bilateral hyperaeration and diffuse infi ltrates with a variety of radiographic patterns including interstitial, reticular nodular, atelectasis, coalescence, and bronchopneumonia (Radkowski et al. 1981 ) (Fig. 13.16 ).

Tuberculosis (TB) is caused by infection with the Mycobacterium tuberculosis complex. Once inhaled, the infected aerosolized droplet in the alveoli cascades a series of infl ammatory reaction, and the bacilli also spread to nearby mediastinal lymph nodes. The alveolar site of infection (Ghon focus), the infected lymph nodes, and the associated lymphangitis form the "primary (Ranke's) complex." In most immunocompetent children, the infection goes into latency and the bacilli become dormant. These children usually have a reactive tuberculin skin test (TST) and/or a positive Interforon-gamma release assay (IGRA) test, but without clinical evidence of TB and generally no abnormalities on chest radiograph apart from the primary complex residual. Primary tuberculosis disease occurs if the host is unable to contain the infection, and disease progression occurs in the lungs, the lymph nodes, and adjacent structures in the thorax or could disseminate in any part of the body.

Lymphadenopathy (present in 92 %) with or without a visible Ghon focus is the radiographic hallmark of TB infection and usually involves the hilar and paratracheal regions. The Ghon focus may be too small to be radiographically visible but can also undergo caseation and calcify ( Fig. 13.17 ). Disease progression may occur at the site of Ghon focus, within the regional lymph nodes, or following disease spread (Fig. 13.18 ). Parenchymal involvement in primary pulmonary TB most commonly appears as homogeneous consolidation, although it can appear patchy, linear, nodular, and mass-like. Caseation necrosis, liquefaction, or calcifi cations can be seen within the consolidation and can progress into extensive lung damage (Marais et al. 2004 ) (Fig. 13.19 ). Enlarged and edematous hilar, paratracheal, and subcarinal lymph nodes may cause compression of the adjacent bronchus and can lead to hyperinfl ation or atelectasis of the affected lung segment. Contrast-enhanced CT shows a characteristic appearance consisting of central areas of low attenuation with peripheral rim enhancement and obliteration of perinodal fat (Kim et al. 1997 ) (Fig. 13.20 ) .

Pulmonary dissemination, usually seen in very young and immunocompromised patients, leads to the formation of pulmonary nodular interstitial granulomas, usually 1-2 mm in size, throughout the lungs. Chest radiographs demonstrate the usual miliary nodular pattern but CT is more sensitive for the detection of miliary TB (Kim et al. 1997 ) (Fig. 13.21 ) . Adult-type disease presentation is common after primary infection in children over 10 years of age or via endogenous reactivation (postprimary TB) or reinfection. Chest radiograph shows ill-defi ned, fi bronodular parenchymal disease and cavitation mainly involving the apical segments of the upper lobes (Perez-Velez and Marais 2012 ) (Fig. 13.22 ).

Aspergillus fumigatus is a ubiquitous saprophytic mold found in many environmental sites, and infection is usually via inhalation of spores, although other routes of entry also occur. Infection can manifest as colonization of airway cavities and necrotic tissue, allergic disease, and invasive disease, which is usually acute and rapidly progressive severe disease (Foster and Alton 2003 ) .

Airway colonization occurs in patients with underlying airway disease such as asthma and bronchiectasis. Intertwined fungal hyphae, called as mycetoma or aspergilloma, form in the pulmonary cavity or ectatic bronchi. Important underlying causes are pulmonary TB with cavitation and cystic fi brosis with bronchiectasis. Rounded soft tissue mass within a cavity forming an "air-crescent" sign is a typical appearance ( Fig. 13.23 ). Allergic bronchopulmonary aspergillosis is characterized by mucoid impaction of the proximal bronchi presenting as fi ngerlike shadows involving the upper lobes on the chest radiograph. CT demonstrates the mucoid impaction of the central airways and the bronchiectasis of the segmental or subsegmental airways (Foster and Alton 2003 ) . Invasive disease is an aggressive, rapidly disseminating and destructive disease and occurs when host defenses are impaired. It is characterized by the occlusion of large-or medium-sized arteries by plugs of hyphae causing pulmonary hemorrhage, arterial thrombosis, and infarction. Radiographic fi ndings are nonspecifi c, with multiple nodules or areas of consolidation (Fig. 13.24 ) . The typical CT fi nding is the halo sign due to ground-glass attenuation representing hemorrhage surrounding the pulmonary nodule or mass (Foster and Alton 2003 ; Eslamy and Newman 2011 ) .

Histoplasmosis, caused by the fungus Histoplasma capsulatum , is usually an asymptomatic and self-limited disease that rarely requires therapy in children other than the very young or immunocompromised. It is found in the soil of endemic areas including Central United States, Central America, and Northern South America but has also been reported in some parts of Asia (Houston 1994 ) . After inhalation, the spores germinate within the alveoli inciting an intense tissue reaction characterized by granulomas, which may calcify. It spreads to the lymphatics and into the hilar or mediastinal lymph nodes, and systemic dissemination may occur in patients with impaired T-cell immunity (McAdams et al. 1995 ) .

Histoplasmosis falls in one of three categories: acute, chronic pulmonary, and disseminated disease. Acute pulmonary histoplasmosis is a self-limited illness. Chronic pulmonary histoplasmosis occurs in patients with chronic lung disease and presents similar to tuberculosis with predilection for apical and posterior segments of the lung. Disseminated histoplasmosis in children is characteristically a fulminant illness, which may or may not have pulmonary involvement. Radiologic manifestations parallel the clinical syndromes. Acute disease usually manifests as focal parenchymal consolidation with or without ipsilateral hilar adenopathy (Fig. 13.25 ) . With healing, a nodule representing a histoplasmoma may result. Chronic histoplasmosis radiographically manifests as an upper lobe fi brocavitary disease indistinguishable from postprimary tuberculosis. Chest radiographs of patients with disseminated disease may show miliary or diffuse reticulonodular pattern that could progress to diffuse airspace opacifi cation (McAdams et al. 1995 ) .

Echinococcosis, also known as hydatid disease or hydatidosis, is a parasitic infection in humans caused by dog tapeworm, Echinococcus granulosus , in its larval stage. It is endemic in many sheep and cattle-raising countries throughout the world. Humans are intermediate hosts and become infected through ingestion of contaminated water or vegetables. When eggs of adult tapeworm are ingested, embryos are freed and migrate through the host's gastrointestinal mucosa and enter the portal vein and lymphatic system to various parts of the body where the embryo develops into a cyst. The wall of the cysts contains three layers: the outermost, pericyst; the middle laminated membrane layer, ectocyst; and innermost germinal layer, endocyst (Czermak et al. 2001 ) . The lungs are the most common sites of infection in children but majority remain asymptomatic until the cyst enlarges to cause symptoms due to mass effect or due to cyst rupture (Santivanez and Garcia 2010 ) .

Diagnosis is obtained by imaging evaluation, supported by serology. A high proportion of lung lesions are discovered incidentally on a routine x-ray, and the most prominent radiological fi nding is a dense, round, well-demarcated opacity that can resemble a neoplasm (Fig. 13.26a ) . When the growth of the cyst produces erosion in the bronchioles, air between the endocyst and pericyst can produce a "crescent or inverse crescent sign." If air continues to enter the cyst cavity, endocyst membrane can be seen fl oating in the most dependent part of the pericyst cavity producing the "waterlily sign" (Fig. 13.26b ) . CT recognizes the appearance of the cystic lesion including smaller cysts, assesses signs of cyst rupture, evaluates the surrounding structures, and helps exclude alternative differential diagnoses (Santivanez and Garcia 2010 ) .

Ascariasis and hookworms remain the most common intestinal nematodes in the world (Sarinas and Chitkara 1997 ) . In the western hemisphere, parasitic pneumonia secondary to Toxoplasma gondii is associated with compromised hosts, particularly acquired immunodefi ciency syndrome (AIDS) patients. Strongyloides stercoralis infestation is seen in patients receiving glucocorticoids or chemotherapy and in patients with AIDS or other causes of T-cell dysfunction (Berk and Verghese 1998 ) . Ascaris infestation generally occurs through hand-to-mouth ingestion of food contaminated with parasite eggs, while hookworms are transmitted through larval penetration of the skin. Symptomatic pulmonary disease may present with fever, cough, chest pain, hemoptysis, dyspnea, and wheezing. These pulmonary symptoms could be due to Loffl er's syndrome, effects of larval tissue migration, airway reactivity or bronchospasm, superimposed bacterial infection, and chronic eosinophilic pneumonia (Sarinas and Chitkara 1997 ) . Ascaris and hookworm infections present with peripheral eosinophilia during larval migration phase. Chest radiographs could be normal or demonstrate nonspecifi c patchy pulmonary infi ltrates (Fig. 13.27 ) . CT scan could depict abnormalities better and could show ground-glass pulmonary lesions with ill-defi ned margins as well as nodules (Sakai et al. 2006 ).

Causes of immunodefi ciency can be divided into congenital (primary) and acquired (secondary). The range of respiratory complications encountered is broad and is infl uenced by both the type and degree of immunodefi ciency. Chest radiographs are insensitive and may show only subtle change.

HRCT detects abnormalities not visible on the plain fi lm such as bronchial wall thickening, bronchial dilatation, and air trapping. Various noninfectious pulmonary processes including alveolar hemorrhage, pulmonary edema, graft versus host disease, and drug reaction are also seen in the immunocompromised hosts, which can mimic pulmonary infection on imaging.

The primary or congenital immunodefi ciency disorders are inherited group of disorders resulting from innate defects of the immune system. Clinical manifestations are diverse and nonspecifi c, which include recurrent infections, infection with opportunistic organisms, failure to thrive, skin rashes, recurrent skin sepsis, and unusual wound healing (Jeanes and Owens 2002 ) . Primary immunodefi ciency can be broadly divided into T-cell (cellular) immune defi ciency versus B cell (humoral defi ciency). Humoral immunodefi ciencies are the most commonly encountered type characterized by defective antibody production with increased susceptibility to pyogenic infections but able to recover from viral infections (Fig. 13.28 ). Examples are X-linked agammaglobulinemia, IgA defi ciency, and common variable immunodefi ciency. Cellular immunodefi ciencies have increased susceptibility to disseminated viral and opportunistic infections. Cellular immune disorders include DiGeorge syndrome and severe combined immunodefi ciency (Collingsworth 2005 ) .

Acquired immunodefi ciencies in childhood can be caused by chemotherapy, radiation therapy, immunosuppressive therapy aimed at treating childhood malignancies, transplant rejection, rheumatologic disorders, or infl ammatory or infectious diseases. It can also be due to human immunodeficiency virus (HIV) infection, malnutrition, or any state of chronic debilitation.

Bone marrow transplant requires complete eradication of the immune system. Early infectious complications are frequently caused by bacteria and fungi, most commonly Gramnegative bacteria ( Pseudomonas and Klebsiella ) and Aspergillus (Fig. 13.29 ). Widespread use of long-term indwelling catheters has led to an increased incidence of both staphylococcal and streptococcal pneumonia. Chest radiographs may show classic focal or lobar consolidation although atypical appearance can also be seen. Children are also at increased risk of viral infections, most importantly RSV, Herpes simplex , Adenovirus , and Varicella . Immunosuppressive therapy following solid organ transplantation predisposes a patient to recurrent pulmonary infections. In these patients, viral infections can be life-threatening, but Pneumocystis and fungal infections ( Aspergillus and Candida ) can also be seen (Collingsworth 2005 ) .

Children represent 2 % of the reported cases of human immunodefi ciency virus (HIV) infection. Most children are infected after vertical transmission from their mother, and majority develop acquired immunodefi ciency syndrome (AIDS) early in life. There is increased susceptibility to bacterial, viral, fungal, protozoal, and opportunistic infections. Lobar or segmental consolidations are the most common patterns (Marks et al. 1996 ) . Mycobacterial infection can be seen in AIDS patients, and the radiographic appearance mimics that seen in immunocompetent children with primary tuberculosis. Mycobacterium avium-intracellulare is also encountered later in the course of disease and imaging fi ndings cannot be distinguished with other forms of mycobacterial infections (Collingsworth 2005 ) . Pneumocystis jiroveci is the most common opportunistic pulmonary infection in children with AIDS, occurring in up to 50 %, and is the leading pulmonary cause of death (Jeanes and Owens 2002 ) . Radiographic appearances are variable and include hyperinfl ation with diffuse bilateral interstitial or nodular infi ltrates from the perihilar region to the periphery, which often progresses to widespread alveolar opacities with air bronchogram (Fig. 13.30 ) . Cavitary nodules and cysts can be seen, with pneumothorax and/or pneumomediastinum as common complications. HRCT fi ndings include patchy or diffuse ground-glass opacity, consolidation, cyst or cavities, centrilobular opacities, nodules, peribronchial cuffi ng, and interlobular septal thickening (Jeanes and Owens 2002 ; Collingsworth 2005 ) . 

Pathological changes in virus infection of the lower respiratory tract in children

Parasitic pneumonia

Infl ammatory lung disease

Interpretation of chest radiographs in infants with cough and fever

Thoracic disorders in the immunocompromised

Pneumonia in children

Echinococcus granulosus revisited: radiologic patterns seen in pediatric and adult patients

Pulmonary infections

Maximizing the usefulness of imaging in children with community-acquired pneumonia

Practical issues concerning imaging of pulmonary infection in children

Pneumonia in children: decreased parenchymal contrast enhancement-CT sign of intense illness and impending cavitary necrosis

Pneumonia in normal and immunocompromised children: an overview and update

Chronic lung infection in children

Obliterative bronchiolitis: individual CT signs of small airways disease and functional correlation

Imaging of cavitary necrosis in complicated pneumonia

Lung abscess versus necrotizing pneumonia: implications for interventional therapy

Histoplasmosis and pulmonary involvement in the tropics

Mycoplasma pneumonia: clinical and radiographic features

Management of empyema in children

Chest imaging in the immunocompromised child

Spectrum of clinical and radiographic fi ndings in pediatric mycoplasma pneumonia

Pulmonary tuberculosis in children: evaluation with CT

High resolution CT fi ndings of varicella zoster pneumonia

Ultrasound in the diagnosis of pediatric chest diseases

Therapy of parapneumonic effusions in children: video-assisted thoracoscopic surgery versus conventional thoracostomy drainage

Swine-origin infl uenza A (H1N1) viral infection in children: initial chest radiographic fi ndings

The natural history of childhood intrathoracic tuberculosis: a critical review of prechemotherapy literature

Thoracic diseases in children with AIDS

Thoracic mycoses from endemic fungi: radiologic-pathologic correlation

Imaging of the pleura

Community acquired pneumonia in infants and children

Pathology and pathogenesis of fatal bordetella pertussis infection in infants

Tuberculosis in children

Chlamydia trachomatis in infants: radiography in 125 cases

Mycoplasma pneumoniae pneumonia: radiographic and HRCT features in 28 patients

Spherical pneumonias in children simulating pulmonary and mediastinal masses

Pulmonary lesions associated with visceral larva migrans due to Ascaris suum and Toxocara canis : imaging of six cases

Pulmonary cystic echinococcosis

Ascariasis and hookworm

Viral versus bacterial infections in children: is roentgenographic differentiation possible?

High resolution fi ndings in community acquired pneumonia

Atypical pathogens and challenges in community acquired pneumonia

What imaging should we perform for the diagnosis and management of pulmonary infections?

Postinfectious bronchiolitis obliterans in children: clinical and radiological profi le and prognostic factors