Journal of the Formosan Medical Association
Volume 110, Issue 12 , Pages 744-749, December 2011

Computed tomography of children with pulmonary Mycobacterium tuberculosis infection

  • Steven Shinn-Forng Peng

      Affiliations

    • Department of Medical Imaging, National Taiwan University Hospital and Medical School, Taiwan
    • Department of Radiology, Medical School, National Taiwan University, Taiwan
    • Corresponding Author InformationCorresponding author. 7 Chung-Shan South Road, Taipei 100, Taiwan.
    • ,
  • Pei-Chun Chan

      Affiliations

    • Center of Disease Control, Department of Health, Executive Yuan Republic of China, Taiwan
    • ,
  • Yeun-Chung Chang

      Affiliations

    • Department of Medical Imaging, National Taiwan University Hospital and Medical School, Taiwan
    • Department of Radiology, Medical School, National Taiwan University, Taiwan
    • ,
  • Tiffany Ting-Fang Shih

      Affiliations

    • Department of Medical Imaging, National Taiwan University Hospital and Medical School, Taiwan
    • Department of Radiology, Medical School, National Taiwan University, Taiwan

Received 7 November 2011; received in revised form 10 November 2011; accepted 21 November 2011. published online 26 December 2011.

Article Outline

Surveillance and control of tuberculous infection in pediatric patients, especially in those with a contact history, is important to prevent tuberculous infection in the general population. Totally 26 patients, younger than 14 years of age, who had a diagnosis of pulmonary Mycobacterium tuberculosis (TB), underwent both chest radiographs and computed tomography (CT), which were retrospectively reviewed and compared with those of 20 patients with community-acquired bacterial pneumonia (CABP). TB patients were commonly afebrile and had less cavitating lesions or pleural fluid than CABP patients had. Focal or sub-segmental lung opacities suggested the diagnosis of TB than of CABP. Chest CT could also help to identify enlarged, calcified, necrotic mediastinal lymph nodes, which are less frequently found in CABP and frequently obscured by thymic shadows on chest radiographs of children. Low-dose CT for children or infants suspected to have pulmonary TB infection could help to make the decision of further antibiotic treatment.

Keywords: children, computed tomography, infection, lung, tuberculosis

 

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Introduction 

The incidence of infection by Mycobacterium tuberculosis (TB) has been decreasing gradually in developed countries since 1970. Control of TB in children and inclusion of children with TB under surveillance are essential to prevent TB infection in the general population.1 Children with TB have different pathophysiologic and immunologic responses than adults in many ways.2, 3, 4 It is often difficult to make the diagnosis of TB infection in children with vague complaints and symptoms. The incidence of TB infection in the pediatric population is estimated by indirect indices. The annual natural infection rate of Taiwanese children younger than 14 years of age is about 3–4 per 100,000 in 2002 to 2008.5, 6, 7, 8 The challenge is to reduce the incidence by half.9, 10, 11

The diagnosis of TB in the pediatric population depends on the meticulous and thorough contact tracing, history taking, and relevant investigations including the tuberculin skin test, sputum smear, and chest radiographs.12 The ability to identify TB infection with active disease by imaging is vital to controlling the spread of TB. Chest radiography in posterior–anterior view is the most commonly used modality to evaluate possible pulmonary TB infection. In the presence of history of exposure to active TB and a positive skin test for tuberculin, chest radiographs are usually used to determine whether the children should receive only preventive or curative antiTB medication.13 In the absence of skin tuberculin reaction, the World Health Organization still suggests diagnosing pulmonary TB based on the findings of chest radiographs in symptomatic children with contact history.10 However, subtle lesions of lung parenchyma and mediastinal lymph nodes are more readily disclosed by thoracic computed tomography (CT) rather than chest radiographs. This study aimed to find out whether thoracic CT will demonstrate active lung lesions, pulmonary hilar, and mediastinal lymph nodes of children, unremarkable on chest radiographs. We also tried to explore the difference between TB and bacterial pneumonia on thoracic CT.

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Materials and methods 

In total 26 patients, aged 8±4 (range, 1–14) years, who had a diagnosis of TB confirmed by the Center for Disease Control of Taiwan and underwent both chest radiographs and CT not more than 1 month apart were retrospectively reviewed. All the 26 patients including 12 girls and 14 boys, in the study group had a history of close contact with TB victims. Ten of the 26 patients had TB cultured from sputum. The other 16 patients had pulmonary TB diagnosis according to the consensus of expert meetings. Another 20 patients, aged 4±3 years (10 girls and 10 boys), who received both chest radiographs and CT for complications related to community-acquired bacterial pneumonia (CABP) including Streptococcus pneumoniae (n=17), Staphylococcus aureus (n=3), and Pseudomonas aeruginosa (n=1) and were included in the comparison group.

CT of TB patients was performed on LightSpeed 16 (General Electric) scanner (n=13), Aquilion S16 (Toshiba) (n=8), HiSpeed Nxi (General Electric) (n=4), and Brilliance 16 (Philips) (n=1). All CABP patients received CT examinations on Sensation 64 (Siemens) scanner. The following parameters were used: kVp, 100–120; mA, 10–100; exposure time, 0.5–1s; and slice thickness, 3–5mm without gaps.

Chest radiographs and CT covering all lung fields of 26 TB patients and 20 CABP patients were analyzed by a radiologist and a pediatric infection specialist, and a consensus was obtained. On the chest radiographs, presence of patchy lung opacities, cavitating or nodular lesions, pleural fluid accumulation, pulmonary hilar enlargement (> 2cm in the superior-inferior diameter), and mediastinal widening were examined in both TB and CABP patients. Lung nodular lesions, appearance of patchy lung opacities, cavitations, pulmonary hilar lymph nodes (> 0.6cm), mediastinal lymph nodes (> 0.6cm), and pleural fluid accumulation were also examined on thoracic CT of TB and CABP patients. All the findings on chest radiographs and CT were compared between the TB and CABP patients. In addition, the detection of lung opacities and lung nodules, hilar lymph nodes or enlargement, and mediastinal lymph nodes or widening were compared between chest radiographs and CT in 26 patients with pulmonary TB. All the comparisons were analyzed by Chi-square test, and p<0.05 were considered significant.

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Results 

Fever was more common in CABP (20/20) than in TB patients (6/20) (p<0.01) (Table 1). However, there was no significant gender difference between TB and CABP groups.

Table 1. Findings of the clinical information, chest radiograph and computed tomography (CT) in TB and community-acquired bacterial pneumonia (CABP) patient groups.
TBCABPpOdds ratio for TB
Gender
Male141011.17
Female1210
Age (y)8±44±3<0.01*
Fever (–/+)20/60/20<0.01*4.33
Patches on X-ray (–/+)17/90/20<0.01*3.22
Nodules on X-ray (–/+)22/420/00.120.52
Cavitation on X-ray (–/+)25/115/50.078.33
Pleural fluid on X-ray (–/+)25/12/18<0.01*225
Hilar enlargement on X-ray (+/–)17/90/20<0.01*3.22
Mediastinal widening on X-ray (–/+)2/240/200.501.83
Nodules on CT(–/+)16/1019/10.01*0.08
Inhomogeneous or subsegmental /segmental or lobar lung patches on CT10/10/20<0.01*21
Cavitation on CT(–/+)25/112/8<0.01*16.67
Enlarged hilar lymph nodes (>0.6cm) on CT (+/–)11/152/180.02*12.27
Enlarged mediastinal lymph nodes (>0.6cm) on CT (+/–)10/164/16<0.01*6.4
Pleural fluid on CT (–/+)24/21/19<0.01*228

∗ = p < 0.05.

On the chest radiographs, patchy lung opacities and pleural fluid accumulation were significantly more common in CABP (20/20 and 18/20, respectively) than in TB (9/26 and 1/26, respectively) patients (p<0.01) (Table 1). Pulmonary hilar enlargement (Fig. 1A, 1B) was significantly less common in CABP (0/20) than in TB (17/26; p<0.01) patients.

  • View full-size image.
  • Figure 1 

    Chest posterior–anterior radiograph of an asymptomatic 11-year-old girl with a history of close contact. (A) Before treatment, the inferior lateral aspect of the enlarged right pulmonary hilar shadow bulges (arrows). (B) The right hilar shadow is no longer prominent after 3 months of antiTB treatment.

TB patients more commonly had lung nodules (6/26) than CABP patients (1/20; p<0.01) did on chest CT (Table 1). TB patients (10/11 and 1/26, respectively) more commonly had focal or sub-segmental patches (Fig. 2A) and less cavitating lung lesions on CT than CABP patients (Fig. 2B) (0/20 and 8/20; p<0.01). On CT, lymph nodes at the pulmonary hila or mediastinal regions were more commonly larger than 6mm in diameter in TB patients (11/26 and 10/26, respectively) than in CABP patients (2/20 and 4/20, respectively; p≤0.02). TB patients had significantly more calcified mediastinal (Fig. 3) or pulmonary hilar lymph nodes (9/26) than CABP patients had (0/20; p<0.01). Caseous necrosis (Fig. 3) was evident in three TB patients, but none of the CABP patients had necrotic mediastinal or hilar lymph nodes. In addition, pleural fluid was more common in CABP patients (19/20) than in TB patients (2/26; p<0.01).

  • View full-size image.
  • Figure 2 

    (A) Chest CT of a 10-year-old boy with pulmonary TB. Axial image through both upper lung fields revealed small nodular opacities (arrows) at the left upper anterior and apicoposterior peripheral lung zones. Subsegmental patchy densities (arrowheads) are scattered at the left upper anterior, posterior, and right posterior lung fields. (B) Another boy, aged 6 years, had CABP involving the right upper posterior lung segment, and cavitation with air-fluid level is evident (black arrows).

  • View full-size image.
  • Figure 3 

    Chest CT of a 14-year-old boy in the coronal plane. Tiny calcification densities (arrows) extend along the aortic arch. Rim enhancement suggesting caseous necrosis is evident at the right lower jugular region and right lower and left upper paratracheal regions after use of intravenous contrast medium (arrowheads).

In 13 of 26 TB patients, chest radiographs showed no evident lung lesions or pleural fluid accumulation. Among the 13 patients, eight patients, including two with lung nodules on CT, had enlarged pulmonary hilar or mediastinal lymph nodes on chest CT. Diagnoses in eight of these 13 TB patients were false negative if pulmonary hilar lesions were not detected by chest radiographs or chest CT. In addition, mediastinal widening was evident in 24 of 26 patients, but only 10 patients had enlargement of mediastinal lymph nodes on chest CT (p<0.01). Furthermore, chest radiographs demonstrated significantly less calcified hilar or mediastinal lymph nodes (1/26) than chest CT (9/26; p=0.01).

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Discussion 

The retrospective study focused on the TB patients younger than 14 years of age. TB patients tended to be more commonly afebrile than CABP patients were. The findings on chest radiographs and CT of TB children were different from those in adults with TB,14 including significantly less pulmonary cavitations and pleural fluid accumulation in patients with TB than with CABP. Focal or sub-segmental lung opacities and enlargement of pulmonary hilar or mediastinal lymph nodes on chest CT more suggested the diagnosis of TB than of CABP. Chest CT could also help to identify enlarged mediastinal lymph nodes, which were frequently obscured by thymic shadows on chest radiographs of children. In addition, calcified or caseous necrotic lymph nodes were only shown in TB patients but not in CABP patients. Furthermore, intrathoracic calcified lymph nodes were more readily revealed by chest CT than by chest radiographs.

Prepubertal pulmonary TB patients tend to have an image pattern of so-called primary TB characterized by mediastinal lymph nodes and involvement of mid or lower lung fields.15, 16, 17 In young children, the cell-mediated immunity is usually less efficient or effective than that of adults2, 3, 4 and cannot restrain the pulmonary alveolar tuberculous infection.18, 19 However, many children with TB infection are relatively asymptomatic, and tuberculin test cannot reliably distinguish between latent and active TB infection.20, 21 The skin reaction can lag far behind the full-blown radiographic pictures in very young patients with fulminating active TB.21, 22, 23, 24 The microbiological diagnosis of primary pulmonary TB is usually difficult in young children who do not expectorate sputum,24, 25 and in whom paucibacillary TB is common. The diagnosis of sputum smear-negative TB frequently relies on the finding of enlarged hilar shadows on a chest radiograph.10 The assessment of diagnostic accuracy of chest radiographs in this study is of clinical significance.

Immunocompetent patients with CABP tend to have high fever. The most common radiographic or CT imaging findings in CABP are air-space consolidation with lobar or segmental distribution, especially in pneumococcal pneumonia.26, 27 In patients with nonresolving or relapsing pneumonia after 4 weeks of effective antibiotic treatment, alternate diagnosis including TB should be taken into differential list.28 Enlargement of intrathoracic lymph nodes are not unusual in patients with CABP but less common than that in TB patients.29, 30 In addition, calcification or caseous necrosis of intrathoracic lymph nodes on thoracic CT is predominantly found in TB patients but seldom in CABP patients.31, 32, 33, 34 Enlarged intrathoracic lymph nodes containing calcification density or caseous necrosis usually suggest the diagnosis of TB in children, but it is difficult to find out on chest radiographs.35 Pleural effusion is more common in CABP patients than in children with TB though frequently (20–60%) reactive.36

A standard chest radiograph is not an optimal indicator of pulmonary TB in children because the radiographic findings are usually difficult to interpret.37 The specificity of chest radiographs is about 70%, but the sensitivity remains at about 40%.22, 23, 37 Absence of radiographic evidence of TB usually leads to the adoption of a latent rather than full-course TB treatment in children with contact history.38 However, our study revealed that more than half (8/13) of the infected children without lung or pleural lesions on chest radiographs had enlarged lymph nodes revealed by CT. Although recognition of asymptomatic infected children without lung or pleural lesions is frequently difficult, pulmonary hilar enlargement could be the only abnormal finding related to TB and chest CT that can help to identify involvement of pulmonary hilar lymph nodes.39 In fact, hilar and mediastinal lymph nodes are usually seeded with bacilli during the initial infection.13 If left untreated, progressive involvement of lung fields and more severe extrapulmonary spread can happen in young children.13, 40

CT scan is more preferred for mediastinal evaluation of TB patients. Airway compression by enlarged mediastinal lymph nodes is especially common in children younger than 5 years of age.41, 42 For those with symptoms attributed to airways, chest CT may be suggested to identify enlarged mediastinal lymph nodes compressing on the major airways of afebrile patients. CT of the chest after contrast injection may help to identify enlarged lymph nodes and lung nodules that are suggested or overlooked by chest radiographs.34, 43 The long delay from the onset of infection to diagnosis TB may be avoided and antiTB treatment can commence as soon as possible in order to cut down the rate of transmission if chest CT is performed appropriately in time.

However, a traditional chest CT examination applies approximately 1–2mSv, which is about 3 to 20 times of the radiation dosage used for chest radiography. If low-dose exam was used, CT exposure can be cut down to about 0.3–0.8mSv or to two times a chest radiography dosage.44, 45 Since it is time to deal with the important issue of children’s TB, 46low-dose CT in conjunction with the suitable good contrast injection can help to demonstrate lung hilar and mediastinal lymph nodes to provide better treatment reference.

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PII: S0929-6646(11)00128-8

doi:10.1016/j.jfma.2011.11.003

Journal of the Formosan Medical Association
Volume 110, Issue 12 , Pages 744-749, December 2011

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