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.
FIG 8.1 Posteroanterior chest radiograph with corresponding cardiovascular structures.
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.
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.