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The chest radiograph in heart disease
INVESTIGATIONS
• localizing an abnormality seen on a PA film • helping to clarify a potential abnormality • detecting disease behind the heart or diaphragm Supine projections are used in ill patients. Detection of conditions such as pleural effusion and pneumothorax is more difficult than with PA or AP films, and cardiac and mediastinal assessment is difficult.
The chest radiograph in heart disease F V Gleeson
Exposures: the degree of ‘blackness’ or ‘whiteness’ of a radiograph is determined by the exposure. Most departments now perform high-kV chest radiography, which enables greater visualization of lung that is normally obscured by the ribs, and greater penetration behind the heart and diaphragm. However, assessment of bones is more difficult on high-kV films and the ability to detect calcification in pulmonary nodules and hilar nodes is reduced, making the diagnosis of calcified granulomatous disease more difficult. Using modern technology, the data generated on a digital chest radiograph can be manipulated to allow interpretation of previously inadequate exposures (see below).
Although the most commonly used imaging techniques in cardiac disease are echocardiography, coronary angiography, MRI and multi-slice CT, conventional chest radiography remains important in detection and monitoring. It must be remembered that exposure of patients to any form of ionizing radiation now requires justification under European Union Ionising Radiation (Medical Exposure) Regulations 2000. Adequate clinical details must be written on the request card, and all radiology departments require written protocols to allow examinations to be performed legally.
Digital chest radiography: conventional chest radiography requires exposure of a film to X-rays. In picture-archiving communication systems (PACS), pictures are taken using modern digital techniques that do not require formation of an image on a film. The patient is still exposed to X-rays, but the information derived by PACS can be imprinted onto a reusable photo-stimulatable plate rather than a conventional film−screen combination. The digital image generated can be printed onto film or stored in a computer system, from where it can be transmitted to workstations in the radiology department, wards and out-patient departments. The radiation dose−response of photo-stimulatable plates differs from that of conventional film, allowing greater latitude in exposure, and images that would be underexposed or overexposed on conventional radiography can be interpreted. The images generated by PACS may be manipulated, allowing magnification, measurement and contrast manipulation, and are easily stored and recalled for comparison with earlier films.
Techniques in chest radiography Projections: the two standard views are the erect posteroanterior (PA) and lateral projections. Additional views such as apical, oblique rib and decubitus are used less commonly than in the past, because of the availability of other modalities such as ultrasonography and CT. Erect PA and anteroposterior (AP) projections – PA projections are preferred to AP because: • cardiac and mediastinal diameter, lung volume and vessel size can be measured on erect PA films (cardiac size is exaggerated on AP, see below) • more lung is visualized on PA films (the scapulae are positioned to the side on PA but are projected over the lungs on AP, making interpretation more difficult) • variable exposures are used for AP radiographs, making comparison of films more difficult. AP projections are used in patients who are unable to stand for an erect PA film in the radiology department. (It is not possible to obtain a PA projection on a ward or in the Accident and Emergency department, unless a dedicated chest radiography room is available.) Whenever possible, a radiology department PA film should be obtained. Each department has a standardized system, including anode-to-film distance and exposure, that enables comparison of sequential films for changes in disease status. The exposure is taken on deep inspiration to achieve the greatest possible visualization of the lungs. Lateral projections are obtained in conjunction with a PA film and are not necessary for every patient. They are useful for:
Interpreting the chest radiograph Whenever possible, chest radiographs should be interpreted in a room with low ambient light, to enable detection of subtle abnormalities such as a small pneumothorax or small pulmonary nodule, and the presence of altered pulmonary vascularity in patients with possible intracardiac shunts. Use of conventional films and viewing boxes will decline with the increasing use of PACS, but interpretation of a chest radiograph on a workstation monitor is the same as for a conventional film. Interpretation should determine whether the film is: • normal • abnormal, but not of clinical significance • abnormal, with a diagnosis • abnormal, but no specific diagnosis possible and further investigation required. Occasionally, a normal chest radiograph is important in guiding further investigation (e.g. perfusion scanning for suspected pulmonary embolic disease). In all cases, the available clinical information should be taken into account. Interpretation should follow a routine, as follows.
F V Gleeson is Consultant Radiologist at the Churchill Hospital, Oxford, UK. He qualified from the Royal London Hospital, London, and trained in cardiology in Cambridge, London and Los Angeles, USA. His special interest is thoracic radiology and his research interest is pleural disease. Conflicts of interest: none declared.
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Check the name, date and side marker. Determine the projection – is the radiograph PA, AP or supine? All non-PA films should be clearly marked as such. In erect films, the gastric bubble is under the left hemidiaphragm. In AP films, the scapulae are projected over the upper zone and mid-zone of the lungs. Assess the technical adequacy of the film – that is, the degree of exposure and whether the film is rotated. In adequate exposures, the pulmonary vessels are readily seen within the lung parenchyma, and the spine and disc spaces are just visible behind the heart. In non-rotated films, the medial ends of the clavicles are equidistant from the spinous processes. Rotation may hinder assessment of the mediastinum and hila, and produce an artefactual radiolucent hemithorax and hilar enlargement. Visible medical devices (drains, catheters, pacemakers, valve prostheses) should be assessed to confirm that they are appropriately positioned and not associated with an iatrogenic complication. The nature of any unknown device should be ascertained; foreign bodies (e.g. swabs, broken pacing wires) can be identified by this means. Detected abnormalities should be categorized as a known diagnosis (e.g. lobar collapse, pleural effusion) and the film assessed for the cause or associated abnormalities. Abnormalities not readily categorized should be evaluated in a standard manner. • Is the abnormality genuine, or can it be explained by normal structures or film quality? • Can it be localized on the frontal film, or is a lateral view needed? Once the abnormality is localized, further interpretation and investigation depend on whether the lesion lies in the lung, the mediastinum, the pleura, or two or more areas (e.g. the differential diagnosis of a mass in the lungs differs from that of a mediastinal mass). The site involved is largely determined using the ‘silhouette sign’, which represents obliteration of a border that is normally visualized on chest radiographs. For example, the right heart border is normally clearly seen where it abuts the lung, but in right middle lobe collapse the border abuts soft tissue and is not clearly visualized. (This may be the only means of identifying right middle lobe collapse on a PA radiograph.) The silhouette sign may be used to identify and localize mediastinal, pleural and pulmonary disease. • Can the abnormality be described in detail? Is it completely or partly calcified? Does it have a specific radiographic appearance (e.g. nodular, air bronchogram). Inspect the film in a systematic manner, regardless of whether an abnormality is detected, with particular attention to the apices, the hila, the retrocardiac region, the lungs below the hemidiaphragms and the lung adjacent to the lateral chest wall. When a previous chest radiograph is available, this should be viewed for comparison, as an aid to detecting new disease (e.g. pericardial effusion), or for confirmation that a long-standing abnormality has changed or is of no significance. If cardiac disease is suspected, this may be directly apparent from an increase in the size of part of or the entire cardiac silhouette, or may be inferred by additional information on the film that may not necessarily be associated with cardiac enlargement. Such additional information includes: • the presence of sternal wires and coronary artery vein graft clips, or valve prostheses
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• changes in the normal mediastinal silhouette (e.g. small aortic knuckle in patients with an atrial septal defect) • enlargement of the pulmonary arteries, perhaps secondary to an intracardiac shunt • the presence of pulmonary oedema. When reviewing the chest radiograph in patients with cardiac disease, it is important to be aware, not only of the parameters used to assess cardiac enlargement, but of the normal mediastinal silhouette, the position of the cardiac valves on chest radiographs, the normal appearance of the pulmonary arteries, and signs of pulmonary oedema.
Normal anatomy on the chest radiograph The radiographic appearance of the lungs comprises the pulmonary arteries and veins, some visualization of the proximal bronchi, and an overall density consequent on the degree of inflation and the absence of significant processes that affect the degree of opacification. The vessels branch outwards from the hilum and are not usually visible 1 cm from the lung edge. The vessels in the upper zones are usually thinner than those a similar distance from the hilum in the lower zones (normal ratio 1:3); this may change in left ventricular (LV) failure (termed ‘upper lobe venous blood diversion’). Poor inspiration may make the lungs appear more opaque. On adequate inspiration, the dome of the diaphragm lies between the anterior ends of the fifth and seventh ribs in the mid-clavicular line. Interstitial lung disease may produce a subtle increase in lung radiopacification. Important anatomical features on frontal and lateral chest radiographs are shown in Figure 1. The mediastinal silhouette is formed by the structures outlined in Figure 2, but may change in appearance through enlargement of a normal structure (e.g. left atrium in mitral valve disease) or a pathological process (e.g. malignant thymoma). Detection of early disease depends on familiarity with the normal silhouette and its component parts. In particular, borders that are recognized as interfaces between normal structures may become obliterated or distorted by disease. Right paratracheal stripe forms the interface between the tracheal wall and the right lung. Lymphadenopathy is often detected at this site. Azygos vein is seen at the caudal end of the right paratracheal stripe. It is the site of entrance of the azygos vein into the superior vena cava (SVC), and its normal maximum diameter is 1 cm. It is commonly enlarged in patients with LV failure and pulmonary oedema, or other causes of fluid overload. It may also be enlarged in patients with increased cardiac output (e.g. pregnancy, postexercise). Nodal enlargement at this site may be indistinguishable from vein enlargement. Azygo-oesophageal stripe is a continuation caudally from the azygos vein and represents the interface of the right lung with the ascending azygos vein and oesophagus. It may become distorted by an oesophageal tumour, left atrial enlargement or subcarinal lymphadenopathy. Aortopulmonary window is the concave area between the caudal surface of the aortic arch and the cranial surface of the left main pulmonary artery. This area is commonly ‘filled-in’ by nodal enlargement in patients with mediastinal lymphadenopathy. Left paraspinal line is formed by the interface of the left lung and the paraspinal soft tissues. It should always lie within 1 cm 137
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INVESTIGATIONS
Mediastinal silhouette and major lines and interfaces seen on a frontal chest radiograph
Principal features on a lateral chest radiograph
Superior vena cava Right brachiocephalic vein and artery
Left subclavian artery
Right paratracheal line
Aortic knuckle
Superior vena cava Azygos vein Ascending aorta Azygo-oesophageal interface Right atrium Cardiophrenic fat pad
Main pulmonary artery Left atrial appendage Left paraspinal interface
Ascending aorta Anterior junctional area Right ventricular outflow tract and main pulmonary artery Right ventricle
Left ventricle Para-aortic interface
Trachea Aortic arch Aortopulmonary window Left pulmonary artery Left main stem bronchus
Left atrium Right hemidiaphragm
Left ventricle
Left hemidiaphragm 1
border formed by the SVC then continues to the clavicle as the right brachiocephalic vein. On the left, the mediastinal border immediately above the diaphragm is formed by the left ventricle, and this then continues superiorly as the lateral border of the main pulmonary artery as it moves posteriorly within the mediastinum. Immediately above this is the concavity of the aortopulmonary window, formed by the superior surface of the pulmonary artery and the inferolateral surface of the aortic arch. The bulge above the aortopulmonary window is formed by the lateral border of the aortic arch, the aortic knuckle, with the left subclavian artery forming the most superior border extending to the left clavicle.
of the vertebrae, but may become distorted by paravertebral soft tissue, vertebral collapse, nodal enlargement, neurogenic tumour or enlargement of the descending aorta. Normal cardiac anatomy The mediastinal border inferiorly on the right is made by the lateral portion of the right atrium, with the inferior vena cava sometimes seen inferior to this, passing through the diaphragm into the posterior aspect of the right atrium. Cranial to the right atrium, the superior mediastinal border is formed by the SVC. The
Cardiac enlargement The cardiac silhouette is magnified on an AP projection, and thus care should be taken when commenting on cardiac size. Measurement of the cardiac diameter and other objective parameters should not be performed on an AP film, but if the silhouette is sufficiently enlarged it is reasonable to comment that there is apparent cardiac enlargement. On a PA chest radiograph, the heart may appear enlarged because of an overall increase in cardiac size, or because of selective chamber enlargement. There are two commonly used methods of objectively assessing cardiac size on the chest radiograph (Figure 3) − measurement of the transverse cardiac diameter, and the cardiothoracic ratio. However, it must be remembered that all methods of assessing cardiac size on PA radiographs are inherently inaccurate, because of uncontrolled parameters such as the degree of rotation of the chest, and the depth of inspiration taken by the patient. • The transverse cardiac diameter is the simplest measurement of cardiac size on the conventional PA radiograph. In about 90%
2 Normal chest radiograph
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3 Cardiac size may be reported as the maximum transverse diameter (solid line) or the thoracic ratio (maximum transverse diameter in relation to thoracic width – dotted line).
4 Markedly enlarged left atrium, showing a double right heart border (short arrows) and splaying of the carina (long arrow).
of normal men, it is less than 13.5 cm; in women, it is less than 12.5 cm. In normal individuals, the measurement should vary less than 1.5 cm between films; this allows for comparison on serial radiographs. • The cardiothoracic ratio is the most commonly used method for assessing cardiac size. The accepted upper limit of normal is 50% in adults and 60% in neonates. It may be falsely increased by rotation, poor inspiratory effort, or patient build and age. Most patients with cardiac enlargement have ischaemic heart disease, initially resulting in LV dilatation, which progresses to involve all four cardiac chambers. On the chest radiograph, it is not possible to assess dilatation of the individual cardiac chambers other than the left atrium. Pericardial effusion is another important cause of enlargement of the cardiac silhouette. Some clinical conditions may lead to selective chamber enlargement, and this may be recognized on the chest radiograph. The chambers that are most commonly enlarged are the left atrium and the left ventricle. Pericardial effusion − a sudden increase in the size of the cardiac silhouette on serial radiographs should raise the possibility of an underlying pericardial effusion. Effacement of the normal cardiac/mediastinal silhouette, a globular configuration, or cardiac enlargement with associated diminution of the pulmonary vessels is suggestive of the diagnosis. Left atrial enlargement − the left atrium is projected over the midline immediately below the carina on the chest radiograph. For an enlarged left atrium to be detected, it must be at least 2.5 times its normal size. Enlargement may involve primarily the body of the atrium, or the body and its appendage. Enlargement of the appendage may be a readily recognizable sign of atrial enlargement and is strongly suggestive of rheumatic heart disease. There are several radiographic signs of left atrial enlargement (Figure 4), some or all of which may be present: • elevation of the left main bronchus with splaying of the carina
• a double right heart border, with displacement of the heart border to the right • enlargement of the left atrial appendage. LV enlargement − the left ventricle forms the left heart border and the cardiac apex. LV enlargement occurs most commonly secondary to cardiac ischaemia, but may also be seen secondary to aortic regurgitation or stenosis, mitral regurgitation, hypertension and cardiomyopathy. Enlargement may initially be recognized by rounding of the cardiac apex (Figure 5), and this may occur before significant ventricular dilatation. The second feature is elongation of the axis of the ventricle downwards and to the left.
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5 A rounded cardiac apex secondary to left ventricular hypertrophy in a patient with an aortic valve prosthesis (long arrow) for aortic stenosis. Note the median sternotomy wires (bold arrow) and incidental ventriculoperitoneal shunt (short arrow).
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aortic knuckle on the chest radiograph, but the most characteristic finding is notching of the undersurface of the third to eighth ribs visible after the age of 10 years (Figure 7). Kartagener’s syndrome − patients have dextrocardia (Figure 8) and situs inversus associated with bronchiectasis and paranasal sinusitis.
Pulmonary oedema Pulmonary oedema is most commonly cardiogenic, but there are non-cardiogenic causes such as fluid overload and increased pulmonary capillary permeability. Cardiogenic pulmonary oedema has different radiographic appearances at different stages, starting with increased opacification of the interstitial tissues of the lungs as they become thickened with fluid, and progressing to consolidation and pleural effusions (Figures 9 and 10). The perihilar regions contain more interstitial tissue than the rest of the lungs; this accounts for the perihilar distribution of opacification in the interstitial phase of pulmonary oedema (‘bat’s-wing’ pattern). Other, more subtle signs of interstitial pulmonary oedema include peribronchial cuffing seen as thickened and blurred walls of end-on bronchi (best seen in the perihilar regions), and septal lines. Septal lines are also known as Kerley B lines (Figure 11); they appear as thin, horizontal lines perpendicular to and abutting the pleural surfaces at the lung bases and are caused by fluid thickening the interstitial tissue in the septa within the lung tissue.
6 Patient with atrial septal defect, showing a small aortic knuckle (thin arrow) and enlarged proximal pulmonary arteries (bold arrows). This patient may be developing an Eisenmenger reaction, because the outer lung vessels are reduced.
Suspected pulmonary embolus A plain chest radiograph is seldom diagnostic of pulmonary embolus and is commonly normal. Its major roles are in the exclusion of other diagnoses (e.g. pneumothorax, pneumonia, rib fractures), and in planning the next appropriate investigation.
7 Patient with coarctation of the aorta, showing rib notching (arrow).
Congenital cardiac abnormalities Although most congenital cardiac anomalies are detected in utero or in the neonatal period, some less severe anomalies may present in adolescence or in adults. In addition, sequelae of corrected anomalies may be apparent on chest radiographs later in life. These conditions include intracardiac shunts (e.g. atrial and ventricular septal defects), aortic coarctation and Kartagener’s syndrome. Cardiac shunts − small shunts with a pulmonary blood flow: systemic blood flow ratio of less than 2:1 usually produce no detectable radiographic abnormality. Larger shunts eventually produce proximal pulmonary artery enlargement and pulmonary plethora (pulmonary vessels extending to within a few centimetres of the pleura). If these large shunts remain uncorrected, the patient may eventually develop an Eisenmenger reaction, with shunt reversal, recognized on the radiograph by enlargement of the proximal pulmonary arteries and reduction in size of the vessels in the mid-third and outer third of the lungs (pulmonary oligaemia). Both atrial and ventricular septal defects may be recognized by the pulmonary artery changes described above, but atrial defects commonly also exhibit a small aortic knuckle and normal cardiac size on chest radiography (Figure 6), whereas ventricular defects show a normal sized aorta, but associated cardiomegaly. Coarctation of the aorta comprises local stenosis of the aorta at the level of the aortic isthmus. Associated congenital anomalies occur in about 75% of patients. There may be a high or double MEDICINE 34:4
8 Kartagener’s syndrome. Note the side marker in the top right-hand corner, confirming that the film is the right way round. There is also bilateral basal bronchiectasis, demonstrated by the increased basal lung markings.
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Chest radiograph in pulmonary oedema Increased volume of blood in pulmonary circulation Upper lobe pulmonary vein enlargement
Fluid leaks into pulmonary interstitial tissue Perihilar haze Peribronchial cuffing Septal lines
Fluid leaks into the alveoli and airspaces Consolidation
a
Fluid leaks into the pleural spaces Effusions 9
b a Bilateral septal lines secondary to cardiac failure and fluid overload. b Close up of septal lines. 11
10 Patient with pulmonary oedema, showing the charateristic bat’s-wing appearance of perihilar opacification.
Patients with a normal chest radiograph and no significant history of asthma or chronic obstructive pulmonary disease may be investigated using radionuclide imaging (ventilation/perfusion (V/Q) scanning). This method detects areas of non-perfusion relative to ventilation (inferring an embolus as the cause) and is reported in terms of the probability that an embolus has occurred. A high-probability scan indicates a likelihood of up to 96%; a normal report effectively excludes pulmonary embolic disease. Intermediate and low-probability scans are now termed ‘nondiagnostic’ and suggest that further investigation is required. Patients with a significantly abnormal chest radiograph or obstructive lung disease are likely to produce a non-diagnostic V/Q scan and should be investigated using spiral CT (Figure 12).
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12 Spiral CT showing a pulmonary embolus (short arrow) on a conventional axial section, with distal consolidation (long arrow).
Rapid scanning techniques including multi-slice CT enable investigation of patients unsuitable for V/Q scanning, and directly visualize thrombus. Multi-slice CT has been shown to be at least as accurate as pulmonary angiography, which is now seldom performed. 141
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• localizing an abnormality seen on a PA film • helping to clarify a potential abnormality • detecting disease behind the heart or diaphragm Supine projections are used in ill patients. Detection of conditions such as pleural effusion and pneumothorax is more difficult than with PA or AP films, and cardiac and mediastinal assessment is difficult.
The chest radiograph in heart disease F V Gleeson
Exposures: the degree of ‘blackness’ or ‘whiteness’ of a radiograph is determined by the exposure. Most departments now perform high-kV chest radiography, which enables greater visualization of lung that is normally obscured by the ribs, and greater penetration behind the heart and diaphragm. However, assessment of bones is more difficult on high-kV films and the ability to detect calcification in pulmonary nodules and hilar nodes is reduced, making the diagnosis of calcified granulomatous disease more difficult. Using modern technology, the data generated on a digital chest radiograph can be manipulated to allow interpretation of previously inadequate exposures (see below).
Although the most commonly used imaging techniques in cardiac disease are echocardiography, coronary angiography, MRI and multi-slice CT, conventional chest radiography remains important in detection and monitoring. It must be remembered that exposure of patients to any form of ionizing radiation now requires justification under European Union Ionising Radiation (Medical Exposure) Regulations 2000. Adequate clinical details must be written on the request card, and all radiology departments require written protocols to allow examinations to be performed legally.
Digital chest radiography: conventional chest radiography requires exposure of a film to X-rays. In picture-archiving communication systems (PACS), pictures are taken using modern digital techniques that do not require formation of an image on a film. The patient is still exposed to X-rays, but the information derived by PACS can be imprinted onto a reusable photo-stimulatable plate rather than a conventional film−screen combination. The digital image generated can be printed onto film or stored in a computer system, from where it can be transmitted to workstations in the radiology department, wards and out-patient departments. The radiation dose−response of photo-stimulatable plates differs from that of conventional film, allowing greater latitude in exposure, and images that would be underexposed or overexposed on conventional radiography can be interpreted. The images generated by PACS may be manipulated, allowing magnification, measurement and contrast manipulation, and are easily stored and recalled for comparison with earlier films.
Techniques in chest radiography Projections: the two standard views are the erect posteroanterior (PA) and lateral projections. Additional views such as apical, oblique rib and decubitus are used less commonly than in the past, because of the availability of other modalities such as ultrasonography and CT. Erect PA and anteroposterior (AP) projections – PA projections are preferred to AP because: • cardiac and mediastinal diameter, lung volume and vessel size can be measured on erect PA films (cardiac size is exaggerated on AP, see below) • more lung is visualized on PA films (the scapulae are positioned to the side on PA but are projected over the lungs on AP, making interpretation more difficult) • variable exposures are used for AP radiographs, making comparison of films more difficult. AP projections are used in patients who are unable to stand for an erect PA film in the radiology department. (It is not possible to obtain a PA projection on a ward or in the Accident and Emergency department, unless a dedicated chest radiography room is available.) Whenever possible, a radiology department PA film should be obtained. Each department has a standardized system, including anode-to-film distance and exposure, that enables comparison of sequential films for changes in disease status. The exposure is taken on deep inspiration to achieve the greatest possible visualization of the lungs. Lateral projections are obtained in conjunction with a PA film and are not necessary for every patient. They are useful for:
Interpreting the chest radiograph Whenever possible, chest radiographs should be interpreted in a room with low ambient light, to enable detection of subtle abnormalities such as a small pneumothorax or small pulmonary nodule, and the presence of altered pulmonary vascularity in patients with possible intracardiac shunts. Use of conventional films and viewing boxes will decline with the increasing use of PACS, but interpretation of a chest radiograph on a workstation monitor is the same as for a conventional film. Interpretation should determine whether the film is: • normal • abnormal, but not of clinical significance • abnormal, with a diagnosis • abnormal, but no specific diagnosis possible and further investigation required. Occasionally, a normal chest radiograph is important in guiding further investigation (e.g. perfusion scanning for suspected pulmonary embolic disease). In all cases, the available clinical information should be taken into account. Interpretation should follow a routine, as follows.
F V Gleeson is Consultant Radiologist at the Churchill Hospital, Oxford, UK. He qualified from the Royal London Hospital, London, and trained in cardiology in Cambridge, London and Los Angeles, USA. His special interest is thoracic radiology and his research interest is pleural disease. Conflicts of interest: none declared.
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Check the name, date and side marker. Determine the projection – is the radiograph PA, AP or supine? All non-PA films should be clearly marked as such. In erect films, the gastric bubble is under the left hemidiaphragm. In AP films, the scapulae are projected over the upper zone and mid-zone of the lungs. Assess the technical adequacy of the film – that is, the degree of exposure and whether the film is rotated. In adequate exposures, the pulmonary vessels are readily seen within the lung parenchyma, and the spine and disc spaces are just visible behind the heart. In non-rotated films, the medial ends of the clavicles are equidistant from the spinous processes. Rotation may hinder assessment of the mediastinum and hila, and produce an artefactual radiolucent hemithorax and hilar enlargement. Visible medical devices (drains, catheters, pacemakers, valve prostheses) should be assessed to confirm that they are appropriately positioned and not associated with an iatrogenic complication. The nature of any unknown device should be ascertained; foreign bodies (e.g. swabs, broken pacing wires) can be identified by this means. Detected abnormalities should be categorized as a known diagnosis (e.g. lobar collapse, pleural effusion) and the film assessed for the cause or associated abnormalities. Abnormalities not readily categorized should be evaluated in a standard manner. • Is the abnormality genuine, or can it be explained by normal structures or film quality? • Can it be localized on the frontal film, or is a lateral view needed? Once the abnormality is localized, further interpretation and investigation depend on whether the lesion lies in the lung, the mediastinum, the pleura, or two or more areas (e.g. the differential diagnosis of a mass in the lungs differs from that of a mediastinal mass). The site involved is largely determined using the ‘silhouette sign’, which represents obliteration of a border that is normally visualized on chest radiographs. For example, the right heart border is normally clearly seen where it abuts the lung, but in right middle lobe collapse the border abuts soft tissue and is not clearly visualized. (This may be the only means of identifying right middle lobe collapse on a PA radiograph.) The silhouette sign may be used to identify and localize mediastinal, pleural and pulmonary disease. • Can the abnormality be described in detail? Is it completely or partly calcified? Does it have a specific radiographic appearance (e.g. nodular, air bronchogram). Inspect the film in a systematic manner, regardless of whether an abnormality is detected, with particular attention to the apices, the hila, the retrocardiac region, the lungs below the hemidiaphragms and the lung adjacent to the lateral chest wall. When a previous chest radiograph is available, this should be viewed for comparison, as an aid to detecting new disease (e.g. pericardial effusion), or for confirmation that a long-standing abnormality has changed or is of no significance. If cardiac disease is suspected, this may be directly apparent from an increase in the size of part of or the entire cardiac silhouette, or may be inferred by additional information on the film that may not necessarily be associated with cardiac enlargement. Such additional information includes: • the presence of sternal wires and coronary artery vein graft clips, or valve prostheses
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• changes in the normal mediastinal silhouette (e.g. small aortic knuckle in patients with an atrial septal defect) • enlargement of the pulmonary arteries, perhaps secondary to an intracardiac shunt • the presence of pulmonary oedema. When reviewing the chest radiograph in patients with cardiac disease, it is important to be aware, not only of the parameters used to assess cardiac enlargement, but of the normal mediastinal silhouette, the position of the cardiac valves on chest radiographs, the normal appearance of the pulmonary arteries, and signs of pulmonary oedema.
Normal anatomy on the chest radiograph The radiographic appearance of the lungs comprises the pulmonary arteries and veins, some visualization of the proximal bronchi, and an overall density consequent on the degree of inflation and the absence of significant processes that affect the degree of opacification. The vessels branch outwards from the hilum and are not usually visible 1 cm from the lung edge. The vessels in the upper zones are usually thinner than those a similar distance from the hilum in the lower zones (normal ratio 1:3); this may change in left ventricular (LV) failure (termed ‘upper lobe venous blood diversion’). Poor inspiration may make the lungs appear more opaque. On adequate inspiration, the dome of the diaphragm lies between the anterior ends of the fifth and seventh ribs in the mid-clavicular line. Interstitial lung disease may produce a subtle increase in lung radiopacification. Important anatomical features on frontal and lateral chest radiographs are shown in Figure 1. The mediastinal silhouette is formed by the structures outlined in Figure 2, but may change in appearance through enlargement of a normal structure (e.g. left atrium in mitral valve disease) or a pathological process (e.g. malignant thymoma). Detection of early disease depends on familiarity with the normal silhouette and its component parts. In particular, borders that are recognized as interfaces between normal structures may become obliterated or distorted by disease. Right paratracheal stripe forms the interface between the tracheal wall and the right lung. Lymphadenopathy is often detected at this site. Azygos vein is seen at the caudal end of the right paratracheal stripe. It is the site of entrance of the azygos vein into the superior vena cava (SVC), and its normal maximum diameter is 1 cm. It is commonly enlarged in patients with LV failure and pulmonary oedema, or other causes of fluid overload. It may also be enlarged in patients with increased cardiac output (e.g. pregnancy, postexercise). Nodal enlargement at this site may be indistinguishable from vein enlargement. Azygo-oesophageal stripe is a continuation caudally from the azygos vein and represents the interface of the right lung with the ascending azygos vein and oesophagus. It may become distorted by an oesophageal tumour, left atrial enlargement or subcarinal lymphadenopathy. Aortopulmonary window is the concave area between the caudal surface of the aortic arch and the cranial surface of the left main pulmonary artery. This area is commonly ‘filled-in’ by nodal enlargement in patients with mediastinal lymphadenopathy. Left paraspinal line is formed by the interface of the left lung and the paraspinal soft tissues. It should always lie within 1 cm 137
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Mediastinal silhouette and major lines and interfaces seen on a frontal chest radiograph
Principal features on a lateral chest radiograph
Superior vena cava Right brachiocephalic vein and artery
Left subclavian artery
Right paratracheal line
Aortic knuckle
Superior vena cava Azygos vein Ascending aorta Azygo-oesophageal interface Right atrium Cardiophrenic fat pad
Main pulmonary artery Left atrial appendage Left paraspinal interface
Ascending aorta Anterior junctional area Right ventricular outflow tract and main pulmonary artery Right ventricle
Left ventricle Para-aortic interface
Trachea Aortic arch Aortopulmonary window Left pulmonary artery Left main stem bronchus
Left atrium Right hemidiaphragm
Left ventricle
Left hemidiaphragm 1
border formed by the SVC then continues to the clavicle as the right brachiocephalic vein. On the left, the mediastinal border immediately above the diaphragm is formed by the left ventricle, and this then continues superiorly as the lateral border of the main pulmonary artery as it moves posteriorly within the mediastinum. Immediately above this is the concavity of the aortopulmonary window, formed by the superior surface of the pulmonary artery and the inferolateral surface of the aortic arch. The bulge above the aortopulmonary window is formed by the lateral border of the aortic arch, the aortic knuckle, with the left subclavian artery forming the most superior border extending to the left clavicle.
of the vertebrae, but may become distorted by paravertebral soft tissue, vertebral collapse, nodal enlargement, neurogenic tumour or enlargement of the descending aorta. Normal cardiac anatomy The mediastinal border inferiorly on the right is made by the lateral portion of the right atrium, with the inferior vena cava sometimes seen inferior to this, passing through the diaphragm into the posterior aspect of the right atrium. Cranial to the right atrium, the superior mediastinal border is formed by the SVC. The
Cardiac enlargement The cardiac silhouette is magnified on an AP projection, and thus care should be taken when commenting on cardiac size. Measurement of the cardiac diameter and other objective parameters should not be performed on an AP film, but if the silhouette is sufficiently enlarged it is reasonable to comment that there is apparent cardiac enlargement. On a PA chest radiograph, the heart may appear enlarged because of an overall increase in cardiac size, or because of selective chamber enlargement. There are two commonly used methods of objectively assessing cardiac size on the chest radiograph (Figure 3) − measurement of the transverse cardiac diameter, and the cardiothoracic ratio. However, it must be remembered that all methods of assessing cardiac size on PA radiographs are inherently inaccurate, because of uncontrolled parameters such as the degree of rotation of the chest, and the depth of inspiration taken by the patient. • The transverse cardiac diameter is the simplest measurement of cardiac size on the conventional PA radiograph. In about 90%
2 Normal chest radiograph
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3 Cardiac size may be reported as the maximum transverse diameter (solid line) or the thoracic ratio (maximum transverse diameter in relation to thoracic width – dotted line).
4 Markedly enlarged left atrium, showing a double right heart border (short arrows) and splaying of the carina (long arrow).
of normal men, it is less than 13.5 cm; in women, it is less than 12.5 cm. In normal individuals, the measurement should vary less than 1.5 cm between films; this allows for comparison on serial radiographs. • The cardiothoracic ratio is the most commonly used method for assessing cardiac size. The accepted upper limit of normal is 50% in adults and 60% in neonates. It may be falsely increased by rotation, poor inspiratory effort, or patient build and age. Most patients with cardiac enlargement have ischaemic heart disease, initially resulting in LV dilatation, which progresses to involve all four cardiac chambers. On the chest radiograph, it is not possible to assess dilatation of the individual cardiac chambers other than the left atrium. Pericardial effusion is another important cause of enlargement of the cardiac silhouette. Some clinical conditions may lead to selective chamber enlargement, and this may be recognized on the chest radiograph. The chambers that are most commonly enlarged are the left atrium and the left ventricle. Pericardial effusion − a sudden increase in the size of the cardiac silhouette on serial radiographs should raise the possibility of an underlying pericardial effusion. Effacement of the normal cardiac/mediastinal silhouette, a globular configuration, or cardiac enlargement with associated diminution of the pulmonary vessels is suggestive of the diagnosis. Left atrial enlargement − the left atrium is projected over the midline immediately below the carina on the chest radiograph. For an enlarged left atrium to be detected, it must be at least 2.5 times its normal size. Enlargement may involve primarily the body of the atrium, or the body and its appendage. Enlargement of the appendage may be a readily recognizable sign of atrial enlargement and is strongly suggestive of rheumatic heart disease. There are several radiographic signs of left atrial enlargement (Figure 4), some or all of which may be present: • elevation of the left main bronchus with splaying of the carina
• a double right heart border, with displacement of the heart border to the right • enlargement of the left atrial appendage. LV enlargement − the left ventricle forms the left heart border and the cardiac apex. LV enlargement occurs most commonly secondary to cardiac ischaemia, but may also be seen secondary to aortic regurgitation or stenosis, mitral regurgitation, hypertension and cardiomyopathy. Enlargement may initially be recognized by rounding of the cardiac apex (Figure 5), and this may occur before significant ventricular dilatation. The second feature is elongation of the axis of the ventricle downwards and to the left.
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5 A rounded cardiac apex secondary to left ventricular hypertrophy in a patient with an aortic valve prosthesis (long arrow) for aortic stenosis. Note the median sternotomy wires (bold arrow) and incidental ventriculoperitoneal shunt (short arrow).
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aortic knuckle on the chest radiograph, but the most characteristic finding is notching of the undersurface of the third to eighth ribs visible after the age of 10 years (Figure 7). Kartagener’s syndrome − patients have dextrocardia (Figure 8) and situs inversus associated with bronchiectasis and paranasal sinusitis.
Pulmonary oedema Pulmonary oedema is most commonly cardiogenic, but there are non-cardiogenic causes such as fluid overload and increased pulmonary capillary permeability. Cardiogenic pulmonary oedema has different radiographic appearances at different stages, starting with increased opacification of the interstitial tissues of the lungs as they become thickened with fluid, and progressing to consolidation and pleural effusions (Figures 9 and 10). The perihilar regions contain more interstitial tissue than the rest of the lungs; this accounts for the perihilar distribution of opacification in the interstitial phase of pulmonary oedema (‘bat’s-wing’ pattern). Other, more subtle signs of interstitial pulmonary oedema include peribronchial cuffing seen as thickened and blurred walls of end-on bronchi (best seen in the perihilar regions), and septal lines. Septal lines are also known as Kerley B lines (Figure 11); they appear as thin, horizontal lines perpendicular to and abutting the pleural surfaces at the lung bases and are caused by fluid thickening the interstitial tissue in the septa within the lung tissue.
6 Patient with atrial septal defect, showing a small aortic knuckle (thin arrow) and enlarged proximal pulmonary arteries (bold arrows). This patient may be developing an Eisenmenger reaction, because the outer lung vessels are reduced.
Suspected pulmonary embolus A plain chest radiograph is seldom diagnostic of pulmonary embolus and is commonly normal. Its major roles are in the exclusion of other diagnoses (e.g. pneumothorax, pneumonia, rib fractures), and in planning the next appropriate investigation.
7 Patient with coarctation of the aorta, showing rib notching (arrow).
Congenital cardiac abnormalities Although most congenital cardiac anomalies are detected in utero or in the neonatal period, some less severe anomalies may present in adolescence or in adults. In addition, sequelae of corrected anomalies may be apparent on chest radiographs later in life. These conditions include intracardiac shunts (e.g. atrial and ventricular septal defects), aortic coarctation and Kartagener’s syndrome. Cardiac shunts − small shunts with a pulmonary blood flow: systemic blood flow ratio of less than 2:1 usually produce no detectable radiographic abnormality. Larger shunts eventually produce proximal pulmonary artery enlargement and pulmonary plethora (pulmonary vessels extending to within a few centimetres of the pleura). If these large shunts remain uncorrected, the patient may eventually develop an Eisenmenger reaction, with shunt reversal, recognized on the radiograph by enlargement of the proximal pulmonary arteries and reduction in size of the vessels in the mid-third and outer third of the lungs (pulmonary oligaemia). Both atrial and ventricular septal defects may be recognized by the pulmonary artery changes described above, but atrial defects commonly also exhibit a small aortic knuckle and normal cardiac size on chest radiography (Figure 6), whereas ventricular defects show a normal sized aorta, but associated cardiomegaly. Coarctation of the aorta comprises local stenosis of the aorta at the level of the aortic isthmus. Associated congenital anomalies occur in about 75% of patients. There may be a high or double MEDICINE 34:4
8 Kartagener’s syndrome. Note the side marker in the top right-hand corner, confirming that the film is the right way round. There is also bilateral basal bronchiectasis, demonstrated by the increased basal lung markings.
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Chest radiograph in pulmonary oedema Increased volume of blood in pulmonary circulation Upper lobe pulmonary vein enlargement
Fluid leaks into pulmonary interstitial tissue Perihilar haze Peribronchial cuffing Septal lines
Fluid leaks into the alveoli and airspaces Consolidation
a
Fluid leaks into the pleural spaces Effusions 9
b a Bilateral septal lines secondary to cardiac failure and fluid overload. b Close up of septal lines. 11
10 Patient with pulmonary oedema, showing the charateristic bat’s-wing appearance of perihilar opacification.
Patients with a normal chest radiograph and no significant history of asthma or chronic obstructive pulmonary disease may be investigated using radionuclide imaging (ventilation/perfusion (V/Q) scanning). This method detects areas of non-perfusion relative to ventilation (inferring an embolus as the cause) and is reported in terms of the probability that an embolus has occurred. A high-probability scan indicates a likelihood of up to 96%; a normal report effectively excludes pulmonary embolic disease. Intermediate and low-probability scans are now termed ‘nondiagnostic’ and suggest that further investigation is required. Patients with a significantly abnormal chest radiograph or obstructive lung disease are likely to produce a non-diagnostic V/Q scan and should be investigated using spiral CT (Figure 12).
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12 Spiral CT showing a pulmonary embolus (short arrow) on a conventional axial section, with distal consolidation (long arrow).
Rapid scanning techniques including multi-slice CT enable investigation of patients unsuitable for V/Q scanning, and directly visualize thrombus. Multi-slice CT has been shown to be at least as accurate as pulmonary angiography, which is now seldom performed. 141
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