nodules, tree-in-bud pattern, and lobular, subsegmental or segmental areas of consolidation [60]. The areas of consolidation may be patchy or conßuent, unilateral or bilateral but usually involve two or more lobes. Segmental atelectasis is common but air bronchograms are infrequent. Common complications are formations of abscess and pneumatoceles (Fig. 12.15). Abscess formation can be identiÞed in about 15Ð30 % of patients [61]. Pneumatocele characteristically increases in size over several days to weeks and is seen more often in children (40Ð75 %) than in adults (15 %) [60, 62].

CT-Pathology Comparisons

Symptoms and Signs

PJP can occur in immunocompromised individuals, especially hematopoietic stem and solid organ transplants, those receiving high-dose corticosteroid therapy and persons with advanced HIV infection [65]. A slow indolent time course with symptoms of pneumonia progressing over weeks to months is characteristic in HIV-infected patients. Fulminant respiratory failure associated with fever and dry cough is typical in non-HIV-infected patients.

CT Findings

The typical histologic pattern of Staphylococcal pneumonia is bronchopneumonia. Because proximal airways are usually Þlled with inßammatory exudates, segmental atelectasis is common, and air bronchograms are infrequent. Staphylococcal pneumonia begins as a focus of consolidation followed by abscess formation, cavitation and pneumatocele formation. Pneumatocele presumably results from drainage of a focus of necrotic lung parenchyma followed by checkvalve obstruction of the airway subtending it, enabling air to enter the parenchymal space during inspiration, but preventing its egress during expiration [63].

On CT of patients with PJP, the prevalence of cysts ranges from approximately 10Ð34 % [42] (Fig. 12.16). Cysts may vary in appearance, with differing shapes and sizes and various degrees of wall thickness. Cysts are usually multiple and bilateral, and they may be found in either a subpleural or an intraparenchymal location. Although upper lobe predominance has been reported, cysts may involve any portion of the lungs. PJP cysts are usually accompanied by various degrees of ground-glass opacity. Cystic pneumocystis pneumonia is associated with an increased incidence of spontaneous pneumothorax, which is believed to occur in association with rupture of subpleural cysts.