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British Medical Bulletin 61:263-279 (2002)
© 2002 Oxford University Press

Application of computed tomography in childhood respiratory infections

S J Copley

Department of Radiology, Hammersmith Hospital, London, UK

	Abstract

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
The role of computed tomography (CT), including high-resolution computed tomography (HRCT), is still evolving in children. Radiation dose is an important consideration, but CT has advantages over chest radiography as it is more sensitive and specific for a variety of conditions affecting the pulmonary parenchyma. Careful attention to CT technique is vital for good quality diagnostic images in the paediatric population. The CT appearances of bacterial, viral, fungal, tuberculous and mycoplasma respiratory tract infections are discussed. The role of CT in specific circumstances such as the investigation of complicated bacterial pneumonia, the immunocompromised child and the sequelae of respiratory infections is addressed.

	Introduction

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Respiratory tract infection represents the most common illness that occurs in childhood¹. Although chest radiography remains crucial in the evaluation of childhood respiratory infections, computed tomography has a role in specific instances. High resolution computed tomography (HRCT) offers several advantages over chest radiography due to its exquisite demonstration of pulmonary architecture without superimposition of overlying structures. Broadly, HRCT allows a better assessment of the type, distribution and severity of parenchymal abnormality than is possible on the plain radiograph. In a study of 36 children, Lynch et al. found the advantages of HRCT over chest radiography were similar to those in adults: HRCT was more sensitive (could detect abnormalities when the chest radiograph was normal); showed greater accuracy in characterizing diseases into interstitial, airway and airspace processes; and gave a more accurate depiction of the extent of disease².

The risk of ionising radiation is a very important consideration in children. The radiation dose from a CT scan is much greater than that from a chest radiograph. However, the radiation dose from HRCT is considerably less than that of standard CT using contiguous sections: HRCT sections are thinner and interspaced, therefore smaller lung volumes are irradiated. Advances in helical and multislice CT detector sensitivity, combined with a reduction in milli-amperage (mA), allow the radiation dose to be kept to a minimum. However, CT should not be regarded as a routine investigation in children and examinations should always be tailored to solve questions not answered by less sophisticated investigations. The risk/benefit ratio of a CT examination in a child should be evaluated in each individual case.

	CT technique in children

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Advances in CT technology, particularly the development of helical and multislice CT, have resulted in faster scan acquisition times, enabling images of sufficient diagnostic quality to be obtained in children during gentle respiration. CT scanning can now be performed under light sedation or after feeding, without the need for general anaesthesia.

CT using contiguous sections is the method of choice for evaluating cases of large airway obstruction and may occasionally be useful in childhood empyema. The use of intravenous contrast medium optimises assessment of the pleura, mediastinum, and pulmonary parenchyma in cases of complicated pneumonia. Section width varies from 5—10 mm depending on the age and size of the child. By contrast, HRCT involves the use of thinly collimated, interspaced 1—3 mm CT sections. HRCT images are also reconstructed with a high spatial frequency or bone algorithm and intravenous contrast is not required for fine detail of the lung parenchyma. Both conventional and spiral CT scanners can produce thin sections, and the terms ‘spiral CT’ and ‘high-resolution CT’ should not be confused.

The advent of helical or spiral CT, particularly multidetector helical CT has significantly reduced acquisition times. The principle of spiral CT involves continuous rotation of the X-ray beam and detectors around the patient while the table moves into the gantry. As yet, there have been few descriptions in the literature for the optimisation of this technique in children, although the applications of ‘dynamic’ helical CT, where a single section is chosen and imaged during an entire respiratory cycle, have been explored³.

The acquisition of CT images without motion artefact is vital. Initial referral to a centre capable of providing adequate CT images with fast acquisition times is preferable, due to the extra radiation dose incurred for a repeat CT when the initial study is technically inadequate. The radiation burden varies depending on the type of scanner used and the scanning protocol. By reducing the tube current to 80 mA for paediatric HRCT, the radiation dose may be decreased by up to 80% without significant impairment of image quality⁴. Calculations from our institution have given a typical effective dose of 1.7 mSv (milliSieverts) for a thin-section CT examination (3 mm section thickness every 10 mm) for a 10-year-old child which is of the same order as background radiation for 1 year (2 mSv per annum).

	The role of CT in childhood respiratory infections

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Due to the extra radiation burden, CT is rarely used in the primary assessment of uncomplicated respiratory infections in the immunocompetent child. However, CT has an important role in specific instances (Table 1): (i) when a complication such as pulmonary abscess is suspected or ultrasound has proved problematic in evaluating an empyema; (ii) to exclude an underlying abnormality in recurrent infections (e.g. pulmonary sequestration); (iii) the investigation of the immunocompromised child; (iv) to guide the type and site of tissue sampling; and (v) to assess the sequelae of respiratory infection (e.g. postviral obliterative bronchiolitis).

View this table: [in this window] [in a new window]   Table 1 Indications for CT in paediatric lower respiratory tract infection

 
Bacterial pneumonia

Most commonly, the CT manifestations of bacterial infection are areas of consolidation with or without air bronchograms, typically with a segmental or lobar distribution and involving the lung periphery⁵. Consolidation on CT is defined as areas of increased pulmonary opacity with obscuration of underlying bronchovascular structures⁶. The pathophysiology of bacterial pneumonia involves rapid outpouring of exudate into the alveoli with spread to adjacent acini and lobules, reflected by homogenous consolidation demonstrated on CT.

CT is rarely used in the initial assessment of uncomplicated bacterial pneumonia. The primary role of CT in bacterial pneumonia is when children fail to respond to treatment and complications are suspected⁷. The imaging features in empyema are discussed earlier in this issue, but pulmonary complications such as lung abscess and necrotizing pneumonia can also be detected on CT more readily than on chest radiography⁸,^,9. Contiguous, intravenous contrast-enhanced sections are usually obtained in preference to HRCT and uncomplicated bacterial pneumonias usually demonstrate homogenous parenchymal enhancement¹⁰. Lung abscesses are characterized by an air-fluid level and a reactive rim (Fig. 1) whereas necrotizing pneumonias are characterized by consolidation with areas of non-enhancement, occasionally with air-fluid levels but without rim enhancement¹⁰. Increasingly, the importance of differentiating between lung abscesses and necrotizing pneumonia is becoming evident. Abscesses not responding to medical therapy may require aspiration or drainage (sometimes with CT guidance), whereas necrotizing pneumonia does not require invasive treatment and intervention may even be harmful resulting in complications such as bronchopleural fistula⁹,^,11.

View larger version (112K): [in this window] [in a new window]   Fig 1. CT of a child with a pulmonary abscess due to Streptococcus pneumoniae infection. An air-fluid level was seen more inferiorly and there is also a pleural effusion. Note the right middle and lower lobe consolidation characteristic of a bacterial pneumonia.

 
The diagnosis of bacterial pneumonia is often straightforward when based on a combination of clinical, radiographic and laboratory findings. However, there are other conditions that can mimic bacterial pneumonia on CT including non-infectious conditions such as acute eosinophilic pneumonia and pulmonary haemorrhage¹². An additional important differential diagnosis to be aware of in children is pulmonary contusion in non-accidental injury. Although ancillary features such as rib fractures may point to the diagnosis, the CT features of subpleural sparing of consolidation and a posterior, cresenteric distribution allow accurate differentiation from bacterial pneumonia in the majority of cases¹³.

Studies that blind the radiologist to the clinical findings and aim to differentiate between the causes of acute parenchymal disease on imaging findings alone are of limited value as they do not necessarily reflect clinical practice. Despite this limitation, a recent study (including adults and children) found that observers could differentiate between infectious (including bacterial pneumonia) and non-infectious causes of acute parenchymal lung disease on HRCT in 90% of cases¹². The most helpful feature in favour of an infectious cause is the presence of patchy centrilobular nodules, but the CT features overlap considerably with non-infectious causes¹²,^,14.

Another important manifestation of bacterial pulmonary infection is blood-borne septic emboli. The typical CT features are multiple pulmonary nodules related to feeding vessels, cavitation and wedge-shaped peripheral consolidation¹⁵.

Viral infection

Viral infection is the commonest cause of lower respiratory tract infection in infants and children¹. Again, HRCT is rarely required in the investigation of viral pneumonia in the immunocompetent child and even recent imaging descriptions are limited to chest radiography¹⁶. The commonest HRCT feature is reported to be peribronchial thickening¹⁷, reflecting the underlying pathophysiological process of inflammation and oedema of bronchial mucosal cells. There are overlapping HRCT features between bacterial and atypical (including viral) pneumonias, but the presence of ground-glass attenuation without consolidation (defined as hazy increased attenuation without obscuration of bronchovascular structures; Fig. 2), peribronchial thickening and a lobular (as opposed to segmental) distribution are in favour of a viral pneumonia⁵. Peripheral segmental or lobar consolidation is rare in pure viral pneumonia, and may suggest secondary bacterial infection¹. The investigation of respiratory infection in the immunocompromised child represents a particular diagnostic challenge, which will be discussed under the relevant section.

View larger version (113K): [in this window] [in a new window]   Fig 2. HRCT of an immunocompromised child (after bone marrow transplant) with lower respiratory infection due to parainfluenza virus. The diagnosis was made by broncho-alveolar lavage. Note the wide-spread areas of ground-glass opacification that do not obscure the bronchovascular markings.

 
An emerging role for CT is in the evaluation of the effects of viral infection on the developing lung, which results in characteristic HRCT appearances. This is discussed later under the heading ‘obliterative bronchiolitis’.

Mycoplasma pneumonia

Mycoplasma pneumoniae typically affects school-aged children¹. HRCT is more sensitive and specific for the diagnosis of mycoplasma pneumonia than chest radiography, but in clinical practice the diagnosis is normally suggested by serological tests¹⁸. The HRCT features of mycoplasma pneumonia are thickened bronchovascular bundles, ground-glass attenuation and consolidation, centrilobular nodules and a lobular distribution (involving whole secondary pulmonary lobules with sparing of adjacent lobules)⁵,^,18. The propensity of bronchial wall and bronchovascular bundle thickening is thought to represent targeting of the ciliated cells of the respiratory tract by the organism⁵. A reticular interstitial pattern (possibly due to visible interlobular septa) is reported to be a characteristic feature of mycoplasma pneumonia on chest radiography. Reittner et al. reported thickened interlobular septa on HRCT in 21% of cases with mycoplasma pneumonia, but interestingly Tanaka et al. found interlobular septal thickening to be present in roughly equal proportions of cases with bacterial pneumonia and atypical pneumonia (16% and 14%, respectively)⁵. Reliance on this sign to diagnose cases of mycoplasma pneumonia on HRCT may, therefore, be over simplistic.

Children who require hospitalisation for mycoplasma pneumonia are reported to demonstrate an increased incidence of HRCT features of obliterative bronchiolitis on follow-up scans than those with a milder clinical course of infection¹⁹. Those most at risk are younger children and those with a high antibody titre at diagnosis.

Tuberculosis

Primary tuberculosis is the commonest form encountered in children²⁰. Chest radiography remains the initial imaging technique and the radiographic features are hilar or mediastinal lymphadenopathy, with or without opacities in the unilateral lung²⁰,^,21. Occasionally, the chest radiograph may be normal and lymphadenopathy may be detected on CT, which is not evident radiographically²²,^,23. In addition, CT features such as low attenuation lymph nodes with peripheral enhancement, lymph node calcification, branching centrilobular nodules and miliary nodules are helpful in suggesting the diagnosis in cases where the radiograph is normal or equivocal²⁴. Other features such as segmental or lobar consolidation and atelectasis are non-specific (Fig. 3). In the study by Kim et al, CT (including HRCT) revealed lymphadenopathy, which was not demonstrated in 21% of radiographs, and parenchymal abnormalities, which were not seen on 35% of radiographs²⁴. Furthermore, clinical management was altered in 37% of cases, including when CT demonstrated complications such as pleural or chest wall involvement.

View larger version (128K): [in this window] [in a new window]   Fig 3. CT of an infant with primary TB. There is right upper lobe consolidation and large volume right paratracheal lymphadenopathy (arrows).

 
Miliary TB represents haematogenous dissemination of disease and is commoner in young infants and immunocompromised children. HRCT is more sensitive than chest radiography for the detection of military TB. The HRCT findings are wide-spread multiple small (< 2 mm diameter) nodules²⁵. The nodules may be so numerous that they coalesce to form larger nodules greater than 2 mm in diameter or even consolidation with air bronchograms. Thickening of the interlobular septa may also be a feature. The presence of these additional HRCT features, as well as the classical widespread ‘miliary’ nodules, led Jamieson and Cremin to advocate the term ‘acute disseminated’ instead of miliary TB in children²⁵. Mediastinal and hilar lymphadenopathy may also be present.

Cavitation is reported to be rare on chest radiography in children with TB. However, children with HIV-related TB may have atypical radiographic features and cavitation has been reported²⁶,^,27. CT may show areas of cavitation that are not apparent on chest radiography²⁵, which may raise the possibility of a previously unsuspected underlying immune disorder.

Fungal infections

Pulmonary fungal infections are rare, but of particular importance in the immunocompromised child. Allergic bronchopulmonary aspergillosis is not strictly an infection, but a hypersensitivity reaction to ubiquitous Aspergillus fumigatus hyphae, which typically may occur in immunocompetent children with chronic airway disease such as asthma or cystic fibrosis. The diagnosis should be suspected in asthmatics demonstrating central bronchiectasis on HRCT²⁸. Traditionally, the distribution of central bronchiectasis is considered to be upper lobe, but lower lobe bronchi are also often affected²⁹. Consolidation and atelectasis are also described²⁹ and in cystic fibrosis consolidation may be peripheral with lobulated borders (Fig. 4).

View larger version (106K): [in this window] [in a new window]   Fig 4. A child with cystic fibrosis and dense, peripheral consolidation due to allergic bronchopulmonary aspergillosis.

 
Aspergillus and Candida albicans are the commonest fungal infections in immunocompromised children, especially neutropenic children with haematological malignancy³⁰. Children treated with corticosteroids and those with childhood chronic granulomatous disease are also at risk. Invasive Aspergillus represents a potentially lethal complication in these children and early diagnosis and treatment are imperative. The hyphae invade pulmonary vessels resulting in pulmonary haemorrhage, arterial thrombosis and infarction, hence the term ‘invasive aspergillosis’. The early HRCT features of invasive aspergillosis are nodules or areas of consolidation with a surrounding ‘halo’ of ground-glass opacity, representing pulmonary haemorrhage³⁰,^,31. These tend to be the cardinal features when children are neutropenic, but when neutrophil recovery occurs, areas of cavitation (thought to represent infarction) with an ‘air crescent’ sign develop³². C. albicans infection also demonstrates nodules on HRCT with surrounding ground-glass attenuation (halo sign); however, the air crescent sign is much more infrequent by comparison with invasive aspergillosis³³. In addition, candidal infection is rarely confined to the lungs and other organs including the liver, spleen and kidneys may demonstrate low attenuation lesions on CT³⁴.

Pulmonary mucormycosis (zygomycosis) is a less common pathogenic finding in the immunocompromised child and CT features include nodules and wedge-shaped areas of consolidation, particularly in the posterior segments of the upper lobes³⁵.

	Applications of HRCT in specific conditions

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Immunocompromised children

Early diagnosis of infection is vital in children with immunodeficiency. Appropriate treatment should be started early as infection can progress rapidly and may be life-threatening. The causes of immunodeficiency can be divided into congenital or acquired. Acquired causes of immunodeficiency include AIDS, haematological malignancies, specifically postchemotherapy or bone marrow transplant and postviral infection. The type of immunocompromise may determine which pathogens are likely to cause infection. For example, primary immunodeficiency can be broadly divided into T-cell (cellular) immune deficiency or B-cell (humoral) deficiency. Children with humoral deficiency are more prone to bacterial infections, whereas viral, fungal and parasitic infections are commoner in children with cellular immune deficiency³⁶. In children with haematological malignancy, the commonest pathogens are fungi, with bacterial and viral infections being less common³⁷. However, it is important to be aware that respiratory infection may be due to several different co-existing pathogens in the same individual.

The advantages of HRCT over chest radiography in immunocompromised adults are the increased sensitivity and specificity of HRCT for respiratory complications³³. However, in a study of children with haematological malignancy, the accuracies of CT and chest radiography were good, but not statistically different, for fungal pneumonia³⁷. The accuracy of chest radiography was satisfactory for viral pneumonia, poor for bacterial pneumonia and CT did not statistically improve accuracy. However, a major limitation of this study was that only 3 of 48 children had thin sections and the detection of features such as ground-glass opacification and characterization of small nodules may be unreliable on conventional CT. Furthermore, nearly half the children in this cohort did not have histopathological correlation. Another important advantage of CT over radiography is observer confidence, which was not assessed.

Although, the role for CT in immunocompromised children is not as well established as in adults, the principles are similar. When respiratory infection is suspected clinically, HRCT may reveal abnormalities despite a normal radiograph. CT may suggest a specific diagnosis when the radiographic findings are equivocal. CT is also useful to guide the type of biopsy (transbronchial or open) and the site of affected lung, when tissue sampling is required to make the diagnosis.

Janzen et al. found that CT is most accurate when the predominant pattern consists of multiple nodules³³. Large nodules (> 3 cm in diameter) are suggestive of either aspergillosis or candidal infection, with other possibilities being nocardia, cryptococcus and bacterial septic emboli, all of which may cavitate³³. Smaller nodules (1—5 mm in diameter) are typically seen in patients with Pneumocystis carinii (PCP), cytomegalovirus (CMV), TB, herpes simplex or herpes varicella-zoster pneumonia (Fig. 5). CMV and PCP pneumonia are also associated with ground-glass opacification (Fig. 5). Although a nodular pattern often is consistent with infection, non-infectious causes such as organizing pneumonia, lymphocytic interstitial pneumonitis (LIP) or lymphoma should also be considered³⁸.

View larger version (113K): [in this window] [in a new window]   Fig 5. HRCT of a child with AIDS and CMV pneumonitis. Note the wide-spread small nodules and ground-glass opacification. The differential diagnosis for this appearance would also include PCP infection, miliary TB and lymphocytic interstitial pneumonitis (LIP).

 
Imaging of children with AIDS represents a particular diagnostic challenge. The primary value of HRCT in paediatric AIDS is in detecting new disease, including infection by bacterial, viral, fungal and protozoal agents, when the chest radiograph is normal or equivocal³⁹. By contrast to adults, young children with HIV infection often present with recurrent bacterial infections as the immune system has not yet been exposed to (and developed immunity to) common bacterial pathogens⁴⁰. Opportunistic infections such as Pn. carinii (PCP) and Mycobacterium avium-intracellulare (MAI) are common later in the course of disease, but unlike adults, children do not tend to present with opportunistic infections. Cytomegalovirus is the commonest cause of viral pneumonia, followed by varicella⁴¹. Tuberculosis has reported to be increasing in children with AIDS and imaging features include massive hilar and mediastinal lympadenopathy, consolidation similar to primary TB, cavitation and miliary nodules²⁶,^,41. Invasive aspergillosis is very rare, but has been described in children with AIDS⁴².

PCP is the commonest opportunistic infection in children with AIDS and the HRCT appearances of PCP pneumonia are similar to those described in adults³⁹. Features are ground-glass attenuation, consolidation, nodules, thickening of interlobular septa and thin-walled cysts³⁹. In children, hilar lymphadenopathy may also be present³⁹. Pneumothoraces have also been reported as a presenting feature⁴³. CMV has very similar appearances to PCP and the two may be indistinguishable³⁹,^,44. Another important differential diagnosis of multiple, small wide-spread nodules in children with AIDS is lymphocytic interstitial pneumonitis (LIP) which has been linked to infection with Epstein-Barr virus and is thought to represent hyperplasia of lymphoid tissue in the lung as a response to the virus⁴⁰. However, LIP rarely co-exists with opportunistic infections and the clinical course is insidious with a slow progression of symptoms⁴⁰.

Due to improved life-expectancy of children with AIDS, the sequelae of pulmonary infection are increasingly recognized on HRCT. Bronchiectasis is a sequela of recurrent and unresolved pneumonias in children with AIDS and has been described after LIP⁴⁵. Repeated infection with PCP has also been reported in resulting in chronic pulmonary fibrosis with extensive cystic disease demonstrated on HRCT⁴⁶.

Recurrent respiratory tract infections

Children with recurrent respiratory tract infection may have an underlying abnormality such as bronchiectasis which is optimally demonstrated with HRCT. Bronchiectasis is characterized by abnormal and irreversible dilatation of bronchi, usually associated with inflammation. Early diagnosis is increasingly important due to the need to investigate underlying causes and rapid institution of appropriate therapy which may improve the clinical course and prevent further deterioration.

HRCT is more sensitive than chest radiography for the diagnosis of bronchiectasis⁴⁷. The CT criteria for the diagnosis of bronchiectasis in children are similar to those in adults: bronchial dilatation in relation to the accompanying pulmonary artery (‘signet ring’ sign), bronchial wall thickening (allowing visualization of airways more distally than usual), crowding of airways and a lack of tapering of airways (Fig. 6)⁴⁷.

View larger version (122K): [in this window] [in a new window]   Fig 6. HRCT of a child with mild cylindrical bronchiectasis of the left lower lobe and lingula. The chest radiograph was normal, but the HRCT shows dilatation of the airways with respect to the accompanying pulmonary arteries (signet ring sign; open arrow), bronchial wall thickening and mucus plugging of the small airways (closed arrow).

 
Overall, cystic fibrosis is the commonest cause of bronchiectasis in children in the UK. Acute infective exacerbations may cause additional morbidity in patients with cystic fibrosis. Although the CT changes in the paediatric population are less well defined, the demonstration of an air-fluid level on CT is the only reliable sign of an acute infective exacerbation in adults⁴⁸. Other features such as centrilobular nodules, mucus plugging and peribronchial thickening may also be reversible, but are not exclusive to patients with an acute exacerbation⁴⁸.

Recurrent aspiration may result in segmental or lobar pneumonia, lung abscess or empyema⁴⁹. A common distribution of consolidation on CT in the supine patient is in the dependent lung, namely the posterior segments of the upper lobes and the superior and posterior basal segments of the lower lobes. Recurrent pneumonias in neonates may be due to a congenital tracheo-oesophageal fistula, whilst the inhalation of foreign bodies in older children results in lobar or segmental hyperinflation due to obstruction or atelectasis⁴⁹.

Developmental abnormalities of the lung such as pulmonary sequestration, congenital cystic adenomatoid malformation (CCAM) and bronchogenic cysts may result in recurrent pneumonias affecting a particular lobe. The lower lobes are most commonly affected and in pulmonary sequestration CT shows cysts, air-fluid levels, areas of decreased attenuation and a systemic arterial supply, and cystic adenomatoid malformations are characterized by multiseptated air-fluid filled cysts.

Sequelae of respiratory infections

Obliterative bronchiolitis (small airways disease)
Inflammatory damage to the small airways is a common occurrence and probably represents the most common injury sustained by the human lung⁵⁰. In children, the most frequently encountered scenario is postviral infection, in particular adenoviral infection, and postmycoplasma infection (Fig. 7). Swyer-James' syndrome or Macleod's syndrome is a variant of postinfectious constrictive obliterative bronchiolitis with special features: by definition, the small airways disease as demonstrated by the chest radiograph is mainly unilateral giving rise to the cardinal sign of unilateral hypertransradiancy.

View larger version (103K): [in this window] [in a new window]   Fig 7. HRCT of a child with postviral obliterative bronchiolitis. Note the differing attenuation of the lung parenchyma (mosaic attenuation pattern) and bronchial wall thickening in both lower lobes (arrows).

 
Chest radiography is poor at demonstrating even advanced obliterative bronchiolitis⁵¹, whereas HRCT can demonstrate indirect signs, although the airways themselves are too small to be visualised directly by current HRCT technique⁵². The key HRCT signs are areas of parenchymal decreased attenuation (giving rise to the so-called mosaic attenuation pattern), pulmonary vascular abnormalities, bronchial abnormalities and air trapping on expiratory CT (Fig. 7)⁵² .

Decreased attenuation is more easily identified using end-expiratory images, which may reveal subtle areas of air trapping due to complete or partial airway obstruction. Air trapping has been defined⁶ as ‘decreased attenuation of pulmonary parenchyma, especially manifest as less than normal increase in attenuation during expiration’. Depending on the age and comprehension of the child, voluntary breath-holding at end-expiration may be feasible. In young children, images are often obtained at different phases of the respiratory cycle, sometimes providing incidental end-expiratory images. Alternatively, dynamic studies may be obtained with CT scanners capable of very fast acquisition times³. However, the majority of children with air trapping also exhibit abnormalities on inspiratory images and, due to the additional radiation burden, expiratory CTs are not routine in children in our institution.

Interstitial lung disease
Lower respiratory tract infections may result in remodelling of pulmonary architecture due to interstitial alveolar pneumonitis resulting in a variable extent of pulmonary fibrosis⁴⁶. This is a very unusual sequel of respiratory tract infection, but has been described with chronic PCP infection⁴⁶. The HRCT features are of parenchymal distortion and cystic pulmonary destruction.

Although not considered a true ‘interstitial’ lung disease by some authorities, organizing pneumonia appears to represent a postinflammatory response and may occur after viral, bacterial or mycoplasma infections in children. Organizing pneumonia is characterized on CT by multifocal areas of consolidation which often have a subpleural or peribronchovascular distribution.

	Conclusions

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Radiation dose is an important consideration in children and, therefore, CT does not have a role in the investigation of a single, uncomplicated lower respiratory tract infection in an immunocompetent child. The HRCT features of lower respiratory tract infections in children often do not allow a definitive diagnosis as to the pathogen involved. However, a simplistic approach is that viral infections are characterized by ground-glass opacification, whereas bacterial infections are usually associated with areas of consolidation, although there may be considerable overlap of these features. The real advantages of CT over chest radiography are in the following circumstances: (i) when a complication such as pulmonary abscess is suspected; (ii) to exclude an underlying abnormality in recurrent infections (e.g. pulmonary sequestration); (iii) the investigation of the immunocompromised child; (iv), to guide the type and site of tissue sampling; and (v) to assess the sequelae of respiratory infection (e.g. postviral obliterative bronchiolitis).

	Key points for clinical practice

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 

• • Radiation dose is an important consideration in children
  
• • It is usually not possible to make a definitive diagnosis as to the pathogen responsible for a lower respiratory tract infection using CT
  
• • CT does have an important role in specific circumstances, e.g. investigation of the immunocompromised child
  

	Acknowledgements

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Thanks to Dr Stephen Evans, Medical Physics Department, Royal Marsden Hospital, London, UK for his help and advice.

	Footnotes

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 
Correspondence to: Dr S J Copley, Department of Radiology, Hammersmith Hospital, Du Cane Road, London W12 0HS, UK

	References

Top Footnotes Abstract Introduction CT technique in children The role of CT... Applications of HRCT in... Conclusions Key points for clinical... Acknowledgements References

 

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