Lung consolidation refers to the replacement of air in the air-filled cavities distal to terminal bronchioles by pathological fluids, cells, or tissues. The lesions may involve acini, lobules, segments, and lobes, as well as normal lung tissue with involved multiple acini and lobules. Common pathological changes are inflammatory exudation, edema fluid, blood, granulation tissue, or tumor tissue. Lung consolidation is mostly in lobar pneumonia, bronchopneumonia, and other types of pneumonia, but also in alveolar pulmonary edema, pulmonary contusion, pulmonary hemorrhage, pulmonary infarction, pulmonary tuberculosis, alveolar carcinoma, and fungal diseases.
X-ray
On the chest x-ray film, the range of consolidation varies. If multiple continuous alveoli develop consolidation, a single patchy dense opacity can be seen; while multiple discontinuous consolidations separated by air-filled lung tissue can form multifocal dense opacities. If consolidation occupies a lung segment or an entire lung lobe, a segmental or lobar dense opacity is formed; the center is predominately hyperdense, and the edge is hazy. When consolidation reaches the interlobar pleura, sharp edges can be seen. When consolidation extends to the vicinity of the hilum, large air-containing bronchi are often distinctly different from the consolidated lung tissue, and air-containing bronchial branches can be seen in the consolidation area, which is termed air bronchogram sign. After treatment, inflammatory consolidation can subside within 1 - 2 weeks. During resorption, the lesions are often heterogeneous. Consolidation caused by pulmonary hemorrhage or alveolar edema resolves faster than inflammatory consolidation, and can completely subside within few hours or 1 - 2 days after treatment.
Figure 1 Lung consolidation
a. Chest plain film shows lung consolidation in the right upper lobe with visible air bronchogram sign (↗); b. CT shows lung consolidation in the right lower lobe with visible air bronchogram sign (↗)
CT
On CT, acute consolidation mainly manifested by exudation is homogeneously hyperdense on the lung window, and is an opacity of soft tissue density on the mediastinal window. Air bronchogram sign is often seen in large lesions. The lesions are mostly homogeneous and poorly marginated. If the lesions are close to the interlobar pleura, the edges can be clear. In the early or resorption stage, due to incomplete consolidation, exudative lesions may be hazy ground-glass opacities, in which the pulmonary vascular markings can be seen, and the lesions are not visible on the mediastinal window. Chronic consolidation is often higher than acute consolidation in density, and the edge of the lesion is also often clearer. When consolidation is localized to alveoli, there may be multiple, approximately 1cm, sharply marginated, nodular opacities
MRI
Exudative consolidation shows poorly marginated, patchy, slightly high signal intensity on T1WI and predominately high signal intensity on T2WI. Sometimes, air-filled bronchial intensities and flow voids can be seen in the lesion area, which resembles air bronchogram sign on CT. Signal intensity depends on the amount of protein contained in the exudate. If alveolar proteinosis is characterized by the deposition of protein and lipid in the alveoli, MRI may show fat signal intensity, which is significantly different from the manifestation of other exudative lesions.