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Imaging has long been important in the evaluation, diagnosis,
management and follow-up of pediatric chest diseases; and it contributes
greatly to our understanding of the pathophysiology of many entities.
The chest is no exception to the increased sophistication and complexity
of imaging techniques, with a myriad of available options involving
a bewildering array of modalities and choices. This section will
provide an overview of these options and their advantages, disadvantages,
and most appropriate use in the pediatric population.
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Children are often unwilling or unable to cooperate with the
maneuvers required to produce high quality imaging. Optimal studies
are most often the product of experienced personnel who work with
children in friendly surroundings with appropriate distraction,
immobilization, analgesic, and sedation techniques.
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Although plain film radiography remains the mainstay for chest
imaging, cross-sectional techniques provide remarkably detailed
images with versatile multiplanar and 3D reconstruction capabilities,
sophisticated angiographic and Doppler techniques, and functional
imaging even at the molecular level. It has become increasingly
important to tailor the imaging studies to the specific clinical
questions and concerns or limitations of individual patients. Imaging
is no longer “one size fits all” and close collaboration
between radiology and clinical colleagues is not just desirable
but essential in making the most appropriate choices and providing
the best patient care.1-12
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Chest Radiographs and
Chest Fluoroscopy
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Plain radiographs of frontal and lateral views of the chest remain the
mainstay of screening imaging of a child with a suspected chest abnormality.
Digital imaging systems are now widely available. These radiographs
usually include portions of the upper airway, larynx, trachea, and
central bronchi as well as the mediastinal structures, heart, lungs,
and bony and soft tissue chest wall. Evaluation of a chest radiograph
should include an overview of the following:
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1. Patient history and specific reason for the current
radiograph
2. Quality of the study: patient positioning, inspiratory effort, motion,
or presence of artifacts (eg, jewelry)
3. Overall pulmonary aeration and symmetry, position, and patency
of the airway
4. Presence and location of any tubes, lines, catheters, foreign
bodies
5. Mediastinum: contour and position; location of the aortic
arch; presence of any mass, displacement, or compression of the
mediastinum and/or airways
6. Heart size and configuration
7. Central and peripheral pulmonary vessels and symmetry
8. Lung parenchyma: symmetry, vascularity, lucency, density,
mass, volume loss, or air trapping
9. Pleura, diaphragm, and bony and soft tissues of the chest
wall
10. Comparison with prior related imaging studies
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Whenever possible, two views of the chest increase diagnostic
confidence, determining the location of a lesion more precisely
and assessing its impact on adjacent structures such as the diaphragm or
airway. Some lesions may look quite different on the frontal and lateral
views and others may only be appreciated on one view or the other. It
is important to be aware that the normal thymus is much larger in
infancy (eFig. 506.1).
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Radiographs of the neck, with both frontal and lateral projections,
are often helpful in evaluating the upper airway for an intrinsic
or extrinsic mass, foreign body, or inflammation such as croup or
epiglottitis. High kilovoltage magnified filtered radiographs may
be used to obtain a more detailed view of the airway and its relationship
to other structures such as abnormal vessels or masses.
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Inspiratory and expiratory radiographs may be useful when evaluating
for air trapping, as is seen with an aspirated foreign body (see eFig. 506.2A), or pneumothorax (see eFig. 506.3A); these are limited to children
who are able to breath-hold. Decubitus views of the chest do not
require cooperation for inspiratory and expiratory breath-holding
and are more useful for infants and young children. In this circumstance,
the dependent lung normally deflates while the opposite lung inflates; the
expected deflation of the dependent lung is not seen when a foreign
body is present. Decubitus views are also useful for evaluating
suspected nonloculated pleural effusion, which layers dependently,
or pneumothorax, which rises superiorly. However, decubitus views
are of no value in assessing for pleural effusion if the lung in question
is completely opacified, as any fluid present cannot be distinguished
from the adjacent, completely opacified hemithorax.
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Fluoroscopy of the chest and airways is useful, because the dynamic
evaluation airway caliber and can demonstrate expiratory airway
collapse in tracheo- or bronchomalacia. Fluoroscopy is also helpful
in evaluating for the presence of an airway foreign body when plain
films are not definitive and for assessing motion of the diaphragm.
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A fluoroscopic contrast esophagram still has significant value,
including evaluating swallowing function, aspiration of liquids
or solid materials, the presence of a tracheoesophageal fistula,
esophageal foreign body and other masses, as well as esophageal
stricture or dysfunction (see eFig. 506.2A).
An esophagram/upper gastrointestinal study has only an
adjunctive role in evaluating gastroesophageal reflux, and it is not
the preferred test to establish this diagnosis. With regard to extrinsic
masses, especially vascular lesions, the esophagram has a limited
role.
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When a vascular ring is suspected as the cause for clinical symptoms,
the best approach is usually to first obtain frontal and lateral
chest radiographs. The presence of a right sided aortic arch along
with anterior bowing of the trachea on the lateral view is highly
suggestive of the presence of a vascular ring. Computerized tomography
(CT) or magnetic resonance imaging (MRI) provides exquisite detail
of the precise anatomy of these lesions. Although an esophagram
may confirm the likelihood of a vascular ring, it usually does not
sufficiently define the exact anatomy and is probably an unnecessary
additional study.
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Ultrasound is especially useful to image the pleural space and
thoracic wall, as the air-filled lungs reflect and distort the ultrasound beam
(see eFig. 506.3C). Sonography can evaluate
diaphragmatic motion, especially in children who cannot easily be transported.
It is also useful in evaluating the amount and nature of pleural
fluid collections. The septations and loculations of exudative effusions
are usually better appreciated on ultrasound than other cross sectional
imaging modalities such as computerized tomography (CT). Ultrasound
can also be used to guide percutaneous drainage of pleural fluid.
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Ultrasound imaging is of some use in evaluating chest masses,
particularly lesions that abut the mediastinum or diaphragm. Ultrasound
is particularly useful in differentiating normal thymus from a mediastinal
mass or fluid collection and has the advantage of ready availability,
lack of ionizing radiation or need for sedation. Although the nature
and anatomic extent of a mass may be better defined on CT or magnetic
resonance imaging (MRI), ultrasound (US) and Doppler screening will
often be helpful in determining whether a lesion is cystic, solid
or vascular, and direct further appropriate imaging. Pericardial
effusion and complex cardiovascular anatomy and function are regularly imaged
and thoroughly evaluated by ultrasound. Depending on local preference,
US, CT, and/or MRI are utilized for image guidance of interventional
procedures such as pericardial drainage and biopsies.
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Computed Tomographic
Imaging
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Computed tomography (CT) of the chest provides far more anatomic detail
than plain radiographs. The use of chest CT has expanded to include
the evaluation of trauma, unusual air collections or densities,
nodules, masses and metastases, suspected pulmonary emboli, vascular
and airway abnormalities, congenital lung anomalies, complicated
or unusual infection, and suspected interstitial lung disease or
bronchiectasis (see eFig. 506.3B). The image
acquisition algorithm may vary depending on the clinical question,
however the images are usually reconstructed so that they can be
optimally viewed for soft tissue (high contrast resolution algorithm),
lung parenchyma, or bone (higher spatial resolution algorithms).
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Modern scanners and computer software now permit image reconstructions
in planes other than the axial plane to which early scanners were limited.
Coronal reconstructions are particularly helpful for lesions especially
in the paramediastinal and paradiaphragmatic regions. Curved planar
reconstructions, maximum and minimum intensity projections, three-dimensional
(3D) volume and surface reconstructions, and “virtual bronchoscopy” programs
are available to display complex anatomy. These reconstruction options require
thin section contiguous slices and are usually performed on an independent
workstation. Subcentimeter volumetric imaging of the entire chest
provides exquisite anatomic detail.
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Modern multidetector CT scanners image the entire chest in just
a few seconds with much less need for patient sedation or anesthesia
than previously. However, breath-hold imaging may be critical and
can only be obtained in a cooperative patient or in an infant who
is sedated or anesthetized. Newer techniques such as controlled
ventilation or spirometry-controlled CT imaging provide excellent
motion-free images of the chest. A controlled ventilation study
relies on imaging during the brief period of apnea that can be induced
in infants following a period of hyperventilation.
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With the many options available for CT imaging, it is increasingly critical
that there is clear communication between clinical and radiologic
colleagues to provide an optimal study. Options include contiguous
thicker slices (typically 3–5 mm, depending on patient size)
or very thin or overlapping volumetric slices (as small as 0.6 mm
with a 50% overlap) when detailed 2D or 3D anatomic reconstructions
are needed. High-resolution chest computed tomography (HRCT) usually
results in thin (1 mm) slices with gap intervals (5–10
mm) and might include both inspiratory and expiratory imaging to
evaluate for lung parenchymal or interstitial disease, bronchiectasis
and expiratory air trapping.
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Depending on the clinical situation, when contiguous images are desired,
thinner slices can often be reconstructed from thicker cuts, with
some loss of spatial resolution detail (eFig.
506.4). The limits of reconstruction depend on the original
detector array used (eg, on the Siemens 64 MDCT scanner, when the
1.2-mm detector array is utilized, slices acquired at 3–5-mm
collimation can be retrospectively reconstructed to 1.2 mm or as
thin as 0.6 mm when the 0.6-mm detector array is used).
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Intravenous dynamic contrast enhancement is often necessary in children,
particularly when evaluation of mediastinal or paramediastinal structures
is required. Contrast enhancement is usually needed when evaluating
vascular lesions, mediastinal masses, adenopathy, and infectious
and neoplastic lesions that involve the chest. HRCT, however, is
usually obtained without IV contrast. Computed tomographic angiography
(CTA) is an emerging technology for improved visualization of vascular
structures. By “gating” CTA to the cardiac cycle,
small vascular structures that move with the heart (eg, coronary
arteries) can be evaluated.
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The contrast used for most studies is a low osmolar nonionic
agent with an iodine concentration of 300 mg/ml, administered
as a dynamic volume bolus of 2 cc/kg. For vascular studies,
the concentration can be increased to 350 mg/ml, and if
needed in infants the bolus can be increased to 3 to 4 cc/kg and/or
diluted with saline to provide a longer bolus profile. Children
receiving iodinated contrast intravenously should be well hydrated
before and after the study to protect their renal function. Special
techniques and drug pretreatment may be needed in the face of decreased
renal function and contrast allergy, the details of these are beyond
the scope of this overview.
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Computed tomographic (CT) angiography consists of contiguous thin
cuts (0.6–1 mm) obtained during administration of a large
rapid bolus of contrast (2–5 cc/sec), optimally
administered via a large bore peripheral IV (18–22 g) using a
mechanical injector. One can however produce high quality CTA studies
in tiny newborn infants using IV contrast administered by hand bolus
technique (1 cc/sec). Whatever the administration technique,
specific timing or real time monitoring of the contrast bolus and
triggering of the CT scan is often utilized for optimal visualization
of the desired vascular structures. Central lines are usually difficult
to inject with a rapid bolus and are mostly not compatible with
the use of an injector and are therefore not useful for CTA and
suboptimal as well for most other chest applications where vascular
opacification is desirable. The technique used for CTA generally
does not provide good quality parenchymal lung imaging and is not
useful as a substitute for thin section high-resolution chest computed
tomography (HRCT) imaging.
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Electrocardiogram (EKG) gated CT angiography is available to
assess very small vascular structures that move with the heart,
such as the coronary arteries, and to evaluate cardiac function
in patients who are not able to undergo MR imaging, for example,
in the presence of a pacemaker. The gating technique increases the
radiation dose approximately fourfold over that used for nongated
CTA.
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Ionizing radiation dose is a major consideration in utilizing
chest CT imaging in children. As techniques become more sophisticated
and slices ever thinner, radiation doses have tended to significantly
increase. Computed tomography (CT) is responsible for the largest percentage
of medical radiation dose to the general population. Children are
particularly vulnerable; not only are their tissues intrinsically
more sensitive to radiation, especially certain organs such as the
thyroid gland and breast, but they also have less surrounding soft tissue
to attenuate the radiation so that their organ dose is higher than
a similar study in an adult. Also, they have a longer time to manifest
the possible cancer induction effect of ionizing radiation. The
risk of cancer from a single CT examination is controversial but
quite small (estimated at about 1 in 1000) relative to the overall
lifetime risk of cancer (1 in 5). However, the risk appears to be
dependent on dose and is cumulative with multiple studies. It is
imperative, therefore, that prospective CT imaging be carefully
assessed for risk versus benefit. When appropriate, other modalities
such as ultrasound (US) or MRI could be substituted. The as low
as reasonably achievable (ALARA) principle should be followed in
regard to decreasing radiation dose in children. Imaging sites must
have specific pediatric protocols that adjust imaging parameters
such as kVp, mAs, slice collimation, gantry rotation, and pitch
based on the size and weight of the child and the requirements of
the study. For example, on follow-up imaging lower dose technique
and limited anatomic coverage may be appropriate. Wherever possible,
CT imaging should be limited to a single series; it is usually unnecessary
to obtain both noncontrast and contrasted images. Although it has
been standard to image at 120 kVp for CT in the past, 80 kVp or
100 kVp are suitable for most pediatric uses and produce significantly
decreased radiation dose. Appropriate shielding of radiation sensitive organs
such as orbits, thyroid, breasts, and gonads should always be provided.
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Magnetic Resonance
Imaging
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Although magnetic resonance (MR) imaging avoids ionizing radiation,
image acquisition is much slower than computed tomography (CT) scanning.
Because of this, MR often requires sedation or anesthesia in young
children. Magnetic resonance (MR) is inferior to CT in evaluating
the lung parenchyma, and its spatial resolution is also less than
CT spatial resolution. However, tissue differentiation such as fluid
and fat is often clearly depicted by MR, and as a result it is most
useful in evaluating mediastinal and spinal or paraspinal lesions,
vascular lesions of the chest, and lesions involving the chest wall.
MR is also well established in evaluating cardiac anatomy and function.
In the heart, MR is used to demonstrate anatomy, as well as cardiac
function and vascular flow using EKG-gated techniques. Contrast
MR is also possible using gadolinium as the contrast agent. The
physics of the multiple imaging modes in MR are beyond the scope
of this discussion.
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The major contraindication to the use of MR is the presence of
a pacemaker and intracranial aneurysm clips. In general other metallic
items such as sutures, clips, staples, and coils are not a definite
contraindication to MR. Often MR can be successfully performed in
such cases although metallic artifact can sometimes be quite marked
and obscure anatomy.
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Nuclear Medicine Imaging
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Nuclear medicine scanning still has considerable value, although many
of it’s prior common uses have been supplanted by other technologies.
For example, computed tomography (CT) angiography to evaluate pulmonary embolism
is faster and more sensitive than nuclear medicine ventilation/perfusion
(V/Q) scanning. Ventilation/perfusion scanning, however,
is still used to evaluate a wide variety of pulmonary and vascular
abnormalities, particularly when differentiating vascular from airway
abnormalities. Perfusion scanning and evaluating differential flow
to the lungs is still most often obtained by nuclear medicine although
phase contrast MR imaging can also provide similar information.
Scintigraphic imaging of bone lesions, occult infections, and cardiac
function are also possible, although such studies are not performed
frequently in children.
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Positron emission tomography (PET) and PET/CT scanning
are used less in children than in adults. The most common use for
PET and/or PET/CT is in evaluating and following
select pediatric tumors, lymphoma in particular. In the future,
combined PET/MR imaging will also become available.
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The development of sophisticated, less invasive computed tomography
(CT) and magnetic resonance (MR) angiographic techniques including
gated coronary angiography has replaced conventional catheter angiography
in many instances. There are, however, limitations to CT and MR,
mainly in the lack of detailed visualization of more distal smaller
vascular branches that can be seen with conventional angiography.
Also, additional data such as vascular dynamics, vascular pressure,
and blood gas measurements are more easily done with conventional
catheter angiography. Catheterization angiography is most often
obtained in cases of complex congenital heart disease, vascular
malformations, and for catheter-based interventions. In addition
to the invasive nature of the catheterization procedure and frequent
need for sedation or anesthesia, ionizing radiation dose and intravenous contrast
load are additional concerns.