CHEST RADIOGRAPHY
TECHNICAL ASPECTS
Chest radiography was one of the first clinical studies to use x-rays, which were discovered in 1895 by Wilhelm Conrad Roentgen. X-rays are typically generated by passing a current across a diode, which results in the generation of electrons. The electron beam is aimed at a metal anode, and the resultant interaction produces x-ray photons. The x-ray beam diverges as it exits from the x-ray tube and pro- duces a conical-shaped beam. When x-rays are captured by film or a digital system, the divergence of the beam can lead to geometric distortion, which is a function of the distance from the x-ray source to the object and from the object to the detector. The further an object is from the radiation source, the less geometric distortion and clearer image that will be produced, but higher levels of energy and longer exposure times are required for adequate image production. More energy and longer exposure lead to an increase in radiation expo- sure for the patient. To balance the competing factors of geometric distortion and radiation exposure, 6 feet is considered the standard source-to-image distance for a typical posteroanterior (PA) and lateral chest x-ray. The radiation exposure of a standard PA and lateral chest x-ray at this distance is approximately 3 millirems or approximately 1/100 of the typical annual rate of celestial radiation for an individual. Because ionizing radiation causes a dose-dependent increase in the risk of genetic alteration and malignancy, as low as reasonably allow-able principles of radiation safety are followed to minimize patient exposure.
Anatomic structures with different tissue compositions will produce
varying degrees of absorption, blocking, and disruption of the x-ray photons,
thereby producing shades of gray or contrast in the image. This contrast allows
differentiation of fluid-filled structures (heart and great vessels) from
air-filled lungs and the much denser bony structures of the thorax. The x-ray image is produced as
x-ray photons strike and alter silver iodide crystals in the x-ray film.
Alternatively, digital radiography (filmless technique) can produce images with
a flat plate that directly converts incident photons to a digital signal.
The quality of the technique used in obtaining a chest radiograph can be
appreciated after a quick survey of the film. The key components are a review
of identification, inspiration, penetration, and rotation. Patient identifiers
and radiographic markers should be noted. PA chest film should demonstrate the
diaphragm along the eighth to tenth posterior rib space or the fifth to sixth
anterior rib space. The film should have adequate penetration to allow the
intervertebral disc space of the thoracic spine to be barely visualized but not
overly penetrated, which would obscure bony details of the spine or
identification of pathology within the pulmonary fields. Rotation should be
evaluated by confirmation that the thoracic spine lies posterior to the
sternum, and that the clavicles are at approximately the first anterior rib.
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FIG 8.1 Posteroanterior chest radiograph with corresponding cardiovascular
structures.
NORMAL ANATOMY
The chest x-ray can be useful at detecting structural abnormalities of the heart and great vessels and sequelae
of cardiovascular disease. It is also frequently used to identify endotracheal
tubes, central venous lines, pacemaker leads, and to evaluate for postprocedure
complications (e.g., pneumothorax). To effectively use the chest x-ray for
these purposes requires an understanding of the normal anatomy as it appears on
a chest x-ray. In the PA projection (Fig. 8.1), the cardiac silhouette
typically is <50% of the transverse diameter of the chest. The heart
overlies the spine with roughly one-quarter of the heart at the right of the
spine and three-quarters to the left of the spine. The right border of the
heart is formed inferiorly by the right atrium and superiorly by the superior
vena cava. Superimposed over the superior vena cava is the aortic arch. The
arch traverses the chest and forms the most superior aspect of the left border
of the cardiac silhouette (the aortic knob). The descending aorta usually can
be visualized near midline from the aortic knob to the diaphragm. Inferior to
the aortic knob is the left pulmonary artery, which is usually
indistinguishable from the mediastinum. In some patients, the left atrial
appendage may be found to be inferior to the pulmonary trunk. The inferior
aspect of the cardiac silhouette is made up of the left ventricle as it rests
on the diaphragm.
In the lateral projection (Fig. 8.2), the right ventricle can be found in
the retrosternal space and is the most anterior chamber. Superiorly, the right
atrium is present but usually not well visualized. The main pulmonary artery
arises from the outflow tract of the right ventricle and courses superiorly and
posteriorly. Superior to the pulmonary artery is the aortic arch, which
continues as the descending aorta, and is visualized down to the diaphragm and
forms the most posterior structure of the intrathoracic cavity.
FIG 8.2 Lateral chest radiographs with corresponding cardiovascular structures.
CLINICAL USEFULNESS
The chest x-ray can be helpful in detecting a multitude of cardiovascular
diseases, quick radiographic assessments after procedures, and for identifying
various intracardiac and intravascular devices. Although the chest radiograph
is frequently normal in coronary artery disease, calcifications of coronary
arteries can be noted and correspond to advanced disease. When congestive heart
failure is present, there may be evidence of chamber enlargement, increased
pulmonary vascularity, and occasionally, pleural effusions. Aneurysmal segments
of myocardium, especially if calcified, may also be distinguished by chest
x-ray. The left ventricle normally has a blurry myocardial border due to
cardiac motion during film exposure and the apical fat pad. Aneurysmal segments
may appear as more distinct myocardial borders arising from the left ventricle.
Disease of the aorta may also be noted on a chest x-ray. Calcifications
in the aorta suggest significant atherosclerotic disease. Enlargement of the aorta can be present with
ascending, arch, and descending aneurysmal segments. Aortic dissection may
present as a widened mediastinum. A PA chest x-ray may suggest a widened
mediastinum (note that the anteroposterior [AP] technique typically used for
portable examination may lead to a false suggestion of a widened mediastinum).
Dissection is suggested on an AP film if the mediastinum is >8 cm wide at
the aortic knob or if aortic calcifications are displaced >1 cm from the
border of the aorta.
A chest x-ray can be valuable in diagnosing valvular heart disease. In
aortic stenosis, findings frequently include calcified aortic annulus,
poststenotic aortic root dilation, and left ventricular hypertrophy. Clinically
significant chronic aortic regurgitation can produce left ventricular
enlargement. Mitral valve annulus calcifications can be present in mitral valve
disease and are typically best seen on overpenetrated films. Mitral
regurgitation can lead to left ventricular dilation, and mitral stenosis may
lead to left atrial enlargement. Aortic and mitral annulus calcifications are
best visualized on lateral radiographs. Left atrial enlargement is also best
seen in the lateral examination, whereas left ventricular changes may be
present in both PA and lateral images.
A variety of congenital defects may be suggested by the chest x-ray.
Coarctation of the aorta may be suggested by “notching” of the third to ninth
ribs because of collateral filling of the distal aorta via the internal mammary
arteries. The figure three sign may also be present in coarctation, because the
dilation of the left subclavian artery, dilation of the proximal aorta, and
dilation of the distal aorta may resemble the number 3 on the chest x-ray.
Tetralogy of Fallot typically presents with a boot-shaped heart due to right
ventricular enlargement and may have a right-sided aortic arch. Dextrocardia
and situs inversus may also be noted from
the standard chest x-ray, but left-sided or right-sided annotation markers are
useful in confirming these lesions from a reversed normal film. The “scimitar
sign,” which is a curved radiodensity lateral to the right heart border, can be
seen in anomalous pulmonary venous return. Pericardial disease may also present
with findings on chest x-ray. A globular-appearing cardiac silhouette, or
“water bottle,” appearance with pulmonary congestion is suggestive of
pericardial effusion. Pericardial calcifications can be suggestive of chronic
pericarditis. An eggshell calcification may be more suggestive of a chronic
inflammatory cause, for which latent tuberculosis typically has a denser,
irregular pattern of calcifications.
Portable chest x-ray examinations are frequently used in postprocedural
care and to identify appropriate placement of devices. PA and lateral chest
x-rays are used to ensure appropriate lead placement of implantable pacemakers
and defibrillators. The leads should course under the clavicle and enter the
superior vena cava. The right atrial lead typically will have an upward
deflection as it inserts into the right atrial appendage. The right ventricular
lead will typically be in the right ventricular apex. Complications from lead
placement, such as a dislodged lead or a pneumothorax, can also be noted on a
chest x-ray. Intraaortic balloon pump positioning is frequently checked with a
portable chest x-ray. The superior radio-opaque marker should be at a level 2
cm above the carina. Central venous catheter and pulmonary artery catheter
positioning can be evaluated on a chest x-ray. Central venous catheters should
be positioned so that the catheter travels from the internal jugular or
subclavian veins into the superior vena cava, with termination of the catheter
tip at or above the junction of the superior vena cava and the right atrium.
Pulmonary artery catheters typically will follow a similar path through the central venous vasculature
and then through the right atrium, right ventricle, the right ventricular
outflow tract, and into the pulmonary artery. Proper positioning of the
catheter should be no more than 1 cm past the mediastinal border in either the
right or left pulmonary arteries.
Finally, chest x-rays may be used to identify unknown implantable cardiac
devices in patients. The presence of coils in leads placed in the superior vena
cava and right ventricular apex are seen in implantable
cardioverter-defibrillator leads. The manufacturer of the device can be
discerned from the chest x-ray by radio-opaque alphanumeric codes, header
orientation, generator can shape, and configuration of the connector pins.
