Pulmonary aspergillosis is a spectrum of mycotic diseases caused by the Aspergillus species, usually A fumigatus. [1, 2] This intensely antigenic and ubiquitous soil fungus is commonly found in the sputum of healthy individuals. However, in susceptible hosts, its ability to invade the arteries and veins facilitates its hematogenous spread.
The development of disease and its histologic, clinical, and radiologic manifestations depend on the virulence and number of spores inhaled and, more importantly, on the patient's immune status. 
Allergic bronchopulmonary aspergillosis (ABPA) - Caused by a hypersensitivity reaction to the fungus; most commonly occurs in persons with asthma
Saprophytic aspergillosis, or aspergilloma - The most common form; noninvasive and involves colonization of preexisting cavities
Chronic necrotizing aspergillosis (also called airway-invasive or semi-invasive aspergillosis) - A chronic, cavitary, pneumonic illness that often affects patients with preexisting chronic lung disease
Angioinvasive aspergillosis - Affects immunocompromised patients and is often fatal
Chest radiography is an initial examination of choice in patients with respiratory symptoms or suspected pulmonary disease. [4, 5] However, many different causes of bronchiectasis, including ABPA, cannot be accurately diagnosed on chest radiographs. Also, radiographic features of pulmonary aspergillosis are generally nonspecific.
Although the computed tomography (CT) scan features of ABPA are not specific, the demonstration of bronchial dilatation, wall thickening, and centrilobular nodules in an asthmatic patient should suggest the diagnosis.  The demonstration of a mobile mass within a cavity on supine and prone scans is virtually diagnostic of a mycetoma.
The CT scan appearances of chronic necrotizing aspergillosis are also nonspecific, but CT does provide useful information regarding the extent of pulmonary disease and any associated pleural thickening. CT scan findings in angioinvasive aspergillosis are more specific, and the presence of nodules with a halo of ground-glass attenuation in the appropriate clinical setting allows confident diagnosis.
Limitations of techniques
The appearances of the different types of thoracic aspergillosis are nonspecific, and a wide variety of lesions can mimic an aspergilloma. Examples of these include chronic necrotizing aspergillosis, angioinvasive aspergillosis, a tuberculous cavity with a Rasmussen aneurysm, cavitating bronchogenic carcinoma, lung abscess, hematoma, and Pneumocystis (carinii) jiroveci pneumonia. Similarly, bronchial dilatation has a variety of causes.
In allergic bronchopulmonary aspergillosis (ABPA), chest radiographic appearances include the following: (1) fleeting alveolar subsegmental or lobar infiltrates, which are usually bilateral (65%) and predominant in the upper lobes (50%); (2) central 1- to 2-cm ring shadows that represent varicose or cystic bronchiectasis; and (3) tram-link bronchial walls caused by edema. [1, 4, 5] The second-order bronchi may become plugged with mucus, and they may be visible as 2.5- to 6-cm long V- or Y-shaped branching tubular opacities that may grow over time and persist for months; this is the so-called finger-in-glove sign (see the image below).
Other features include lobar consolidation, atelectasis, postobstructive pneumonia, cavitation, air trapping, and parenchymal scarring or fibrosis, all of which are more pronounced in the upper lobes. Focal pleural thickening is also reported. Occasionally, mycetomas develop in ectatic bronchi.
The characteristic chest radiographic appearance of an aspergilloma is that of a round or oval mass with the opacity of that of a soft-tissue mass. Often, an adjacent crescent-shaped air space (ie, the air-crescent sign) separates the fungal ball from the cavity wall (see the image below).
The mycetoma may rarely contain amorphous or rimlike calcification. The fungal ball is usually mobile and moves when the patient changes position. Often, extensive, adjacent, apical pleural thickening may be present; this finding may herald the development of the mycetoma (see the image below).
The radiologic manifestations of chronic necrotizing aspergillosis include unilateral or bilateral segmental areas of consolidation that are predominant in the upper lobes; frequently, these progress to cavitation (see the image below). Pleural thickening is also a recognized feature.
The most common chest radiographic appearance of invasive aspergillosis is that of patchy areas of consolidation, which progress despite the use of broad-spectrum antibiotics. Multiple nodules and peripheral, wedged-shaped lesions caused by hemorrhagic infarcts are also observed as the disease progresses. These frequently become cavitated, and an air-crescent sign that mimics mycetoma may also be observed.
Degree of confidence
The many different causes of bronchiectasis, including ABPA, cannot be accurately diagnosed with radiographs. Also, the radiographic features of pulmonary aspergillosis are generally nonspecific.
Cortese et al were able to show the limited usefulness of conventional chest radiographs in the diagnosis of ABPA in patients with cystic fibrosis.  The authors found the most significant abnormalities were nonspecific, and they were commonly seen in cystic fibrosis without ABPA that persisted after treatment in most cases.
An air-crescent sign may occur in the following: aspergilloma, chronic necrotizing aspergillosis, angioinvasive aspergillosis, tuberculous cavity with a Rasmussen aneurysm, cavitating bronchogenic carcinoma, lung abscess, hematoma, and P (carinii) jiroveci pneumonia.
Bronchiectasis and peribronchial thickening are the most common CT scan findings in allergic bronchopulmonary aspergillosis (ABPA; see the image below). Appearances tend to be more severe than in those of chronic, uncomplicated asthma. [7, 8]
ABPA typically involves the segmental and subsegmental bronchi, particularly those in the upper lobes. However, studies have shown that central bronchiectasis simply indicates long-standing severe inflammation; as a marker, it is not as specific for ABPA, as was once thought.
High-attenuating mucoid impaction, present in as many as 30% of patients, is a characteristic finding. One study suggests that a CT density value of 70 Hounsfield units is an adequate cutoff value for high-attenuation mucoid impaction.  High-attenuation mucoid impaction is associated with initial serologic severity and frequent relapses, but it does not influence complete remission. [9, 10] Occasionally, lobar or segmental atelectasis may be a feature. Mucus plugging of the small airways can be observed on high-resolution CT scans, with resultant centrilobular nodularity and the tree-in-bud sign.  Abnormalities of lung attenuation due to either mosaic perfusion or air trapping may also be identified. Scans obtained during expiration are useful in differentiating the findings in this instance.
The CT scan and chest radiographic appearances of an aspergilloma are similar (see the first 2 images below). The fungal ball is seen as a mass of soft-tissue attenuation within a pulmonary cavity. An anterior air crescent is visible if the patient is supine (see the third image below). The mobile nature of the mass can be demonstrated by scanning the patient in the prone position; the fungal ball falls to the dependent portion of the cavity. The cavity wall and adjacent pleura are frequently thickened, although these findings have been shown to resolve with successful treatment or with the spontaneous resolution of the infection.
CT scan findings in chronic necrotizing aspergillosis include areas of chronic, progressive, peripheral consolidation; multiple nodular opacities; and low-attenuating, masslike lesions. Abnormalities may be unilateral or bilateral, with an upper-lobe predilection. Cavitation is a common feature, and this often leads to the development of an intracavitary segment of sequestrated lung, which may mimic a mycetoma. Extension into the chest wall and mediastinum are also described.
CT scan findings of angioinvasive aspergillosis include multiple nodules associated with a halo of ground-glass attenuation, which represents adjacent hemorrhage, and pleural-based, wedge-shaped areas of consolidation, which correspond to hemorrhagic infarcts. The air-crescent sign may be observed in the recovery phase.
Degree of confidence
Although the CT scan features of ABPA are not specific, the demonstration of bronchial dilatation, wall thickening, and centrilobular nodules in an asthmatic patient should suggest the diagnosis. The presence of ABPA is even more likely if bronchiectasis is severe, affects 3 or more lobes, and has a central distribution. Many pulmonary lesions become cavitated; however, the demonstration of a mobile mass within a cavity on supine and prone scans is virtually diagnostic of a mycetoma.
CT scan appearances of chronic necrotizing aspergillosis are nonspecific, although CT scanning does provide accurate information regarding the distribution and extent of pulmonary disease and any associated pleural thickening.
CT scan findings in angioinvasive aspergillosis are more specific, and the presence of nodules with a halo of ground-glass attenuation in the appropriate clinical setting allows confident diagnosis.
The causes of false-positive and false-negative results are the same as those with chest radiography. In one report  , high-resolution chest CT was negative in 22 of 60 patients with glucocorticoid-naive ABPA.