oreign body aspiration is a frequently
encountered pediatric emergency and a common cause of accidental death,
with incidence peaking at two years of age [1]. More commonly,
aspiration is not fatal and symptoms are nonspecific. This often leads
to delayed diagnosis and increased morbidity. Therefore, imaging
evaluation and familiarity with the imaging findings in foreign body
aspiration is essential.
Three main imaging modalities that are currently
utilized in the evaluation of suspected tracheobronchial foreign body
are radiography, fluoroscopy and computed tomography (CT). Radiopaque
foreign bodies are easily identified on any of these modalities.
However, 90% of aspirated foreign bodies are radiolucent and detection
is often difficult [2,3]. Plain radiographic findings of radiolucent
tracheobronchial foreign bodies rely on secondary phenomena, including
air trapping, atelectasis and consolidation. Unlike easily identifiable
radiopaque foreign bodies, radiolucent foreign bodies are much more
difficult to detect and chest radiographs are frequently interpreted as
normal, with standard anterior-posterior (AP) and lateral chest
radiographs having a reported sensitivity of 55%, specificity of 50%,
positive predictive value (PPV) of 80% and negative predictive value
(NPV) of 23% [4]. Inspiratory and expiratory radiographs in older
cooperative children and bilateral decubitus radiographs in younger
non-cooperative children are often performed in an attempt to improve
diagnostic accuracy. However the utility of these additional views has
been questioned [4,5]. Assefa, et al. [4] found lateral decubitus
radiographs to have a sensitivity of 27%, specificity of 67%, PPV of 75%
and NPV of 20%. Brown, et al. [5] reported that the addition of
decubitus views increased false positives by decreasing the specificity
of standard chest radiographs without changing the sensitivity, and
adding expiratory views increased the number of true positives without
reducing the number of false positives. Fluoroscopy has traditionally
been used to better detect secondary signs of air trapping by
visualizing contralateral mediastinal shift and ipsilateral decreased
diaphragmatic excursion [6], however, interpretation is
operator-dependent and is increasingly being replaced by CT [7].
Findings of air trapping, atelectasis and consolidation are easily and
accurately demonstrated by CT [8], and CT has a reported sensitivity
close to 100% and specificity between 66.7% and 100% in studies
utilizing cine CT and 3D volume-rendered imaging of the large airway
[2,9,10]. However, CT must be utilized judiciously given concerns about
radiation exposure particularly in the pediatric population, and plain
radiographs remain the first line imaging modality.
In this issue of Indian Pediatrics, Song,
et al. [11] take a novel approach to the interpretation of the chest
radiograph in cases of foreign body aspiration and apply a quantitative
tool to measure lung radiodensity in order to objectively assess the
radiolucency caused by air trapping. The authors retrospectively
reviewed standard AP chest radiographs of 59 children who had confirmed
tracheobronchial foreign bodies seen on bronchoscopy, and found that in
cases of bronchial foreign body, the radiodensity of the ipsilateral
lung is significantly lower than the contralateral lung. Thirty patients
also underwent CT, and density measurements on chest radiograph
correlated well with density measurements on CT. An objective and
quantitative tool, such as the one that the authors of this article
suggest, has the great potential to improve diagnostic accuracy and
confidence in cases of suspected bronchial foreign body with unilateral
air trapping. This study is particularly interesting because it
investigates a way to improve accuracy of the simple plain radiograph,
but further investigation is needed to determine whether this
quantitative density metric can prospectively aid in the diagnosis of
aspirated foreign body. Much recent work has directed focus on CT, and
has shown that new tools like cine CT imaging, 3-D reconstructions and
4-D imaging can be very useful [2,8-10]. As these tools become more
widely available, they will certainly play a larger role in the
diagnosis of tracheobronchial foreign body. However, they must be used
judiciously given concerns about radiation exposure. Due to these
concerns, MRI is an attractive modality for future investigation given
its lack of radiation. Several papers in the 1990s described the use of
MRI in diagnosis of radiolucent tracheobronchial foreign bodies [12-15];
however little work has been published on this topic in the past 15
years, and this is an important direction for future research.
In summary, foreign body aspiration is a frequent
cause of morbidity and mortality in children. Imaging plays an important
role in the diagnosis and management of tracheobronchial foreign bodies.
Plain radiographs are the first line imaging modality, but they have a
well-known limited diagnostic accuracy when foreign bodies are
radiolucent. Song, et al. [11] describe a quantitative measure of
lung radiodensity which can be used to detect air trapping in cases of
aspirated radiolucent foreign body. This technique may improve the
diagnostic accuracy of plain radiographs and have a potential beneficial
effect on diagnosis and management of this common pediatric condition.
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