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Imaging in chest disease
Investigations
Imaging in chest disease
What’s new?
Fergus V Gleeson
• Picture archiving and communication systems are a recent technological development enabling easy digital film storage and transmission • Multislice CT provides excellent contrast enhancement and has revolutionized the diagnosis of pulmonary embolic disease • High-resolution CT has had a major impact on diagnosis of diffuse lung disease
Abstract Imaging is critically important in the diagnosis and management of patients with chest disease, and the chest X-ray remains the most commonly performed imaging examination in medicine. Its correct interpretation is important in not only the diagnosis and exclusion of disease, but also in the choice of the next imaging investigation performed. As picture archiving and communication systems increase more doctors will have access to images and will need improved skills in their interpretation. The variety of chest imaging techniques available for patient investigation continues to increase, most importantly with the advent of positron emission tomography and computerized tomography (PET-CT), and there has also been a substantial improvement in pre-existing techniques. The decision on which technique to employ in disease investigation requires knowledge of their benefits and disadvantages, and information on how successfully these tests have been employed. Additionally, there is increasing evidence that clinician-performed imaging (ultrasound) provides substantial benefit to patients, with improved speed of access and safety. This chapter will provide information on the normal chest X-ray and its differential diagnosis when abnormal, and will discuss the use of a variety of imaging techniques in investigating common diseases.
• PET-CT has improved the accuracy of lung cancer staging, and should be performed on all patients prior to radical treatment, surgery or radiotherapy
that exposure of patients to any form of ionizing radiation now requires justification under the new 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.
Techniques in chest radiography Projections: the two standard views are the erect postero-anterior (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)
Keywords chest X-ray; computerized tomography; lung cancer; multislice CT; positron emission tomography; pulmonary embolic disease
Imaging in chest disease may be performed: • to investigate symptoms (e.g. high temperature, cough) • to confirm or exclude suspected diagnoses (e.g. pulmonary embolic disease, cancer) • in the further investigation of known diseases (e.g. diffuse lung disease) • to monitor disease status (e.g. consolidation in pneumonia, pleural effusion in empyema, mediastinal lymphadenopathy in lung cancer). A combination of different imaging modalities may be used to aid diagnosis and follow-up. Chest radiography remains the mainstay of imaging in the diagnosis and management of chest disease; 50% of all radiographs taken are of the chest (Tables 1–5). It must be remembered
Causes of an air bronchogram • Pneumonia – infective, eosinophilic • Pulmonary oedema • Haemorrhage – traumatic/contusion, infarction, diffuse pulmonary haemorrhage • Radiation pneumonitis • Non-obstructive pulmonary collapse – compressive atelectasis (e.g. pleural effusion), bronchiectatic collapse • Bronchoalveolar cell carcinoma • Lymphoma • Sarcoidosis • Cryptogenic organizing pneumonia • Crytogenic fibrosing alveolitis • Alveolar proteinosis
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Fergus V Gleeson FRCP FRCR is Consultant Radiologist at the Churchill Hospital, Oxford, UK. He qualified from the Royal London Hospital, London, and trained in radiology in Cambridge, London and Los Angeles, USA. His special interest is thoracic radiology and his research interest is pleural disease. Competing interests: none declared. Q1
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Most common causes.
Table 1
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Causes of a solitary pulmonary nodule
Causes of unilateral transradiancy on chest radiographs
Neoplasm • Bronchial carcinoma • Metastasis • Lymphoma • Benign neoplasm – hamartoma, adenoma Granuloma • Prior tuberculous infection • Prior Histoplasma infection • Rheumatoid nodule • Wegener’s granulomatosis Infection • Abscess – pyogenic • Round pneumonia • Hydatid Congenital • Sequestration • Bronchogenic cyst • Brochial atresia with mucoid impaction Vascular • Pulmonary infarction • Haematoma • Arteriovenous malformation Others • Amyloid • Rounded atelectasis • Progressive massive fibrosis
Technical Chest wall • Mastectomy • Poland’s syndrome • Poliomyelitis Pleura • Pneumothorax Lung • Emphysema • Bullae • Macleod’s syndrome • Compensatory hyperinflation – post-lobectomy, lobar collapse • Post-obstructive – has a vasoconstrictive component • Pulmonary embolus Most common causes.
Table 4
should be obtained. Each radiology department has a standardized system, including anode-to-film distance and exposure, enabling 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.
Most common causes.
Causes of mediastinal masses
Table 2
Anterior mediastinum • Retrosternal goitre • Lymph node enlargement – particularly lymphoma • Thymic mass • Germ cell tumour • Vascular enlargement – aneurysm of the ascending aorta • Sternal tumour – metastasis, sarcoma, myeloma • Morgagni hernia • Pericardial cyst • Pericardial fat Middle mediastinum • Lymph node enlargement – secondary to bronchial carcinoma, lymphoma, Castleman’s disease • Bronchogenic carcinoma • Bronchogenic cyst • Aortic aneurysm Posterior mediastinum • Neurogenic tumour • Paravertebral – abscess, metastasis, extramedullary haemopoiesis • Anterior thoracic meningocele • Oesophageal disease – achalasia, malignancy • Aortic aneurysm • Neuroenteric cyst • Hernia – hiatus, Bochdalek
• 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 an Accident and Emergency department, unless a dedicated chest radiography room is available.) Whenever possible, a radiology department PA film
Causes of predominantly upper zone fibrosis • Tuberculosis • Extrinsic allergic alveolitis • Silicosis • Sarcoidosis • Allergic bronchopulmonary aspergillosis • Radiation fibrosis • Ankylosing spondylitis Most common causes.
Table 3
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Lateral projections are obtained in conjunction with a PA film and are not necessary for every patient. They are useful for: • localizing an abnormality seen on a PA film • helping to clarify a potential abnormality • detecting disease behind the heart or diaphragm • in patients presenting with haemoptysis and a normal PA radiograph. Supine projections are used in ill patients. Detection of conditions such as pleural effusion and pneumothorax are more difficult than with PA or AP films, and cardiac and mediastinal assessment is difficult.
• 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, available clinical information should be taken into account. Interpretation should follow a routine, as follows. 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) 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 a chest radiograph. For example, the right heart border is normally clearly seen where it abuts 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 – Figure 1.) The silhouette sign may be used to identify and localize both mediastinal and pleural disease, and pulmonary disease (Figure 2). • 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, paying 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 subtle new disease, or for confirmation that a long-standing abnormality has changed or is of no significance.
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. 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.1 The patient is still exposed to X-rays, but the information derived by PACS can be imprinted onto a reusable photostimulable 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 outpatient departments. The radiation dose response of photostimulable plates differs from that of conventional film, allowing greater latitude in exposure, and images that would be under-exposed or over-exposed 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, and are never lost, as PACS servers are installed with a secure back-up. A further advantage of PACS is that the images are easy to copy, as with all digital data, and they can be sent via email to other radiologists and clinicians elsewhere for a second opinion. It is of course imperative that access to PACS is secure, and this is usually by personal password protection. Within a few years it is likely that all hospitals in Europe and America will be PACS based, and hard-copy films will be historical.
Interpreting the chest radiograph Whenever possible, chest radiographs and images on workstations 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. Use of conventional films and viewing boxes will decline with 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:
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Important anatomical features on frontal and lateral chest radiographs are shown in Figure 3. The mediastinal silhouette is formed by the structures outlined in Figure 3, but may change in appearance through enlargement of a normal structure (e.g. aortic aneurysm) 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, and its normal maximum diameter is 1 cm. It is commonly enlarged in patients with left ventricular failure and pulmonary oedema, or other causes of fluid overload. It may also be enlarged in patients with increased cardiac output (e.g. pregnancy, post-exercise). 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 of the vertebrae, but may become distorted by paravertebral soft tissue, vertebral collapse, nodal enlargement, neurogenic tumour or enlargement of the descending aorta.
Figure 1 Right middle lobe collapse, showing loss of the right heart border (arrow) as a result of replacement of aerated lung by collapsed lung, and locating its position using the silhouette sign.
Normal anatomy 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 failure, known as 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.
The chest radiograph in lung disease The basic patterns seen on chest radiographs in lung disease are: • air bronchogram • pulmonary collapse • pulmonary nodule/mass • multiple pulmonary nodules • ring shadows and cysts • line shadows • nodular, reticular, honeycombing • increased transradiancy. Air bronchogram: the intrapulmonary airways distal to the proximal segmental bronchi are not normally visible on a chest radiograph. When the normally aerated pulmonary parenchyma is replaced by non-aerated tissue, the bronchi and bronchioles become visible as branching, linear lucencies – the air bronchogram (Figure 4); this may occur because the alveolae are filled with fluid or cellular material, or because the interstitium expands and compresses the alveolae. The most common causes are pneumonia and pulmonary oedema (Table 1).
Figure 2 Increased radio-opacity (long arrow) is seen separate from left heart border (short arrow) and, using the silhouette sign, must lie posteriorly (behing the heart). The diagnosis was posterior loculated pleural effusion.
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Pulmonary collapse: the terms ‘collapse’ and ‘atelectasis’ are synonymous. However, ‘atelectasis’ is often used to denote 135
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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 Figure 3
incomplete collapse, and often implies that this is non-obstructive (e.g. linear atelectasis – band-like shadows at the lung bases in patients who are unable to take a deep breath in, or following, pneumonia or infarction). Atelectasis is also used to describe loss of lung volume secondary to a non-obstructive cause (e.g. passive or compressive atelectasis secondary to pleural effusion). Pulmonary collapse secondary to an obstructive cause may involve a subsegment or segment of a lobe, a complete lobe or an entire lung. The most common cause is bronchial carcinoma, but it is important to exclude other causes (e.g. mucus plug, foreign body, extrinsic compression by lymphadenopathy (Figures 5 and 6).
appearance, but 95% are caused by malignant neoplasm, infective granulomata or benign neoplasm (Table 2).
Pulmonary nodule/mass: a pulmonary nodule is defined as an opacity with a definable edge up to 3 cm in diameter; a mass is a lesion greater than 3 cm. In practice, however, this distinction is of no clinical significance. When a nodule or mass is identified on a chest radiograph, it is important to determine the following. • Is it single or multiple? • Is its contour smooth (raising the possibility of benign disease such as hamartoma or adenoma) or spiculated (more suggestive of malignant disease)? • What is its structure? Is it of soft tissue density, or does it contain air bronchograms? Calcification may suggest benign disease (e.g. a hamartoma –‘popcorn calcification’) or, if central, an infective granuloma. CT can often more clearly identify the features of a nodule or mass detected on chest radiography. Specific features on chest radiography or CT may be diagnostic (e.g. a feeding vessel in an arteriovenous malformation – Figure 7, a peripheral air halo in mycetoma). It is also important to review the radiograph and CT scans for associated abnormalities (e.g. lymphadenopathy, pleural effusion, bone destruction) that may provide a unifying diagnosis such as bronchogenic carcinoma. Most nodules have a nonspecific
Ring shadows and cysts: a ring shadow is an annular opacity with a transradiant centre. The ring may be hollow, or may contain an air–fluid level or a mass. Rings of more than 1 cm in diameter with thin walls are termed ‘cysts’. Air-filled cysts that have developed after trauma or infection are termed ‘pneumatoceles’ and should be differentiated from bullae, which are similar in appearance but occur in association with emphysema. Thicker-walled ring shadows are termed ‘cavities’; these are most commonly seen in infective consolidation, classically in the upper lobes in post-primary tuberculosis, and in anaerobic and staphylococcal pneumonia. Neoplastic lesions may also cavitate (Figure 8) and tend to have irregular, thick walls not associated with consolidation; this helps to distinguish them from infective cavities, though there are exceptions. The presence of an air–fluid level is unhelpful in determining the diagnosis. Other causes of cavities are Wegener’s granulomatosis, infarcts and contusion.
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Multiple pulmonary nodules may be divided into nodules of diameter 0.5–2 mm, 2–5 mm and more than 5 mm. Nodules of more than 5 mm in diameter may be considered individual lesions. In general, granulomatous and infective nodules tend to be of uniform size, whereas metastases tend to vary. It is often difficult to decide whether the chest radiograph genuinely shows multiple small nodules, or whether the appearance is within normal limits. It may be helpful to assess whether the nodules are too peripheral or too large to be vessels.
Line shadows: linear opacities are elongated shadows that are termed ‘lines’ when thin, ‘reticular’ when multiple and interwoven, and ‘bands’ when more than 5 mm thick. It can be difficult to decide whether lines are normal structures; it may be 136
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Figure 6 The left lower lobe is collapsed and is clearly seen as a triangular opacity (arrow) behind the left heart border.
thickened interlobular septa secondary to fluid or tissue accumulation. Normal vessels are not visualized this peripherally. Septal (Kerley) A lines are less common than Kerley B lines, but also represent thickened interlobular septae. They are seen in the mid-zone and upper zone. They appear similar to B lines, but lie away from the pleural surfaces and point to the hila. Parallel line shadows result from thickening of the bronchial walls or an increase in fluid or tissue in the peribronchial interstitium. The distal segmental and subsegmental bronchial walls are not visualized on normal chest radiographs. Peribronchial ‘cuffing’ is a feature in patients with heart failure and is also seen in malignant infiltration; in both cases, the airways are not dilated. Bronchial wall thickening is characteristically seen in bronchiectasis, and in these cases is associated with airway dilatation. Other features may be crowding of the bronchi, and mucus plugs seen as nodules, thick branching lines, or (if seen end-on) ring structures that may contain air–fluid levels. Ring shadows adjacent to smaller vessels end-on are termed ‘signet rings’. CT and, particularly, high-resolution CT (HRCT), now often performed as a multislice CT scan (MSCT), are more sensitive means of detecting bronchiectasis (Figure 10). HRCT and MSCT are the imaging modalities of choice for confirming the degree and distribution of suspected bronchiectasis.
a Air bronchogram (arrow) with the bronchi surrounded by consolidated lung. b Air bronchogram (arrow) on CT. Figure 4
helpful to determine whether there are too many lines, whether they are too peripheral, and whether they do not branch in a normal pattern. Septal (Kerley) B lines are 1–2 mm thick, 1–2 cm long horizontal lines, abut the pleura at right angles (Figure 9), and represent
Nodular, reticular and honeycombing patterns: many pulmonary diseases cause small opacities on the chest radiograph. In most cases, the opacities probably represent the summation of individual lesions, because a miliary (< 1 mm) nodule or an interstitial line would not be resolved by eye. Multiple small nodules may themselves produce a reticulonodular pattern. When deciding whether a genuine abnormality is present, it may be helpful to determine whether the pulmonary vessels are less clearly visualized than normal, and particularly whether their edges are less well defined. Use of a lateral film may help. Increasingly, HRCT and MSCT are often used to confirm any abnormality, to provide a more specific diagnosis than that possible on chest radiography, and to assess the severity. The descriptive terms used in both chest radiography and HRCT/MSCT are as follows.
Figure 5 Right upper lobe collapse (long arrow). Note that the right hemidiaphragm is elevated (juxtaphrenic peak sign, short arrow), pulled upwards by the collapsed lobe.
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Figure 9 Septal lines, seen as fine lines perpendicular to the chest wall.
blacker than normal on HRCT/MSCT. It may be caused by a fine increase in interstitial thickening, subtle air-space opacification, or a combination of the two. When examining the chest radiograph for diffuse interstitial lung disease, it is helpful to assess the zonal predominance (craniocaudally) and the axial distribution (upper lobe, perihilar, subpleural or diffuse). Table 3 lists causes of upper lobe fibrosis. Lung volume is reduced in most fibrotic lung diseases, and normal or increased in cystic fibrosis, emphysema, Langerhans’ cell histiocytosis, lymphangioleiomyomatosis, tuberous sclerosis and neurofibromatosis. Septal lines are a useful discriminatory finding most commonly seen in heart failure and malignancy. Fine, linear opacities are more commonly seen in fibrotic lung diseases such as fibrosing alveolitis, asbestosis and extrinsic allergic alveolitis. Associated abnormalities (e.g. pleural plaques in asbestosis, hilar and mediastinal lymphadenopathy in sarcoidosis and malignancy) should be noted.
a Nodule with a smooth border (long arrow) and a vessel passing to it (short arrow). b The nodule is seen to have a feeding vessel, an artery and a draining vessel (arrows). This is characteristic of an arteriovenous malformation. Figure 7
• Nodular pattern comprises multiple, apparently defined nodules of diameter 1–5 mm. • Reticular pattern comprises fine, linear shadows, often ‘crisscrossing’ each other to form a net-like pattern. • Reticulonodular pattern comprises lines and nodules. This is probably the most common diffuse interstitial lung disease pattern seen on chest radiography. • Honeycombing is a reticulonodular pattern with clearly visualized cystic air-spaces within the net-like pattern. • Ground-glass opacification is a subtle haze or increase in attenuation that makes the outline of the pulmonary vessels less distinct than normal on chest radiography and the bronchi
Increased transradiancy can affect both lungs, one lung, or variable portions of one or both lungs (Table 4). • Increased transradiancy of both lungs is seen in pulmonary disease such as emphysema, bronchiolitis and asthma, and in cardiovascular disease such as right-to-left cardiac shunts. • In most cases, unilateral changes in transradiancy represent an abnormality of the affected side. It may be secondary to a
Figure 8 CT scan showing a cavitating mass with an irregular wall and an airfluid level (arrow). On biopsy, it was found to be a squamous cell carcinoma.
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Figure 10 CT scan showing bronchiectasis.
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chest wall abnormality such as congenital absence of pectoralis muscle (Poland’s syndrome), mastectomy or chest wall tumour (reduced transradiancy). • Pleural abnormalities such as effusion in supine patients may cause a subtle decrease in transradiancy, or pneumothorax causing increased transradiancy.
Diffuse lung disease Although all investigations of diffuse lung disease are initially undertaken via the chest radiograph, HRCT or thin section volumetric MSCT are increasingly used to confirm a suspected diagnosis (Figures 11 and 12), or to make a diagnosis when the radiographic appearances are non-specific.2 HRCT or thin section MSCT obtains thin sections through the chest that enable assessment of detailed structures within the pulmonary parenchyma down to a spatial resolution of 0.1–0.2 mm.3 In addition to the resulting greater diagnostic capability, there is a greater correlation between appearances and lung function than on chest radiography, and is of greater value in assessing the extent of disease and response to treatment. HRCT and MSCT are also valuable in guiding lung biopsy.
Figure 12 Cryptogenic fibrosing alveolitis on high-resolution CT, showing characteristic subpleural distribution of reticulation and honeycombing.
Pleural disease Pleural effusion and pneumothorax are the most common pleural diseases requiring radiological investigation. In both cases, the diagnosis is usually obvious from the chest radiograph; further investigations are often not required. The cause of secondary spontaneous pneumothorax may be identified on chest radiography, but if investigated further is more commonly seen on CT or
Mediastinal disease Neoplasms, vascular lesions, developmental abnormalities and inflammatory masses account for most mediastinal diseases. Classification of mediastinal masses (Table 5) by location is not based on true anatomical boundaries that limit disease spread, but is a useful means of remembering which disease is most likely to occur in which site. The mediastinum can be divided into the following areas: • anterior (anterior to the pericardium and trachea) • middle (between the anterior and posterior mediastinum – contains the heart, great vessels and pulmonary roots) • posterior (posterior to the pericardial surface). When describing mediastinal masses, it is important to accurately describe their position, size, character (e.g. containing fat, fluid or calcification) and borders. In almost all cases, CT or MRI is used to help characterize mediastinal masses. CT is of greatest value in the anterior and middle mediastinum, and MRI in the posterior mediastinum (Figure 13).
a Soft tissue mass (arrow) lying towards the apex in the posterior mediastinum. b The mass is seen to be high-signal on this coronal STIR sequence MRI scan. This is a characteristic feature of benign neurogenic tumours.
Figure 11 Lymphangitis carcinomatosis on high-resolution CT. The patient has previously undergone left mastectomy. The interlobar septal thickening (arrows) is characteristic of lymphangitis carcinomatosis.
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Figure 13
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HRCT/MSCT. It is more difficult to make the diagnosis in supine patients, and confirmation by CT is often required in trauma and ICU patients. The presence of a pleural effusion seen on the chest radiograph can be readily confirmed by ultrasonography (Figure 14). If necessary, aspiration or drain placement can be performed at the same time. Both ultrasonography and CT can also be used to further evaluate the character and cause of a pleural effusion. Effusions can be non-loculated and able to move freely within the pleural space, dependent on the patient’s position, or loculated as a result of pleural adhesions. Occasionally, pleural fluid is collected under the lung (subpulmonic) and mimics an elevated hemidiaphragm. In supine patients, pleural effusions produce a general increase in the opacification of the hemithorax, but the vessels remain distinct because they are still surrounded by aerated lung. Primary (mesothelioma) and metastatic pleural tumours are readily investigated using CT (Figure 15), and targeted biopsy can be performed.4 Q3
Figure 15 CT scan showing a right-sided pleural effusion with multiple pleural metastases (short arrows). The long arrow indicates the collapsed (passive) right lower lobe.
Lung cancer
• CT, MRI and ultrasonography have all been used to determine the presence of mediastinal or chest wall invasion. Ultrasonography is the most sensitive method for chest wall invasion. CT and MRI lack sensitivity and specificity in the detection of invasion, and patients should not be denied surgery on the basis of CT or MRI unless the results are categorically positive. • Positron-emission tomography has recently been shown to be valuable in detecting metastatic disease not seen on CT, and in helping determine which patients would most benefit from surgery. It uses a radiolabelled derivative of glucose (18fluorodeoxyglucose; FDG) that is avidly taken up and trapped in malignant cells, where it breaks down to produce a positron that instantly collides with an electron, producing two photons that are detectable as a signal. This is described as “activity or avidity”, with the degree of activity broadly reflecting the metabolic activity of the tumour and this roughly equates to its malignant potential. Very FDG avid lung cancers have a less good prognosis than minimally FDG avid tumours. PET appears to be reasonably specific for malignancy, with only infection and granulomatous disease producing false-positive results but these are often less
Although most lung cancers are seen on the chest radiograph at presentation, a small proportion (2–5%) are visible only on bronchoscopy or CT. Patients with suspected bronchial carcinoma and a normal chest radiograph therefore warrant further investigation, particularly if they present with haemoptysis. It has been shown from screening programme data that, in up to 90% of patients with an asymptomatic carcinoma, the tumour was visible but not reported on prior chest radiographs. CT is required for staging. It should include the entire chest and liver, and is performed after administration of intravenous contrast. Up to 30% of patients have metastatic disease on CT at presentation. CT also helps in determining whether bronchoscopy or imageguided biopsy will best provide a tissue diagnosis (Figure 16). When staging lung cancer, the aim is to triage patients into a group in which surgery may achieve a cure and a group in which surgery would be futile. Assessment of local disease extent, nodal spread and extrathoracic metastatic disease is necessary. • CT is routinely used to assess local disease extent and nodal disease. Mediastinal nodes are readily detected and are regarded as enlarged if more than 1 cm in short-axis diameter.
Figure 16 CT scan showing a bronchogenic carcinoma into which a subsegmental bronchus is passing (arrow), suggesting that a diagnosis could be made on bronchoscopy.
Figure 14 Ultrasound scan showing a pleural effusion containing echogenic fluid (star). The right lower lobe is passively collapsed (arrow).
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FDG avid than tumour. The absence of FDG activity in areas of abnormality detected on CT is a good marker for the absence of malignancy. Recently PET scanners have been combined with CT scanners as PET-CT scanners, and this has enabled the production of anatomical and functional (metabolic) information (Figure 17). This combination appears to enable more accurate staging and may also provide prognostic information.5
Suspected pulmonary embolus A plain chest radiograph is seldom diagnostic of pulmonary embolus and is commonly normal. Its major role is in the exclusion of other diagnoses (e.g. pneumothorax, pneumonia, rib fractures), and it is helpful in planning the next appropriate investigation. 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 ‘non-diagnostic’ 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 18). Rapid scanning techniques including multislice CT enable investigation of patients unsuitable for V/Q scanning, and directly visualize thrombus. Multislice CT has been shown to
Figure 18 Spiral CT showing a pulmonary embolus (short arrow) on a conventional axial section, with distal consolidation (long arrow).
be at least as accurate as pulmonary angiography, which is now seldom performed.6
Interventional radiology The advent of more flexible imaging techniques and new needle and drain technology has promoted greater use of image-guided intervention. Image-guided drain placement for pleural effusion has reduced the requirement for surgical treatment of empyema, and lung and mediastinal needle biopsy is increasingly used to establish a tissue diagnosis before thoracotomy and in patients unsuitable for surgery. ◆
References 1 McAdams HP, Samei E, Dobbins J, Tourassi GD, Ravin CE. Recent advances in chest radiography. Radiology 2006; 241: 663–81. 2 Aziz AA, Wells AU, Bateman ED, et al. Interstitial lung disease: effects of thin-section CT on clinical decision making. Radiology 2005; 238: 725–33. 3 Webb WR. Thin-section CT of the secondary pulmonary lobule: anatomy and the image – the Fleischner lecture. Radiology 2006; 239: 322–38. 4 Adams RF, Gleeson FV. Percutaneous image-guided cutting-needle biopsy of the pleura in the presence of a suspected malignant effusion. Radiology 2001; 219: 510–14. 5 Van Tinteren H, Hoekstra OS, Smit EF, et al. Effectiveness of positron emission tomography in the preoperative assessment of patients with suspected non-small cell lung cancer: the PLUS multicentre randomised trial. Lancet 2002; 3569: 1388–92. 6 Schoepf UJ, Costello P. CT angiography for diagnosis of pulmonary embolism: state of the art. Radiology 2004; 230: 329–37.
Further reading British Thoracic Society guidelines on the investigation of pleural disease. Thorax 2003; 58(suppl 2). British Thoracic Society guidelines on the investigation of pulmonary embolic disease. Thorax 2003; 58: 470–83. British Thoracic Society guidelines on the selection of patients with lung cancer for surgery. Thorax 2001; 56: 89–109 Hansell DM, Armstrong P, Lynch DA, McAdams HP. Imaging of diseases of the chest, 4th edn. Chicago: Mosby, 2000.
Figure 17 Coronal image from a PET-CT scan demonstrating a right upper lobe tumour (large arrow) with adjacent involved right hilar nodes (small arrow).
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Imaging in chest disease
What’s new?
Fergus V Gleeson
• Picture archiving and communication systems are a recent technological development enabling easy digital film storage and transmission • Multislice CT provides excellent contrast enhancement and has revolutionized the diagnosis of pulmonary embolic disease • High-resolution CT has had a major impact on diagnosis of diffuse lung disease
Abstract Imaging is critically important in the diagnosis and management of patients with chest disease, and the chest X-ray remains the most commonly performed imaging examination in medicine. Its correct interpretation is important in not only the diagnosis and exclusion of disease, but also in the choice of the next imaging investigation performed. As picture archiving and communication systems increase more doctors will have access to images and will need improved skills in their interpretation. The variety of chest imaging techniques available for patient investigation continues to increase, most importantly with the advent of positron emission tomography and computerized tomography (PET-CT), and there has also been a substantial improvement in pre-existing techniques. The decision on which technique to employ in disease investigation requires knowledge of their benefits and disadvantages, and information on how successfully these tests have been employed. Additionally, there is increasing evidence that clinician-performed imaging (ultrasound) provides substantial benefit to patients, with improved speed of access and safety. This chapter will provide information on the normal chest X-ray and its differential diagnosis when abnormal, and will discuss the use of a variety of imaging techniques in investigating common diseases.
• PET-CT has improved the accuracy of lung cancer staging, and should be performed on all patients prior to radical treatment, surgery or radiotherapy
that exposure of patients to any form of ionizing radiation now requires justification under the new 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.
Techniques in chest radiography Projections: the two standard views are the erect postero-anterior (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)
Keywords chest X-ray; computerized tomography; lung cancer; multislice CT; positron emission tomography; pulmonary embolic disease
Imaging in chest disease may be performed: • to investigate symptoms (e.g. high temperature, cough) • to confirm or exclude suspected diagnoses (e.g. pulmonary embolic disease, cancer) • in the further investigation of known diseases (e.g. diffuse lung disease) • to monitor disease status (e.g. consolidation in pneumonia, pleural effusion in empyema, mediastinal lymphadenopathy in lung cancer). A combination of different imaging modalities may be used to aid diagnosis and follow-up. Chest radiography remains the mainstay of imaging in the diagnosis and management of chest disease; 50% of all radiographs taken are of the chest (Tables 1–5). It must be remembered
Causes of an air bronchogram • Pneumonia – infective, eosinophilic • Pulmonary oedema • Haemorrhage – traumatic/contusion, infarction, diffuse pulmonary haemorrhage • Radiation pneumonitis • Non-obstructive pulmonary collapse – compressive atelectasis (e.g. pleural effusion), bronchiectatic collapse • Bronchoalveolar cell carcinoma • Lymphoma • Sarcoidosis • Cryptogenic organizing pneumonia • Crytogenic fibrosing alveolitis • Alveolar proteinosis
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Fergus V Gleeson FRCP FRCR is Consultant Radiologist at the Churchill Hospital, Oxford, UK. He qualified from the Royal London Hospital, London, and trained in radiology in Cambridge, London and Los Angeles, USA. His special interest is thoracic radiology and his research interest is pleural disease. Competing interests: none declared. Q1
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Most common causes.
Table 1
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Causes of a solitary pulmonary nodule
Causes of unilateral transradiancy on chest radiographs
Neoplasm • Bronchial carcinoma • Metastasis • Lymphoma • Benign neoplasm – hamartoma, adenoma Granuloma • Prior tuberculous infection • Prior Histoplasma infection • Rheumatoid nodule • Wegener’s granulomatosis Infection • Abscess – pyogenic • Round pneumonia • Hydatid Congenital • Sequestration • Bronchogenic cyst • Brochial atresia with mucoid impaction Vascular • Pulmonary infarction • Haematoma • Arteriovenous malformation Others • Amyloid • Rounded atelectasis • Progressive massive fibrosis
Technical Chest wall • Mastectomy • Poland’s syndrome • Poliomyelitis Pleura • Pneumothorax Lung • Emphysema • Bullae • Macleod’s syndrome • Compensatory hyperinflation – post-lobectomy, lobar collapse • Post-obstructive – has a vasoconstrictive component • Pulmonary embolus Most common causes.
Table 4
should be obtained. Each radiology department has a standardized system, including anode-to-film distance and exposure, enabling 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.
Most common causes.
Causes of mediastinal masses
Table 2
Anterior mediastinum • Retrosternal goitre • Lymph node enlargement – particularly lymphoma • Thymic mass • Germ cell tumour • Vascular enlargement – aneurysm of the ascending aorta • Sternal tumour – metastasis, sarcoma, myeloma • Morgagni hernia • Pericardial cyst • Pericardial fat Middle mediastinum • Lymph node enlargement – secondary to bronchial carcinoma, lymphoma, Castleman’s disease • Bronchogenic carcinoma • Bronchogenic cyst • Aortic aneurysm Posterior mediastinum • Neurogenic tumour • Paravertebral – abscess, metastasis, extramedullary haemopoiesis • Anterior thoracic meningocele • Oesophageal disease – achalasia, malignancy • Aortic aneurysm • Neuroenteric cyst • Hernia – hiatus, Bochdalek
• 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 an Accident and Emergency department, unless a dedicated chest radiography room is available.) Whenever possible, a radiology department PA film
Causes of predominantly upper zone fibrosis • Tuberculosis • Extrinsic allergic alveolitis • Silicosis • Sarcoidosis • Allergic bronchopulmonary aspergillosis • Radiation fibrosis • Ankylosing spondylitis Most common causes.
Table 3
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Lateral projections are obtained in conjunction with a PA film and are not necessary for every patient. They are useful for: • localizing an abnormality seen on a PA film • helping to clarify a potential abnormality • detecting disease behind the heart or diaphragm • in patients presenting with haemoptysis and a normal PA radiograph. Supine projections are used in ill patients. Detection of conditions such as pleural effusion and pneumothorax are more difficult than with PA or AP films, and cardiac and mediastinal assessment is difficult.
• 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, available clinical information should be taken into account. Interpretation should follow a routine, as follows. 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) 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 a chest radiograph. For example, the right heart border is normally clearly seen where it abuts 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 – Figure 1.) The silhouette sign may be used to identify and localize both mediastinal and pleural disease, and pulmonary disease (Figure 2). • 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, paying 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 subtle new disease, or for confirmation that a long-standing abnormality has changed or is of no significance.
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. 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.1 The patient is still exposed to X-rays, but the information derived by PACS can be imprinted onto a reusable photostimulable 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 outpatient departments. The radiation dose response of photostimulable plates differs from that of conventional film, allowing greater latitude in exposure, and images that would be under-exposed or over-exposed 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, and are never lost, as PACS servers are installed with a secure back-up. A further advantage of PACS is that the images are easy to copy, as with all digital data, and they can be sent via email to other radiologists and clinicians elsewhere for a second opinion. It is of course imperative that access to PACS is secure, and this is usually by personal password protection. Within a few years it is likely that all hospitals in Europe and America will be PACS based, and hard-copy films will be historical.
Interpreting the chest radiograph Whenever possible, chest radiographs and images on workstations 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. Use of conventional films and viewing boxes will decline with 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:
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Important anatomical features on frontal and lateral chest radiographs are shown in Figure 3. The mediastinal silhouette is formed by the structures outlined in Figure 3, but may change in appearance through enlargement of a normal structure (e.g. aortic aneurysm) 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, and its normal maximum diameter is 1 cm. It is commonly enlarged in patients with left ventricular failure and pulmonary oedema, or other causes of fluid overload. It may also be enlarged in patients with increased cardiac output (e.g. pregnancy, post-exercise). 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 of the vertebrae, but may become distorted by paravertebral soft tissue, vertebral collapse, nodal enlargement, neurogenic tumour or enlargement of the descending aorta.
Figure 1 Right middle lobe collapse, showing loss of the right heart border (arrow) as a result of replacement of aerated lung by collapsed lung, and locating its position using the silhouette sign.
Normal anatomy 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 failure, known as 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.
The chest radiograph in lung disease The basic patterns seen on chest radiographs in lung disease are: • air bronchogram • pulmonary collapse • pulmonary nodule/mass • multiple pulmonary nodules • ring shadows and cysts • line shadows • nodular, reticular, honeycombing • increased transradiancy. Air bronchogram: the intrapulmonary airways distal to the proximal segmental bronchi are not normally visible on a chest radiograph. When the normally aerated pulmonary parenchyma is replaced by non-aerated tissue, the bronchi and bronchioles become visible as branching, linear lucencies – the air bronchogram (Figure 4); this may occur because the alveolae are filled with fluid or cellular material, or because the interstitium expands and compresses the alveolae. The most common causes are pneumonia and pulmonary oedema (Table 1).
Figure 2 Increased radio-opacity (long arrow) is seen separate from left heart border (short arrow) and, using the silhouette sign, must lie posteriorly (behing the heart). The diagnosis was posterior loculated pleural effusion.
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Pulmonary collapse: the terms ‘collapse’ and ‘atelectasis’ are synonymous. However, ‘atelectasis’ is often used to denote 135
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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 Figure 3
incomplete collapse, and often implies that this is non-obstructive (e.g. linear atelectasis – band-like shadows at the lung bases in patients who are unable to take a deep breath in, or following, pneumonia or infarction). Atelectasis is also used to describe loss of lung volume secondary to a non-obstructive cause (e.g. passive or compressive atelectasis secondary to pleural effusion). Pulmonary collapse secondary to an obstructive cause may involve a subsegment or segment of a lobe, a complete lobe or an entire lung. The most common cause is bronchial carcinoma, but it is important to exclude other causes (e.g. mucus plug, foreign body, extrinsic compression by lymphadenopathy (Figures 5 and 6).
appearance, but 95% are caused by malignant neoplasm, infective granulomata or benign neoplasm (Table 2).
Pulmonary nodule/mass: a pulmonary nodule is defined as an opacity with a definable edge up to 3 cm in diameter; a mass is a lesion greater than 3 cm. In practice, however, this distinction is of no clinical significance. When a nodule or mass is identified on a chest radiograph, it is important to determine the following. • Is it single or multiple? • Is its contour smooth (raising the possibility of benign disease such as hamartoma or adenoma) or spiculated (more suggestive of malignant disease)? • What is its structure? Is it of soft tissue density, or does it contain air bronchograms? Calcification may suggest benign disease (e.g. a hamartoma –‘popcorn calcification’) or, if central, an infective granuloma. CT can often more clearly identify the features of a nodule or mass detected on chest radiography. Specific features on chest radiography or CT may be diagnostic (e.g. a feeding vessel in an arteriovenous malformation – Figure 7, a peripheral air halo in mycetoma). It is also important to review the radiograph and CT scans for associated abnormalities (e.g. lymphadenopathy, pleural effusion, bone destruction) that may provide a unifying diagnosis such as bronchogenic carcinoma. Most nodules have a nonspecific
Ring shadows and cysts: a ring shadow is an annular opacity with a transradiant centre. The ring may be hollow, or may contain an air–fluid level or a mass. Rings of more than 1 cm in diameter with thin walls are termed ‘cysts’. Air-filled cysts that have developed after trauma or infection are termed ‘pneumatoceles’ and should be differentiated from bullae, which are similar in appearance but occur in association with emphysema. Thicker-walled ring shadows are termed ‘cavities’; these are most commonly seen in infective consolidation, classically in the upper lobes in post-primary tuberculosis, and in anaerobic and staphylococcal pneumonia. Neoplastic lesions may also cavitate (Figure 8) and tend to have irregular, thick walls not associated with consolidation; this helps to distinguish them from infective cavities, though there are exceptions. The presence of an air–fluid level is unhelpful in determining the diagnosis. Other causes of cavities are Wegener’s granulomatosis, infarcts and contusion.
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Multiple pulmonary nodules may be divided into nodules of diameter 0.5–2 mm, 2–5 mm and more than 5 mm. Nodules of more than 5 mm in diameter may be considered individual lesions. In general, granulomatous and infective nodules tend to be of uniform size, whereas metastases tend to vary. It is often difficult to decide whether the chest radiograph genuinely shows multiple small nodules, or whether the appearance is within normal limits. It may be helpful to assess whether the nodules are too peripheral or too large to be vessels.
Line shadows: linear opacities are elongated shadows that are termed ‘lines’ when thin, ‘reticular’ when multiple and interwoven, and ‘bands’ when more than 5 mm thick. It can be difficult to decide whether lines are normal structures; it may be 136
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Figure 6 The left lower lobe is collapsed and is clearly seen as a triangular opacity (arrow) behind the left heart border.
thickened interlobular septa secondary to fluid or tissue accumulation. Normal vessels are not visualized this peripherally. Septal (Kerley) A lines are less common than Kerley B lines, but also represent thickened interlobular septae. They are seen in the mid-zone and upper zone. They appear similar to B lines, but lie away from the pleural surfaces and point to the hila. Parallel line shadows result from thickening of the bronchial walls or an increase in fluid or tissue in the peribronchial interstitium. The distal segmental and subsegmental bronchial walls are not visualized on normal chest radiographs. Peribronchial ‘cuffing’ is a feature in patients with heart failure and is also seen in malignant infiltration; in both cases, the airways are not dilated. Bronchial wall thickening is characteristically seen in bronchiectasis, and in these cases is associated with airway dilatation. Other features may be crowding of the bronchi, and mucus plugs seen as nodules, thick branching lines, or (if seen end-on) ring structures that may contain air–fluid levels. Ring shadows adjacent to smaller vessels end-on are termed ‘signet rings’. CT and, particularly, high-resolution CT (HRCT), now often performed as a multislice CT scan (MSCT), are more sensitive means of detecting bronchiectasis (Figure 10). HRCT and MSCT are the imaging modalities of choice for confirming the degree and distribution of suspected bronchiectasis.
a Air bronchogram (arrow) with the bronchi surrounded by consolidated lung. b Air bronchogram (arrow) on CT. Figure 4
helpful to determine whether there are too many lines, whether they are too peripheral, and whether they do not branch in a normal pattern. Septal (Kerley) B lines are 1–2 mm thick, 1–2 cm long horizontal lines, abut the pleura at right angles (Figure 9), and represent
Nodular, reticular and honeycombing patterns: many pulmonary diseases cause small opacities on the chest radiograph. In most cases, the opacities probably represent the summation of individual lesions, because a miliary (< 1 mm) nodule or an interstitial line would not be resolved by eye. Multiple small nodules may themselves produce a reticulonodular pattern. When deciding whether a genuine abnormality is present, it may be helpful to determine whether the pulmonary vessels are less clearly visualized than normal, and particularly whether their edges are less well defined. Use of a lateral film may help. Increasingly, HRCT and MSCT are often used to confirm any abnormality, to provide a more specific diagnosis than that possible on chest radiography, and to assess the severity. The descriptive terms used in both chest radiography and HRCT/MSCT are as follows.
Figure 5 Right upper lobe collapse (long arrow). Note that the right hemidiaphragm is elevated (juxtaphrenic peak sign, short arrow), pulled upwards by the collapsed lobe.
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Figure 9 Septal lines, seen as fine lines perpendicular to the chest wall.
blacker than normal on HRCT/MSCT. It may be caused by a fine increase in interstitial thickening, subtle air-space opacification, or a combination of the two. When examining the chest radiograph for diffuse interstitial lung disease, it is helpful to assess the zonal predominance (craniocaudally) and the axial distribution (upper lobe, perihilar, subpleural or diffuse). Table 3 lists causes of upper lobe fibrosis. Lung volume is reduced in most fibrotic lung diseases, and normal or increased in cystic fibrosis, emphysema, Langerhans’ cell histiocytosis, lymphangioleiomyomatosis, tuberous sclerosis and neurofibromatosis. Septal lines are a useful discriminatory finding most commonly seen in heart failure and malignancy. Fine, linear opacities are more commonly seen in fibrotic lung diseases such as fibrosing alveolitis, asbestosis and extrinsic allergic alveolitis. Associated abnormalities (e.g. pleural plaques in asbestosis, hilar and mediastinal lymphadenopathy in sarcoidosis and malignancy) should be noted.
a Nodule with a smooth border (long arrow) and a vessel passing to it (short arrow). b The nodule is seen to have a feeding vessel, an artery and a draining vessel (arrows). This is characteristic of an arteriovenous malformation. Figure 7
• Nodular pattern comprises multiple, apparently defined nodules of diameter 1–5 mm. • Reticular pattern comprises fine, linear shadows, often ‘crisscrossing’ each other to form a net-like pattern. • Reticulonodular pattern comprises lines and nodules. This is probably the most common diffuse interstitial lung disease pattern seen on chest radiography. • Honeycombing is a reticulonodular pattern with clearly visualized cystic air-spaces within the net-like pattern. • Ground-glass opacification is a subtle haze or increase in attenuation that makes the outline of the pulmonary vessels less distinct than normal on chest radiography and the bronchi
Increased transradiancy can affect both lungs, one lung, or variable portions of one or both lungs (Table 4). • Increased transradiancy of both lungs is seen in pulmonary disease such as emphysema, bronchiolitis and asthma, and in cardiovascular disease such as right-to-left cardiac shunts. • In most cases, unilateral changes in transradiancy represent an abnormality of the affected side. It may be secondary to a
Figure 8 CT scan showing a cavitating mass with an irregular wall and an airfluid level (arrow). On biopsy, it was found to be a squamous cell carcinoma.
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Figure 10 CT scan showing bronchiectasis.
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chest wall abnormality such as congenital absence of pectoralis muscle (Poland’s syndrome), mastectomy or chest wall tumour (reduced transradiancy). • Pleural abnormalities such as effusion in supine patients may cause a subtle decrease in transradiancy, or pneumothorax causing increased transradiancy.
Diffuse lung disease Although all investigations of diffuse lung disease are initially undertaken via the chest radiograph, HRCT or thin section volumetric MSCT are increasingly used to confirm a suspected diagnosis (Figures 11 and 12), or to make a diagnosis when the radiographic appearances are non-specific.2 HRCT or thin section MSCT obtains thin sections through the chest that enable assessment of detailed structures within the pulmonary parenchyma down to a spatial resolution of 0.1–0.2 mm.3 In addition to the resulting greater diagnostic capability, there is a greater correlation between appearances and lung function than on chest radiography, and is of greater value in assessing the extent of disease and response to treatment. HRCT and MSCT are also valuable in guiding lung biopsy.
Figure 12 Cryptogenic fibrosing alveolitis on high-resolution CT, showing characteristic subpleural distribution of reticulation and honeycombing.
Pleural disease Pleural effusion and pneumothorax are the most common pleural diseases requiring radiological investigation. In both cases, the diagnosis is usually obvious from the chest radiograph; further investigations are often not required. The cause of secondary spontaneous pneumothorax may be identified on chest radiography, but if investigated further is more commonly seen on CT or
Mediastinal disease Neoplasms, vascular lesions, developmental abnormalities and inflammatory masses account for most mediastinal diseases. Classification of mediastinal masses (Table 5) by location is not based on true anatomical boundaries that limit disease spread, but is a useful means of remembering which disease is most likely to occur in which site. The mediastinum can be divided into the following areas: • anterior (anterior to the pericardium and trachea) • middle (between the anterior and posterior mediastinum – contains the heart, great vessels and pulmonary roots) • posterior (posterior to the pericardial surface). When describing mediastinal masses, it is important to accurately describe their position, size, character (e.g. containing fat, fluid or calcification) and borders. In almost all cases, CT or MRI is used to help characterize mediastinal masses. CT is of greatest value in the anterior and middle mediastinum, and MRI in the posterior mediastinum (Figure 13).
a Soft tissue mass (arrow) lying towards the apex in the posterior mediastinum. b The mass is seen to be high-signal on this coronal STIR sequence MRI scan. This is a characteristic feature of benign neurogenic tumours.
Figure 11 Lymphangitis carcinomatosis on high-resolution CT. The patient has previously undergone left mastectomy. The interlobar septal thickening (arrows) is characteristic of lymphangitis carcinomatosis.
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Figure 13
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HRCT/MSCT. It is more difficult to make the diagnosis in supine patients, and confirmation by CT is often required in trauma and ICU patients. The presence of a pleural effusion seen on the chest radiograph can be readily confirmed by ultrasonography (Figure 14). If necessary, aspiration or drain placement can be performed at the same time. Both ultrasonography and CT can also be used to further evaluate the character and cause of a pleural effusion. Effusions can be non-loculated and able to move freely within the pleural space, dependent on the patient’s position, or loculated as a result of pleural adhesions. Occasionally, pleural fluid is collected under the lung (subpulmonic) and mimics an elevated hemidiaphragm. In supine patients, pleural effusions produce a general increase in the opacification of the hemithorax, but the vessels remain distinct because they are still surrounded by aerated lung. Primary (mesothelioma) and metastatic pleural tumours are readily investigated using CT (Figure 15), and targeted biopsy can be performed.4 Q3
Figure 15 CT scan showing a right-sided pleural effusion with multiple pleural metastases (short arrows). The long arrow indicates the collapsed (passive) right lower lobe.
Lung cancer
• CT, MRI and ultrasonography have all been used to determine the presence of mediastinal or chest wall invasion. Ultrasonography is the most sensitive method for chest wall invasion. CT and MRI lack sensitivity and specificity in the detection of invasion, and patients should not be denied surgery on the basis of CT or MRI unless the results are categorically positive. • Positron-emission tomography has recently been shown to be valuable in detecting metastatic disease not seen on CT, and in helping determine which patients would most benefit from surgery. It uses a radiolabelled derivative of glucose (18fluorodeoxyglucose; FDG) that is avidly taken up and trapped in malignant cells, where it breaks down to produce a positron that instantly collides with an electron, producing two photons that are detectable as a signal. This is described as “activity or avidity”, with the degree of activity broadly reflecting the metabolic activity of the tumour and this roughly equates to its malignant potential. Very FDG avid lung cancers have a less good prognosis than minimally FDG avid tumours. PET appears to be reasonably specific for malignancy, with only infection and granulomatous disease producing false-positive results but these are often less
Although most lung cancers are seen on the chest radiograph at presentation, a small proportion (2–5%) are visible only on bronchoscopy or CT. Patients with suspected bronchial carcinoma and a normal chest radiograph therefore warrant further investigation, particularly if they present with haemoptysis. It has been shown from screening programme data that, in up to 90% of patients with an asymptomatic carcinoma, the tumour was visible but not reported on prior chest radiographs. CT is required for staging. It should include the entire chest and liver, and is performed after administration of intravenous contrast. Up to 30% of patients have metastatic disease on CT at presentation. CT also helps in determining whether bronchoscopy or imageguided biopsy will best provide a tissue diagnosis (Figure 16). When staging lung cancer, the aim is to triage patients into a group in which surgery may achieve a cure and a group in which surgery would be futile. Assessment of local disease extent, nodal spread and extrathoracic metastatic disease is necessary. • CT is routinely used to assess local disease extent and nodal disease. Mediastinal nodes are readily detected and are regarded as enlarged if more than 1 cm in short-axis diameter.
Figure 16 CT scan showing a bronchogenic carcinoma into which a subsegmental bronchus is passing (arrow), suggesting that a diagnosis could be made on bronchoscopy.
Figure 14 Ultrasound scan showing a pleural effusion containing echogenic fluid (star). The right lower lobe is passively collapsed (arrow).
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Investigations
FDG avid than tumour. The absence of FDG activity in areas of abnormality detected on CT is a good marker for the absence of malignancy. Recently PET scanners have been combined with CT scanners as PET-CT scanners, and this has enabled the production of anatomical and functional (metabolic) information (Figure 17). This combination appears to enable more accurate staging and may also provide prognostic information.5
Suspected pulmonary embolus A plain chest radiograph is seldom diagnostic of pulmonary embolus and is commonly normal. Its major role is in the exclusion of other diagnoses (e.g. pneumothorax, pneumonia, rib fractures), and it is helpful in planning the next appropriate investigation. 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 ‘non-diagnostic’ 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 18). Rapid scanning techniques including multislice CT enable investigation of patients unsuitable for V/Q scanning, and directly visualize thrombus. Multislice CT has been shown to
Figure 18 Spiral CT showing a pulmonary embolus (short arrow) on a conventional axial section, with distal consolidation (long arrow).
be at least as accurate as pulmonary angiography, which is now seldom performed.6
Interventional radiology The advent of more flexible imaging techniques and new needle and drain technology has promoted greater use of image-guided intervention. Image-guided drain placement for pleural effusion has reduced the requirement for surgical treatment of empyema, and lung and mediastinal needle biopsy is increasingly used to establish a tissue diagnosis before thoracotomy and in patients unsuitable for surgery. ◆
References 1 McAdams HP, Samei E, Dobbins J, Tourassi GD, Ravin CE. Recent advances in chest radiography. Radiology 2006; 241: 663–81. 2 Aziz AA, Wells AU, Bateman ED, et al. Interstitial lung disease: effects of thin-section CT on clinical decision making. Radiology 2005; 238: 725–33. 3 Webb WR. Thin-section CT of the secondary pulmonary lobule: anatomy and the image – the Fleischner lecture. Radiology 2006; 239: 322–38. 4 Adams RF, Gleeson FV. Percutaneous image-guided cutting-needle biopsy of the pleura in the presence of a suspected malignant effusion. Radiology 2001; 219: 510–14. 5 Van Tinteren H, Hoekstra OS, Smit EF, et al. Effectiveness of positron emission tomography in the preoperative assessment of patients with suspected non-small cell lung cancer: the PLUS multicentre randomised trial. Lancet 2002; 3569: 1388–92. 6 Schoepf UJ, Costello P. CT angiography for diagnosis of pulmonary embolism: state of the art. Radiology 2004; 230: 329–37.
Further reading British Thoracic Society guidelines on the investigation of pleural disease. Thorax 2003; 58(suppl 2). British Thoracic Society guidelines on the investigation of pulmonary embolic disease. Thorax 2003; 58: 470–83. British Thoracic Society guidelines on the selection of patients with lung cancer for surgery. Thorax 2001; 56: 89–109 Hansell DM, Armstrong P, Lynch DA, McAdams HP. Imaging of diseases of the chest, 4th edn. Chicago: Mosby, 2000.
Figure 17 Coronal image from a PET-CT scan demonstrating a right upper lobe tumour (large arrow) with adjacent involved right hilar nodes (small arrow).
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