Introduction

Pneumonia is the sixth most common cause of death, and the leading cause of death from infection in the US. The diagnosis of pneumonia requires combination of careful clinical evaluation, appropriate laboratory investigations including microbiological tests and radiological confirmation of pneumonia. The chest radiograph is the cornerstone of radiological diagnosis, and the imaging modality of choice in establishing the presence of pneumonia, including its severity and extent. The efficacy of the clinical examination in detecting pneumonia is acknowledged to be less sensitive, with auscultatory evidence of pneumonia reported to be absent in a quarter of patients. In addition, Osmer and Cole found that stethoscopic findings were often not in concordance with radiographic findings.

As this book deals with imaging of severe acute respiratory syndrome (SARS), a predominantly pneumonic illness, this chapter serves to provide a background to the radiological appearances of pneumonias of different aetiology. The main radiographic feature of any pneumonia is consolidation, defined as an opacity that obscures vascular markings, which can range from small ill-defined areas to larger areas involving one or more lobes. The pulmonary acinus is the smallest airspace unit visible on the radiograph (4-9 mm in diameter) and is that part of the lung distal to the terminal bronchiole including respiratory bronchioles, alveolar ducts and alveolar sacs.