PathologyLung infections
Lung infection (pneumonia) may be bacterial, viral or fungal. Various kinds of protozoa can also cause lung infection. In pneumonia, opacities develop in the affected segments of the lung. The opacities may vary
considerably in appearance and distribution. In some types of pneumonia, the opacities consolidate as the disease progresses. The appearance of this type of consolidation may vary considerably, both in a macroscopic lung preparation and in a radiograph, depending on the etiology and distribution. Alveolar consolidation is the most common, and may be segmental or lobar. The appearance varies considerably depending on
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Figure 54.
Bronchopneumonia, right lower lobe, PA (a) and lateral views (b). Relatively homogenous opacity. Air-filled bronchial branches are seen in the opacity.
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whether only single segments, or whole lobes of the lung are affected. The opacities in pneumonia are often bilateral and are often accompanied by collections of fluid in the pleural cavity.
Earlier, it was common to differentiate between the terms lobar pneumonia (Fig. 53 a, b) and bronchopneumonia (Fig. 54). Lobar pneumonia is usually unifocal and concentrated to a single lobe where the opacity will be homogenous with sharp outlines which follow the borders of the affected lobe. The volume of the affected area is not reduced, the bronchial branches may be aerated, and a so-called air bronchogram visible bronchial branches in an opacity - is common. In bronchopneumonia, the opacities are more scattered and are often seen in several lobes at the same time. Atelectasis (volume reduction) is common. The opacities in bronchopneumonia are thus much less homogenous than in lobar pneumonia, and seem less consolidated. The air bronchogram is not normally seen.
It is no longer usual to maintain the differentiation between lobar pneumonia and bronchopneumonia as the radiological picture may vary considerably.
 Figure 55.
Pneumococcal pneumonia - massive opacity left lung with air bronchogram.
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Bacterial infections
Bacterial pneumonia may occur in normal "healthy" individuals, but in today's modem society, bacterial pneumonia often occurs in patients with reduced resistance to infection. Special risk groups will be patients with advanced cancer, nutritional disturbances, or immunodeficiency caused either by immunological disease or various drugs.
Gram positive infections
Pneumococcal pneumonia (Fig. 55)
Streptococci pneumoniae are the bacteria that most often cause lung infections (80-90%). Pneumococci are part of the normal bacterial flora in the respiratory tract, and can be isolated from larynx swabs in up to 60 % of all normal individuals.
Pneumococcal pneumonia is most frequently seen in older, frail patients, alcoholics, and patients with chronic obstructive lung disease. The typical radiological finding is a homogenous, non-segmental, circumscribed consolidation of lung tissue. The opacities are situated anywhere in the lungs, although they are located most often in the basal segments, and are unifocal or multifocal. An air bronchogram is often seen and is considered to be characteristic of pneumococcal pneumonia. The affected segments of the lung are not reduced in volume. Pleural involvement as pleural effusion is not usual.
Small amounts of fluid that can only be diagnosed in lateral decubitus views can, however, be seen in up to 15% of the cases. Empyema is unusual, as is necrosis of the infected lung tissue with abscess formation.
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Figure 56.
a) Staphylococcal pneumonia with mottled multifocal bilateral pulmonary opacities.
b) Staphylococcal pneumonia late stage. The opacities coalesce and become homogenous in character.
c) Staphylococcal pneumonia late stage with consolidated pulmonary opacities. Bilateral pneumothorax has developed.
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Staphylococcal pneumonia (Fig. 56 1 - c)
Staphylococcus aureus is normally present in the nose and throat of 20% of healthy adults. Staphylococcal pneumonia is most often seen in children, frail old people, and in patients who have aspirated stomach contents into the respiratory tract. As staphylococcus aureus produces
 Figure 57.
Staphylococcal pneurnonia, right upper lobe. Volume reduction of the upper lobe is seen as the horizontal fissure has been pulled cranially.
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toxins, the infection frequently spreads to large parts of the lungs. Complications such as lung abscess, empyema, and bronchopleural fistulae are frequently seen.
The radiological appearance is that of typical bronchopneumonia with segmental and mottled opacities, which are usually multifocal and bilateral. In the later stages of the disease, the opacities may coalesce and become homogenous. The volume of the affected lung segment is usually reduced (Fig. 57). An air bronchogram is seldom seen.
Gram negative infections (Fig. 58)
The most common Gram negative infections are caused by Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Haemophilus influenzae, and Proteus. The radiological picture is almost identical, and it is not possible to differentiate between the different bacteria. Gram negative lung infections are most often seen in chronically ill patients who are weakened by other diseases such as alcoholism, chronic lung diseases, diabetes, or different types of cancer. Aspiration to the respiratory tract is the commonest single cause of Gram negative pneumonia.
Opacities caused by Gram negative bacteria are usually localized in the basal lung segments. It is not possible to differentiate these opacities from those caused by staphylococcus aureus. Complications such as empyema and lung abscess are frequently seen (Fig. 59 a, b). The
 Figure 58.
Homogenous consolidated in flltrate right lower lobe. Klebsiella pneumonia.
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Figure 59.
a) Gram negative pneumonia with abscess formation. Considerable volume reduction of the affected lobe. Compensatory emphysema left lung.
b) CT thorax. Gram negative pneumonia. Opacity with abscess formation in right lower lobe. Pleural fluid right side.
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 Figure 60
Tuberculosis with segmental opacities in the left upper lobe.
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mortality in Gram negative pneumonia is relatively high in spite of adequate treatment with antibiotics.
Anaerobic infections
The most common anaerobic infection, tuberculosis, is caused by mycobacterium tuberculosis. Tuberculosis is most often seen as limited epidemics in densely populated are as. The primary infection is almost always due to inhalation of mycobacterium tuberculosis, usually to the middle or basal segments of the lung. After this, spread of bacteria occurs internally in the lung, possibly also via lymph channels, to lymph nodes in the mediastinum. The immunity reaction tends to encapsulate the actual tubercle bacillus, which then enters an inactive "dormant" stage. The tubercle bacilli are most often implanted in areas of the lung with a high oxygen tension, usually in the apical segments (Fig. 60). At this stage, the tuberculous opacity looks like a segmental or lobar opacity with
 Figure 61.
Reactivated old tuberculosis with cavitation and fluid levels in the left upper lobe.
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 Figure 62.
Reactivated old tuberculosis with fibrous, striped opacities and less extensive infiltrates in right upper lobe.
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consolidated lung tissue, Accompanying enlargement of lymph nodes in the hilus is usual. Sometimes pleural effusion occurs. These changes often disappear spontaneously, sometimes after chemotherapy. The consolidated lung tissue shrinks so that the remaining lesion is a pulmonary nodule, which may become calcified at a later stage. The same process takes place in the lymph nodes in the hilum. The combination of peripheral pulmonary calcification and calcification of the hilar lymph nodes is called a primary complex, and is a typical finding in primary tuberculosis. It can be seen in about 50 % of infected patients.
Reactivation of old tuberculosis infection (Fig. 61) may occur many years after the primary infection, and is most frequently localized to the apical and posterior segments of the upper lobes and to the superior segments of the lower lobes. It is se en as a mottled, poorly defined alveolar infiltrate, often with the formation of cavities. In most cases, only one lung is involved, but bilateral changes are seen in the more advanced cases. At this stage, the lung changes are very variable with cavities, single or multiple nodular opacities, pleural fluid, large consolidating infiltrates (both lobar an diffusely scattered), or pneumothorax (Fig. 62). The great variability in the radiological picture at this stage makes it difficult to make a diagnosis on the basis of the radiological picture alone. Infectious diseases and lung cancer may be relevant differential diagnoses.
Tuberculosis occurs infrequently outside the lungs. Tuberculous lymphadenopathy is common, as is involvement of the visceral and parietal pleura, the pericardium, and the peritoneum. The tuberculous infection may be disseminated and involve both lungs (miliary tuberculosis). In these cases characteristic changes are seen with small miliary opacities diffusely scattered over both lungs.
Tuberculomas in the lungs are seen both in the primary and the later phases of the disease. A tuberculoma represents a limited, localized condition of the parenchyma which heals and later shrinks. Finally, a sharply limited node-like opacity, usually 1-5 cm in diameter, is seen. This is usually localized in the upper segment of the lung. Tuberculomas are usually solitary, but may be multiple and often calcify. Formation of cavities is rare. The differential diagnosis between tuberculoma and a primary lung tumor, or possibly a metastasis may be difficult.
The lung changes in tuberculosis will improve after adequate treatment, but often leave considerable changes with fibrosis, infiltrates, calcification, shrinking, and pleural thickening and pleural adhesions (Fig. 63). Response to treatment is indicated by shrinking of infiltrates, reduced wall thickness of the cavities, and fibrosis.
However, any residual tuberculous process may become reactivated at any time in the later stages of the disease. Reactivation often occurs many years after the initial primary tuberculous infection.
Before chemotherapy of tuberculosis became usual, pulmonary tuberculosis was often treated surgically. One of the most common types of operation was thoracoplasty, where the upper ribs were removed in order to collapse the upper segments of the lung, where the disease was most frequently located (Fig 64). Other forms of treatment not infrequently used were induction of pneumothorax and injection of
 Figure 63.
Old tuberculosis, predominantly right-sided, with fibrous infiltrates, calcification, pleural thickening with adhesions, and shrinking.
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 Figure 64.
Left-sided thoracoplasty with resection of cranial ribs.
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sclerosing substances into the pleural cavity.
Viral and viral-like infections
Viral pneumonia and pneumonia with viral-like aetiology are common in children and young people, but are less frequent in adults. In children, pneumonia is most often caused by the respiratory syncytial virus and parainfluenza virus. In older children and adults, non-bacterial pneumonia is most often caused by mycoplasma pneumoniae, less often by the
 Figure 65.
Varicella pneumonia; alveolar opacities symmetrical distributed in both lungs.
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adenovirus and parainfluenza virus.
Influenza virus
There are a series of different viruses of this type (influenza A, B, and C), with several variants within each group- The antigen component may thus vary considerably, and immunological resistance to the influenza virus is therefore infrequent. In uncomplicated influenza, the chest radiograph is normal. In complicated lung infections caused by the influenza virus, signs of bilateral pneumonia with pronounced consolidation of the lung parenchyma are usually seen. Bacterial pneumonia is often seen in connection with influenza. Radiologically, it is not possible to differentiate between pneumonia caused by the influenza virus and bacterial pneumonia.
In rare cases, a number of other types of virus such as parainfluenza, respiratory syncytial-, adeno-, rhino-, and herpes viruses may cause pneumonia. None of these types have special radiological characteristics, and it is not possible to differentiate these infections from bacterial pneumoma.
Varicella virus (Fig. 65)
Varicella and herpes zoster are caused by the same virus (varicella-zoster virus), and occur mainly in children (varicella) and in older patients (herpes zoster). The lung picture shows mottled alveolar opacities, which can vary in size from node-like opacities to extensive changes which may involve most of the lungs. The opacities are usually from 5-10 mm in
 Figure 66.
Mycoplasma pneumonia pneumonic opacities in both lower lobes, most pronounced on the right side.
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diameter, and normally disappear within a week. In a few patients, the opacities may persist for several months. In about 2 % of the patients, the opacities may resemble small nodules which calcify causing permanent changes.
Mycoplasma pneumonia
Mycoplasma pneumonia is most often seen in children and young people. Up to 15 % of all cases of pneumonia in patients younger than 40 years are caused by mycoplasma pneumoniae. Most mycoplasma infections are manifested clinically as bronchitis and/or pharyngits. Pneumonia develops in between 3 and 10% of the patients.
The radiological picture is extremely variable, but one or both lower lobes are usually involved. The opacities usually start as partly mottled, partly node-like peribronchial opacities, which may gradually develop to involve whole segments or lobes. Consolidation is frequently seen (Fig. 66). This typical progression is often seen in mycoplasma pneumonia, and the diagnosis may therefore sometimes be made on the radiological picture alone. The upper lobes are seldom affected. A volume reduction of the affected segments or lobes of up to 10% is usual. Pleural effusion and abscess formation are seldom seen. The opacities that develop with mycoplasma pneumonia usually take far longer to disappear than opacities caused by bacteria or viruses.
 Figure 67.
Actinomycosis - pneumonic opacity, right up per lobe, with volume reduction of the upper lobe.
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Fungal pneumonia, protozoal pneumonia
The commonest of this type of lung infection is caused by Histoplasma capsulatum, and is most frequently seen in USA. The radiological changes in the lungs caused by this bacteria have no typical characteristics, but have many features in common with tuberculosis both in the early active and in the late stages.
The opacities are non-specific, and in most cases they heal with fibrosis and calcification. In late stages with extensive calcification, the changes may be difficult to differentiate from the calcification which may occur after varicella pneumonia.
Other fungal infections such as cryptococcosis, nocardiosis, blastomycosis, and actinomycosis (Fig. 67) are very seldom seen.
Aspergillus infections are encountered a little more frequently, and most often as secondary changes in lung cavities.
The most important protozoal infections are caused by pneumocystis carinii (see lung diseases with immunosuppresion), toxoplasma gondii and entamoeba histolytica. There are also a number of metazoan infections caused by different organisms, of which the most usual are echinococci (Fig. 68 a, b).
The radiological changes seen in diffuse generalized pulmonary disease can be divided into two main groups - alveolar and interstitial opacities.
Alveolar pulmonary opacities affect the aerated segments of the Iungs - the trachea, bronchi and alveoli. These changes may be found only in the alveolar area, but larger parts of the aerated bronchial tree including
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Figure 68
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PA and lateral views - echinococcal cyst right lower lobe. Sharply defined homogenous opacity. Minimal amounts of pleural fluid.
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the alveoli will usually be involved.
Alveolar opacities, opacities with air bronchogram, and atelectasis are thus all variants within the same group, i.e. involvement of the aerated parts of the lung.
lnterstitial opacities can be divided into four main types - reticular, nodular, reticulonodular, and linear. These are changes seen in the lung tissue itself - i.e. the lung tissue located between the alveoli (in the interstitium). Reticular opacities resemble a network, and are often graded from fine reticular to coarse reticular. Fine reticular changes can be compared with the network seen in nylon stockings, while coarse reticular opacities are characterized by larger, almost cyst-like cavities of 1 cm or more in diameter, with a rough, thickened periphery surrounding the cavities. Nodular opacities are seen when more node-like lesions arise in the interstitium. These opacities are homogenous, well-defined, and many vary considerably in size.
Reticulonodular opacities may be seen when several nodular opacities coalesce forming network-like opacities. Linear, striped opacities may represent thickening of interalveolar clefts or interlobular septa (Kerley's A and B lines). In interstitial opacities, a combination of these four main types is often seen.
Alf Kolbenstvedt, Arnulf Skjennald and Charles B. Higgins