Special Situations

Pulmonary Embolism

The CXR is often abnormal in pulmonary embolism. Atelectasis and other focal pulmonary parenchymal abnormalities are the most common findings. Pleural effusions are also common,but usually small and unilateral. Other plain film findings associated with pulmonary embolism are:

i) Westermark sign: Dilatation of the pulmonary vessels at or proximal to an embolism.

ii) Fleischner’s sign: The combination of enlargement of the pulmonary artery due to thrombus, with distal oligemia.

iii) Hampton hump: Hampton hump, a triangular or rounded pleural-based infiltrate with the apex pointed toward the hilum represents infarcted lung in pulmonary embolism.

Pericardial Effusion

The configuration of the heart in pericardial effusion depends on the volume of fluid and its distribution. It may have a globular or non-specific shape. In large effusions, there is very often a localized bulge in the left upper cardiac border. There is striking absence of abnormalities of the pulmonary vasculature. The combination of a large heart, with clear rather than congested lungs suggests a pericardial effusion. A rapid change in heart size over serial films also may be seen.

Dissection of the Aorta
The CXR may be abnormal in upto 80 per cent of patients. The abnormalities include:

i) Widened mediastinum: This is defined as a mediastinal width of more than 8 cm on the AP CXR. A tortuous aorta may be difficult to distinguish from a widened mediastinum. If in doubt, a good PA view is recommended.

ii) Abnormal aortic knob: Loss of definition or a focal widening of the mediastinal contour in the region of the aortic knuckle may be seen.

iii) Ring sign: This describes the displacement of the aorta, > 5 mm past the calcified aortic intima. The presence of this as a new finding on CXR, is considered a very specific radiographic sign.

iv) Left apical cap: This is a result of the pleural effusion that often accompanies an acute dissection.

v) Tracheal deviation and depression of left main stem bronchus or displacement of an NG tube.

Several studies have concluded that it is a combination of several of these findings that lead to suspicion of dissection.

Aneurysm of Aorta
CXR findings are an enlargement of the involved portion of the aorta. A focal dilatation may simulate a mass or adenopathy. A more generalized dilatation leads to widening of the mediastinal contours. It may be indistinguishable from an unfolded arch. In the acute situation, left pleural effusion may be present.

Pleural Effusion
Fluid has a density indistinguishable from soft tissue on a radiograph. Pleural fluid tends to accumulate in the deepest part of the posterior costophrenic angle. Small effusions are thus identified earlier on a lateral view. Ultrasonography is also capable of picking up very small effusions.

An effusion may not be recognized on a PA view until 100 to 200 ml of fluid has accumulated and has caused blunting of the costophrenic angle. Larger effusions have a fairly well defined concave upper edge (which is higher laterally than medially) and obscure the diaphragm, and later the mediastinal borders.
 

Atypical Distribution of Pleural Fluid
 

i)Lamellar effusion: These are shallow collections of fluid between the chest wall and the lung surface.
 

ii) Subpulmonic effusion: Fluid accumulating between the lung and the diaphragm will cause the contour of the “diaphragm” to be altered, its apex being more lateral than expected. There may be associated blunting of the CP angle. On the left side, a subpulmonic effusion may result in an increased distance between the fundic air bubble and the lung base.
 

iii) Loculated effusion: Fluid may be loculated along the lung periphery due to fusion of the visceral and parietal pleura. These collections often have a biconvex shape and when viewed in profile have a sharp outline, with tapered margins forming an obtuse angle with the chest wall. Fluid may be loculated in the interlobar fissures, most commonly seen in heart failure.Fluid in the horizontal fissure is well defined and more easily recognized to be an interlobar
effusion. In the major fissure, PA and lateral views may be necessary to make the diagnosis. Typically, these collections disappear rapidly after treatment for heart failure, and are known as pseudo or vanishing tumours.
 

Pneumothorax
 

Air in the pleural cavity manifests in a number of ways on the CXR, depending on the volume of air and position of the patient. The typical findings of a pneumothorax are an area of marked radiolucency, with absent vascular markings, and visibility of the adjacent lung margin. On an erect film, a small pneumothorax would be identified at the lung apex. Larger pneumothoraces are easily identified by their radiolucency and the adjacent collapsed lung. A tension
pneumothorax would increase the volume of the ipsilateral thoracic cavity by flattening the diaphragm, widening the rib interspaces, displacing the mediastinum to the opposite side and causing complete collapse of the lung.

Cardiac Calcification

Pericardial

Calcium is most dense in the atrioventricular grooves and is seen as thick oblique circles or arcs of calcification. From the grooves, calcification spreads over the surface of the atria/ventricles.Calcium localized to the left AV groove may be mistaken for mitral valve calcification.Pericardial calcification is better seen on a lateral view.

Aortic Valve

This valve lies in the centre of the heart, so that calcification overlaps the spine on the anterior view. On the lateral view, it lies midway between the anterior and posterior cardiac borders,largely anterior to a line drawn from the tracheal bifurcation to the anterior costophrenic angle.As the two sides of diaphragm are rarely at the same level, the lower reference pt should be midway between the domes.

Mitral Valve

On the PA view, mitral valve calcification is seen just to the left of the spine, below the position of the aortic valve. The larger the left atrium, the further leftward and caudal is the position of valve calcification. On the lateral view, the valve lies posteroinferior to the aortic valve, below a line drawn form the tracheal bifurcation to the anterior costophrenic angle.Coronary Arteries Coronary artery calcification is seen as a plaque or a double line. It is most frequently seen on CXR in the proximal part of the left coronary artery.

Pulmonary Vasculature

Normal Pulmonary Vasculature

Pulmonary vessels are seen in the medial 2/3 of the lung. Vessels are generally not identified in the lateral third. The radiographic appearance of pulmonary vasculature is dependent on technique. Underexposure will lead to prominence of the vasculature, while overexposure will cause an apparent decrease in the vasculature.

Patient position may vary appearances greatly. In the erect position, there is reduced low to the upper lobes due to gravity. Alveolar pressure tends to collapse the upper lobe veins. The normal size ratio of upper to lower lung vessels is 1/2 or 1/3 on the erect PA radiograph. The hilar angle,which is the angle between the superior pulmonary vein and the descending pulmonary artery is normally concave.

a) Arterial

The main pulmonary artery is seen on the PA view, as its left border forms the pulmonary bay.The right pulmonary artery runs horizontally to the right within the mediastinum and is not seen on the frontal view. Its upper lobe branch is given off in the mediastinum. Thus, the descending branch of the right pulmonary artery is the vessel first identified, as it forms the lower part of the right hilum. On the lateral view, it is seen as a rounded density just anterior to the carina. The left pulmonary artery, along with its descending branch are identified on the plain film as it forms part of the left hilum and continues into the lower lobe. On the lateral view, it is seen over the left main bronchus, superior and posterior to the right pulmonary artery. The pulmonary arteries
within the lung parenchyma, are closely related to the bronchi and taper gradually, as they branch.

b) Pulmonary Veins

The right and left upper lobe or superior pulmonary veins descend lateral to the arteries, cross in front of the hilum, and enter the left atrium. The right inferior veins can be distinguished from the arteries as they follow a more horizontal course to the left atrium. On the left side, the inferior veins are more vertical.

Pulmonary Arterial System Changes
a) Pulmonary Plethora
With increased pulmonary arterial blood flow, pulmonary branches are visualized beyond the inner 2/3 of the lungs. Vessels in upper and lower lobes are dilated to the same degree. The number of end on vessels seen is 5 or more in both lung fields (or 3 or more in one lung field).

b) Pulmonary Arterial Hypertension

The features of pulmonary arterial hypertension are:

i)Central arterial enlargement, manifesting as an increased convexity of the pulmonary conus.

ii) Enlarged descending pulmonary artery, of more than 16 mm. Right descending arterial calibre of more than 25 mm is in keeping with Primary Pulmonary Hypertension.

iii) Sharp pruning of peripheral vasculature.

iv) Features of right ventricular hypertrophy and dilatation.

c) Pulmonary Oligemia
With reduced pulmonary flow, pulmonary vascular markings are markedly reduced, with vessels appearing attenuated. This is diagnosed radiographically only in patients with markedly decreased pulmonary flow.

Pulmonary Venous Congestion
In pulmonary venous hypertension, the earliest change is an increase in calibre of the upper lobe vessels. If the upper lobe veins measure more than 3 mm in the first interspace, they reflect an increase in pulmonary venous pressure.

Grading of Pulmonary Venous Hypertension
Grade 1: Diameter of upper zone vessels greater than or equal to lower zone; right hilar angle obliterated.

Grade 2: Interstitial pulmonary edema or pleural effusion; right hilar angle straightened;Kerley B lines and later Kerley A lines.

Grade 3: Alveolar edema; Right hilar angle convex.

Pulmonary Edema

When the capillary pressure exceeds the plasma osmotic pressure, fluid first accumulates in the interstitial spaces. The components of the interstitium (central and peripheral) are shown in Fig.

Components of the Interstitium

The central interstitium invests the bronchovascular bundle and extends from centre to periphery.Fluid accumulating in this perivascular and peribronchial interstitium causes an apparent increase in the size of vessels at the hilum, as well as loss of definition of vessels on the CXR.

The peripheral interstitium consists of subpleural, inter and intralobular septal components. One of the early manifestations of interstitial edema on the CXR are septal lines, commonly Kerley B lines. These are short, straight, horizontal lines, best seen in the lower zones, representing thickening of the interlobular septae .

Other lines described are Kerley A lines (4 to 6 cm, radiating from the hilum, more in the upper zones) and Kerley C lines (short, crisscrossing lines), all representing thickened interstitium.Further fluid accumulation results in edema of alveolar walls and alveolar edema. This characteristically has an “air-space” appearance with coalescent pulmonary opacities, resembling cotton wool. Air space opacification creates a contrast between air filled bronchi and the surrounding lung, and this may produce an air-bronchogram. Typically, there is a perihilar distribution, resulting in a “bats wing appearance”. Rapid clearing with antifailure measures is seen.