AJR 2000; 174:235-241
© American Roentgen Ray Society
Expiratory Chest CT in Children
Javier Lucaya1,2,
Pilar García-Peña2,
Lilian Herrera2,
Goya Enríquez2 and
Joaquim Piqueras2
1
Institute of Diagnostic Imaging and Hospital Materno-infantil, Vall d'Hebron
Hospitals, ps Val d'Hebron 119-129, E-08035 Barcelona, Spain.
2
Department of Radiology, Hospital Materno-infantil, Vall d'Hebron Hospitals,
E-08035 Barcelona, Spain.
Received April 1, 1999;
accepted after revision June 17, 1999.
Address correspondence to J. Lucaya.
Introduction
Expiratory chest CT can improve recognition of anomalies not seen on
inspiratory examinations [1].
Whenever we study cooperative patients with clinical features or CT findings
suggestive of air trapping, we complete the examination with three expiratory
slices: one in the upper, one in the middle, and one in the lower third of the
chest. A useful method of obtaining expiratory CT scans in patients not
following breath-holding commands is to use the lateral decubitus technique,
which Capitanio and Kirkpatrick
[2] describe for conventional
chest radiography. When the child is placed on one side, the dependent
hemithorax will be on expiration and the hemithorax facing up will be on
inspiration (Fig. 1). This
method can be applied to perform expiratory or inspiratory chest CT scans.
When air trapping is suspected, we image one or two slices of the suspicious
portion of the lung with the patient in the lateral decubitus position. If air
trapping is present, the dependent lung, lobe, or segment will remain
hyperlucent (Fig. 2A,
Fig. 2B). In this pictorial
essay, we review the chest CT features of several pediatric lung disorders in
which expiratory radiographs provide information of diagnostic value.
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Fig. 1. —12-month-old girl with previous episodes of bronchiolitis. CT scan
with patient in left lateral decubitus position shows normal findings. When
left lung is in full expiration, right lung is well aerated.
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Fig. 2 .—4-year-old boy with immunodeficiency and repeated episodes of left
lower lobe pneumonia.
A, Inspiratory CT scan with patient supine shows mosaic perfusion
pattern in both lower lobes and questionable bronchiectasis in left lower
lobe.
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Fig. 2 .—4-year-old boy with immunodeficiency and repeated episodes of left
lower lobe pneumonia.
B, Right lateral decubitus chest CT scan at same level as A
confirms air trapping in right middle and lower lobes (arrows) and
improves depiction of left lower lobe bronchiectasis.
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Congenital Malformations
Air trapping is a common feature in several pulmonary malformations. Type I
or type II cystic adenomatoid malformations usually present multiple
thin-walled air- or fluid-filled cysts
[3]. These lesions will often
present air trapping on expiration and will, therefore, be easier to detect
with expiratory scans (Fig. 3A,
Fig. 3B).
Bronchial atresia, which most frequently involves the left upper lobe,
obviously impedes ventilation of the affected part of the lung. Chest CT will
usually show hyperexpansion of the affected lobe with some central densities
corresponding to mucoceles. Occasionally, air-or air-and-fluid-filled cysts or
tubular densities or both will be seen
[4]. Expiratory examinations
facilitate identification of the air trapping distal to the atretic bronchus
(Fig. 4A,
Fig. 4B).
An appearance simulating emphysema can be seen in pulmonary sequestration.
It has been suggested that this appearance results from collateral ventilation
and air trapping [5]
(Fig. 5A,
Fig. 5B). Lobar emphysema
usually presents on the radiograph as overdistended upper or right middle
lobes. An expiratory CT scan is helpful in delineating the extent of the
malformation (Fig. 6A,
Fig. 6B).
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Fig. 5 .—15-year-old boy with intralobar pulmonary sequestration in left
lower lobe.
A, Enhanced CT scan shows hyperlucency in left lower lobe. Systemic
vessel (arrow), originating from aorta and feeding sequestration, is
well defined.
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Fig. 5 .—15-year-old boy with intralobar pulmonary sequestration in left
lower lobe.
B, Expiratory high-resolution CT scan at same level as A
shows significant air trapping within sequestered lung.
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Fig. 6 .—9-year-old asymptomatic boy with long-standing lobar emphysema of
right upper lobe.
B, Expiratory CT scan at same level as A. Emphysema is better
delineated. Note decreased vascularity of affected lobe. This finding, seen on
inspiratory or expiratory scans, should suggest that associated emphysema is
obstructive rather than compensatory.
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Bronchiolitis Obliterans
Bronchiolitis obliterans is a lung disease characterized by granulation
tissue that may lead to extensive scarring and obliteration of the small
airways [6]. In children most
cases of bronchiolitis obliterans are of infectious origin and occur after
pulmonary infections by adenovirus, measles, pertussis, tuberculosis, and
mycoplasma. Other causes include toxic gas inhalation, connective tissue
disease, drug reaction, and chronic graft-versus-host disease in patients
after transplantation [7]. A
mosaic perfusion pattern of lung attenuation, air trapping, peribronchial
thickening, and bronchiectasis is the most common CT feature of bronchiolitis
obliterans. Expiratory chest CT can improve the detection of this disease
(Figs. 7A,
Figs. 7B, 8A,
8B,
9A,
9B). Furthermore, it may
reveal bilateral pulmonary involvement where chest radiographs and inspiratory
CT scans had shown unilateral disease
(Figs. 8A,
Figs. 8B and
9A,
9B).
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Fig. 7 .—6-year-old boy with postinfectious bronchiolitis obliterans.
A, Inspiratory CT scan shows hyperlucent left lung with left lower
lobe bronchiectasis and mosaic perfusion pattern of lung attenuation in both
lungs.
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Fig. 9 .—11-year-old girl with cystic fibrosis. Chest CT scans performed 6
months after bilateral lung transplantation.
B, Expiratory CT scan at same level as A shows mosaic
perfusion pattern. This pattern is common CT finding in children after lung
transplantation and, although it may be seen in asymptomatic patients, should
be regarded as suggestive of bronchiolitis obliterans.
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Asthma and Reactive Airways Disease
Asthma is usually diagnosed on the basis of clinical findings, and
high-resolution CT (HRCT) is not indicated. Sometimes children with asthma
will present with recurrent bouts of pulmonary infection, and HRCT will be
requested to show or rule out predisposing pulmonary abnormalities. Common CT
findings in asthma include bronchial wall thickening, cylindric
bronchiectasis, small centrilobular opacities, a mosaic perfusion pattern of
lung attenuation, mucoid impaction, and right middle lobe collapse. Expiratory
HRCT can show air trapping in asthma patients who have normal findings on
inspiratory scans (Fig. 10A,
Fig. 10B). Contrary to what
occurs in patients with bronchiolitis obliterans, these areas of air trapping
can completely disappear on CT scans obtained after therapy with
bronchodilators.
Bronchiectasis
HRCT is more sensitive than unenhanced radiography or conventional CT scans
and is the imaging technique of choice for the diagnosis of bronchiectasis.
Airway obstruction distal to classic postinfectious bronchiectasis is
extremely common; therefore, areas of pulmonary hyperexpansion with air
trapping on expiration will almost always be seen in the lobes harboring the
bronchiectasis [8]. Sometimes,
when interpreting the HRCT scans, one may doubt whether some of the visualized
linear densities correspond to normal vascular structures or to mucous-filled
dilated bronchi. In such instances, identification of associated air trapping
will favor the diagnosis of bronchiectasis
(Fig. 11A,
Fig. 11B). In the follow-up
HRCT scans of patients with cylindric bronchiectasis responding to longterm
therapy, it is not unusual to observe persistent areas of air trapping on
expiration long after the previous bronchiectasis has become unidentifiable
(Fig. 12A,
Fig. 12B).
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Fig. 12 .—11-year-old boy with left lower lobe bronchiectasis treated with
long-term antibiotics and physiotherapy. Patient was clinically well.
A, Inspiratory CT scan shows slight hypoattenuation of left lower
lobe.
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Fig. 12 .—11-year-old boy with left lower lobe bronchiectasis treated with
long-term antibiotics and physiotherapy. Patient was clinically well.
B, Expiratory CT scan at same level as A shows significant
air trapping.
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Mediastinal Masses
Some mediastinal masses, particularly those located in the middle
mediastinum, can compress the adjacent bronchi and, depending on the degree of
bronchial obstruction, cause either pulmonary collapse or obstructive
emphysema (Fig. 13A,
Fig. 13B,
Fig. 13C). One or two
expiratory CT slices of the suspected site may reveal the presence of
obstructive emphysema, suspected with inspiratory scans. Expiratory slices
should not be obtained routinely.
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Fig. 13 .—12-year-old girl with neuroectodermal tumor of right thigh that had
been treated with chemotherapy. One year after initial diagnosis, she
developed chest pain. Unenhanced radiograph (not shown) showed left upper lobe
collapse.
A, Chest CT scan shows left upper lobe collapse and increased
hypoattenuation of left lower lobe. Reduced number of vessels in left lower
lobe suggests obstructive emphysema.
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Fig. 13 .—12-year-old girl with neuroectodermal tumor of right thigh that had
been treated with chemotherapy. One year after initial diagnosis, she
developed chest pain. Unenhanced radiograph (not shown) showed left upper lobe
collapse.
B, Expiratory chest CT scan reveals obstructive emphysema of left
lower lobe.
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Fig. 13 .—12-year-old girl with neuroectodermal tumor of right thigh that had
been treated with chemotherapy. One year after initial diagnosis, she
developed chest pain. Unenhanced radiograph (not shown) showed left upper lobe
collapse.
C, Mediastinal contrast-enhanced chest CT scan shows multiple
mediastinal lymph nodes (arrows) completely obstructing left upper
lobe and partially obstructing left lower lobe bronchi. Note significant
enhancement of collapsed left upper lobe (arrowheads).
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Systemic Diseases
Cystic Fibrosis
HRCT provides more information than conventional radiographs. We usually
examine patients with cystic fibrosis with limited-slice low-dose HRCT. One of
the earliest HRCT findings in cystic fibrosis is the presence of sharply
defined lobular areas of decreased attenuation, presumably representing
lobular or subsegmental air trapping, which can be seen only on expiratory
HRCT scans (Fig. 14A,
Fig. 14B). Other early
findings include peribronchial thickening, bronchiectasis, and bronchiolar
impaction or "tree-in-bud."
Langerhans' Cell Histiocytosis
Langerhans' cell histiocytosis of the lung is an idiopathic disease usually
occurring in children and young adults. In most patients, HRCT scans show
peribronchial and centrilobular nodules of 1-5 mm in diameter or air-filled
thinwalled cysts that are usually smaller than 10 mm in diameter or both.
Because the normal lung becomes dense on expiration, expiratory slices can
facilitate visualization of small cysts
(Fig. 15A,
Fig. 15B).
In our experience additional expiratory slices should only be obtained in
cases in which the radiologist thinks such slices can provide information not
clearly depicted on the routine CT scans. To minimize the radiation dose, we
usually obtain these slices using a low-milliamperage (34-50 mAs) HRCT
technique.
Acknowledgments
We thank C. O'Hara for her help with manuscript preparation.
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