Airspace disease is usually caused by bacterial infections. However, it can be seen in viral, protozoal, and fungal infections as well as malignancy, typically brochioloalveolar carcinoma [37]. Acute airspace pneumonia is characterized by a mostly homogeneous consolidation of lung parenchyma and well-defined borders; it does not typically respect segmental boundaries. An air bronchogram is very common. Progression to lobar consolidation may occur. As with lung nodules, whether consolidation is detected or missed on the plain chest radiograph may depend on a combination of the same factors of size, density, location, and overlying structures. Location is a significant factor for missed consolidations [38, 39]. Consolidations in the middle lobe and both lower lobes can be difficult to diagnose, especially when only the posterioranterior view is obtained [40]. Figure 4 shows the radiographs from a 46-year-old woman with cough and right-sided chest pain. The posteroanterior radiograph was interpreted as normal (Fig. 4a). Due to a clinical suspicion of pulmonary embolism, MDCT was requested, which showed consolidation in the anterior segment of the right lower lobe. The coronal and sagittal reformats demonstrated the extent of the consolidation (Fig. 4b, c). There were no signs of pulmonary embolism on the contrast medium study. A diagnosis of right lower lobe pneumonia was established and the patient was treated successfully with antibiotics.

Chest radiography is the most frequently performed diagnostic investigation requested by general practitioners in Europe [41]. Chest radiography is considered the gold standard for the diagnosis of pneumonia. It can be used to diagnose pneumonia when an infiltrate is present and to differentiate pneumonia from other conditions that may present with similar symptoms, such as acute bronchitis. The results of the chest radiograph may in some cases suggest a specific etiology (for example, a lung abscess) or reveal a complication (empyema) or coexisting abnormalities (bronchiectasis, bronchial obstruction, interstitial lung disease). Chest radiography remains a valuable diagnostic tool in primary-care patients with a clinical suspicion of pneumonia as it substantially reduces the number of misdiagnosed patients [42]. MDCT can help in differentiating infectious from non-infectious abnormalities and may detect empyema, cavitation, and lymphadenopathy when the chest radiograph cannot. MDCT imaging is useful in patients with community-acquired pneumonia when there is an unresolving or complicated chest radiograph and should be performed in immunocompromised patients with a clinical suspicion of pneumonia when the chest radiograph is normal. This is especially true when the early diagnosis of pneumonia is critical, as is the case with immunocompromised and severely ill patients [43].

Figure 5 shows the imaging studies from a 38-year-old immunocompromised man with fever. The posteroanterior and lateral radiographs (Fig. 5a, b) were interpreted as showing a peri-hilar reticular infiltrate with right upper lobe consolidation. MDCT was requested to further characterize the infiltrate and revealed ground-glass opacification with bilateral lung cysts (Fig. 5 c–e). Pneumocystis pneumonia was confirmed by bronchoalveolar lavage.

Diffuse lung disease presenting with widely distributed patchy infiltrates or interstitial reticular or nodular abnormalities can be produced by a number of disease entities. An attempt is usually made to separate the group of idiopathic interstitial pneumonias from known causes, such as infections, associated systemic disease, or drug-related. The most common infectious organisms are viruses and protozoa. In general, the etiology of an underlying pneumonia cannot be specifically diagnosed because the patterns overlap. It is beyond the aim of this chapter to discuss in detail the contribution of MDCT to the diagnosis of diffuse infiltrative lung disease. The development of HRCT has resulted in markedly improved diagnostic accuracy in acute and chronic diffuse infiltrative lung disease [44–47]. The chest radiograph remains the preliminary radiological investigation of patients with diffuse lung disease but is often non-specific. Pattern recognition in diffuse lung disease has been the subject of controversy for many years. Extensive disease may be required before an appreciable change in radiographic density or an abnormal radiographic pattern can be detected on the plain chest radiograph. At least 10% of patients who are ultimately found to have biopsy-proven diffuse lung disease have an apparently normal chest radiograph [48]. HRCT, and now MDCT, have become integral components of the clinical investigation of patients with suspected or established interstitial lung disease. These techniques have had a major impact on clinical practice [49, 50].

The deep sulcus sign (Fig. 6) is seen on chest radiographs obtained with the patient in the supine position [51]. It represents lucency of the lateral costophrenic angle extending toward the abdomen. The abnormally deepened lateral costophrenic angle may have a sharp, angular appearance. In the supine patient, air in the pleural space (pneumothorax) collects anteriorly and basally within the nondependent portions of the pleural space; when the patient is upright, the air collects in the apicolateral location. If air collects laterally rather than medially, it deepens the lateral costophrenic angle and produces the deep sulcus sign. In Fig. 6, a deep sulcus sign is seen on the left, in addition to a continuous diaphragmatic sign, present when there is air between the diaphragm and the heart.

On the normal lateral chest radiograph, the attenuation decreases (the lucency increases) as one progresses down the thoracic vertebral bodies [52]. If the attenuation increases, locally or diffusely, there must be a posteriorly located lesion (Fig. 7). This lesion might not be seen on the frontal view, as it might be hidden by the heart or the hila.

In a chest X-ray, non-visualization of the border of an anatomic structure that is normally visualized shows that the area neighboring this margin is filled with tissue or material of the same density [53]. The silhouette sign is an important indicator of the presence and localization of a lesion.