The diagnostic sensitivity, specificity, positive predictive value, and negative predictive value of high-resolution CT for Pneumocystis carinii pneumonia are 100 %, 83.3 %, 90.5 %, and 100 %, respectively. And the most characteristic demonstration is intrapulmonary ground glass opacity, which represents the early stage of alveolar infiltration (Fig. 5.14).

It was once believed that lung air sac is uncommon in patients with HIV/AIDS complicated by Pneumocystis carinii pneumonia. However, with the development of detection techniques, the occurrence of lung air sac in patients with HIV/AIDS complicated by PCP is increasing, with an incidence rate of 10–34 %. It is, therefore, another common CT demonstration in addition to ground glass opacity characteristically of Pneumocystis carinii pneumonia. The demonstration of lung air sac in such patients is diverse, with thin sac wall, intact and smooth lining and outer wall, no liquid, and other substances in the sac cavity. The lung air sac may be demonstrated as singular, multiple, or honeycomb-like lesion that is commonly located at the subpleura or pulmonary parenchyma. The lung air sac is commonly distributed at the upper lung lobes, possibly with any lung lobes involved (Fig. 5.15). The mechanism underlying its occurrence remains unknown, which is speculated to be a pulmonary parenchymal response to Pneumocystis carinii. The response exhibits pulmonary alveolar or interstitial inflammation and different degrees of fibrosis that leads to remodeling of lung structures to develop cystic or honeycomb-like lesion. Lung air sac may occur at any stage of infection, commonly surrounded by different degrees of ground glass opacity. After effective treatment, lung air sac can repair itself and be cured.

The atypical demonstrations of Pneumocystis carinii pneumonia are various and non-characteristic, which present challenge for its radiological diagnosis. The abnormalities at pulmonary parenchyma include:

Lu PX et al. categorized Pneumocystis carinii pneumonia into 5 types based on its CT demonstrations, including ground glass opacity type, lung air sac type, consolidation type, interstitial type, and mixed type. The ground glass opacity type is the most common characteristic, with symmetrically distributed opacities with increased density in the lungs and bronchovascular bundles inside. The lung air sac type shows thin-wall air sac, with intact and smooth lining and outer wall but no liquid level in the sac. The sac is commonly surrounded by different degrees of ground glass opacity and flaky infiltrative opacity. Lung air sac type has multiple lesions at both lungs that are more commonly found at upper lung lobes (Fig. 5.16). The consolidation type displays uneven patches of opacities at both lungs and segmental pneumonia with one lobe or segment as a unit. The interstitial type displays thickened interlobular septum and bilaterally asymmetric diffuse reticular or network-like nodular opacities, which may be complicated by consolidation (Fig. 5.17). The mixed type shows concurrence of above lesions, which is commonly found in patients at the advanced stage.

The categorization entails the pathological changes and the dynamic CT findings of Pneumocystis carinii pneumonia, which is significant for the diagnosis and differential diagnosis of Pneumocystis carinii pneumonia.

Hilar and mediastinal lymphadenectasis may be singular or multiple. In the cases with multiple hilar and mediastinal lymphadenectasis, the lesions are extensive and may integrate into mass. By CT scan, 70–90 % of patients with primary tuberculosis have hilar and mediastinal lymphadenectasis, and the calcification rate of enlarged lymph nodes is much lower than that of simplex tuberculosis. CT plain scan demonstrated the enlarged lymph nodes with low density. By contrast scan, marginal ring-shaped enhancement is commonly shown, with central low-density caseous necrosis (Fig. 5.18). Intrapulmonary infiltration commonly occurs at vulnerable sites of non-simplex tuberculosis, such as the lower lung lobes or right middle lung lobe. Multiple lung segments and lobes may be concurrently involved. The lesions progress rapidly that are commonly exudative consolidation. In some cases, signs of bronchial dissemination may be demonstrated, such as acinar nodular opacity and tree-bud sign. However, the signs of fibrosis and calcification are rarely shown.

At acute and asymptomatic stages of HIV infection, the radiological demonstration of secondary tuberculosis resembles to tuberculosis in patients with no HIV infection due to slightly suppressed immunity of the patients. The lesions are often located at the apicoposterior segment of the upper lung lobe or dorsal segment of the lower lung lobe, displayed as flaky focal consolidation and nodule opacities. Cavity has an incidence rate of 20–25 %, while lymphadenopathy and extrapulmonary lesions have an incidence rate of 10–15 %. Along with continual decrease of peripheral CD4 + T-cell count, patients with AIDS experience increasingly serious cellular immunodeficiency. Therefore, the complication of tuberculosis may be radiologically demonstrated as more extensively severe and atypical lesions. The lesions are radiologically characterized by no definite location, diversified morphology, common progression into thin-wall or irregular no-wall cavity, and common occurrence of hilar and mediastinal lymphadenectasis. CT scans show involvement of multiple lung lobes or segments. Lesions with various morphologies can be simultaneously demonstrated, including infiltrations and military and acinar nodule dissemination. In addition, caseous pneumonia is common (Fig. 5.19). Radiologically, the lesions are generalized into “three multis and three rares”: concurrence of multiple lesions (including exudation, proliferation, and cavity), multiple morphology, and multi-segmental distribution but rare occurrence of fibrosis, calcification, and mass-like opacity. The cavity may be singular or multiple and multiple cavities are more common. The cavities commonly scatter within the lungs, especially the upper and middle lung lobes (Fig. 5.20). Lymphadenectasis at lung hilus and mediastinum has a high incidence rate (Fig. 5.21), which is 17–69 % in patients with AIDS complicated by tuberculosis, but only 3–23%in patients with simplex tuberculosis. CT plain scan showed enlarged lymph nodes with homogeneous and slightly low density. By contrast CT scan, irregular ring-shaped enhancement can be displayed but no enhancement of the central caseous necrosis.

It is a common deep fungal infection and commonly concurs with meningitis. Radiologically, the lesions are characterized by various shapes, various kinds, and multiple different-sized foci.

By CT scan, pulmonary cryptococcosis manifests solitary mass-like opacity, singular or multiple nodular opacities, singular or multiple patches of infiltration opacity, diffusive miliary nodule opacity, interstitial pneumonia, and diffusive mixed lesions. The nodular opacities are commonly well defined but morphologically irregular with different sizes and a diameter of about 5–60 mm or larger. Some of the nodular opacities are lobulated and have spikes. The patches of infiltration opacity may be demonstrated as small strips or flakes with the singular or multiple lung lobes involved. The opacity is poorly defined with inhomogeneous density, possibly with observable bronchus sign or vacuole sign. By HRCT, the diffuse miliary nodules are displayed as poorly defined acinar nodules with homogeneous density and a size of 3–5 mm, which resemble to lesions of pneumonia or miliary tuberculosis. Diffuse mixed lesions refer to coexistence of nodules, patches, mass, and lobar consolidation, in which solitary or multiple lung nodules are the most common sign by CT scan. The second common signs include mass and consolidation. The rare CT signs include cavity, calcification, hydrothorax, and hilar/mediastinal lymphadenectasis (Fig. 5.22).

Simplex involvement of the lungs is rare, accounting for about 5–10 %. About 40 % of the patients also experience pleuritis.

In patients with AIDS, histoplasmosis may be hematogenously disseminated to involve multiple organs and induce pneumonia. The condition is commonly accompanied by systemic lymphadenectasis, hepatosplenomegaly, meningitis, pericarditis, or endocarditis.